Daniele Basso

Francesco Patuzzi

Daniele Castello

Marco Baratieri

Elena Cristina Rada

Elsa Weiss-Hortala

Luca Fiori
email: luca.fiori@unitn.it

Agro-industrial waste to solid biofuel through hydrothermal carbonization

Keywords:

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Nowadays, the need for energy is a strong and relevant issue worldwide. The international community agrees that a strong effort should be made looking for alternative energy resources, capable to lower as much as possible the amount of fossil fuels from the energy portfolio and to develop a sustainable supply chain. At the same time, another important issue concerns waste management. As a matter of fact, proper waste management and treatment may allow both greenhouse gases (GHGs) mitigation and possibilities for bioenergy production [START_REF] Pyo | Developing a process-level vulnerability assessment index and evaluating mitigation strategies of GHG emission in waste treatment plants for climate change[END_REF][START_REF] Rada | Energy from municipal solid waste[END_REF]. These constraints are a strong driving force for the investigation on new and affordable technologies, able to combine proper and sustainable waste management with the production of clean and renewable energy.

During the last decades, many processes have been proposed which could successfully address such issues. Such technologies can be classified as biological or thermochemical. In the general case, biological processes are quite sensitive with respect to the inlet feedstock as, potentially, microorganisms could be easily inhibited by toxic substances present in the substrate. Additionally, they need very long residence times (i.e. from one day to several weeks). Therefore, besides being slower, these processes entail bigger volumes, which cause higher design and manufacturing costs. On the contrary, the time scale of thermochemical processes generally ranges from a few minutes to some hours. Moreover, since such processes do not rely on microorganisms, they can overcome the limitations due to chemical lag and the possible inhibition by toxic agents, as described above. This is a noticeable advantage for the treatment of heterogeneous feedstock or substrates with a low degree of purity (e.g. organic wastes, sewage sludges, etc.). Several thermochemical processes have been proposed and discussed for the treatment of different kind of biowaste [START_REF] Khoo | Life cycle impact assessment of various waste conversion technologies[END_REF][START_REF] Iakovou | Waste biomass-to-energy supply chain management: a critical synthesis[END_REF][START_REF] Devesa-Rey | Valorization of winery waste vs. the costs of not recycling[END_REF].

Referring to the type of feedstock, thermochemical processes can be divided into two main groups: dry and wet processes. The former, such as combustion or pyrolysis, are suitable when the feedstock has low water content. For example, several authors have discussed the application of these processes to woody agricultural biomass waste [START_REF] Barbieri | Management of agricultural biomass wastes: preliminary study on characterization and valorisation in clay matrix bricks[END_REF] or municipal solid wastes [START_REF] Ragazzi | Multi-step approach for comparing the local air pollution contributions of conventional and innovative MSW thermo-chemical treatments[END_REF][START_REF] Abnisa | A review on co-pyrolysis of biomass: an optional technique to obtain a high-grade pyrolysis oil[END_REF][START_REF] Chen | Pyrolysis technologies for municipal solid waste: a review[END_REF]. When the feedstock presents high water content (i.e. moisture higher than 60%), hydrothermal processes become more appropriate. These processes make use of hot pressurized water to convert wet substrates. Pressure is always held high enough to keep water in its liquid or, possibly, supercritical state. Among them, a classification on the basis of the process temperature and the type of obtained products can be done. When the temperature values range between 180 °C and 250 °C, the main product is a solid material (referred to as hydrochar) and the process is called hydrothermal carbonization (HTC) [START_REF] Castello | Supercritical water gasification of hydrochar[END_REF]. If the temperatures are increased from 250 °C to about 373 °C, hydrothermal liquefaction (HTL) is performed, with the main production of a liquid phase [START_REF] Elliott | Hydrothermal liquefaction of biomass: developments from batch to continuous process[END_REF]. Finally, rising the temperature and the pressure above the critical values for water (373.95 °C and 22.06 MPa), the process is called hydrothermal gasification (HTG) or supercritical water gasification (SCWG), which allows for a combustible gas as main product [START_REF] Castello | Supercritical water gasification of hydrochar[END_REF]. The application of these hydrothermal processes to several typologies of biomass has been discussed by many authors (Fiori et al., 2012a;[START_REF] Lu | Thermal conversion of municipal solid waste via hydrothermal carbonization: comparison of carbonization products to products from current waste management techniques[END_REF][START_REF] Pala | Hydrothermal carbonization and torrefaction of grape pomace: a comparative evaluation[END_REF][START_REF] Xiao | Hydrothermal carbonization of lignocellulosic biomass[END_REF][START_REF] Subagyono | Pressurized thermal and hydrothermal decomposition of algae, wood chip residue, and grape marc: a comparative study[END_REF][START_REF] Yedro | Hydrothermal fractionation of grape seeds in subcritical water to produce oil extract, sugars and lignin[END_REF].

The type of feedstock is another important aspect to be considered in order to guarantee an appropriate degree of both sustainability and profitability. In particular, the ready availability of the biomass on the territory around the treatment plant, as well as its cost, is crucial. As a matter of fact, when a substrate is produced as a by-product of agricultural and industrial activities established nearby the conversion plant, its supply and transportation costs are lowered, which is beneficial to the overall economic balance of the energy production process. Additionally, such choice improves the sustainability of the process by reducing the polluting emissions due to biomass transportation over long distances. Moreover, if the company that produced that by-product is also in charge of its disposal, the possibility to collect this by-product as a feedstock for energy purposes can represent an interesting, viable and advantageous alternative.

In the region in which the present research group is operating (i.e. the North-East of Italy), grape marc is widely available, as it is an important by-product of the wine making industries. Grape marc already has a utilization for the production of spirits through distillation. However, downstream of such process, most residual grape marc is still found as a wet ligno-cellulosic residue, which must be properly disposed of. HTC could be an effective technology to achieve both the disposal and valorisation of such feedstock.

Considering that HTC is performed at milder operational conditions than the other hydrothermal processes, this process has been investigated in the present paper as a suitable way to energetically valorise the grape marc by-product which is affordable also at the SME (Small and Medium Enterprise) level.

In this paper, the results of HTC of grape marc performed in a lab scale batch reactor are presented. Several reaction conditions were explored, including different reaction temperatures and residence times. Both solid and gaseous products were sampled and analyzed. The energy content, the elemental analysis and the thermal stability of the solid was determined, showing the great potentiality of hydrochar as a fuel. Analyses on the gaseous phase were performed both to try to understand the chemical processes occurring during HTC and to get information on the gaseous emissions on the environment.

Materials and methods

The feedstock

Grape marc collected after wine production consists of seeds, skins and, possibly, stalks. Actually, stalks are collected together with seeds and skins or, vice versa, separately according to the different wine-making procedures. Usually, when grape marc originates from the wine-making process, it is referred to as ''fresh grape marc'', while after it has been used for spirits production, it is referred to as ''exhausted grape marc''. In the case of fresh grape marc, the moisture content ranges between 60% and 70% [START_REF] Fiori | Gasification and combustion of grape marc: comparison among different scenarios[END_REF]. Thus, after a drying pre-treatment, which could be addressed also through innovative technologies such as bio-drying [START_REF] Rada | Bio-drying of grape marc and other biomass: a comparison[END_REF], grape marc can be exploited for energy purposes through conventional thermochemical processes such as combustion or pyrolysis (Fiori et al., 2012b). Alternatively, it can undergo hydrothermal treatments, like the HTC here investigated.

For the present research, fresh grape marc was collected at a wine-production site located in Trentino (North-East of Italy). The raw feedstock was dried in a ventilated oven at 65 °C for about 48 h. Subsequently, grape marc was let cooling down to room temperature and finally stored into plastic bags, until use. The final average moisture was measured to be about 5%. This kind of pre-treatment was performed in order to collect data regardless of the degree of moisture of the incoming feedstock, which could be variable. Furthermore, this drying pre-treatment was necessary to prevent the degradation of the feedstock during the period between its collection and its actual utilization for the experiments (up to some weeks). The presence of water would have caused undesired degradation reactions, as well as mould formation, which would have altered the quality and composition of the substrate in a significant way. Nevertheless, when applying HTC to grape marc in a real industrial process, wet grape marc would most likely be used as received and possibly applying additional quantities of water, in order to reach the optimal biomass to water (B/W) ratio.

The HTC experimental apparatus

As explained above, the hydrothermal carbonization is usually performed in a temperature range of 180-250 °C. Since water has to be maintained in its liquid state, pressures reach values of 40-50 bar. Therefore, a 50 mL stainless steel (AISI 316) batch reactor was designed and built, with a design temperature and pressure of 300 °C and 140 bar, respectively. For further technical information, the reader can refer to [START_REF] Basso | Hydrothermal carbonization of off-specification compost: a byproduct of the organic municipal solid waste treatment[END_REF]. The experimental apparatus was equipped with a thermocouple embedded inside the reactor. For the measurement of the pressure, both a pressure gauge and a pressure recorder were used.

Experimental procedure

The reactor was filled with 5.4 ± 0.1 g of feedstock and 27.0 ± 0.1 g of distilled water, obtaining a biomass to water ratio of about 0.19 (dry basis). The reactor was sealed with a copper gasket and then it was closed. Before each test, nitrogen was flushed within the reactor to purge it from the presence of air. The reactor was warmed up through a band heater. At the end of each test, the reactor was quenched with the help of a stainless steel block kept at À26 °C which was put underneath the reactor itself, while fresh compressed air was blown onto it. To both warm up and quench the reactor, about 15-20 min were required. The residence times here considered refer to the period between the reaching of the set temperature and the starting of the quenching. Three temperatures and three residence times were chosen for the experiments: 180 °C, 220 °C, 250 °C at 1 h, 3 h and 8 h. Test runs corresponding to all the 9 combinations of such parameters were executed.

The experimental apparatus did not allow for the simultaneous measure of both the volume and the composition of the gas produced during the process. Thus, tests were repeated twice at the same conditions in order to measure separately the volume of the produced gas and its composition. All the gas composition measurements were repeated three times. The volume of the produced gas was measured directly in one series of tests, while in the other series (where the gas composition was measured), it was calculated considering the measured reactor residual pressure after quenching. In all the repetitions, temperature and pressure profiles showed a deviation never higher than 1.0% and 1.4%, respectively.

Measurements and analyses of the solid phase

At the end of each run, the reactor was opened. Its content was filtered and both liquid and solid products were collected. The wet filter and the reactor, both holding a certain amount of solids, were dried in an oven at 105 °C for at least 8 h. After drying, the filter and the reactor were weighed again. The hydrochar yield was then determined in terms of amount of solid recovered with respect to the amount of substrate initially loaded into the reactor, i.e. kg hydrochar per kg dry feed .

The energy content of the hydrochar was assessed measuring the higher heating value (HHV) of powdered samples in a calorimetric bomb (IKA 200C). Isoperibolic method was applied in accordance with UNI EN 14918:2010.

The elemental analysis was carried out in a Thermo NA 2100 to obtain C, H, N, and S mass fractions. The ash content was then determined by incineration at 550 °C according to EN 14775 procedure. The O content was deduced by difference.

The thermo-gravimetric analysis (TGA) of solid samples was carried out in a TG-DTA LabSys Evo (Setaram). About 20 mg of each sample were heated from 30 to 800 °C in a nitrogen atmosphere at 5 °C min À1 .

Measurements and analyses of the gaseous phase

To measure the amount of gas formed during the HTC process, a simple experimental apparatus consisting of a plastic graduated cylinder with the lower part submerged into water was used. Before each measurement, vacuum was created inside the cylinder through a pump, to raise the water meniscus up to the zero of the graduated scale. The zero was positioned on the upper part of the cylinder, with the graduated scale (in cm) facing downwards. Thus, a needle valve was opened and the gas flowed inside the cylinder, causing the water level to decrease. The difference between the initial and the final position of the meniscus allowed the calculation of the volume of formed gas.

For the analyses of the gaseous phase composition, a mobile micro-GC (Agilent 3000), equipped with a MOLSIEVE and a PLOT U column was used. Detectable components were H 2 , N 2 , O 2 , CO, CH 4 , CO 2 , C 2 H 6 and C 2 H 4 .

Statistical analysis of data

ANOVA was performed to get insights on the influence of both temperature and residence time on three different variables: hydrochar yield, hydrochar HHV and gas composition. As far as hydrochar yield is concerned, the data used for the statistical analysis are relevant to experimental tests repeated twice. Thus, for this variable two-way ANOVA with two replications was performed (with significance level a = 0.05).

On the contrary, the hydrochar HHV was measured only once. Thus, two-way ANOVA without replications (with a = 0.05) was performed for HHV.

The gas composition was measured three times by the micro-GC, although these measurements are relevant to the same HTC test (see Section 2.3). For the gas composition two-way ANOVA without replications (with a = 0.05) was performed, taking the values of the gas composition as the average (arithmetic mean) of the three measurements.

To perform ANOVA the following Ms Excel Data Analysis tools were used: ''Anova: Two-Factor With Replications'' for hydrochar yield, and ''Anova: Two-Factor Without Replication'' for both HHV and gas composition.

Results and discussion

Experimental runs

In Fig. 1 typical temperature and the pressure profiles are represented, with reference to an experimental run performed for 8 h at 250 °C.

In about 20 min the reactor reaches the set point temperature (in the case shown, 250 °C), that is then kept constant for the selected residence time (in this case, 8 h). At the end of the process, the reactor is quenched and this operation takes again about 20 min. An initial increase of the temperature above 250 °C can be observed: this is due to the time delay in heat transfer from the heat source (band heater) to the thermocouple providing the temperature value to the temperature controller.

It can be observed that the temperature along the whole run is kept constant, while the pressure slightly increases with time. This increase is due to the formation of gas during the HTC process. An interesting comparison can be made with the NIST steam tables data (NIST, 2015), represented by the light gray solid line in Fig. 1. The gas formed from the feedstock is responsible for the difference between the vapor pressure (39.762 bar at 250 °C, NIST, 2015) and the actual pressure measured during HTC. It can be appreciated that gas is formed in the early stages of the process: immediately after the heating up phase, differences between the actual pressure and the vapor pressure become evident. The gas production does not follow a linear trend, but it is more remarkable at the beginning of the process, while it decreases increasing the residence time. Finally, when the system is cooled down, the residual pressure within the reactor testifies once more that a non-negligible amount of gas formed during HTC.

Hydrochar yield

Fig. 2 shows the hydrochar yield for all the performed tests and its dependence on process temperature and residence time. Hydrochar yield ranges from 0.76 (temperature: 180 °C; residence time: 1 h) to 0.56 (temperature: 250 °C; residence time: 8 h). Similar results have been obtained by [START_REF] Pala | Hydrothermal carbonization and torrefaction of grape pomace: a comparative evaluation[END_REF], performing HTC on grape pomace. The increase in process temperature and residence time causes the hydrochar yield to decrease (and the production of gas to increase, Fig. 4). In the range of operating conditions investigated, the effect of the process temperature is much greater than that of the residence time. As a matter of fact, the two-way ANOVA results confirmed that the temperature affects the hydrochar yield (F > Fc and p = 0.002), while the residence time cannot be considered statistically relevant (F < Fc and p = 0.173). Moreover, it is not possible to affirm that the effect of interactions between temperature and residence time is statistically significant (F < Fc and p = 0.514).

The minor influence of the residence time on the hydrochar yield is particular evident at 180 °C: the hydrochar yields are nearly coincident for all residence times analyzed. At 220 and 250 °C, the system seems more reactive and the effect of residence time can be noticed (in terms of the average values).

This could be explained by referring to the main reactions typically taking place during HTC. Some authors [START_REF] Funke | Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering[END_REF][START_REF] Libra | Hydrothermal carbonization of biomass residuals : a comparative review of the chemistry, processes and applications of wet and dry pyrolysis[END_REF] suggested that such reactions include: hydrolysis of the feedstock, dehydration, decarboxylation, decarbonylation, condensation, and polymerization. In particular, [START_REF] Funke | Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering[END_REF] reported that, when carbonizing lignocellulosic materials, hemicellulose readily hydrolyzes from 180 °C, while both cellulose and lignin are hydrothermally degraded at temperatures higher than 200 °C. Therefore, the almost constant yields at 180 °C shown in Fig. 2 could be due to the fact that only hemicellulose is degraded and that such conversion is achieved in the first stage of reaction (i.e. up to 1 h). Thus, at 180 °C the residence time does not contribute to the degradation of the feedstock to both liquid and gaseous products.

Gas phase: composition and production

Fig. 3 reports the data obtained from the measurements with the portable micro-GC. CO 2 is the gas formed the most during HTC, with molar fractions ranging from 94% to 99% (Fig. 3a). At 180 °C almost all the gas formed is composed of CO 2 (98-99%). Increasing the temperature, relatively less CO 2 is formed while the CO production increases. Referring to Fig. 3a andb, the influence of the residence time at 220 °C is negligible, while it is more pronounced at 250 °C. Moreover, an interesting effect can be appreciated from both Fig. 3a andb. At 180 °C, increasing the reaction time causes the molar fraction of CO 2 to decrease slightly and the molar fraction of CO to increase slightly. At 220 °C, the reaction time has no effect of the relative amount of CO 2 and CO. At 250 °C, increasing the reaction time causes the molar fraction of CO 2 to increase and the molar fraction of CO to decrease, thus the trend is opposite to that detected at 180 °C. A possible explanation to this phenomenon could be that, at low residence times, de-carboxylation reactions are favored at low temperatures, while high temperatures tend to favor de-carbonylation. At 250 °C, the decrease of CO at increasing residence time, and the corresponding increase of H 2 and CO 2 , are most likely an effect of the water-gas shift reaction [START_REF] Castello | Low temperature supercritical water gasification of biomass constituents: glucose/phenol mixtures[END_REF] which can effectively take place at the considered reaction conditions. At 180 °C, the decrease of CO 2 at increasing residence time could be related to some reductive processes taking place in the aqueous phase. The tests at the intermediate temperature (220 °C) do not show a relevant dependence on the residence time since, probably, such two tendencies balance each other. Further work should be performed to better explain this issue.

Fig. 3c andd shows that, in all process conditions, both H 2 and CH 4 are produced in trace amounts.

The two-way ANOVA performed on the gas composition data allowed to state that both temperature and residence time affect the gas production for all the gases (F > Fc and p < a).

Fig. 4 reports the total gas production at the different conditions. The gas production trends are clear: the increase of both temperature and residence time results in an enhancement of the gaseous phase produced by HTC. This result is in agreement with that obtained for the hydrochar yield (Fig. 2): increasing the severity of the process causes higher degradation of the solid feedstock in favor of the gaseous products. Furthermore, Fig. 4 shows how the effect of temperature is stronger if compared to that of residence time. As a matter of fact, the increase of the produced gas at increasing residence time (at fixed temperature) is lower than that at increasing temperatures (at fixed residence time). This is further evidence that the main reactions involved in HTC mostly take place in the first stage of the process. Hence, further research should focus on the reaction steps that characterize the first hour of operations. Finally, the results shown in Fig. 4 confirm that the amount of gas produced during HTC is smaller if compared to that of other thermochemical processes. Hence, when considering HTC industrial applications, this fact will ease the management of environmental issues.

Hydrochar characterization

In Fig. 5 the HHVs of the hydrochar obtained at different HTC conditions were reported. On the x-axis, the label ''RF'' stands for ''raw feedstock'', i.e. dried grape marc.

The increase in HHV due to HTC is significant, ranging from 24% (residence time: 1 h; temperature: 180 °C) to 54% (residence time: 8 h; temperature: 250 °C). These values of energy densification seem to be consistent with those obtained by [START_REF] Pala | Hydrothermal carbonization and torrefaction of grape pomace: a comparative evaluation[END_REF]. Performing HTC of several municipal solid wastes, [START_REF] Lu | Effects of hydrothermal treatment on characteristics and combustion behaviors of municipal solid wastes[END_REF] obtained an increase in HHV ranging from 1% to 41% (residence time: 30 min; temperature: 220 °C), values comparable to those here obtained.

The two-way ANOVA allows to state that both temperature and residence time affect the HHV of the hydrochar (F > Fc and p < a).

Also the HHV data of Fig. 5 corroborate the conclusions above reported when discussing on hydrochar and gas yields (Sections 3.2 and 3.3): the influence of process temperature is greater than that of residence time. This is in clear agreement with previous literature results [START_REF] Funke | Heat of reaction measurements for hydrothermal carbonization of biomass[END_REF][START_REF] Gaji | Microcosm study on the decomposability of hydrochars in a Cambisol[END_REF][START_REF] Liu | Hydrothermal carbonization of waste biomass for energy generation[END_REF]. As a matter of fact, an increase in temperature favors dehydration and the decarboxylation reactions, which corresponds to a decrease in the content of both oxygen and hydrogen in the feedstock. Similar results were found also by other authors [START_REF] Heilmann | Hydrothermal carbonization of distiller's grains[END_REF][START_REF] Dinjus | Hydrothermal Carbonization -1. Influence of lignin in lignocelluloses[END_REF][START_REF] Mumme | Hydrothermal carbonization of anaerobically digested maize silage[END_REF].

The elemental analysis (Table 1) clearly shows that the carbon content of the hydrochars increases with temperature and residence time and, correspondingly, the oxygen content decreases. Vice versa, the relative abundance of hydrogen, nitrogen, and ash seems not affected by the severity of the process. The content of sulfur, extremely small and often measured equal to zero, was not reported in Table 1.

The compositional data reported in Table 1 support the energy densification results previously discussed: the higher is the carbon content of the substrate, the higher its HHV. Importantly, the atomic ratio O/C decreases drastically with process time and temperature: from 0.54 (raw biomass) to 0.21 (residence time: 8 h; temperature: 250 °C). Referring to the van Krevelen's diagram [START_REF] Basso | Hydrothermal carbonization of off-specification compost: a byproduct of the organic municipal solid waste treatment[END_REF], the HTC process provides hydrochars in the region of biomass/peat when the process temperature is 180 °C, and in the region of peat for more severe process conditions. Actually, the hydrochar obtained after 8 h at 250 °C is in the region of lignite.

Results from TGA testify that the mass loss of the hydrochar samples during analysis greatly reflects the HTC operational conditions: the mass loss is in the range of 65.7-67.9 wt.% for the hydrochars obtained at 180 °C, while it is comprised between 51.8 and 57.2 wt.% for the hydrochars obtained at 250 °C (complete set of results not shown).

The derivative mass loss (DTG) is reported in Fig. 6 for five samples including the raw material and the hydrochars obtained after 1 or 8 h HTC treatment at 180 or 250 °C.

For the raw sample, a large peak of DTG is observed at low temperature with three relative minima at 80, 102 and 134 °C. These peaks are due to dehydration and release of volatile compounds. After HTC treatment, the peak appears to be only centred at 80 °C, likely due to residual water content. Therefore, the HTC treatment impacts the presence of volatile compounds in the remaining solid.

Raw sample shows three other main peaks centred at 262, 310, and 408 °C. These peaks correspond to the thermo-degradation of organic compounds present in the grape marc.

Considering hydrochars obtained at low temperature (180 °C), the main mass loss is represented by a peak centred at a temperature of about 337 °C. Interestingly, such peak is more intense than the peaks of grape marc, meaning that new compounds are produced during HTC. Moreover, this peak is slightly shifted to a higher temperature if compared to the peak detected for the raw material (310 °C), meaning that the new compounds are slightly more stable. At the highest treatment temperature (temperature: 250 °C; residence time: 1 h), the peak is shifted to 371 °C, testifying that the new formed compounds are even more stable.

HTC has a clear impact on the compounds which are degraded at 262 °C, as the peak is much less intense after HTC. Thus, a large part of these compounds were removed during HTC.

The peak centred at 410 °C is also impacted by HTC, as its intensity increases with the increase of temperature and residence time. The highest impact is found for the most severe conditions (temperature: 250 °C; residence time: 8 h).

To summarize the information contained in Fig. 6, the increase of temperature and residence time causes a decrease in the amount of volatile compounds and thermo-sensitive molecules, and results in an increase in thermal stable compounds in the hydrochar. This is in complete agreement with the compositional data of Table 1: the increase in carbon content indicates an increase in ''fixed carbon'', which is testified by the increased thermal stability proved by TGA. In addition, as previously discussed, the increase of carbon content sustains the higher HHV of the hydrochar produced at the most severe conditions, as shown in Fig. 5.

Conclusions

In this paper, hydrothermal carbonization was studied as a viable process to energetically valorise grape marc. In particular, the research was performed with an experimental apparatus, through which several tests were carried out at different temperatures and residence times.

The results obtained for both solid and gaseous products showed that temperature is the main parameter affecting the process. An increase in temperature reduces hydrochar mass yields in favor of the gaseous phase. Despite less solid formed, the hydrochar produced at higher temperature has a higher carbon content and thus a higher HHV. At 250 °C, the HHV increase due to HTC is significant, and varied from 43% to 54%, depending on residence time. Conversely, the increase in HHV was only equal to 24% at the less severe process conditions (1 h, 180 °C). In addition, increasing the severity of the process greatly increases the thermal stability of the hydrochar produced.

The gaseous phase is mainly composed by CO 2 , with some CO being generated at higher temperatures. Other molecules, such as H 2 and CH 4 , are only produced in traces. Their amounts tend to increase when more severe process conditions are adopted.

HTC seems to have great potentialities for meeting the challenges of both energetically valorising winery industry residues and improving their management. Future research will include in-deep analyses of the liquid phase, in order to better understand the chemical reactions occurring during HTC. Moreover, to address real industrial applications of the HTC process, an economic evaluation is mandatory.

Fig. 1 .

 1 Fig. 1. Trends of temperature and pressure during an experimental run (set point temperature: 250 °C; residence time: 8 h); vapor pressure data are also reported (NIST, 2015).

Fig. 2 .

 2 Fig. 2. Hydrochar yield (average of two replications ± standard deviation) at different temperatures and residence times.

Fig. 3 .

 3 Fig. 3. Gas phase molar composition (average of three replications ± standard deviation) at different temperatures and residence times: (a) CO 2 ; (b) CO; (c) H 2 ; (d) CH 4 .

Fig. 4 .

 4 Fig. 4. Gas yield (average of two replications ± standard deviation) at different temperatures and residence times.

Fig. 5 .

 5 Fig. 5. HHVs at different temperatures and residence times.

Table 1

 1 Elemental analysis: values in wt.%.

	Residence time (h)	Temperature (°C)	C	H	O	N	Ash	H/C	O/C
	Grape marc (dried)		49.71	6.15	35.57	2.43	6.00	1.49	0.54
	1	180	56.21	5.88	31.99	2.53	3.38	1.26	0.43
	3	180	56.98	5.64	30.18	2.58	4.62	1.19	0.40
	8	180	57.23	5.71	29.67	2.78	4.62	1.20	0.39
	1	220	59.85	5.74	28.00	2.59	3.83	1.15	0.35
	3	220	62.47	5.43	25.85	2.72	3.53	1.04	0.31
	8	220	64.10	5.66	24.02	2.24	3.98	1.06	0.28
	1	250	64.94	5.80	22.40	2.77	4.09	1.07	0.26
	3	250	65.58	6.00	20.35	2.81	5.26	1.10	0.23
	8	250	68.07	5.81	19.36	2.56	4.20	1.02	0.21


M Rouleux
email: rouleux@univ-tln.fr

Generalized Pareto optimum and semi-classical spinors

In 1971, S.Smale presented a generalization of Pareto optimum he called the critical Pareto set. The underlying motivation was to extend Morse theory to several functions, i.e. to find a Morse theory for m differentiable functions defined on a manifold M of dimension ℓ. We use this framework to take a 2 × 2 Hamiltonian H = H(p) ∈ C ∞ (T * R 2 ) to its normal form near a singular point of the Fresnel surface. Namely we say that H has the Pareto property if it decomposes, locally, up to a conjugation with regular matrices, as

, where u : R 2 → R 2 has singularities of codimension 1 or 2, and C(p) is a regular Hermitian matrix ("integrating factor"). In particular this applies in certain cases to the matrix Hamiltonian of Elasticity theory and its (relative) perturbations of order 3 in momentum at the origin.

Matrix valued Hamiltonian systems and crossing of modes

Here we recall some well known facts about generic normal forms for a matrix Hamiltonian near a crossing of modes. Consider a real valued symmetric Hamiltonian H ∈ C ∞ (T * R d ) ⊗ R n . We may also replace H(x, ξ) by a more general symbol (in a suitable class) with asymptotic sum H(x, ξ; h) = H 0 (x, ξ) + hH 1 (x, ξ) + • • • , and consider its semi-classical Weyl quantization H w (x, hD x ; h)U (x; h) = (2πh) -d e i(x-y)η/h H( x + y 2 , η; h)U (y) dy dη

This defines an operator essentially self-adjoint on the space of square-integrable "spinors" L 2 (R d ) ⊗ C n . For simplicity we keep denoting by H the principal symbol H 0 which actually plays the main role in the present analysis. Let ∆(x, ξ) = det H(x, ξ), and N = {(x, ξ) ∈ T * R d , ∆(x, ξ) = 0} be the characteristic variety. At points of N , the polarisation space ker H(x, ξ) has positive dimension k ≥ 1.

For k = 1, {(x, ξ) ∈ N : d∆ = 0} is a smooth hypersurface, and modulo an elliptic factor, we can reduce H to a scalar symbol with simple characteristics; the corresponding operator, of real principal type, can again be locally conjugated to hD x 1 by an elliptic Fourier integral operator (FIO); this induces well-known polarization properties for solutions of the original system.

Effects which are truly specific for systems occur therefore when k ≥ 2; let Σ = {(x, ξ) ∈ N : d∆ = 0} be the singular part of N .

For generic Hamiltonian H, Σ is a stratified set. Splitting off elliptic summands, H can be (generically) reduced to a 2 × 2 system. For a 2 ×

2 symmetric system a + f b b -a + f , one
should generically move 2 parameters to bring it to a matrix with an eigenvalue of multiplicity 2, and one more to bring this eigenvalue to 0; thus the top (larger) stratum of Σ is of codim 3.

On directions transverse to Σ, N looks like a quadratic cone. In the next item we review some known results, making these observations more precise.

a) Classical and semi-classical normal forms for generic H

We first mention Arnold's [START_REF] Arnold | Singularities of Caustics and Wave Fronts[END_REF] normal forms for (generic) ∆ = det H near the stratum of Σ of codim 3: at a conical point (i.e. where a = b = c = 0), there are symplectic coordinates (p, q) such that modulo an elliptic factor, we can bring ∆ to one of the following expressions :

∆(x, ξ) ≡ p 1 q 1 -p 2 2 hyperbolic conical point ∆(x, ξ) ≡ p 2 1 + q 2 1 -p
A * HA = -p 1 + p 2 q 1 p 3 q 1 p 3 p 1 + p 2 hyperbolic conical point A * HA = p 1 -p 2 q 2 p 3 q 2 p 3 p 1 + p 2 elliptic conical point
Moreover these normal forms extend naturally to quantization of Weyl symbols, composing A by a suitable FIO U microlocally defined near the conical point, and associated with the canonical change of coordinates (x, ξ) → (q, p). Namely, in the hyperbolic case U * (A * HA) w U = -hD q 1 + hD q 2 q 1 hD q 3 q 1 hD q 3 hD q 1 + hD q 2 and similarly in the elliptic case. Normal forms are useful for studying the Hamiltonian flow of H(x, ξ) or finding quasi-modes for H(x, hD x ) in terms of classical functions. These normal forms are structurally stable, i.e. they are not affected by small perturbations of a generic H in the C ∞ (T * R d )⊗R n topology. Even more precise results are available in 1-D, see e.g. [START_REF] Rouleux | Tunneling effects for h-Pseudodifferential Operators, Feshbach resonances and the Born-Oppenheimer approximation[END_REF] in the framework of Born-Oppenheimer approximation, or [8], [START_REF] Bensouissi | Proc. "Days of Diffraction 2011[END_REF] for Bogoliubov-de Gennes Hamiltonian.

b) Particular cases: k = 2, codim 3 singularities

In many physical situations however, the genericity assumption is not fulfilled, and special reductions should be carried out :

• Conical refraction in 3-D Recall from [START_REF] Ivrii | [END_REF] in dimension d = 3 that the normal form is given by the symmetric matrix

H = p 1 + p 2 p 3 p 3 p 1 -p 2 (1)
which is independent of space variables. This is not structurally stable, because the singular part Σ of N is involutive.

• Graphene Hamiltonian in 2-D. The Hamiltonian is a complex Hermitian matrix [9]:

H(p) = 0 f (p 1 , p 2 ) f (p 1 , p 2 ) 0 = |λ(p)| 0 e -iθ(p) e -iθ(p) (2)
Here p = (p 1 , p 2 ) ∈ R 2 are quasi-momenta, and the eigenvalues are explicitely given by

λ(p) = ±t 3 + 2 cos( √ 3p 1 a) + 4 cos( √ 3p 2 a/2) cos(3p 2 a/2)
Energy vanishes at Dirac points (2 Dirac points per hexagonal cell). The linearization at the Dirac point is of the form

H(p) = v 0 k 1 -ik 2 k 1 + ik 2 0 , v = 3t/(2a) (3)

c) Particular case: higher order singularities

Here we are concerned in the case where ∆ vanishes of order 4 at the conical point (leaving open the case k = 3). The physical example is the Hamiltonian quadratic in the momenta from Elasticity Theory [START_REF] Bogaevsky | [END_REF] in (2+1)-D, that is obtained in the following way.

On the set of maps Γ : R t ×R 2 x → R 2 , (t, x, y) → (φ(x, y, t), ψ(x, y, t)) with Sobolev regularity H 1 (R 3 ), we consider the Lagrangian density:

L( x y , φ t ψ t , φ x ψ x , φ y ψ y ) = T -V = 1 2 (φ 2 t + ψ 2 t ) - 1 2 (φ 2 x + ψ 2 y ) - 1 2 A(x, y)(φ y + ψ x ) 2
Euler-Lagrange equations from the variational principle δ L dt dx dy = 0 lead to extremal curves (rays) in the (t, x, y)-space, and the set of points (t, x, y) connected to the origin in R 3 by such a ray is called the "world front". A matter of interest are the singularities of the world front.

Applying Fourier transformation with respect to (x, y, t) φ → φ, φ x → ξ φ, φ y → η φ, φ t → τ φ, etc. . . switches from Lagrangian formulation to Hamiltonian formulation and leads to a Pseudodifferential Hamiltonian system (as a quadratic form in ( φ, ψ)) with principal symbol

H = τ 2 0 0 τ 2 - ξ 2 + A(x, y)η 2 A(x, y)ξη A(x, y)ξη A(x, y)ξ 2 + η 2 (4)
Genericity properties for this Hamiltonian imply as above that the singular set Σ is of codimension 3, so that it can be brought to one of Arnold-Braam-Duistermaat normal forms (in 2-D) near Σ. However, genericity breaks down in the case of constant coefficient A(x, y) = Const., which justify a direct approach.

In particular, for A = 1 2 , the spatial part has determinant

∆(ξ, η) = 1 2 (ξ 2 + η 2 ) 2
which vanishes of order 4 at (ξ, η) = 0. Our purpose, precisely in this case, is to provide a normal form for H near τ = ξ = η = 0. This could be achieved by a straightforward diagonalisation of H, but our method carries naturally to (relative) perturbations of this Hamiltonian, depending on (ξ, η) alone; more naively this example is intended as an illustration of the role played by the generalized Pareto set.

Generalized Pareto optimum

A central problem in Differential Calculus consists in maximizing a function: Morse theory on a smooth manifold provides a globalization of this problem.

Economists are rather concerned in maximizing simultenaously several "utility functions", obtaining this way the notion of Pareto optimum in a free exchange economy.

In 1971, S.Smale presented a generalization of Pareto optimum he called the critical Pareto set [START_REF] Smale | Global Analysis and Economics I: Pareto Optimum and a Generalization of Morse Theory Proc[END_REF]. The main mathematical motivation is to find a Morse theory for m differentiable functions defined on a manifold M of dimension ℓ. Note that this problem is distinct from this of relative extrema of a single function, which is relevant to Lagrange multipliers.

a) "Classical" Pareto optimum in a free exchange economy

Consider a free exchange economy consisting in m consumers i = 1, • • • , m, and for each i, let

x i = (x (1) i , • • • , x (J) i ) represent the (positive) amount of goods j = 1, • • • , J, with x (j)
i ≥ 0. We define the "commodity space" as P = {x = (x (1)

, • • • , x (J) ) ∈ R J ; x (j) ≥ 0}. An unrestricted state of the economy is a point x = (x 1 , • • • , x m ) ∈ P m ,
but we may restrict to the affine space

M = { x ∈ P m : m i=1 x (j) i = C (j) , j = 1, • • • , J}
with total ressource C (j) of good j. Each consumer is supposed to have an utility function u i : P → R (generally an homogenous function of x (1)

, • • • , x (J) ). Thus consumer i prefers x i to x ′ i iff u i (x i ) > u i (x ′ i ).
The level sets u -1 i (c), c > 0 are called "indifference surfaces". One considers exchanges in M which will increase each u i on M . A state x ∈ M is called "Pareto optimal" iff there is no x ′ ∈ M such that u i (x i ) ≥ u i (x ′ i ) for all i, and u j (x j ) > u j (x ′ j ) for some j. If x ∈ M is not Pareto, x is not economically stable. For m = 1 Pareto optimum equals the usual notion of a (constrained) maximum of u : M → R, x → u(x).

Physicists would replace everywhere the words "maximizing" by "minimizing", and "total resource" by "total energy".

b) Generalized Pareto optimum in the sense of Smale.

Here we do not only consider (joined) maxima, but also (joined) critical points. Let M be a smooth manifold, dim M = ℓ, and u : M → R m be m smooth functions defined by u = (u 1 , • • • , u m ) (vector of "utilities").

Let

H(x) = (u ′ (x)) -1 (R m + ), where u ′ (x) : T x M → R m + denotes the derivative (Jacobian) of u at x ∈ M , and R m + the set of v = (v 1 , • • • , v m ) ∈ R m with v j > 0 all j.
Definition 1 [START_REF] Smale | Global Analysis and Economics I: Pareto Optimum and a Generalization of Morse Theory Proc[END_REF]: We call Θ = {x ∈ M : H(x) = ∅} the Pareto critical set.

Thus the relation x ∈ Θ means that there is no smooth curve γ :]ǫ, ǫ[→ M starting at x, and such that t → u i • γ(t) increases for all i's (gradient flow dynamics). For m = 1, Pareto critical set is just the set of critical points of u.

If u is a (single) Morse function on M , Θ is a discrete set, and the Hessian u ′′ (x) is defined intrinsically on Θ. For m > 1, Θ need not be discrete; but when it consists, as is usual, of a submanifold of dim m -1, then u ′′ (x) is still intrinsically defined on Θ and valued in the 1-D space R m /Ranu ′ (x).

The open subset Θ S ⊂ Θ of stable points (classical Pareto set), which reduces for m = 1 to the set of local maxima of u, plays a special rôle.

• Paradigm of Pareto critical in 2-D: the immersed Klein bottle in R 3 .

The paradigm of a Morse function on a compact 2-manifold is the "height function" on the embedded torus, and its (Pareto) critical points are the minimum, 2 saddles, and maximum. Similarly Pareto critical set for 2 functions of 2 variables will be obtained from Klein bottle [Wan], by projecting the immersed bottle in R 3 onto a suitable plane, and looking at the "joined extrema" of the coordinates functions on the projection plane. Another, more convenient immersion of Klein bottle in R 3 (though not so easy to visualize) is the so called "figure eight" or "bagel" immersion, given by

x 1 = (r + cos θ 2 sin v -sin θ 2 sin 2v) cos θ x 2 = (r + cos θ 2 sin v -sin θ 2 sin 2v) sin θ x 3 = sin θ
2 sin v + cos θ 2 sin 2v with r > 2 a parameter, -π ≤ θ < π, 0 ≤ v < 2π are the variables. It is obtained by gluing two Möbius bands along their edges. Then the map with typical critical Pareto set is given by u = ( x 2 1 + x 2 2 , x 3 ). Θ is a stratified set, consisting in a finite number of segments (codimension 1 strata) where rank(u ′ ) = 1, terminating at codimension 2 strata (isolated points) where rank(u ′ ) = 0.

We will only consider ℓ = m which leads to the simplest topology.

• Elementary Pareto sets: the case of quadratic polynomials in 2-D.

As in the one-dimensional case, quadratic polynomials in R ℓ provide useful examples of maps with a typical critical Pareto set. For m = ℓ = 2 we may take

u 1 (x) = 1 2 (x 2 1 + x 2 2 ), u 2 (x) = 1 2 (x 2 1 + x 1 x 2 + x 2 2 ), Θ = {0} u 1 (x) = x 1 x 2 , u 2 (x) = 1 2 (x 2 1 -x 2 2 ), Θ = {0} u 1 (x) = 1 2 (x 2 1 + x 2 2 ), u 2 (x) = 1 2 (x 2 1 -x 2 2 ), Θ = {x 1 = 0}
More generally, the critical Pareto set of two elliptic or two hyperbolic quadratic linearly independent polynomials reduces to the origin, and to a line for a "mixed" pair.

Matrix valued Hamiltonian systems with the Pareto property

Since we are interested in 2-spinors, we work with m = ℓ = 2 and H(x, p) = H(p).

Definition 2: Let H ∈ C ∞ (R 2 ) ⊗ R 2 ⊗ R 2 be
a (real) Hermitian matrix. We say that H has the Pareto property iff there exists a smooth map (but with possibly degenerate derivative) u : R 2 → R 2 , and a (regular) Hermitian matrix C(p) such that, locally, and up to conjugation with an elliptic factor of the form H(p) → A(p) * H(p)A(p), we have:

H(p) = u ′ (p)C(p)(u ′ (p)) *
Remark: A hint on this definition is the following: let p ∈ R 2 such that u ′ 1 (p) = 0, then the "pure classical state" 1 0 ∈ N (p), and similarly for 0 1 when u ′ 2 (p) = 0. So any "classical state"

x y is a superposition of "classical pure states".

These decompositions are local and sometimes can be checked only in the sense of germs. If exact, we say that H is "integrable in Pareto sense". Only in 1-D problems, one can consider general Hamiltonians of the form H(x, p).

Pareto property and the quadratic Hamiltonian of Elasticity Theory

Because Hamiltonians depending on p alone are not structurally stable, few Hamiltonian systems verify Pareto property. We have : Theorem 1: For A = 1/2, the quadratic Hamiltonian of Elasticity Theory (4) is integrable in Pareto sense: with

u 1 (ξ, η) = 1 2 √ 2 (ξ 2 -η 2 ), u 2 (ξ, η) = 1 √ 2 ξη, we have H(ξ, η) = u ′ (ξ, η)diag(2, 1)(u ′ (ξ, η)) * with u ′ (u ′ ) * = 1 2 (ξ 2 + η 2 )Id. Let σ 2 = 0 i -i 0 , we have the "skew- diagonalization" 1 2 σ 2 (H(ξ, η) -τ 2 )σ * 2 = u ′ (ξ, η) diag( 1 2 , 1) -(ξ 2 + η 2 ) -1 τ 2 Id (u ′ (ξ, η)) * (5)
From this we can construct by inverse Fourier transformation semi-classical spinors t (φ, ψ)(x, y; τ ) that verify near "Helmholtz equation" corresponding to (4), τ 2 standing for the energy parameter. We write φ for the Fourier transform w.r.t the space variables. There are two linearly independent solutions t (φ 1 , ψ 1 )(x, y; τ ) of the equation

diag( 1 2 , 1) -(ξ 2 + η 2 ) -1 τ 2 t ( φ 1 , ψ 1 ) = 0
given by

φ 1 (x, y, τ ) = J 0 ( x 2 + y 2 √ 2τ h ), ψ 1 = J 0 ( x 2 + y 2 τ h ) (6)
and φ, ψ are derived from (5) by convolution integrals. From this it is standard to deduce the spectral properties of H.

• Relative stability of the Pareto property

Theorem 2: Let H(ξ, η) = ξ 2 + 1 2 η 2 1 2 ξη 1 2 ξη 1 2 ξ 2 + η 2 + O((ξ, η) 3
) Then H(ξ, η) has the Pareto property (at least in the sense of germs at 0), with C(ξ, η) = C 0 + O(ξ, η), u(ξ, η) = u 0 (ξ, η) + O((ξ, η) 2 ). Moreover there is a skew-diagonalization of type (5), and we can find a set of solutions to the "Helmholtz equation" as in (6).

The proof goes as in Poincaré-Dulac theorem [START_REF] Arnold | Geometrical methods in the theory of ordinary differential equations[END_REF]: Namely we seek for a "new" u of the

form v = u + f , with f = O(|ξ, η| 3 ), and a "new" C of the form diag(2, 1) + a b b d . The upper-left matrix element of v ′ H(v ′ ) * is given by √ 2L 1 f 1 + (ξ 2 + η 2 2 ) + 1 2 (aξ 2 -2bξη + dη 2 ), with L 1 = 2x ∂ ∂x -y ∂
∂y , which has a resonance 2:1. Its lower-right matrix element is given by

√ 2L 2 f 2 + (η 2 + ξ 2 2 ) + 1 2 (aη 2 + 2bξη + dξ 2 ) with L 2 = 2y ∂ ∂x + x ∂ ∂y . The off-diagonal terms involve the term L 1 f 2 + L 2 f 1 .
It turns out that we can solve (at least perturbatively) this system, the coefficients a = a 1 ξ + a

2 η + • • • , b = b 1 ξ + b 2 η + • • • , d = d 1 ξ + d 2 η + • • •
being determined to fulfill the compatibility conditions. The skew-diagonalization of type [START_REF] Bogaevsky | [END_REF] follows from the fact that v ′ (ξ, η)(v ′ (ξ, η)) * = 1 2 (ξ 2 + η 2 ) 2 (Id + O(|ξ, η|) is close to a multiple of Id. We can still construct semi-classical solutions, and their asymptotics in h for small E, obtained by varying the argument of the Bessel functions.

Conclusion

We have made an attempt to extend the notion of "non-degenerate critical point" for a scalar Hamiltonian to the notion of "Pareto critical set" Θ for a 2×2 Hamiltonian system. We focussed to the case where Θ reduces to a point. Our analysis applies to the Hamiltonian H(p) quadratic in momenta arising in Elasticity theory, for a particular value of the coefficients. It allows to account for the spectral properties of H together with the semi-classical spectral asymptotics of (relative, i.e. depending again only on p) perturbations of H near E = 0. These Hamiltonians present a codimension 3 singularity of order 4 at Θ = {0}. It seems difficult to extend this purely algebraic approach for general Hamiltonians of the form H(x, p): namely, when trying a map u : R 4 → R 2 , eigenvectors of H are given from those of C(x, p) by applying a pseudo-inverse of (u ′ ) * , which is not uniquely defined.

Acknowledgements: I thank Ilya Bogaevsky for interesting discussions. This work has been partially supported by the grant PRC CNRS/RFBR 2017-2019 No.1556 "Multi-dimensional semi-classical problems of Condensed Matter Physics and Quantum Dynamics".


Jun Dong

Yong Chi
email: chiyong@zju.edu.cn

Yuanjun Tang

Mingjiang Ni

Ange Nzihou

Elsa Weiss-Hortala

Qunxing Huang

Partitioning of Heavy Metals in Municipal Solid Waste Pyrolysis, Gasification, and Incineration

Heavy metal emission is a great environmental concern for the development of municipal solid waste (MSW) thermal treatment techniques. In this study, both experimental investigations and theoretical simulations are carried out to identify the partitioning of heavy metals between the gaseous phase and solid fractions during pyrolysis, gasification, and incineration of simulated MSW. Two types of incinerators are used. A tubular furnace is applied to evaluate the evaporation of metals from residues, whereas the metal distribution among bottom ash, cyclone fly ash, and filter fly ash is further examined in a fluidized bed. Six target metals (Cd, Pb, Zn, Cu, Cr, and Ni) are studied. Results show that a reductive atmosphere favors the evaporation of Cd and Zn but refrains Cu, Ni, and Cr volatilization, because metals are mainly reduced to their elemental form or sulfide, according to thermodynamic equilibrium calculation. Oxides are the dominant species under oxidizing condition due to the abundance of alkalis. Pb behaved differently, most probably by forming stable metal-matrix compounds such as Pb 3 Ca 2 Si 3 O 11 and PbZnSiO 4 . The cyclone ash is then separated into different sizes. The metal concentrations recorded reveal that most of the vaporized metals are transferred to the cyclone at its working temperature of 350-600 °C by an evaporation and condensation process; however, entrainment is also a determining factor for the transfer of less-volatile metals. Overall, parameters determining the transfer of heavy metals during MSW thermal treatment can be summarized as (i) metal speciation affected by redox atmosphere, temperature, and the presence of alkalis, chloride, sulfur, and other mineral substances; (ii) system characteristics, such as furnace type and cyclone temperature; and (iii) mechanical entrainment of particles caused by gas velocity.

INTRODUCTION

Guided by the waste management hierarchy, [START_REF]Council of European Communities[END_REF] municipal solid waste (MSW) incineration has played a gradually more important role over the past few decades. Compared to landfill, incineration benefits from considerable waste volume reduction, completely organic destruction, and the added ability to reclaim a significant amount of energy. [START_REF] Arena | Element partitioning in combustionand gasification-based waste-to-energy units[END_REF] In China, a total of 166 MSW incineration plants with treatment capacity of 492 300 tons•day -1 were in operation by the end of 2013, accounting for 30% of the total MSW treated. 3 The proportion of incineration is expected to achieve 35% in 2015 according to the National Twelfth Five-year Plan. [START_REF]The National Twelfth Fiveyear Plan on the construnction of urban solid waste safety disposal facilities; General Office of the State Council[END_REF] In spite of the advantages derived from incineration, toxic emission of dioxin remains a thorny issue, and the energy efficiency is relatively low, e.g., 16-24%. In recent years, advanced MSW thermal technologies, pyrolysis and gasification, have received increasing attention. It is proven that, in an oxygen-deficient atmosphere, the formation of dioxin and NO x can both be diminished significantly. [START_REF] Weber | Mechanism of the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans from chlorophenols in gas phase reactions[END_REF] Moreover, the generated synthesis gas can be utilized in different technical approaches, offering a potential of higher energy efficiency. Therefore, MSW pyrolysis and gasification are regarded as viable alternatives for traditional incineration even though few plants really operate. [START_REF] Arena | Process and technological aspects of municipal solid waste gasification. A review[END_REF] However, metal species contained in MSW will not be destroyed during high-temperature conversion but will instead be concentrated in the solid residues or escape as airborne aerosols exiting the stack. Heavy metal is one of the most toxic contaminants that can accumulate in the human body such as kidneys, bones, and liver, causing serious health disorders. [START_REF] Singh | Health risk assessment of heavy metals via dietary intake of foodstuffs from the wastewater irrigated site of a dry tropical area of India[END_REF][START_REF] Nzihou | The fate of heavy metals during combustion and gasification of contaminated biomassA brief review[END_REF] Certain metals such as Cu are proven to act as catalysts for dioxin formation. [START_REF] Olie | Metals as catalysts during the formation and decomposition of chlorinated dioxins and furans in incineration processes[END_REF] Heavy metal emissions thus become a great environmental concern. [START_REF] Lu | Comparison of trace element emissions from thermal treatments of heavy metal hyperaccumulators[END_REF][START_REF] Saffarzadeh | Characterization study of heavy metal-bearing phases in MSW slag[END_REF] Previous studies have revealed that the partitioning of heavy metals is affected by various factors such as (i) feedstock characteristics, i.e., physical and chemical properties of the heavy metals, the content of chlorine, sulfur, and alkalis; (ii) operating conditions, such as temperature, redox environment, residence time, incinerator type, and so on; and (iii) entrapment techniques, including flue gas treatment facilities, catalysts, and sorbents. [START_REF] Zhang | Fate of heavy metals during municipal solid waste incineration in Shanghai[END_REF][START_REF] Belevi | Factors determining the element behavior in municipal solid waste incinerators. 2. Laboratory experiments[END_REF][START_REF] De Boom | Belgian MSWI fly ashes and APC residues: A characterisation study[END_REF][START_REF] Huang | Effect of HCl/SO 2 /NH 3 /O 2 and mineral sorbents on the partitioning behaviour of heavy metals during the thermal treatment of solid wastes[END_REF] Because the oxidation/ reduction atmosphere and the operation parameters are quite different during MSW pyrolysis, gasification, and incineration, it is necessary to understand the transfer of heavy metals in these three generic groups of processes.

To date, numerous studies have discussed the fate of heavy metals during MSW incineration. [START_REF] Zhang | Fate of heavy metals during municipal solid waste incineration in Shanghai[END_REF][START_REF] Zhang | Experimental and thermodynamic investigation on transfer of cadmium influenced by sulfur and chlorine during municipal solid waste (MSW) incineration[END_REF][START_REF] Wang | Effects of chlorides on emissions of toxic compounds in waste incineration: study on partitioning characteristics of heavy metal[END_REF][START_REF] Belevi | Factors determining the element behavior in municipal solid waste incinerators. 1. Field studies[END_REF][START_REF] Sun | Volatilization of heavy metals during incineration of municipal solid wastes[END_REF] Nevertheless, the effect of MSW pyrolysis and gasification processes on the migration of heavy metals has not been investigated clearly. Yu et al. investigated the vaporization of metals for different oxygen concentrations (0%, 5%, and 20%), [START_REF] Yu | Vaporization of heavy metals during thermal treatment of model solid waste in a fluidized bed incinerator[END_REF] showing that the evaporation of Cd, Pb, Cu, and Zn was enhanced with decreasing oxygen content. However, Wu et al. drew an opposite conclusion. [START_REF] Wu | Effect of waste incineration and gasification processes on heavy metal distribution[END_REF] They mentioned that gasification could lower the evaporation of metals; Cd, Pb, and Cr concentration in the fly ash after gasification was about 3.3-10% of that after incineration. Jakob et al. analyzed the thermal treatment of MSW fly ash, [START_REF] Jakob | Evaporation of heavy metals during the heat treatment of municipal solid waste incinerator fly ash[END_REF] observing that a reductive atmosphere was effective for Zn evaporation but restrained Cu volatilization. On the other hand, to clarify the effect of redox atmosphere, previous studies are mostly based on theoretical calculation. [START_REF] Abanades | Fate of heavy metals during municipal solid waste incineration[END_REF][START_REF] Elled | Fate of zinc during combustion of demolition wood in a fluidized bed boiler[END_REF][START_REF] Enestam | Evaluation of the condensation behavior of lead and zinc in BFB combustion of recovered waste wood[END_REF] Studies that incorporate both theoretical and experimental investigations are quite lacking, especially taking into account the complex composition of the reaction systems and feedstock characteristics. To better reveal the tendency toward metal volatilization and the accompanying reaction mechanisms during MSW thermal treatment, such a quantitative study is in urgent demand.

Accordingly, the objective of the present work is to determine the effect of redox atmosphere on the partitioning of heavy metals during MSW thermal treatment. Six target heavy metals, including Cd, Zn, Pb, Cu, Ni, and Cr, are investigated in a temperature range of 550-850 °C. MSW pyrolysis, gasification, and incineration tests are performed in both a tubular furnace and a fluidized-bed reactor, which serve as models of the two most representative types of incinerator. Besides, thermodynamic equilibrium is also calculated, using the same reaction conditions as those applied in the experiments. The knowledge gained would assist in understanding the metal speciation and transfer mechanism during MSW thermal treatment and provide scientific experience to develop effective heavy metal control strategies.

MATERIALS AND METHODS

2.1. Sample Preparation. The handling of actual MSW in laboratory scale is difficult due to the heterogeneity of the feedstock and the uncontrollability in measurement. [START_REF] Patumsawad | Experimental study on fluidised bed combustion of high moisture municipal solid waste[END_REF] Thus, simulated MSW (S-MSW) is used in each test. The S-MSW is prepared based on the typical MSW physical composition in China, [START_REF] Dong | Comparison of municipal solid waste treatment technologies from a life cycle perspective in China[END_REF] consisting of food waste, paper, plastic, textile, timber, and rubber. The inert portion is added separately, either as SiO 2 powder in the tubular furnace or as bed material (silica sand) in the fluidized bed. The composition, elementary analysis, and heating value of the S-MSW are shown in Table 1.

The S-MSW is crushed and sieved to ∼2 mm. Because the background level of heavy metals in the S-MSW is negligible, target metals are added using impregnation method. [START_REF] Yu | Vaporization of heavy metals during thermal treatment of model solid waste in a fluidized bed incinerator[END_REF] Weighted metal chlorides are dissolved in distilled water. The S-MSW is immersed and stirred in this solution for 12 h to achieve good mixing and then dried at 85 °C for 24 h. The metal concentration (mg•kg -1 ) of the acquired feedstock is 575.7 (Cd), 3926 (Zn), 864.8 (Pb), 2301 (Cu), 931.8 (Ni), and 615.1 (Cr), respectively.

2.2. Tubular Furnace Experiments. A horizontal tubular furnace is used to investigate the volatilization of heavy metals. The experimental system comprises an electrically heated tubular furnace, a gas source and flow controller, and a flue gas absorption train (Figure 1). The tube is 40 mm in inner diameter and 700 mm in length, with a thermocouple inserted at the center monitoring the furnace temperature. The flue gas passes successively through three impingers immersed in an ice-water bath to capture heavy metals. The first and second impringers contain 5% HNO 3 and 10% H 2 O 2 as absorption solution, and the third one is filled with silica gel for moisture removal. [START_REF]Method 29 Determination of Metals Emissions from Stationary Sources; Office of Air Quality Planning and Stardards[END_REF] Reductive and oxidative conditions are supplied by N 2 , model synthetic gas, and air, respectively. The model gas is composed of 1.0% H 2 , 1.5% CH 4 , 3.0% CO, 6.5% CO 2 , and 88.0% N 2 , a composition typical for MSW air gasification. [START_REF] Xiao | Study on MSW fluidized-bed gasification and swirlmelting technology[END_REF] A total of 12 tests are conducted, with their detailed working conditions listed in Table 2. For each test, 3 g of S-MSW together with 0.5 g of SiO 2 is mixed and placed in a dried quartz boat. The gas is introduced into the tube at the heating stage of the experiment. When the furnace is heated to the desired temperature, the boat is pushed to the center of the combustion chamber. The combustion time is determined beforehand to ensure complete reaction. At the end of the experiment, the heating source and air supply are turned off. The boat is pulled out of the tube carefully after the furnace has cooled down to room temperature. The heavy metal contents in both solid residues and gaseous phase are detected. Connection pipes and the tube furnace are flushed by absorption solution, and then this solution is mixed with the gas sample.

2.3. Fluidized-Bed Experiments. A schematic diagram of the fluidized-bed experimental apparatus is depicted in Figure 2. The furnace is 60 mm in inner diameter and 1100 mm in height. It is electrically heated by three stages of resistance wire. Three K-type thermocouples are installed to monitor the reaction temperature at the top, middle, and bottom of the furnace. A speed-adjustable screw feeder is feeding the S-MSW into the furnace continuously. The fluidizing gases are air and N 2 ; the fluidizing flow rate is kept the same (U m = 0.183 N•m•s -1 = 1.5U mf ). The flue gas is treated by a cyclone separator, a glass-fiber filter, and the impinger train. The filter is maintained at 200 °C using heating tape to prevent vapor condensing in the pipe. Heavy metals in the flue gas are sampled by the same aforementioned method. For each test, the S-MSW feeding rate is varied to satisfy pyrolysis, gasification, and incineration conditions. The equivalence ratios for gasification and incineration are set at 0.4 and 1.2, respectively. Bed material, with particle size of 0.28-0.45 mm, is put into the furnace before heating. When the desired temperature is reached, the S-MSW begins to be fed into the furnace. Flue gas (100 L) is taken isokinetically once the system achieves stability. Gas bags are used to collect the dry and incondensable gas; the total generated gas is calculated based on N 2 balance. Heavy metals in the bottom ash, cyclone fly ash, and filter fly ash are all detected.

2.4. Thermodynamic Simulation. Thermodynamic simulation software, FactSage, is used to predict the equilibrium metal volatilization tendencies under different thermal conditions. The program uses the principle of minimizing the total Gibbs free energy of the system to calculate the thermodynamic equilibrium. The final equilibrium state is obtained by determining all possible species that are derived from the input elements. The modeling could provide a better understanding of the experimental results. When both experimental and simulative data are assimilated, the major metal species and possible reaction mechanisms during the specific thermal process can be proposed.

The calculation conditions are set similar to those applied in the experiments. The S-MSW used in the simulation is assumed to consist of the basic elements C, H, O, N, S, Cl, and H 2 O. The six heavy metals are input as metal chlorides. Besides, both alkali metals (K, Ca, Na, and Mg) and inert (SiO 2 ) are considered, since they have proven to be important in the vapor of heavy metals. [START_REF] Abanades | Fate of heavy metals during municipal solid waste incineration[END_REF][START_REF] Elled | Fate of zinc during combustion of demolition wood in a fluidized bed boiler[END_REF] Products output includes gas, pure solids, and pure liquids. The air supply strategies are set the same as that in the tubular furnace to represent reductive and oxidative conditions. The air is assumed to be composed of 79% N 2 and 21% O 2 . Equilibrium calculations are performed in the temperature range of 300-1000 °C, and the total pressure is 1 atm.

2.5. Analysis Method. To ensure the reliability of test data, each experiment is repeated three times and the average value is adopted. The gas composition resulting from fluidized-bed tests is determined by gas chromatography (micro GC-490 analyzer, Agilent). To analyze heavy metal concentrations, solid samples as the S-MSW, bottom ash, cyclone fly ash, and filter fly ash are digested with a mixture of HNO 3 , HCl, and HF in a microwave oven. The digested samples together with gas samples are then analyzed by inductively coupled plasma mass spectrometry (Xeries II; Thermo Fisher Scientific, Inc., U.S.A.).

RESULTS AND DISCUSSION

Evaporation of Heavy Metals in Tubular Furnace.

Under the test conditions in the tubular furnace, the mass balance closure of heavy metals is in a range of 84-97%, which is quite acceptable. [START_REF] Pohorěly | Behavior of heavy metals in steam fluidized bed gasification of contaminated biomass[END_REF] Heavy metals evaporation under different redox atmospheres and temperatures is depicted in Figure 3.

Generally, the proportion of heavy metals in the gaseous phase is increased gradually with temperature, because a higher temperature could raise the vapor pressure of metal compounds and enhance the rates of diffusion. [START_REF] Lu | Comparison of trace element emissions from thermal treatments of heavy metal hyperaccumulators[END_REF][START_REF] Liu | Volatilization behavior of Cd and Zn based on continuous emission measurement of flue gas from laboratory-scale coal combustion[END_REF] The redox atmosphere poses great influences on the evaporation of heavy metals: Cd and Zn are found to be more volatile under reductive atmosphere, whereas an oxidative atmosphere effectively promotes the volatilization of Pb, Cu, Ni, and Cr. When N 2 is injected instead of air, the evaporation of Cd and Zn is increased by 2-10% and 11-34% under experimental temperature ranges, respectively. On the contrary, Pb has a higher tendency to evaporate in oxidative conditions, increasing by 5-23% compared to N 2 atmosphere. As least volatile metals, 29% Cu, 22% Ni, and 19% Cr are released in an oxidative atmosphere at 850 °C, which are ∼2-9 times higher than those in N 2 .

To better understand the effect of redox atmosphere on the volatility of metals, a thermodynamic equilibrium calculation is carried out, with some of the most typical results presented in Figure 4. It is shown that metals are alternatively present as elemental form or sulfide under reducing conditions. At lower temperatures, sulfides are the dominant species and metals are mainly retained in the solid phase. The mass of elemental metals increases with elevated temperature. Cd, Pb, and Zn vaporize and thus are found in the flue gas; nevertheless, the majority of metallic Cu, Ni, and Cr remain in the solid residues due to their relatively high boiling point. In the case of oxidizing conditions, oxides are the dominant metal species. Part of Pb and Cu is found as gaseous PbCl, PbCl 2 , and CuCl when the temperature is >650 °C. The retention of Zn in the solid phase remains strong at higher temperatures due to the formation of complex zinc-matrix compound (Ca 2 ZnSi 2 O 7 ). The thermodynamic simulation results are in good accordance with the experimental observations. The significant difference in metal volatility characteristics under redox atmosphere can be explained by the formation of different chemical species. The evaporation of Cd and Zn is retrained under oxidative atmosphere, because the boiling points of CdO, ZnO, or zinc-matrix compounds are higher than those of their metallic forms. On the other hand, PbO is volatile to promote the evaporation of Pb under oxidative atmosphere. The boiling points of elemental Cu, Ni, and Cr are significantly higher than their oxides or chlorides, so that these metals can hardly be released under reducing conditions.

Besides, theoretical calculations also indicate that certain metal-matrix compounds such as (CdO) (SiO 2 ), Pb 3 Ca 2 Si 3 O 11 , Zn 2 SiO 4 , etc. are formed under oxidative atmosphere; however, these products are not found under reducing conditions. The results could also partly account for the low volatilization of Cd and Zn during S-MSW incineration experiments. Especially, Zn evaporation shows a strongly declining tendency under oxidative atmosphere. Consulting the vapor pressure of Zn compounds, it is expected that the volatility of Zn should be in a same level as Cd. The observed phenomena may be caused by the incorporation reaction of metals with SiO 2 , which will be enhanced with increasing oxygen concentration (eq 1). [START_REF] Belevi | Factors determining the element behavior in municipal solid waste incinerators. 2. Laboratory experiments[END_REF][START_REF] Huang | Effect of HCl/SO 2 /NH 3 /O 2 and mineral sorbents on the partitioning behaviour of heavy metals during the thermal treatment of solid wastes[END_REF][START_REF] Abanades | Kinetics of heavy metal vaporization from model wastes in a fluidized bed[END_REF] Under the incineration process, the formation of binary or ternary metal oxides is favored. These nonvolatile compounds can fill the pores and inhibit the volatilization of non-reacted metals, resulting in less metal evaporation. [START_REF] Yu | Vaporization of heavy metals during thermal treatment of model solid waste in a fluidized bed incinerator[END_REF][START_REF] Verhulst | Thermodynamic behavior of metal chlorides and sulfates under the conditions of incineration furnaces[END_REF]

+ + → • + MCl SiO O MO SiO Cl 2 2 2 2 2 2 (1)
It is well-recognized that the presence of chlorine would have a positive effect on the evaporation of heavy metals by forming volatile metal chlorides. [START_REF] Yu | Vaporization of heavy metals during thermal treatment of model solid waste in a fluidized bed incinerator[END_REF][START_REF] Jakob | Evaporation of heavy metals during the heat treatment of municipal solid waste incinerator fly ash[END_REF] Although ∼1.6% of Cl is contained in the S-MSW and metal chlorides are added as metal source, the evaporation of metals is not as much as expected, i.e., >70% of Cu, Ni, and Cr are retrained in the solid residues under oxidative atmosphere. This fact is verified by theoretical simulation that oxides are the major compounds under oxidizing condition. Figure 5 exhibits the equilibrium distribution of chlorine compounds under oxidizing condition. It is found that the total amounts of alkali chlorides produced are several orders of magnitude higher than heavy metal chlorides. Previous studies have proposed the preferential reaction of Cl with alkali metals. [START_REF] Pohorěly | Behavior of heavy metals in steam fluidized bed gasification of contaminated biomass[END_REF][START_REF] Zhong | Thermal characteristics and fate of heavy metals during thermal treatment of Sedum plumbizincicola, a zinc and cadmium hyperaccumulator[END_REF] Correlating with our experiments, the alkali metals are in strong abundance in the S-MSW (Cl/alkali metals is ∼0.08); therefore, the suppression of the Cl effect on heavy metal volatility can be expected.

3.2. Partitioning of Heavy Metals in Fluidized-Bed Furnace. Figure 6 shows the effects of equivalence ratio and temperature on the phase transfer of metals from fluidized-bed experiments. Results are presented as a mass percentage of the input metals to reflect the recovery rate. The total concentrations of metals cannot make 100%; possibilities for this incomplete closure will be discussed later in section 3.3. Metal partitioning is split over bottom ash, cyclone fly ash, and filter fly ash, while essentially no metals are detected in the flue gas. With respect to the relative distribution of metals over different ash fractions, more volatile metals such as Cd and Pb are enriched in filter ash to a large extent. The majority of Cu, Ni, and Cr are retained in bottom ash or cyclone ash.

Consulting the distribution of metals in the bottom ash, the obtained results exhibit a quite similar changed tendency with tubular furnace tests. Reductive atmosphere has a positive effect on the evaporation of Cd, Zn, and Pb. In contrast, Cu, Ni, and Cr in the bottom ash under reductive atmosphere are ∼1-7 times of those under oxidative atmosphere. However, the observed Pb vaporization is opposite compared to that in tubular furnace. It is found that, under fluidized-bed condition, Pb becomes less volatile during the incineration process. By checking the equilibrium prediction in Figure 4, it is speculated that the incorporation reaction (eq 1) may be favored. Because the fluidized bed affords more intense mixing and contact between feedstock and bed materials, [START_REF] Basu | Combustion of coal in circulating fluidized-bed boilers: a review[END_REF] the formation of matrix species such as Pb 3 Ca 2 Si 3 O 11 and PbZnSiO 4 may be enhanced and postpone or reduce the evaporation of Pb compounds. [START_REF] Yu | Vaporization of heavy metals during thermal treatment of model solid waste in a fluidized bed incinerator[END_REF][START_REF] Folgueras | Effect of inorganic matter on trace element behavior during combustion of coalsewage sludge blends[END_REF][START_REF] Van Lith | Release to the gas phase of inorganic elements during wood combustion. Part 2: Influence of fuel composition[END_REF] Liu et al. investigated the sewage sludge incineration ash by X-ray diffraction (XRD) analysis and found that Pb was primarily formed as PbSiO 4 in the bottom ash, which is nonvolatile. [START_REF] Liu | An experimental and thermodynamic equilibrium investigation of the Pb, Zn, Cr, Cu, Mn and Ni partitioning during sewage sludge incineration[END_REF] Zhang et al. conducted a 2-year field study on two large-scale incinerators, [START_REF] Zhang | Fate of heavy metals during municipal solid waste incineration in Shanghai[END_REF] observing that only 21-54% of Pb was transferred to the gaseous phase despite its high volatility. On the other hand, Zn also exhibits abilities to react with other metals to form complex compounds. [START_REF] Abanades | Fate of heavy metals during municipal solid waste incineration[END_REF][START_REF] Verhulst | Thermodynamic behavior of metal chlorides and sulfates under the conditions of incineration furnaces[END_REF] It is thus suggested that the metal-matrix reaction is one of the most important aspects affecting the transfer of metals and is correlated to the presence of mineral substances, alkalis, and the interactions among different existing metals.

As least-volatile metals, lower volatility is found for Cu, Ni, and Cr in a reductive than in an oxidative environment. The result is in good agreement with the tubular furnace tests, which confirms that metals are mainly reduced to elemental form or sulfide under reducing atmosphere. The proportion of Cu, Ni, and Cr in the cyclone ash is increasing with elevated equivalence ratio. However, no obvious trend is found for their transfer to the filter. The proportion of Cu, Ni, and Cr in the filter ash is found similar (1-9%) and fluctuates with equivalence ratio and temperature. It can be postulated that the condensation of Cu, Ni, and Cr occurs most probably in the cyclone; their transfer to the filter ash is primarily due to entrainment. Meanwhile, results also reveal that ∼13-84%, 8-72%, and 21-72% of the detected Cu, Ni, and Cr, respectively, is transferred to the cyclone ash. The values are much higher than those from tubular furnace tests and literature. [START_REF] Zhang | Fate of heavy metals during municipal solid waste incineration in Shanghai[END_REF][START_REF] Vervaeke | Fate of heavy metals during fixed bed downdraft gasification of willow wood harvested from contaminated sites[END_REF][START_REF] Jung | Metal distribution in incineration residues of municipal solid waste (MSW) in Japan[END_REF] The high gas velocity used in the fluidized bed is responsible for this distinction, which has also influence on the transfer of heavy metals by entrainment.

To verify the metal migration mechanism, the cyclone ash samples are then separated into different particle sizes by vibration sieve (>250, 150-250, 106-150, 62-106, 48-62, and <48 μm). The metal concentration of the fractioned ashes is detected and illustrated in Figure 7. Results demonstrate that the concentration of Cd increases with decreasing particle sizes no matter whether under reductive or oxidative atmosphere, indicating that the transfer of Cd to the cyclone is mainly from evaporation. Similar distribution behavior of Pb is observed. However, Cu and Ni are distributed uniformly with different sizes under pyrolysis and gasification cases. It reveals that although part of Cu and Ni is found in the cyclone ash under reductive condition, they are dominantly transferred by entrainment. On the contrary, these less-volatile metals exhibit enrichment in finer particles under incineration process, proving that not only entrainment but also evaporation determines their migration. Table 3 presents the concentration of metals in different ash fractions. Most heavy metals are enriched in the cyclone. This confirms that most of the vaporized metals are condensed upon cooling at the working temperature of the cyclone, i.e., 350-600 °C. Besides, entrainment also contributes to the high metal concentrations in the cyclone, which is especially outstanding for less-volatile metals based on the analysis in Figure 7. The concentration of Cd and Pb is relatively high in the filter ash under reductive atmosphere, which is comparable to or even exceeds that content in the cyclone ash. More volatile Cd and Pb species easily form under reductive atmosphere and will remain in the gaseous phase after passing through the cyclone. Therefore, for both pyrolysis and gasification processes, effective cleaning prior to syngas utilization should be concerned. Meanwhile, a particular enrichment of Zn in the bottom ash is observed under oxidative atmosphere. Special attention should be paid to this, because the metal concentration may exceed the limited value if the incineration bottom ash is further recycled and reutilized.

3.3. Discussion. The average recovery rate of heavy metals under fluidized-bed tests is <50%, which is significantly lower than that from the tubular furnace. Several possibilities may account for this. Some metals may be condensed, depositing on the furnace wall, cyclone, and piping. Lower recoveries of 25-74% are found for Cd, Zn, and Pb; on the contrary with 48-103% for Cu, Ni, and Cr. This indicates that the production of the generated gas and filter fly ash may be underestimated, because the high volatility may have resulted in a higher proportion of metals escaping from the furnace. Besides, incineration exhibits a lower metal recovery rate than pyrolysis and gasification processes. Part of the metals may be adsorbed or bounded by the bed materials, which will be sieved from the bottom ash and missed. Because the particle size of the incineration bottom ash is much smaller due to more thorough decomposition of the feedstock, an elevated proportion of metals will thus be excluded from the results.

Comparing the results from a tubular furnace and a fluidized bed, a similar transfer tendency of heavy metals is obtained. Generally, the volatility of target metals in decreasing order is Cd > Pb > Zn > Cu > Ni and Cr (the volatility of Ni and Cr is comparable in the analyzed temperature ranges). The experimental observations accord well with the thermodynamic equilibrium results. Tubular furnace is effective to reflect the evaporation of metals, whereas fluidized bed further reveals the effect of several factors, such as flue gas condensation and entrainment, on the distribution of metals over different ash fractions. Nevertheless Pb vaporization behaves differently from the two reactors. This may be attributed to the enhanced metalmatrix reaction as analyzed from the thermodynamic simulation, which emphasizes the influence of incinerator type on the transfer of metals. Overall, during MSW thermal treatment processes, parameters determining the partitioning of heavy metals can be summarized as (i) metal speciation, which depends primarily on the redox atmosphere, temperature, and presence of alkalis, chloride, sulfur, and mineral substances; (ii) system characteristics, such as furnace type and working temperature of the cyclone; (iii) mechanical entrainment of particles caused by gas velocity.

A key aspect to optimize the MSW thermal techniques will therefore be focused on reducing the environmental impact of the contaminated ashes. To improve the quality of residues, methods should be proposed aiming at concentrating most of the heavy metals in a tiny ash fraction and recycling the remainder of the ashes safely as secondary materials. [START_REF] Vervaeke | Fate of heavy metals during fixed bed downdraft gasification of willow wood harvested from contaminated sites[END_REF][START_REF] Narodoslawsky | From waste to raw materialthe route from biomass to wood ash for cadmium and other heavy metals[END_REF] Practices have proven that the typical operating temperature of gasification is ∼700 °C, [START_REF] Arena | Gasification of a solid recovered fuel in a pilot scale fluidized bed reactor[END_REF] lower than direct MSW incineration at ca. 850 °C. Consulting the experimental data from this study, most of the less-volatile metals, such as Cu, Ni, and Cr, are immobilized under the working condition of gasification. It indicates the recycling potential of the gasification bottom ash. Studies have proven that via the gasification-melting process, 2 metals could be separated and recovered from the residues. The melting slag is stable and contains few harmful heavy metals, which could be reused as road and construction material. Realizing these advantages, more in-depth investigation is necessary to develop gasification as a potential optimum for the current MSW incineration method.

CONCLUSIONS

The partitioning of heavy metals during S-MSW pyrolysis, gasification, and incineration are investigated. Tubular furnace tests show that a reductive atmosphere promotes the evaporation of Cd and Zn but inhibits the volatilization of Pb, Cu, Ni, and Cr. Thermodynamic equilibrium calculation reveals that the metal speciation differs a lot under redox atmosphere. Metals are alternatively reduced to elemental form or sulfide under reducing conditions. In the case of oxidizing conditions, oxides are the dominant species because of the preferential reaction of chloride with alkalis. Fluidized-bed results exhibit a quite similar changed tendency with that of the tubular furnace. However, Pb behaved oppositely due to the formation of stable metal-matrix compounds. Consulting the metal concentration of the fractioned cyclone ash, most of the vaporized heavy metals are condensed upon cooling at the working temperature of the cyclone (350-600 °C); however, entrainment is also a determining factor for the transfer of lessvolatile metals under reductive atmosphere. Cd and Pb are volatile and enriched in the filter ash; thus, effective flue gas cleaning is essential for emission control. Gasification traps more of the less-volatile metals in the bottom ash; their recycling potential should be of concern to better develop heavy metal control strategies.

Figure 2 .

 2 Figure 2. Schematic diagram of the fluidized-bed furnace: 1, screw feeder; 2, furnace; 3, thermocouples; 4, controller; 5, cyclone separator; 6, induced draft fan; 7, glass-fiber filter covered with a heating tape; 8, absorption solution with 5% HNO 3 + 10% H 2 O 2 ; 9, silica gel; 10, rotor flowmeter; 11, vacuum pump; 12, accumulative flowmeter.

Figure 3 .

 3 Figure 3. Evaporation S-MSW of heavy metals from tubular furnace experiment. "Syn." is short for synthetic gas.

Figure 4 .

 4 Figure 4. Thermodynamic equilibrium distribution of heavy metals under different redox atmosphere. "Re." represents reductiveand the results under N 2 atmosphere are presented; "Ox." is short for oxidative atmosphere.

Figure 5 .

 5 Figure 5. Thermodynamic equilibrium distribution of chlorine compounds under oxidizing condition.

Figure 6 .

 6 Figure 6. Partitioning S-MSW of heavy metals from fluidized-bed furnace experiment. Equivalence ratio 0, 0.4, and 1.2 represents pyrolysis, gasification, and incineration, respectively.

Figure 7 .

 7 Figure 7. Concentration of heavy metals in the cyclone ash with different particle sizes. Tests at 850 °C. ER = 0, 0.4, and 1.2 represents pyrolysis, gasification, and incineration, respectively.

Table 1 .

 1 Composition and Characteristics of the Simulated Municipal Solid Waste

	composition (as received basis)	elementary analysis (air-dried basis)
	rice	29%	C	48.4%
	cabbage	29%	H	6.3%
	cardboard	13%	O	29.2%
	polyethylene (PE)	17%	N	0.8%
	polyvinyl chloride (PVC)	3%	S	0.3%
	sawdust	5%	Cl	1.6%
	cotton fabric	2%	moisture	9.2%
	scrap tire	2%	ash	4.2%
			heating value	22.5 MJ•kg -1

Figure 1. Schematic diagram of the horizontal tubular furnace: 1, gas source; 2, gas flow controller; 3, horizontal quartz tubular furnace; 4, quartz boat; 5, electric heater and temperature controller; 6, absorption solution with 5% HNO 3 + 10% H 2 O 2 ; 7, silica gel; 8, ice-water bath.

Table 2 .

 2 Working Conditions Used for Tubular Furnace Experiment

	reaction time (min)

atmosphere gas flow rate (l•min -1 ) temperature (°C)

Table 3 .

 3 Concentration of Heavy Metals in Different Ash Fractions; Tests at 850 °C (mg•g -1 ; dry weight basis)

			Cd	Zn	Pb	Cu	Ni	Cr
	bottom ash	ER = 0 a	0.03	7.44	0.36	9.75	6.49	2.84
		ER = 0.4	0.11	17.65	0.27	4.00	2.75	2.61
		ER = 1.2	0.22	29.87	0.77	4.50	3.02	2.09
	cyclone fly ash	ER = 0	7.05	22.10	6.13	11.77	7.32	3.18
		ER = 0.4	9.65	25.41	9.65	39.73	11.74	7.72
		ER = 1.2	4.44	14.45	3.11	29.22	11.23	6.91
	Filter fly ash	ER = 0	7.01	14.25	7.51	1.01	0.59	0.79
		ER = 0.4	4.14	9.21	6.95	0.42	1.72	0.56
		ER = 1.2	0.73	1.65	2.15	0.33	0.20	0.17
	a ER = equivalence ratio; ER = 0, 0.4, and 1.2 represents pyrolysis, gasification, and incineration, respectively.

■ ACKNOWLEDGMENTS

This project is supported by the National Basic Research Program of China (no. 2011CB201506), the National Natural Science Foundation of China (no. 51276168), and the Program of Introducing Talents of Discipline to University (B08026).