Jun Dong
email: zd_dongjun@zju.edu.cn

Yuanjun Tang

Ange Nzihou

Yong Chi

Elsa Weiss-Hortala

Mingjiang Ni

Life cycle assessment of pyrolysis, gasification and incineration waste-to-energy technologies: theoretical analysis and case study of commercial plants

Keywords: Waste-to-energy technology, Environmental sustainability, Life cycle assessment, Non-toxic and toxic impacts, Large-scale commercial plants, Improvement and impediments

Municipal solid waste (MSW) pyrolysis and gasification are in development, stimulated by a more sustainable waste-to-energy (WtE) option. Since comprehensive comparisons of the existing WtE technologies are fairly rare, this study aims to conduct a life cycle assessment (LCA) using two sets of data: theoretical analysis, and case studies of large-scale commercial plants. Seven systems involving thermal conversion (pyrolysis, gasification, incineration) and energy utilization (steam cycle, gas turbine/combined cycle, internal combustion engine) are modelled. Theoretical analysis results show that pyrolysis and gasification, in particular coupled with a gas turbine/combined cycle, have the potential to lessen environmental loadings. The benefits derive from an improved energy efficiency leading to less fossil-based energy consumption, and reduced process emissions by syngas combustion. Comparison among the four operating plants (incineration, pyrolysis, gasification, gasification-melting) confirms a preferable performance of the gasification plant attributed to syngas cleaning. The modern incineration is superior over pyrolysis and gasification-melting at present, due to the effectiveness of modern flue gas cleaning, use of combined heat and power (CHP) cycle, and ash recycling. Sensitivity analysis highlights a crucial role of the plant efficiency and pyrolysis char land utilization. The study indicates that the heterogeneity of MSW and syngas purification technologies are the most relevant impediments for the current pyrolysis/gasification-based WtE. Potential development should incorporate into all process aspects to boost the energy efficiency, improve incoming waste quality, and achieve efficient residues management.

Introduction

In the transition towards more sustainable development, treatment technologies for municipal solid waste (MSW) have made considerable progress [START_REF] Zhao | Life cycle assessment of municipal solid waste management with regard to greenhouse gas emissions: Case study of Tianjin[END_REF].

The last decades witnessed a gradually decreased proportion of landfill as required by the European Landfill and Waste Framework Directives (Council of European Communities, 1999[START_REF] Amsa | Silla 2 waste to energy plant[END_REF]. In contrast, waste-to-energy (WtE) is gaining increasing interest. Until recently, incineration is the most widespread WtE technology with more than 1400 incineration plants in operation around the world [START_REF] Leckner | Process aspects in combustion and gasification Waste-to-Energy (WtE) units[END_REF].

However, even the last generation of MSW incinerators is limited by a low electricity efficiency up to about 22-25% [START_REF] Panepinto | Environmental performances and energy efficiency for MSW gasification treatment[END_REF], due to the limitation in the maximum steam temperature of the boiler, normally less than 450 o C to prevent corrosion by gaseous HCl [START_REF] Belgiorno | Energy from gasification of solid wastes[END_REF]. Although modern and well-operated incinerators can fulfil the requirements of an environmentally sound technology, potential risk of PCDD/Fs still present as a debate for the public. As a consequence, technological development towards more environmental-friendly and energy-efficient alternative WtE options are still required.

In recent years, there is considerable interest in new WtE technologies particularly pyrolysis and gasification, which attain the possibility to obtain a syngas suitable for different applications [START_REF] Funari | The potential impact of municipal solid waste incinerators ashes on the anthropogenic osmium budget[END_REF][START_REF] Khoo | Life cycle impact assessment of various waste conversion technologies[END_REF]. About energetic use in WtE plants, there is a general perception that pyrolysis and gasification could achieve a higher efficiency by supplying the syngas with a more efficient energy conversion device such as a gas turbine/combined cycle (gas turbine/CC) or an internal combustion engine [START_REF] Arena | Process and technological aspects of municipal solid waste gasification[END_REF]. Even if in a steam cycle plant, the limitation of efficiency could be overcome by adding gas pre-treatment before it goes into the burner, to allow the removal of HCl and an improvement in steam temperature of 520-540 o C [START_REF] Belgiorno | Energy from gasification of solid wastes[END_REF]. Besides, pyrolysis and gasification have the potential to diminish PCDD/Fs [START_REF] Noma | Development of waste gasification and gas reforming system for municipal solid waste (MSW)[END_REF], thus reducing the total generation of pollutants if the downstream syngas oxidization is processed efficiently. However, using of the newly developed WtE options does not automatically guarantee the total sustainability of the whole multi-stage thermal conversion and energy utilization chain [START_REF] Ning | Benefit assessment of cost, energy, and environment for biomass pyrolysis oil[END_REF][START_REF] Wang | Life cycle assessment of fast pyrolysis of municipal solid waste in North Carolina of USA[END_REF]. The "raw" syngas, which contains a variety of contaminants such as H 2 S, tar, NH 3 and particulate matter (PM), needs to be purified to meet the stringent requirement of entering an engine [START_REF] Wood | Review of state-of-the-art waste-to-energy technologies[END_REF].

The configuration of different energy cycles downstream may also influence the overall environmental effects: the consumptions and losses of gasification and syngas clean-up may cause the overall energy efficiency be close or lower to incineration. It is not a simple procedure to select an optimal WtE technology. A comprehensive assessment of different WtE process configurations is necessary to understand if pyrolysis/gasification-based WtE may become potential alternative or improvement for the current incineration.

Guided by ISO standards (ISO, 1997), life cycle assessment (LCA) is benefited from the quantification of the entire life cycle impacts. This can help identify the most critical process for environmental burdens [START_REF] Millward-Hopkins | Fully integrated modelling for sustainability assessment of resource recovery from waste[END_REF], and provide a benchmark for new technologies. LCA has provided reliable evaluation of MSW treatment technologies [START_REF] Kaplan | Is it better to burn or bury waste for clean electricity generation?[END_REF][START_REF] Lundie | Life cycle assessment of food waste management options[END_REF][START_REF] Morselli | Environmental impacts of waste incineration in a regional system (Emilia Romagna, Italy) evaluated from a life cycle perspective[END_REF][START_REF] Wäger | Life cycle assessment of post-consumer plastics production from waste electrical and electronic equipment (WEEE) treatment residues in a Central European plastics recycling plant[END_REF]. However, LCA of WtE technologies is rarely performed other than incineration. This is mainly because the operational practice using pyrolysis and gasification is quite limited despite that a number of applications do exist [START_REF] Molino | Biomass gasification technology: The state of the art overview[END_REF][START_REF] Panepinto | Environmental performances and energy efficiency for MSW gasification treatment[END_REF], making comparisons very difficult. The existing studies are focused mainly on the thermal conversion process itself, while few of them examine the downstream use of syngas in detail. The environmental performance of WtE options depends on many factors such as emission levels, energy efficiencies, type of end-use applications, and energy source. However, the LCA studies available on pyrolysis and gasification are often based on varying assumptions and insufficient to thoroughly study these issues. This may limit the LCA comparisons between different WtE technologies on a consistent and common basis.

The goal of this work is to provide a detailed life cycle investigation of different WtE technologies. In response to the incompletion and scarcity of data on pyrolysis and gasification, this study is striving to conduct both theoretical analysis of the possible configuration of WtE technologies and real case studies of several commercial plants. In the first part, a general and extensive theoretical analysis of seven multi-stage WtE systems involving thermal conversion (pyrolysis, gasification, incineration) and energy utilization (steam cycle, gas turbine/CC, internal combustion engine) is modelled, using the most typical and well-accepted reported data. In the second part, four large-scale commercial operation WtE plants (pyrolysis, gasification, gasification-melting, modern incineration) are compared. Besides, a sensitivity analysis is carried out to identify key parameters responsible for the environmental impacts. This study aims at understanding how the current WtE could get a benefit towards a more environmentally sustainable technology. Potential improvements and impediments to the further development of pyrolysis and gasification-based WtE technologies are also discussed and suggested.

Methodology

System definition

The system boundaries (Fig. 1) of the study attain at the moment when MSW enters the WtE plant. Four basic processes are included: (1) MSW pre-treatment, (2) thermal conversion, (3) utilization of acquired products, and (4) ash and air pollution control (APC) residues management. MSW can either be thermally converted by adding sufficient amount of air (incineration), where the MSW is fully oxidized into process heat; or by supplying an air deficiency, where the waste is pyrolyzed (in the absence of air) or gasified (in a partial oxidant amount lower than stoichiometric combustion).

The latter case produces intermediated products including syngas, tar and char, which can recover energy in several pathways [START_REF] Molino | Biomass gasification technology: The state of the art overview[END_REF]: to be combusted in a boiler and connected with a steam turbine; or, after a purification step, to be used in a gas turbine/CC or an internal combustion engine. Thus a total of seven scenarios are formed. S1 is defined as MSW direct incineration to represent the current WtE. S2, S3

and S4 represents pyrolysis coupled with steam turbine, gas turbine/CC and internal combustion engine, respectively; gasification combined with those energy devices are defined as S5, S6 and S7. MSW pre-treatment mainly refers to drying and shredding with the aim of size reduction and homogenization. While incineration plants could process MSW directly [START_REF] Evangelisti | Life cycle assessment of conventional and two-stage advanced energy-from-waste technologies for municipal solid waste treatment[END_REF], pre-treatment is basically needed prior to pyrolysis/gasification [START_REF] Mckendry | Energy production from biomass (part 3): gasification technologies[END_REF]. Detailed flowchart of each system is illustrated in the Supplementary Material (Fig. S1).

The functional unit is set at one ton of MSW as received at the plant. Upstream production of fuels and materials including diesel, electricity, lime, etc. is considered as the 'cradle to grave' type of calculation. The benefits from useful co-products, such as electricity and heat, are allocated by system expansion. The recovered electricity is assumed to substitute that provided by the "energy mix" of a specific region, here the European average (42.7% fossil fuels, 26.5% nuclear, 30.0% renewable energies, 0.7% waste and 0.1% other in 2015) is selected (Eurostat). The produced heat displaces an equal amount of heat generated by "heat mix", of which the heat production data based on European average is again used (69.3% fossil fuels, 0.2% nuclear, 22.9% renewable energies, 4.8% waste and 2.7% other in 2015) (Eurostat).

The database Gabi 7.0 provides the remaining, mainly indirect burdens, of the background system.

MSW characteristics

The MSW typically treated in the WtE plant is the residual from the source-separated collection of dry recyclables and wet organic fractions. The waste characteristics in different countries have a high variability depending on the culture, climate and socioeconomic [START_REF] Vergara | Municipal solid waste and the environment: a global perspective[END_REF]. Therefore, a typical MSW, reflects the average waste composition in Europe [START_REF] Arena | A life cycle assessment of environmental performances of two combustion-and gasification-based waste-to-energy technologies[END_REF], is selected as the basis for comparison (Table 1).

Data source for theoretical analysis

The data utilized for theoretical analysis are mainly derived from industrial practice, peer-reviewed literature, standards, and recent research reports. The data are regionalized in the sense that they refer to the situation of Europe. For each of the WtE system, the modelling of material consumptions, emissions and energy recovery is analyzed on basis of mass and energy balance, the detailed calculations are available in the Supplementary Material (Section SM-8). Each unit process and the main data source are presented as following.

MSW pre-treatment

A pre-treatment step is assumed to be conducted before pyrolysis and gasification process. In order to facilitate homogenization, the incoming MSW is shredded to an average size of a few hundred millimeters (e.g. a size of around 100 mm in practice).

The estimated energy use for mechanical treatment is set at 100 kWh of electricity and 25 kWh of natural gas per ton of MSW [START_REF] Kourkoumpas | Comparison of waste-to-energy processes by means of life cycle analysis principles regarding the global warming potential impact: applied case studies in Greece, France and Germany[END_REF]. The waste then undergoes drying to a final moisture content of around 10%. The heat required by the dryer is internally supplied with a thermal efficiency of 90% [START_REF] Roberts | Life cycle assessment of biochar systems: estimating the energetic, economic, and climate change potential[END_REF]. For systems using gas turbine/CC and internal combustion engine (S3, S4, S6 and S7), the heat derives from the syngas purification unit which recovers the sensible heat of the hot syngas during cooling. For S2 and S5, the heat is supplied by the hot flue gas.

Thermal conversion (pyrolysis, gasification and direct incineration)

For pyrolysis, the proportion of each product (syngas, tar and char) is strongly dependent on the reaction temperature, residence time and heating rate ( [START_REF] Van De Velden | Fundamentals, kinetics and endothermicity of the biomass pyrolysis reaction[END_REF]. For waste processing, a running temperature of 500-550 °C is widely used in industrial plants [START_REF] Chen | Pyrolysis technologies for municipal solid waste: a review[END_REF]. This pyrolysis technology, represented by the RWE-ConTherm â process [START_REF] Hauk | ConTherm-thermal utilisation of waste in a power plant by integrated waste pyrolysis of high-calorific residual derivate fuels[END_REF], is considered in this analysis, since it is the most typical pyrolysis process presently available in the European market.

The pyrolysis reactor is a rotary kiln type, with a residence time of approximately 1 hour. About 85% of the energy will be converted into the hot gas (i.e., hot gas efficiency), with cold gas efficiency attaining around 50%. The cold gas efficiency can be defined as the ratio of the energy content of the cold syngas to that of the feedstock. The balance is char, and its mass proportion is around 30%. The data are based on average reported values of the industrial plants (DGEngineering -The rotary kiln engineers, July 2009a, b). We assume the reliability is high because they can be cross-checked extensively.

Gasification owns the sole objective to produce syngas, although the generation of tar is inevitable along with the gas. In comparison to pyrolysis, gasification occurs at a generally higher temperature: 550-900 °C in air gasification and 1000-1600 °C if using pure oxygen, oxygen-enrich gas or steam [START_REF] Arena | Process and technological aspects of municipal solid waste gasification[END_REF]. Based on several operation data from the existing plants, the cold gas efficiency is in a range of 50-80% [START_REF] Arena | Process and technological aspects of municipal solid waste gasification[END_REF]. Here a cold gas efficiency of 70% is used as a conservative estimate [START_REF] Panepinto | Environmental performances and energy efficiency for MSW gasification treatment[END_REF][START_REF] Yassin | Techno-economic performance of energy-from-waste fluidized bed combustion and gasification processes in the UK context[END_REF]. A hot gas efficiency of 90% is assumed in the case syngas is directly used in a boiler without any pre-cooling.

MSW direct incineration is well-proven and has greater operational reliability than pyrolysis and gasification. The assumed incineration is based on a moving grate. The waste is directly combusted to heat up water in the boiler to generate steam. A heat loss is also inevitable, for example the discharge of the ash and flue gas will cause a high loss of the sensible heat. However, we do not tend to assume this efficiency, since it will be reflected in the overall plant efficiency.

While the incineration process is exothermic, gasification can also achieve heat self-sustaining around an equivalence ratio of 0.3-0.4 [START_REF] Zhang | Thermodynamic evaluation of biomass gasification with air in autothermal gasifiers[END_REF], i.e. no any external thermal assistance is needed, the same under which in the real plants [START_REF] Arena | Gasification of a solid recovered fuel in a pilot scale fluidized bed reactor[END_REF]. Nevertheless, pyrolysis requires an additional thermal energy to maintain the reaction. The input energy is around 9% of the MSW energy according to the research of Baggio et al. [START_REF] Baggio | Energy and environmental analysis of an innovative system based on municipal solid waste (MSW) pyrolysis and combined cycle[END_REF]. The heat is assumed to be supplied by the hot flue gas as it is commonly preferred in the plants.

Energy utilization cycles

Each WtE plant in this study is assumed to be an integrated facility, in which the final energy utilization is operated on-site. The electrical efficiency of the incineration plant is set at 22.5% [START_REF] Arena | Process and technological aspects of municipal solid waste gasification[END_REF][START_REF] Morris | Energy recovery from solid waste fuels using advanced gasification technology[END_REF], which represents an average of the modern dedicated waste combustion systems. For pyrolysis and gasification systems, steam cycle is the simplest option because the hot syngas could undergo combustion in the gas boiler without purification. A higher efficiency can be achieved (set at 27.8% in this study), since the homogenous and gas-phase combustion is more controllable and effective [START_REF] Consonni | Waste gasification vs. conventional Waste-To-Energy: A comparative evaluation of two commercial technologies[END_REF]. The syngas can also be burned in a gas turbine/CC or an internal combustion engine.

Potentially, the electrical efficiencies would be higher (set at 35.5% and 25.0% for gas turbine/CC and engine, respectively [START_REF] Belgiorno | Energy from gasification of solid wastes[END_REF][START_REF] Morris | Energy recovery from solid waste fuels using advanced gasification technology[END_REF]). However, the syngas needs to be cooled and purified to meet the stringent inlet gas quality requirement. To ensure the transparency of the data, the values of plant efficiencies are determined by extensively searching and comparing with similar set-up in the literature and reports (see details in Table S2). Additionally, a range of variations of each plant efficiency will be discussed in the sensitivity analysis. For all the systems analyzed, 20% of the generated electricity is assumed to be self-consumed in the plant, with the remaining 80% sent to the power grid.

For systems using gas turbine/CC and internal combustion engine (S3, S4, S6 and S7), cleaning the syngas allows the chemical energy to be conserved. The sensible heat is recovered assuming an efficiency of 75% [START_REF] Yi | Energy evaluation for lignite[END_REF]. As stated earlier, the heat is used for MSW pre-treatment; the excessive amount is transferred to the needs of the end user. The formed pyrolysis char can either be combusted at the facility to generate more energy or be used as a product (biochar). The former application is considered as the baseline, while the latter case will be discussed in the sensitivity analysis. The pyrolysis char is assumed to be sent into the boiler and combusted together with the gas in the S2 system, which is in accordance with the real operation in reference plants. If a gas turbine/CC or internal combustion engine is used, the char is assumed to be combusted in a separated boiler for heat production at a thermal efficiency of 75%, which is a typical value for industrial heating boilers in operation [START_REF] Roberts | Life cycle assessment of biochar systems: estimating the energetic, economic, and climate change potential[END_REF].

Emissions at the stack

In attempt to better perform a transparent evaluation, the emission factors used in this theoretical analysis are estimated using the European pollution control standards, i.e., the exhaust flue gas from each WtE system is assumed to meet the requirements of specified emission standards (Directive 2007/76/EC (The Commission of the European Communities, 2007) and Directive 2010/75/EU with some adaptions (Directive, 2010)). The real emission data from industrial plants will be analyzed in the second part (case studies). Table 2 summarizes the related emission factors. These data have been used in conjunction with estimates of flue gas volumes per functional unit of MSW produced to derive the final mass release rates. Details on the standards, adaptations and calculations can be found in the Supplementary Material (Section SM-5).

Ash and air pollution control residues management

The amount of solid resides produced by incineration and pyrolysis/gasification plants are assumed to be 180 kg/t-MSW and 120 kg/t-MSW, respectively, as reported by UK's waste report (DEFRA UK, 2004). The solid residues may be recycled as road construction materials or concrete aggregate [START_REF] Sakai | Municipal solid waste incinerator residue recycling by thermal processes[END_REF]. However, only landfill is considered in the theoretical analysis and the potential benefit will be included in the case studies of the commercial plants. The APC residues, including mainly fly ashes and exhausted sorbents, are assumed to be stabilized before final disposal in landfill. Emissions, mainly heavy metals to the soil, are estimated according to the UK's waste report (DEFRA UK, 2004). 2 and Table S5 summarizes the related emission factors and information of these plants, respectively; with a brief introduction of each plant presented as following.

l Incineration plant (C1): Silla 2 incineration plant, located in Milan, Italy, is studied as a typical case of the modern incineration. The plant is equipped with 3 moving grate combustion lines, having a treatment capacity of 450,000 t/a. MSW is incinerated at 850 °C to produce electricity and district heating at an efficiency of 24% (net) and 6%, respectively [START_REF] Turconi | Life cycle assessment of waste incineration in Denmark and Italy using two LCA models[END_REF]. The flue gas cleaning includes electrostatic precipitator, acid gas neutralization (NaHCO 3 injection), fabric filter and a SCR unit for NO x abatement [START_REF] Amsa | Silla 2 waste to energy plant[END_REF]. After combustion, metals are sorted from the bottom ash and recycled. 88% of the bottom ash is utilized in road construction, while the remaining fraction is landfilled and the APC residues are safety disposed.

l Pyrolysis plant (C2): The selected plant, located in Hamm, Germany, has a capacity of 100,000 t/a, although it is no longer in operation after the chimney collapse in 2009. The pyrolysis process belongs to the RWE-ConTherm â technology (DGEngineering -The rotary kiln engineers, July 2009a). After shredded to 200 mm, the MSW is decomposed in the absence of air in a rotary kiln at 500 o C with a residence time of 1 hour, using natural gas as the heating source. The products, hot syngas and char, are incinerated in the boiler of a coal-fired plant for electricity production. The residues are considered to be landfilled and the metals are recycled.

The plant electricity efficiency (gross) is around 22% [START_REF] Stein | Review of Small Scale Waste Conversion Systems[END_REF].

l Gasification plant (C3): The selected plant, Lahti II, located in Finland, has started its commercial operation in 2012 with an annually capacity of 250,000 tons (Lahti Energia). The feedstock is solid recovered fuels (SRF), i.e., high calorific waste unsuitable for recycling. The gasifier is a circulating fluidized bed operated at 850-900 o C. The syngas generated undergoes cooling at 400 o C to remove heavy metals and PM. The cleaned syngas enables a more efficient heat recovery boiler at 121 bar and superheated steam at 540 o C. The plant attains final 27% of electricity efficiency (net) and 61% of heat efficiency [START_REF] Savelainen | Kymijärvi II plant: High-efficiency use of SRF in power production through gasification[END_REF]. The flue gas cleaning system consists of a bag house filter with additive injections (NaHCO 3 and activated carbon) and a SCR for NO x reduction. From the plant outlet, the bottom ash is removed to landfill disposal and the APC residues are safety disposed.

l Gasification-melting plant (C4): The reason to select this technology is its possibility to recover materials effectively [START_REF] Tanigaki | Co-gasification of municipal solid waste and material recovery in a large-scale gasification and melting system[END_REF]. The selected plant, having a total throughput of 80 MW, is located in Japan and is one of the largest gasification-melting facilities in the world. The MSW is charged into a shaft-furnace type gasifier from the top with coke and limestone, and the ash is melt at the bottom by O 2 -rich air at 1000-1800 o C. No pre-treatment of the incoming waste is required.

The syngas is transferred to be combusted to generate steam at 400 o C and 3.92 MPa.

The electricity efficiency (gross) attains at 23% [START_REF] Tanigaki | Co-gasification of municipal solid waste and material recovery in a large-scale gasification and melting system[END_REF]. The flue gas cleaning applies a quencher, a baghouse with Ca(OH) 2 injection for desulfurization, a re-heater and a SCR for NO x reduction. The molten materials from the gasifier are magnetically separated into slag and metals, which can be completely recycled; while the APC residues are further treated.

Life cycle inventory

By combining all unit processes input-output data, a detailed LCI table is compiled (see Table S6 andTable S7). Biogenic CO 2 is assumed to be carbon neutral to global climate change. For the specific MSW in this study, the fraction of biogenic carbon contributes 64% of the received MSW. Emissions to the water are not included, since modern WtE systems are commonly designed with wastewater treatment and reused equipment to meet a 'zero discharge' target [START_REF] Chen | Life-cycle assessment (EASEWASTE) of two municipal solid waste incineration technologies in China[END_REF].

Life cycle impact assessment

The well-accepted Danish EDIP methodology is used to aggregate the LCI data [START_REF] Hauschild | Spatial differentiation in Life Cycle impact assessment-The EDIP2003 methodology[END_REF][START_REF] Wenzel | Environmental Assessment of Products, Volume 1: Methodology, tools and case studies in product development[END_REF]. Seven impact categories are considered: global warming (GW), acidification (AC), terrestrial eutrophication (TE), photochemical ozone formation to human health (POFh), human toxicity via air (HTa) and solid (HTs), and ecotoxicity via solid (ETs). Results based on normalized values are used to reflect the relative magnitude of different impacts into person equivalence.

A summary of the normalization references is available in Table S8.

Results

Theoretical analysis results

Fig. 2 reports the overall environmental performance of different systems.

Compared to direct incineration (S1), pyrolysis and gasification are effective to lessen the environmental impacts of TE, POF, HTa and ETs, yet increase the burdens of GW and HTs. For a direct comparison of different WtE processes, gasification systems (S5-S7) lead to a lower impact than pyrolysis systems (S2-S4). For systems using different energy cycles, gas turbine/CC (S3, S6) has surpassed steam turbine (S2, S5) and internal combustion engine (S4, S7) and becomes the most preferred energy utilization approach.

To give a clear and transparent explanation of the aforementioned results, the overall impact is divided into four stage-wise contributors: energy input, direct emissions, ash management and energy recovery. As shown in Fig. 3, the environmental savings for non-toxic impacts are primarily brought by energy recovery, which compensates a significant amount of emissions generated by fossil fuel-based energy production. In particular, negative values appear for several systems regarding AC, TE and POFh, indicating that the environmental benefit has balanced the loading and a net environmental saving is achieved. The highest recovered energy has been found for systems equipped with gas turbine/CC (S3, S6). This reveals the advantage brought by a more efficient energy device that is able to counterbalance an increasing amount of emissions. Besides, pyrolysis equipped with combustion engine (S4) also exhibits significant avoided impacts due to the additional savings from process heat (mainly from tar and char combustion), which highlights the importance of heat recovery in improving the total recovered energy.

Direct emissions also have a large influence to the total impacts. Different systems show negligible difference of GW, because CO 2 emission is decisively contributed to GW and it mainly derives from the fossil-origin carbonaceous compounds contained in MSW. However, there is a dramatic difference in direct emissions among all the systems, if consulting the impacts of AC, TE and POFh. Compared with incineration (S1), 21-34% and 28-83% decrease in those indicators are achieved for pyrolysis and gasification systems, respectively. The principal contributors for AC, TE and POFh are acid gases including NO x , SO 2 , HCl and HF. The reduced emissions by pyrolysis and gasification can in fact be ascribed to, on one hand, a lower amount of flue gas as a consequence of the lower excess air required for syngas combustion; on the other hand, the limited NO x generation as a result of the homogeneous gas-gas reaction [START_REF] Consonni | Waste gasification vs. conventional Waste-To-Energy: A comparative evaluation of two commercial technologies[END_REF]. It shows also a further reduction of emissions from gasification systems using gas turbine/CC and internal combustion engine (S6, S7), because purifying the syngas allows the removal of a part of acid gases; and, the syngas volume is much smaller to limit the total flue gas. Conversely the direct emissions from pyrolysis systems (S3, S4) tend to increase due to char and tar combustion.

All systems contribute positive impacts to toxic categories including HTa, HTs and ETs. Fig. 2 reveals that HTs and HTa are the highest burden categories, being 1-2 orders of magnitude more significant than non-toxic impacts. Direct emissions and ash management are the main contributors. The avoided emissions are insignificant, which is opposite to that of non-toxic impacts. The toxic impacts are decisively due to heavy metals, PCDD/Fs and PM emissions for their relatively high equivalent factors. Ash management takes a crucial effect to HTs and ETs, since heavy metals contained in the ash is liable to be transferred into the soil after landfill, or released during the solidification/stabilization process of the APC residues.

Consequently, it could be concluded from the theoretical analysis that compared with incineration, both pyrolysis and gasification own the potential to have a better environmental performance due to two-folds benefits: the reduced process emissions as well as a substantial increase in the amount of energy recovered. However, the important input energy demand, for example waste pre-treatment, syngas cleaning and endothermic pyrolysis reaction, may on the other hand become additional burdens especially regarding GW. This is also one reason for an inferior performance from pyrolysis systems in comparison to gasification. Overall, gasification equipped with gas turbine (S6) is observed to be the most environmentally preferable system.

Case studies for commercial operation WtE plants

Fig. 4 summarizes the environmental impacts from four large-scale commercial operation WtE plants, where all impacts experience a significant drop compared with the theoretical analysis. The benefit is mainly due to the reduction in the process direct emissions, revealing that plants based on all the technologies in connection with a steam boiler can comfortably meet the required emission limits. The environmental sustainability of each plant in descending order is: gasification > incineration > (pyrolysis, gasification-melting); while it is difficult to figure out the relative superiority between pyrolysis and gasification-melting. It reveals that the modern incineration could fulfil an environmentally sound technology, i.e., better than pyrolysis and gasification-melting plants at present. The emission factors reported in Table 2 indicate that the actual emissions from the reference incineration and pyrolysis/gasification plants are quite similar due to the technological performance of the modern flue gas cleaning devices (fabric filters, desulfurization, NO x abatement, activated carbon injection, etc.). The improved performance of incineration could also be attributed to the use of the more efficient combined heat and power (CHP) cycle, which has achieved an additional 6% of heat production. On the other hand, gasification reaches the best performance among the four plants. This fact again verifies the positive role of syngas cleaning, which allows the gas clean enough to employ higher steam data (540 o C, 121 bar compared with 400 o C, 40 bar in conventional waste boiler) for an increased electricity efficiency (27% net compared with 24% in incineration). Additionally, this gasification plant shows further advantage by an abundance of heat production (61%), significantly larger than in the incineration plant (6%). Those together have resulted in a significant environmental saving from the avoided heat and electricity production in the gasification plant.

Contrarily, pyrolysis and gasification-melting plants show an inferior performance.

The increased environmental burdens are either due to a high amount of auxiliary fuel used, or a low amount of net energy recovered. Particularly, gasification-melting plant shows an important internal parasitic energy demand reaching 24% of the total energy production, mainly due to the use of O 2 -rich air for ash melting.

An obvious reduction in HTs and ETs is achieved in the incineration and gasification-melting plants. The offset impacts are mainly attributed to the recycling of the bottom ash, slag and metals owing to two-aspects benefits: the reduced amount of ash to be treated, which is the main cause of solid heavy metals leaching; and, the avoided manufacture of road construction materials and metals from their virgin materials.

Sensitivity analysis

A sensitivity analysis has been carried out to identify key process parameters as well as to seek for potential improvements. The evaluation is based on the data from the theoretical analysis considering two variations: changing of the plant efficiency, and alternative utilization of the pyrolysis char as soil amendment.

A ±10% variation of the plant efficiency for each system is conducted. Results in Table 3 show an up to ±665% variation in the environmental impacts, of which non-toxic impacts appear of remarkable relevance. The variation is primarily related to the amount of energy recovered as it could replace the associated emissions from the burning of fossil fuels. The results confirm a crucial role of the energy recovery efficiency in determining the total sustainability of a WtE plant.

For pyrolysis systems (S2-S4), the sensitivity analysis considers also the case where the char is used as soil amendment. In such case, the pyrolysis char is considered to have two additional merits [START_REF] Harder | A critical review of developments in the pyrolysis of automotive shredder residue[END_REF][START_REF] Roberts | Life cycle assessment of biochar systems: estimating the energetic, economic, and climate change potential[END_REF]: substitution of fertilizer (N, P and K) and carbon sequestration. Key assumptions and calculations are presented in Table S10. Fig. 5 indicates that this assumption has exhibited an obvious reduction on the majority of impacts except for HTs and ETs. The benefit is dominantly attributed to the reduced airborne emissions from char combustion, together with a small portion of avoided emissions from fertilizer substitution and carbon sequestration. However, a non-negligible increase of the HTs and ETs loadings are observed due to the increased heavy metals to soil, which should be controlled effectively apart from the associated potential benefits of land application.

Discussions

Pyrolysis and gasification have been applied to waste treatment since 1970s, however their commercial application does not achieve widespread so far [START_REF] Panepinto | Environmental performances and energy efficiency for MSW gasification treatment[END_REF]. One of the main impediments is the heterogeneity of MSW, i.e., inconstant on size and highly variable on composition, which could not easily run stable. Despite this challenge, after years of practical experience, the main technical difficulties seem to be solved and innovative plants started to be operated [START_REF] Panepinto | Environmental performances and energy efficiency for MSW gasification treatment[END_REF].

The theoretical analysis of this study shows that using pyrolysis/gasification to supply a gas turbine/CC may achieve higher energy efficiencies and lower emissions than the current incineration. However, its application has not yet overcome many obstacles. For example, the state-of-the-art syngas purification technologies do not achieve the required quality standards. Also running gas turbines require complex maintenance. These reasons have in fact caused a very limited application of the gas turbine/CC in pyrolysis/gasification-based WtE plants [START_REF] Panepinto | Environmental performances and energy efficiency for MSW gasification treatment[END_REF]; while the most common configuration today is to burn the syngas in a steam boiler, namely, "two-step oxidation" [START_REF] Consonni | Waste gasification vs. conventional Waste-To-Energy: A comparative evaluation of two commercial technologies[END_REF].

In recent years, development of the pyrolysis/gasification-based WtE technologies has become a focus of attention, stimulated by the search for more efficient energy recovery and environmentally sustainable waste management. However, case studies results based on the current large-scale commercial plants reveal that the modern incineration could fulfil an environmentally sound technology, which performs better than the selected pyrolysis and gasification-melting plants. To be commercially successful, the pyrolysis/gasification-based WtE must develop the whole process chain (pre-treatment, thermal conversion, products utilization, residues management).

Those potential areas of development could include:

1. Boost the plant efficiency. The superior performance of the Lahti gasification plant attains at its effective syngas cleaning, which facilitates increasing the steam parameters while avoiding the corrosion problem. It could serve as a demonstration for designing the next generation of WtE configuration. The overall energy efficiency could also be increased by the utilization of the CHP system, or syngas co-incineration in a higher efficiency power station.

2. Use of selected waste streams. Pyrolysis and gasification plants tend to require very careful feedstock pre-treatment. To be more effective, solutions could be the use of SRF, refuse derived fuel (RDF), or residuals from mechanical biological treatment (MBT) systems, which are more homogenous than the raw MSW.

3. Efficient residues management. Recycling materials from WtE solid residues, particularly metals and bottom ash, may result in two main benefits: a decrease in waste landfill; and, a reduction in the consumption of virgin raw materials. Pyrolysis plant could also consider the use of char in land application. The specific properties of a Results represent a percentage increase or decrease of the environmental impacts in the base case scenarios.

Figure captions

2. 4 .

 4 Data source for commercial operation WtE plants Four large-scale commercial operation WtE plants (pyrolysis, gasification, gasification-melting, modern incineration) are modeled as case studies. The selected plants could represent the most typical modern state-of-the-art plants, therefore reflecting the actual environmental sustainability of different WtE technologies. The selected plants are all in connection with a steam turbine cycle, i.e., in a similar configuration of the S1, S2 or S5 system. Table

Fig. 1 .

 1 Fig. 1. System boundaries of the study.

Fig. 2 .

 2 Fig. 2. Normalized environmental impacts of different systems based on the

Fig. 3 .

 3 Fig. 3. Contributional analysis for each environmental impact based on the theoretical

Fig. 4 .

 4 Fig. 4. Environmental impacts of different systems based on case studies of the

Fig. 5 .

 5 Fig. 5. Sensitivity analysis by alternative utilization of the pyrolysis char, based on the

Table 1

 1 Characteristics of the MSW as received at the plant.

	Characteristics (wt. %, as received basis)
	C	25
	H	4
	N	0.84
	S	0.13
	O (by difference)	12
	Moisture	34
	Ash	24
	Lower heating value, MJ/kg	9.8

Table 2

 2 List of emission factors used in theoretical analysis and case studies of commercial WtE plants (Unit: mg/Nm 3 ). MSW incineration accords with the Directive 2007/76/EC; while pyrolysis and gasification plants meet the limits of the Directive 2010/75/EU with some adaptions (see detailed assumptions in Supplementary Material). Data based on four commercial operated WtE plants (see plant information and data source in Supplementary Material).

			Theoretical analysis a			Commercial WtE plants b
			Gas	Gas	Internal
									Gasification-
		Incinerator	boiler-steam	turbine/	combustion	Incineration Pyrolysis Gasification
									melting
			turbine	CC	engine
	CO	50	50	100	100	5.5	10	2	6.2
	SO 2	50	35	15	15	0.44	8	7	3.3
	NO x	200	200	120	100	41.4	166.9	161	20.9
	HCl	10	10	0	0	1.9	5.1	1	3.7
	PM	10	5	0	0	0.09	1.4	2	1
	PCDD/Fs
		0.1	0.1	0.1	0.1	0.005	0.001	0.002	0.006
	(ng-TEQ/m 3 )
	Hg	0.05	0.05	0.05	0.05	0.002	0.011	0.0001	0.02
	Cd	0.05	0.05	0.05	0.05	0.001	0.006	n.a. c	n.a. c

a b c Data not available.

Table 3

 3 Sensitivity analysis by changing of the plant efficiency by ±10%, based on the data from theoretical analysis.

				Environmental impacts change a (%)
		S1	S2	S3	S4	S5	S6	S7
	GW	±50.2	±19.2	±24.9	±20.4	±26.9	±110.9	±18.3
	AC	±19.2	±23.5	±17.8	±18.4	±20.8	±12.7	±14.0
	TE	±9.8	±12.1	±12.2	±11.9	±15.5	±23.9	±34.6
	POFh	±14.6	±17.8	±665.5	±480.2	±24.5	±22.0	±32.2
	HTa	±2.8	±3.6	±2.0	±1.9	±4.1	±11.9	±6.4
	HTs	±0.4	±0.3	±0.2	±0.2	±0.3	±0.4	±0.3
	ETs	±0.1	±0.1	±0.1	±0.1	±0.1	±0.2	±0.1

Acknowledgments

This project is supported by the French Centre National de la Recherche Scientifique (CNRS), the Labex SOLSTICE (Agence Nationale de la Recherche, ANR), and the Chinese Program of Introducing Talents of Discipline to University (B08026).

Appendix A. Supplementary data

Supplementary data to this article (detailed process description, data acquisition and calculation, inventory analysis, and the sensitivity analysis used in the LCA) are available.


Mirvat Omar

Khalil Haidar

Ahmed Loukili

Gilles Pijaudier

Keywords: flexural creep, loading level, residual capacity, acoustic emission

come

Creep loading influence on the residual capacity of concrete structure : Experimental investigation

INTRODUCTION

In common practice, it is usually assumed that linear visco-elasticity takes place for low load levels and that the instantaneous mechanical behaviour of concrete is elastic. For high load levels, deviation from linearity of the creep behaviour of concrete is expected. Under high sustained loads, cracks grow and interact with visco-elasticity. Some experimental and analytical results concerning non-linear creep can be found in the literature (see among others Bazant, 1988[START_REF] Gettu | Rate effects and load relaxation: static fracture of concrete[END_REF][START_REF] Mazzotti | Nonlinear creep damage model for concrete under uniaxial compression[END_REF][START_REF] Rüsch | Experimental Determination of the effect of the Duration of Loading on Strength and Deformation[END_REF][START_REF] Rüsch | Investigations on the Strength of Concrete under Sustained Load[END_REF]) but much remains to be learned.

Modelling non linear creep of concrete is of fundamental importance for severely loaded structures as creep may decrease the material strength with increasing time. The evaluation of the residual carrying capacity of structural components is a problem of growing importance for civil engineering structures such as nuclear power plants. Actually, these structures are often exposed to high stresses for a long time, due to what damage develops. Therefore, the results arising from short time tests cannot be considered a sufficient basis for judging the safety of such structures and it is of great interest to devise methods for evaluating their residual capacity.

This problem has drawn the attention of many authors, among them [START_REF] Rüsch | Experimental Determination of the effect of the Duration of Loading on Strength and Deformation[END_REF]co-workers (1957, 1958), who have carried out the first experiments in order to determine the effect of continuous loading duration on the resistance and deflections of concrete specimens under compression tests. They found that the capacity of a structure subjected to creep loads seems to be 70 to 80% of that observed in short time compression tests.

In the present paper, the main results of a set of experimental creep tests on concrete three points bend specimens at different load levels are presented. The objective is to investigate the ranges of variation of the time response under constant load due to variations of the load level. For each creep test, a different load level has been applied, starting from low levels (36% of peak load) where linear visco-elasticity applies, to high levels (80% of peak load) where tertiary creep causing failure at a finite time can be observed.

Also, this paper deals with the evaluation of the residual capacity of concrete after creep. Concrete B11 used for the construction of the Civaux power Creep load influence on the residual capacity of concrete structure: Experimental investigation Mirvat Omar, Khalil Haidar, Ahmed Loukili, and Gilles Pijaudier-Cabot R&DO, Institut de recherches en Génie civil et Mécanique, Ecole Centrale de Nantes, BP 92101, F-44321 Nantes cedex 3, France ABSTRACT: Recent experiences and results investigating flexural creep tests with various loading levels are presented. The aim of the study is to analyse the influence of creep on the residual capacity of concrete beams. The load level effects on creep strain are also discussed. Results show that for bending beams subjected to 60% of their load capacity during 40 days, the residual carrying capacity is reduced about 20%. This effect is essentially due to drying creep. Basic creep experiments show that for high load levels there is a coupled effect between creep and damage as tertiary creep is observed. Acoustic emission analyses underline also that damage occurs during creep. These results, which include size effect fracture tests, may serve for the development and the validation of coupled creep-damage models.

plant in France and tested by [START_REF] Granger | Comportement différé du béton dans les enceintes de centrales nucléaires (analyse et modélisation)[END_REF] The third kind of tests consisted on three points bend size effect experiments on prismatic geometrically similar notched beams. Three different sizes were used, the depths were D1 = 10cm, D2= 20cm, and D3= 40 cm with respective lengths 35, 70, and 140 cm while the thickness was kept constant for all the specimens b = 10 cm. The span to depth ratio was l/D = 3 for all specimens. One notch of depth 0.15D and thickness 3 mm (same for all dimensions) was performed in each specimen by placing a plastic plate at the midpoint perpendicular to the long direction of the mould before casting (Fig. 1). Nine prismatic specimens were cast, three for each size. Size effect tests followed the guidelines established by RILEM (1990) using a closed -loop testing machine, (160 KN capacity INSTRON machine), under crack mouth opening displacement (CMOD) control. A general view of the experimental set-up is provided in figure 2. Prior to the test, two steel plates with lips to fit into the notch were glued on both sides of the notch. The clip gage used to measure the CMOD was then placed between knife edges attached to the steel plates. In order to measure the deflection of a central point of the beam, a laser extensometer was used. This extensometer was attached at mid -span to a steel beam which was fixed to the concrete beam on two supports at half height and at the ends of it. A metallic plate glued on the specimen at mid span and half height reflect the ray emitted by the laser extensometer and permit to measure the deflection. Deflection was measured in this way so as not to include the effect of local deformations at the support and load points. A data acquisition system was used to record the load, the CMOD, and the deflection with time. Another set of three points bending tests was carried out on beams of size D2:10x20x70 cm without notches. These tests were aimed at determining the maximum load of unotched beams in order to calibrate the applied load in the creep tests, where beams having the same geometry have been used. The average maximum load determined from flexural tests on three specimens is about 2409 daN.

Creep tests

The creep tests were performed on frames designed at R&DO. The aim is to load concrete flexural beams with a constant force. The frames have a capacity ranging from 5 to 50 KN and can accommodate geometrically similar specimens of three different sizes. The load is applied by gravity with a weight and counter-weight system which enables a fine tuning of the load. Figure 4 shows a general view of these creep frames. The specimens tested have the same thickness of 10 cm. The smallest D1 is 10 cm high and 35 cm long. The medium D2 is 20 cm high and 70 cm long and the largest D3 is 40 cm high and 140 cm long. Two kinds of creep tests have been performed. The first one is intended to study the influence of the load level on creep. In the second kind of test (which is still in progress), the interaction between creep and fracture of concrete is considered via the associated size effect in structures and the decrease of the fracture energy due to creep. The applied loads in the creep tests have been determined from the previous fracture tests on the same material and the same geometry.

Load level tests

It is expected that visco-elasticity and crack growth interaction is related to the solicitation level. Thus, under high sustained loads, damage occurs and the role of microcracking on creep evolution is exhibited. It has also been verified that creep in The first set consists of 4 unotched beams of dimension D2 loaded at 50%, 60%, 70% and 80% of the maximum load. These tests are performed at 20°C and 50% RH and the specimens are not protected. Therefore, basic creep and drying creep occur at the same time. The age of concrete at the beginning of the creep tests is 28 days. Figure 5 presents the total displacement (creep displacement plus instantaneous displacement) measured on the specimens versus time for the first three loading levels. These curves indicate that for all loading levels creep kinetics are comparable, with the exception of the specimen loaded at 70% from peak load, where failure due to tertiary creep occurs after 8 days. In this test, we observed an important acceleration of the creep kinetic and then failure. Concerning the specimen loaded at 80% from peak load, total failure occured 2 minutes after applying the load. The second test series consists of two notched beams of dimension D1 loaded at 60 % and 36 % of the maximum load and one notched beam of dimension D2 loaded at 60% of the maximum load. These tests were followed by a third set consisting of two notched beams of dimension D2 and three notched beams of dimension D1 loaded at 80% of the peak load, one of these beams failed after 4 days from loading. Results obtained for this test are not reported here.

Specimens in the second and third test series were protected from desiccation by a double layer of self-adhesive aluminium paper. Hence basic creep was considered only. The experimental measurement of basic creep of concrete requires also that drying shrinkage be prevented. The curing conditions (3 months at 100% RH and 20°C) guaranteed to avoid early age autogeneous shrinkage so that basic creep could be measured only. The basic creep displacement was determined by subtracting the instantaneous elastic displacement from the total displacement.

Figure 6 compare the displacements due to basic creep for the smaller specimens (size D1). It shows the influence of the load level on the basis creep evolution versus the elapsed time. As expected, increasing the applied load increases the basic creep magnitude. Moreover, creep develops very fast in the first days of loading and stabilises after a few weeks for the two lower loading levels. The specimens loaded at 36% and 60% of the maximum load were cast from the same batch. The results show a variation of the amplitude of basic creep of the 36% and 60% loaded specimens, which increases and decreases again between 40 days and 50 days of loading. This variation is due to a temperature regulation problem, which occurred on the climate control system and which lead to this elevation in the magnitude of deflections measured on loaded specimens. The specimen loaded at 80% is part of the third set of creep tests. It was made from another batch and loaded at a different time but followed the same curing conditions. For this specimen, basic creep increases rapidly. Figure 7 shows the creep test results for the beams of size D2. These plots exhibit the same trend as those in Fig. 6. This difference is more obvious in the case of a 80% loading level as seen in figure 9. On this plot, we show also the results obtained for the two specimens of each size subjected to the same creep load. The reproducibility of the test is quite satisfactory.

Acoustic Emission in basic creep tests

Acoustic emission (AE) occurs when there is a sudden change in energy or energy release, due to flaws and defects nucleation or propagation in a specimen. Most studies have focused on relating acoustic emission characteristics to the properties of the fracture process zone [START_REF] Maji | Process zone and acoustic emission in concrete[END_REF], and using AE source location analysis to evaluate damage localisation [START_REF] Berthelot | Modelling concrete damage by acoustic emission[END_REF]. We applied this technique to the creep tests presented above.

The AE system used was comprised of an eightchannel MISTRAS system manufactured by Physical Acoustic Corporation. Piezoelectric transducers (R15/C, resonant frequency of 150 KHz) were used. Transducers were placed around the expected location of the process zone, on one side of the specimen, and in a linear array (Fig. 10).

In the present study, the primary aim is to correlate the basic creep evolution and the total number of acoustic events. In other terms, this study is intended to clarify the qualitative relationship between basic creep strain and damage occurring inside the material. More accurate analyses might be performed which include acoustic event localisation in order to visualise the locations of damage. Such tests which require a large number of transducers placed on the same beam will be performed in future experiments. We may observe that the number of AE events increases as basic creep develops. The evolution of the AE number is almost linear. There is a change of slope of the curve, which seems to correspond to a small peak in Fig. 12. This peak is due again to a slight defect in the room temperature control.

If one admits that AE events are due to microcracking, these results show clearly that there is a coupling between creep and damage. It follows that the mechanical responses of beams subjected to creep compared to those not subjected to creep, ought to exhibit a difference due to initial damage accumulated during creep loads. A reduction of the residual carrying capacity due to basic creep has to be expected in particular.

RESIDUAL CAPACITY TESTS

In order to investigate the variation of the residual capacity due to creep, comparison specimens cast at the same time than those subjected to creep were kept under the same conditions of temperature and relative humidity. Three points bending experiments till failure were carried out on the creep loaded and on the comparison specimens.

The first series of test results deals with unotched beams under combined drying and basic creep. After 40 days of loading, beams of dimension D2 subjected to loads of 50% and 60% of the maximum load were removed from the creep frames and then immediately subjected to three point bending loading up to failure with a constant loading rate. The chosen rate of loading is 0,1µm/s, which is the same rate used as in the flexural experiments intended to determine the average maximum load for this kind of specimens (see section 2.2). Figure 13 displays the load-displacement curves obtained for the specimens after creep loading and for the unloaded comparison specimens. The maximum carrying force of the specimens subjected to creep initially is reduced of about 20% in comparison to those of the unloaded specimens. Furthermore, it is found that there is no significant impact of the loading level on the response of beams. The 50% and 60% loaded specimens have approximately the same peak load. Figure 14 presents the results obtained on three points bend experiments carried out after 60 days of basic creep tests. These are notched specimens of dimension D1 loaded at 36% and 60% of the maximum load. Contrarily to previous results, it is observed that there is no influence of the 60 days of basic creep loading on the residual capacity of bending beams, when compared to the response obtained with the comparison specimens. The differences are within the experimental dispersion.

DISCUSSION AND CLOSURE

The following conclusions can be drawn from the above test results:

• For high load level, tertiary creep i.e. a viscoelastic response of the material coupled with an evolution of damage, occurs. In our bending tests, tertiary creep is observed for load levels that are above 60% of the maximum carrying capacity of the beam measured on initially unloaded specimens. This limit is consistent with other experimental data, namely on compression tests [START_REF] Mazzotti | Nonlinear creep damage model for concrete under uniaxial compression[END_REF].

• Basic creep on bending beams seems to be size dependent. For two geometrically similar specimens (sizes D1 and D2), the displacement due to creep is neither equal nor multiplied by two (which is the ratio between the sizes of the two beams).

• AE shows that there is some activity during the creep phase. Such an activity should correspond to the development of microcracks.

It is an indication that damage should be coupled to creep.

• Residual capacity tests show that drying creep has a great influence on the residual carrying capacity of the beams. For the tests presented, which are limited to low creep levels (50% and 60% of the maximum load), basic creep does not influence the structural strength of the beam.

These test results have now to be used for the development of a constitutive relation in which creep and damage are coupled. Some proposals exist in the literature (see for instance [START_REF] Mazzotti | An isotropic damage model for non linear creep behavior of concrete in compression[END_REF][START_REF] Mazzotti | Nonlinear creep damage model for concrete under uniaxial compression[END_REF][START_REF] Zi | Continuous relaxation rpectrum of concrete creep and its incorporation incorporation into microplane model M4[END_REF], Bazant and Jirasek 1992[START_REF] Omar | Etude du couplage endommagementfissuration[END_REF][START_REF] Benboudjema | A basic creep model for concrete subjected to multiaxial loads, Fracture Mechanics of Concrete Structures[END_REF], and could be compared with the above results as well. Furthermore, additional test data dealing with larger sizes of specimen, higher load levels in basic creep experiments followed by size effect tests are expected to provide a more comprehensive understanding of the creep damage interaction and of the influence of creep on the fracture energy of concrete.

ACKNOWLEDGMENTS

Financial

  Figure 1. (a) Three point bend experiments on notched specimen; (b) Description of the instrumentation of the mechanical test

  Figure3shows the average responses for all dimensions. Curing specimens were 28 days at 50% RH and controlled temperature of 20° C. Mean values of physical and mechanical properties are summarised in table 2.

Figure 2 .

 2 Figure 2. general view of experimental fracture test set-up

Figure 4 .

 4 Figure 4. general view of creep frames

Figure 5 .

 5 Figure 5. Deflection of unotched beams D2 for different load levels

Figure 6 .

 6 Figure 6. Basic creep displacement of notched beams D1 for 36% , 60% , and 80% of peak load

Figure 7 .

 7 Figure 7. Basic creep displacement of notched beams D2 for 60% , and 80% of peak load Figure 8 presents a comparison between basic creep displacements obtained on two specimens of size D1 and D2, both of them loaded at 60% from their corresponding maximum load. These results show that the basic creep kinetic for dimension D2 is greater than that of dimension D1.

Figure 8 .

 8 Figure 8. Basic creep displacement of notched beams D1 and D2 loaded at 60% of peak load

Figure 9 .

 9 Figure 9. Basic creep displacement of notched beams D1 and D2 loaded at 80% of peak load

Figure 10 .

 10 Figure 10. Specimen geometry and AE transducers position Figure 11 presents the total number of acoustic events versus time and figure 12 shows the corresponding basic creep evolution occurring at the same time.

Figure 11 .

 11 Figure 11. Total number of AE versus time for the 80% basic creep loaded specimen D2

Figure 13 .

 13 Figure 13. Residual capacity of unotched beams of dimension D2

Figure 14 .

 14 Figure 14. Residual capacity of notched beams of dimension D1

Table 2 .

 2 Physical and mechanical properties of concrete

	_________________________________________ _______________________________________
	Properties _________________________________________________ Mean values
	f ' c (MPa)	41.25
	f ' t (MPa)	3.48
	E dyn (MPa)	39000
	D1 : F max (daN) σ N (MPa)	863.42 5.61
	D2 : F max (daN) σ N (MPa)	1401.13 4.42
	D3 : F max (daN) σ N (MPa)	2377.31 4.81
	G f (N/m) _________________________________________ _______________________________________ 180

Table 3 .

 3 compression is significantly affected by the applied load amplitude. The greater the load level, the larger the creep strain magnitude.Twelve prismatic beams were used. For each creep test, a different load level has been applied, starting from low levels to very high levels. The applied loads are given in table3. _______________________________________ Applied loads in creep tests

	Applied load	notched beams	unotched beams
		D1 _______________ _______________ D2 D2 ___________ _________________
	daN _________________________________________________ daN daN
	36% F peak	313	-	-
	50% F peak	-	-	1198
	60% F peak	510	746	1444
	70% F peak	-	-	1678
	80% F peak _________________________________________ _______________________________________ 672 1033 1940

  support from the European Commission through the MAECENAS project (FIS5-2001-00100) and from the partnership between Electricité de France and the R&DO group through the MECEN project are gratefully acknowledged. 6 REFERENCES Bazant Z.P. 1988, Mathematical modeling of creep and shrinkage of concrete , John Wiley & Sons Ltd. Bazant, Z.P. and Jirasek, M. 1992, R-curve modeling of rate effect in static fracture and its interference with size effect, in Fracture Mechanics of Concrete Structures. Proc. Int. Conf. On Fracture Mechanics of Concrete Structures. Breckenridge, Colorado, June, ed. by Bazant Z.P., Elsevier Applied Science, London, pp. 918-923.

		1000
		800
	Load (daN)	400 600			without creep after creep at 36% after creep at 60%
		200
		0
		0	50	100	150	200	250
				Displacement (µm)


Evaluating the value of Bias Correction of High-Resolution Satellite Rainfall Product (CHIRP) to simulate Stream flow into Lake Ziway, Ethiopia Demelash Goshime (1), Rafik Absi (2), Béatrice Ledesert (4), and Alemseged Haile (3)

(1) University of Cergy Pontoise, Doctoral Studies, Civil Engineering, Cergy, France (d.goshime@hubebi.com), (2) Ecole de Biologie Industreille, 49 Avenue Genottes, 95895 CERGY CEDEX France, (4) Université de Cergy-Pontoise Laboratoire Géosciences et Environnement Cergy,1, rue Descartes 95000 NEUVILLE SUR OISE France, (3) International Water Management Institute, Nile Basin and East Africa Sub-Regional Office, P.O. Box 5689 Addis Ababa, Ethiopia Significant lowering has been observed in the Lake Ziway volume, which hinders the lake services for a wide variety of ecosystems. However, the contribution of water withdrawal, land cover change and climate change has not been quantified yet due to scarce data for rainfall-runoff modeling. Hydrological modeling for runoff simulation requires long-term accurate and consistence rainfall data both in space and time. Satellite hydro-meteorological estimate may serve as important inputs for modeling in an area of data scarce and poorly gauged region. In this study, we evaluated the runoff simulation from near-real time, Climate Hazards Group InfraRed Precipitation (CHIRP) satellite rainfall product at daily temporal and 0.05 • X 0.05 • spatial resolution for a period from 1984 to 2016. The study area is Lake Ziway watershed situated in Ethiopian Central Rift Valley lakes basin (CRV), Eastern Africa, the highest in a chain of four lakes. As evidenced from other studies, the bias of satellite rainfall estimate must be removed before estimates receive hydrological and water resources applications. We applied a non-linear power bias correction method to obtain bias corrected precipitation using in-situ rainfall data as references. The bias-corrected CHIRP data were used to calibrate and simulate rainfall-runoff using Hydrologiska Byråns Vattenbalansavdelning (HBV) model at Katar and Meki catchments, two major river inflows into the lake.

The HBV model revealed very good performance when evaluated using Nash-Sutcliffe model efficiency (NSE) and relative volume error (RVE) objective functions for the calibration and validation periods. For calibration period run 1986 to 1991, the value of NSE was 0.78 and 0.65, and the RVE was -2.49 and -4.45 for Katar and Meki catchments respectively. The verification of model performance revealed NSE values of 0.73 and 0.61, and RVE values of 1.93 and 5.56 for both catchments during a period from 1996 to 2000. Simulation result indicates that, Katar and Meki rivers contribute a runoff volume of 614.6MCM (61% for the Katar and 39% for the Meki) to Lake Ziway for the period from 1986 to 2016. On average a change of up to 9% annual Lake runoff volume reduction obtained during simulation over time. Our study shows that bias corrected CHIRP satellite rainfall estimates play a major role in estimating catchment runoff and lake inflows. Further study should incorporate lake level simulation and climate change impact assessment to assess the likely current and future behavior of lake.