Claire Douillard
email: claire.douillard@chru-lille.fr

Arnaud Jannin
email: arnaud.jannin.aj@gmail.com

Marie-Christine Vantyghem
email: mc-vantyghem@chru-lille.fr

Pr Mc Vantyghem

RARE CAUSES OF HYPOGLYCEMIA IN ADULTS CAUSES RARES d'HYPOGLYCEMIES de L'ADULTE

Keywords: hypoglycemia, inborn errors of metabolism, glucokinase mutation, anti-insulin receptor and insulin antibodies, insulin receptor mutation, IGF2, NICTH, type B insulin resistance, HIRATA syndrome, glycogen synthesis disorder, beta-oxidation disorders, neoglucogenesis disorders, hereditary fructose intolerance

Hypoglycemia is defined by a low blood glucose level associated to clinical symptoms. Hypoglycemia may be related to treatment of diabetes, but also to drugs, alcohol, critical illness, cortisol insufficiency including hypopituitarism, insulinoma, bariatric or gastric surgery, pancreas transplantation or glucagon deficiency, or may be surreptitious. Some hypoglycemic episodes remain unexplained, and genetic, paraneoplastic and immune causes should be considered. Genetic causes may be related to endogenous hyperinsulinism and to inborn errors of metabolism (IEM). Endogenous hyperinsulinism is related to monogenic congenital hyperinsulinism, and especially to mutations of the glucokinaseactivating gene or of insulin receptors, both characterized by post-prandial hypoglycemia with major hyperinsulinism. In adulthood, IEM-related hypoglycemia can persist in a previously diagnosed childhood disease or may be a presenting sign. It is suggested by systemic involvement (rhabdomyolysis after fasting or exercising, heart disease, hepatomegaly), sometimes associated to a family history of hypoglycemia. The timing of hypoglycemic episodes with respect to the last meal also helps to orientate diagnosis. Fasting hypoglycemia may be related to type 0, I or III glycogen synthesis disorder, fatty acid oxidation or gluconeogenesis disorder. Postprandial hypoglycemia may be related to inherited fructose intolerance. Exercise-induced hyperinsulinism is mainly related to activating mutation of the SLC16A1 gene.

Besides exceptional ectopic insulin secretion, paraneoplastic causes involve NICTH (Non-Islet-Celle Tumor Hypoglycemia), caused by Big-IGF2 secretion by a large tumor, with low blood levels of insulin, C-peptide and IGF1. Autoimmune causes involve antibodies against insulin (HIRATA syndrome), especially in case of Graves' disease, or against the insulin receptor. Medical history, timing, and insulin level orientate the diagnosis.

à des médicaments, à l'alcool, à des maladies débilitantes, à une insuffisance en cortisol, à un insulinome, une chirurgie bariatrique ou gastrique, une transplantation pancréatique, un déficit en glucagon, ou une cause factice. Lorsque les hypoglycémies restent inexpliquées, des causes rares génétiques, paranéoplasiques et immunitaires doivent être envisagées. Les causes génétiques sont les hyperinsulinismes endogènes et les erreurs innées du métabolisme (IEM). L'hyperinsulinisme endogène est lié à un hyperinsulinisme congénital monogénique, en particulier aux mutations activatrices du gène de la glucokinase, ou aux mutations du récepteur de l'insuline, les deux pathologies étant caractérisées par une hypoglycémie post-prandiale avec hyperinsulinisme majeur.

Les hypoglycémies liées à une IEM peuvent persister à l'âge adulte, alors qu'elles ont été diagnostiquées dans l'enfance ou sont parfois révélatrices de l'IEM. Elles sont suggérées par un contexte d'atteinte systémique (rhabdomyolyse après jeûne ou exercice, cardiopathie, hépatomégalie, avec parfois antécédents familiaux d'hypoglycémie ...). L'horaire de l'hypoglycémie par rapport au dernier repas permet également d'orienter le diagnostic. En effet, une hypoglycémie de jeûne oriente vers une glycogènose de type 0, I ou III, un trouble de l'oxydation des acides gras ou un trouble de la gluconéogenèse. Les hypoglycémies postprandiales orientent vers une intolérance au fructose. L'hyperinsulinisme induit par l'exercice est principalement engendré par une mutation activatrice du gène SLC16A1. Les causes paranéoplasiques correspondent parfois à une sécrétion ectopique d'insuline et le plus souvent à un NICTH (Non-Islet-Cell Tumor Hypoglycemia). Ces NICTH, liées à la sécrétion de big-IGF2 par une volumineuse tumeur, sont caractérisées par des hypoglycémies avec taux effondrés d'insuline, de peptide C et d'IGF1. Les causes auto-immunes sont liées aux anticorps anti-insuline (syndrome HIRATA) ou anti-récepteur de l'insuline. Les antécédents médicaux, l'horaire de survenue des hypoglycémies et leur caractère hypo ou hyperinsulinique permettent d'orienter le diagnostic.

Mots-clés : hypoglycémie, erreur métabolique innée, mutation de la glucokinase, récepteur antiinsuline et anticorps anti-insuline, mutation du récepteur de l'insuline, IGF2, NICTH, insulinorésistance de type B, syndrome HIRATA, trouble de la synthèse du glycogène, troubles de la beta-oxydation, troubles de la néoglucogenèse, intolérance héréditaire au fructose

I. Introduction

Hypoglycemia is defined by a low venous blood glucose level, most often <0.50 g / L (2.75 mmol / L) associated with signs:

-adrenergic, linked to the catecholaminergic response, such as sweating, tremors and palpitations;

-neuroglucopenic, caused by a drop in the concentration of cerebral glucose, ranging from dysarthria, confusion, epilepsy, visual and behavioral disorder to coma.

According to the consensus, the hypoglycemia threshold is 55 mg/dL or 3 mmol/L, but what is important is not so much the threshold as the fact that a low blood sugar level is associated with concomitant clinical signs.

Apart from the iatrogenic causes of hypoglycemia in a diabetic person (1), the most common etiologies are (2) (Figure 1):

-other iatrogenic causes (sulfonamides, tramadol, topiramate, tyrosine kinase inhibitors (3), etc.);

-surreptitious hypoglycemia (sulfonamides, glinides, insulin); -acute alcoholism;

-critical illness, visceral failure or cachectic state;

-cortisol insufficiency (peripheral adrenal insufficiency) (4), sometimes associated with GH deficiency, (hypopituitarism): repeated hypoglycemia blunts the corticosteroid response and differential diagnosis between primary hypoglycemia and adrenal insufficiency is sometimes difficult (5);

-hypoglycemia after gastric or bariatric surgery [START_REF] Øhrstrøm | Evidence for Relationship Between Early Dumping and Postprandial Hypoglycemia After Roux-en-Y Gastric Bypass[END_REF] or after pancreas transplantation;

-glucagon deficiency;

-or insulinoma, characterized by fasting symptomatic hypoglycemia, relieved by food (Whipple's triad). In terms of hypoglycemia, insulinoma is characterized by insulinemia ≥ 3.0 μIU/mL (18 pmol/L) and plasma C-peptide ≥0.6ng/mL (0.2 nmol / L) in absence of sulphonylureas [START_REF] Caron | Consensus of the French Endocrine Society: Hypoglycaemia in non-diabetic adult patients[END_REF][START_REF] Agin | French Endocrine Society. Fast test: clinical practice and interpretation[END_REF][START_REF] Challis | Adult-onset hyperinsulinaemic hypoglycaemia in clinical practice: diagnosis, aetiology and management[END_REF].

Pancreatic computed tomography and MRI, endosonography, 18F-DOPA (6-[18F] fluoro-levodopa)

PET and especially PET or scintigraphy with analogs of GLP1 (glucagon-like peptide type 1) help locate and enucleate the pancreatic tumor.

Besides these classic causes, rarer etiologies may be involved: genetic (congenital hyperinsulinism, insulin receptor mutation, inborn error of metabolism (IEM)), paraneoplastic (NICTH for Non-Islet-Cell Tumor Hypoglycemia, sometimes ectopic insulin secretion) or autoimmune (Figure 1). The anamnestic and clinical approach is not very different from the usual strategy: family and personal history, time since last meal, effort, pigmentation change, weight loss, medication. A family context of hypoglycemia, sudden death, myopathy including rhabdomyolysis after fasting or exercise, cardiomyopathy or hepatomegaly, or history of lactic acidosis, spontaneous eviction of fruit, etc.

suggest IEM [START_REF] Shah | Hyperinsulinaemic hypoglycaemia in children and adults[END_REF]. When IEM is suspected, laboratory investigations should include specific assessments performed as much as possible during hypoglycemia (Figure 2); metabolic fasting test requires a specialized environment.

II. Genetic causes

A. Monogenic congenital hyperinsulinism

Congenital hyperinsulinism (CHI) can be monogenic or syndromic. Syndromic CHI will not be considered here, but includes Beckwith-Wiedemann, Perlman, Simpson-Golabi-Behmel, Kabuki, Sotos, Timothy, Costello (mutation of the HRAS gene), Ondine, Usher type 1C, congenital glycosylation disorder type 1 or CDG I [START_REF] Shah | Hyperinsulinaemic hypoglycaemia in children and adults[END_REF][START_REF] Stanley | Perspective on the Genetics and Diagnosis of Congenital Hyperinsulinism Disorders[END_REF]. Monogenic CHI is linked to mutations in genes that regulate the insulin secretion of pancreatic β cells such as:

-Channel anomalies: ABCC8 (most frequent case) and KCNJ11, coding for the subunits of potassium channels of type SUR-1 and Kir6.2 respectively; channelopathy is a classic etiology in children, although mutation screening is negative in more than 30% of cases;

-Enzyme or transport anomalies: GLUD1 coding for glutamate dehydrogenase, GCK coding for glucokinase, HADH1 coding for the Short-Chain L-3-Hydroxyacyl-coenzyme A Dehydrogenase (SCHAD), UCP2 for mitochondrial uncoupling protein 2, SLC16A1 coding for Monocarboxylate Transporter 1 (MCT1);

-Transcription factor alterations: Hepatocyte Nuclear Factor (HNF4A, HNF1A);

-Enzymes: hexokinase 1 (HK1), phosphoglucomutase 1 (PGM1) (Figure 3).

Among these genes, glucokinase (GCK) mutations induce MODY2 diabetes in their inactivating forms and CHI in their activating forms [START_REF] Jannin | Hyperinsulinemic hypoglycemia without insulinoma: Think of activating glucokinase mutation[END_REF]. Seven percent of CHIs are linked to an activating mutation in glucokinase, which is the most common abnormality -although still rare -in adults. A glucokinase mutation should be suspected in case of hyperinsulinemic hypoglycemia mimicking insulinoma with negative imaging, and frequent hypoglycemia in relatives, although not always identified as such, in this autosomal dominant form of familial hyperinsulinism. Although symptomatology varies with the type of mutation (around 15 reported) [START_REF] Martínez | Heterogeneity in phenotype of hyperinsulinism caused by activating glucokinase mutations: a novel mutation and its functional characterization[END_REF], revealing hypoglycemia in adulthood may be fasting but is more often postprandial. These hypoglycemias are never very deep (0.35 to 0.55 g / L) even on fasting test and are often pauci-symptomatic without neurological or weight repercussions. Mirroring MODY2 type diabetes, it is more a readjustment of the blood sugar regulation threshold close to 0.45 g/L than severe hypoglycemia [START_REF] Basco | α-cell glucokinase suppresses glucose-regulated glucagon secretion[END_REF]. These hypoglycemic episodes are usually sensitive to diazoxide ( 15), but pauci-symptomatic patients often do not seek treatment.

B. Insulin receptor mutations

Insulin receptor mutations can be diagnosed in adulthood in a heterozygous state, most often in case of late postprandial hypoglycemia, in a patient without severe overweight [START_REF] Preumont | Hypoglycaemia revealing heterozygous insulin receptor mutations[END_REF]. Hypoglycemia can be reproduced by oral glucose tolerance test, and generally occurs 3 hours after the start of the test in case of major hyperinsulinism. Sometimes, a symptomatic homozygous newborn with Rabson-Mendenhall syndrome leads pauci-symptomatic heterozygosity being detected in the parents.

C. Inborn errors of metabolism

Inborn errors of metabolism are rare diseases affecting the metabolism of carbohydrates, proteins, lipids, steroids, etc. They are usually classified into 3 groups:

-1) intoxication diseases (amino-acidopathy, urinary organic aciduria, certain pathologies of sugar metabolism, metal overload, porphyria);

2) diseases of energy metabolism (mitochondrial cytopathy, glycogenosis, free fatty acid oxidation disorder, etc.);

3) diseases of complex molecules metabolism, usually autosomal recessive and associated with inbreeding. They are very often diagnosed in childhood but can sometimes be revealed in adulthood, possibly on family survey. Given the rarity of these conditions, it is often useful to refer to a reference center. The diseases responsible for hypoglycemia in adulthood are classified according to time of onset relative to meals (Figure 1) (17):

-fasting hypoglycemia suggests glycogenosis, fatty acid oxidation disorder or neoglucogenesis disorder (Figure 4); -postprandial hypoglycemia suggests hereditary fructose intolerance, other causes being insulin receptor mutation or congenital hyperinsulinism;

-hypoglycemia under effort suggest mutations of the SLC16A1 gene coding for monocarboxylate (MCT1) transporter.

1) Glycogenosis

Glycogenoses are hereditary diseases of glycogen metabolism, with incidence of 1 in 20,000 births.

There are at least 15 subtypes, classified according to the type of deficient enzyme and the affected tissue (liver, muscle or both), usually diagnosed in childhood [START_REF] Laforêt | What have we learned about glycogenosis in recent years?[END_REF]. In children, hepatic (0, I, VI, XI) or hepato-muscular (III, IV, VIII, IX) glycogenoses manifest as hypoglycemia of varying severity after a shorter or longer period of fasting, depending on the enzyme deficit. The risk of hypoglycemia in adulthood persists for types 0, I and III. The most severe in terms of hypoglycemic risk is type I glycogenosis (Von Gierke disease), with incidence <1/100,000, linked to a glucose-6-phosphatase deficiency blocking the last stages of glycogenolysis and neoglucogenesis (Figure 5). These conditions are characterized by fasting hypoglycemia with lactate elevation (after fasting in type 1, postprandial in type 3), and early gout with hyperuricemia, hypertriglyceridemia, hepatomegaly with frequent adenomas, high liver enzyme levels, and high CPK for type 3 [START_REF] Kaiser | Glycemic control and complications in glycogen storage disease type I: Results from the Swiss registry[END_REF].

Positive diagnosis is based on metabolic assessment and definitive diagnosis is molecular. In adulthood, diagnosis of hepatic or hepatocellular glycogenosis is rare and is founded not on hypoglycemia but on organ complications (nodular hepatomegaly, elevated CPK, heart disease, etc.).

The main differential diagnosis is Mauriac syndrome resulting in hepatomegaly with steatosis in a young diabetic patient with severe glucose imbalance [START_REF] Pinto | Mauriac syndrome: a rare complication of type 1 diabetes mellitus[END_REF].

In women, estrogen-progestogen contraception should be avoided, given its role in hepatic adenoma and its contraindication in case of dyslipidemia. There have been a few reports of favorable pregnancies. Maternal nutritional status is important because of the risk of hypoglycemia, and may require fractionated glucose supplementation, especially from the second semester and during breastfeeding.

2) Fatty-acid oxidation disorders Disorders of fatty acid oxidation are IEMs which alter the synthesis of acetylcoenzyme A in the mitochondrial energy pathway: this results in a deficit in activation of pyruvate carboxylase and a deficit in ATP (adenosine triphosphate) synthesis, these two elements contributing to non-ketotic hypoglycemia since ketogenesis is impaired (Figure 5). These autosomal recessive conditions are among the most common, taken together, and are classified into 4 subgroups:

-carnitine cycle abnormalities;

-electron transfer abnormalities;

β-oxidation cycle abnormalities: o acyl-coenzyme A dehydrogenase

• very long chain fatty acids (VLCAD)

• medium chain fatty acids (MCAD)

• short chain fatty acids (SCAD) o 3-hydroxyacyl coenzyme A dehydrogenase

• long chain fatty acids (LCHAD)

• short chain fatty acid (SCHAD)

• deficiency in the most rare tri-functional mitochondrial protein (MTP),

-abnormalities in the synthesis of ketone bodies related to HMG coenzyme A, synthase or lyase deficiencies.

These fatty acid oxidation disorders are classically revealed in the neonatal period or infancy, or during intercurrent disease causing catabolism (for example: prolonged fasting during gastroenteritis).

In adulthood, some of these pathologies can be revealed by hypoglycemia associated with global energy failure (rhabdomyolysis, hepatic insufficiency, hyperammoniemia, hyperlactatemia) or hypertrophic cardiomyopathy. Positive diagnosis is based on carnitine/acylcarnitine profile, in vitro beta-oxidation study, urinary organic acid analysis, and molecular study.

Late presentations include myopathy, arrhythmia and neuropathy, and also retinitis pigmentosa which can lead to blindness in LCHAD and MTP. Sudden death occurs at all ages. In pregnant women, intrauterine growth retardation, prematurity, pre-eclampsia or acute hepatic steatosis of pregnancy can occur in heterozygous mothers if the fetus has LCHAD or MTP deficiency.

a. Beta -oxidation disorders

In adults, various β-oxidation disorders have been more specifically reported, in particular some cases of acyl-coenzyme A dehydrogenase 9 (ACAD 9) deficiency, VLCAD or MCAD deficiency, or in contexts of energy failure (rhabdomyolysis, hepatic failure, hyperammoniemia, hyperlactatemia, hypoglycemia). Hypoglycemia is not indicative of the pathology and is sometimes even absent from the inaugural presentation.

b. Carnitine cycle disorders

Carnitine ensures the transfer of long-chain fatty acids to the mitochondria. This requires enzymes and transporters which:

1. accumulate carnitine in the cell (OCTN2 carnitine transporter);

2. conjugate with long-chain fatty acids (acylCoActivated, carnitine palmytol transferase 1, CPT1);

3. transfer the acylcarnitines to the internal plasma membrane (carnitine acyl-carnitine translocase, CACT);

4. conjugate fatty acids to coenzyme A for β-oxidation (carnitine palmytol transferase 2, CPT2).

Clinical cases of CPT2 deficiency (in its juvenile form) are regularly reported in adulthood, revealed more by circumstantial rhabdomyolysis (in case of caloric insufficiency, intercurrent disease with catabolism, etc.) than by hypoglycemia. On the other hand, hypoglycemia is classic in the neonatal period and infancy. In children, defects in the hepatic isoforms of CPT1 appear as recurrent bouts of non-ketotic fasting hypoglycemia, which rarely persist into adulthood; CACT deficit manifests during intercurrent illness with episodes of hypoglycemia, with sometimes very severe rhabdomyolysis [START_REF] Vitoria | Carnitine-acylcarnitine translocase deficiency: experience with four cases in Spain and review of the literature[END_REF].

3) Hereditary fructose intolerance

Hereditary fructose intolerance is linked to fructose-1-phosphate aldolase B deficit, manifesting during food diversification (introduction of fruit fructose or sucrose which contains fructose and glucose).

The accumulation of fructose-1-phosphate inhibits the glycogenolysis and neoglucogenesis pathways, which leads to late postprandial hypoglycemia (Figure 5). In infants, in addition to postprandial hypoglycemic discomfort, the picture can be complicated by liver failure and tubular dysfunction if diagnosis is not made and the child remains subject to intoxication by fructose. Several cases of diagnosis have been reported in adulthood because of diet naturally devoid of any sugary substances or fruit and/or because of liver enzyme elevation. Diagnosis in adulthood is suggested by anamnesis of difficult weaning, abdominal discomfort with food diversification, liver enzyme elevation, or hypoglycemia [START_REF] Morales-Alvarez | Fructosuria and recurrent hypoglycemia in a patient with a novel c.1693T>A variant in the 3' untranslated region of the aldolase B gene[END_REF]. Diagnosis is confirmed by molecular analysis; fructose load test is now only exceptionally used, given the risk of acute hepatocellular insufficiency. Prognosis after fructose eviction is usually favorable, despite the risk of vitamin C deficiency, which requires supplementation [START_REF] Guery | Doctor, my son is so tired... about a case of hereditary fructose intolerance[END_REF]. Fructose metabolism disorder may, however, induce fatty liver disease [START_REF] Simons | Patients with aldolase B deficiency are characterized by an increased intrahepatic triglyceride content[END_REF].

4) Neoglucogenesis disorders

Neoglucogenesis disorders are mainly related to fructose-1,6-biphosphatase deficiency. This pathology is revealed in very young children by hypoglycemia with prolonged fasting accompanied by lactic acidosis. Fasting hypoglycemic episodes can persist into adulthood in conditions of major catabolism. This condition is sometimes diagnosed in adulthood in case of lactic acidosis with hypoglycemia and liver enzyme elevation [START_REF] Pinto | International practices in the dietary management of fructose 1-6 biphosphatase deficiency[END_REF]. One case diagnosed in adulthood has been reported in a 36-year-old woman with recurrent hypoglycemic attacks that went unnoticed in childhood and increased during pregnancy [START_REF] Krishnamurthy | Three successful pregnancies through dietary management of fructose-1,6-bisphosphatase deficiency[END_REF]. Any case of hypoglycemia with digestive intolerance must suggest neoglucogenesis disorder

III. Paraneoplastic causes

Hypoglycemia secondary to extra-pancreatic tumor was identified in 1929 (Doege-Potter syndrome).

A. NICTH

Initially attributed to glucose consumption by the tumor, then to tumor secretion of an "insulin-like" factor which has been found to correspond to a high molecular weight precursor of IGF-2, called Big-IGF-2, this paraneoplastic hypoglycemia is now known as NICTH (Non-Islet-Cell Tumor Hypoglycemia). While secretion of Big-IGF2 is the most common cause, secretion of somatostatin or IGF1 may also be responsible [START_REF] Daughaday | Hypoglycemia due to paraneoplastic secretion of insulin-like growth factor-I[END_REF].

IGF-2 is a hormone secreted mainly by the liver, where it acts on cell proliferation, differentiation and apoptosis. The IGF-2 gene is overexpressed in various tumor types, especially large liver and mesenchymal (pleural solitary fibrous) tumors. Increased production of pre-pro-IGF-2 exceeding the capacity of the enzymes that metabolize it, incomplete cleavage of pro-IGF-2 leading to an excess of Big IGF-2 compared to IGF-2, and the binding of IGF-2 and Big IGF-2 to receptors that also bind insulin , explain the hypoglycemia [START_REF] Jannin | Non-islet-cell tumour hypoglycaemia (NICTH): About a series of 6 cases[END_REF][START_REF] Kawai | Imbalanced expression of IGF2 and PCSK4 is associated with overproduction of Big IGF2 in SFT with NICTH: a pilot study[END_REF].

Fasting hypoglycemic symptoms reveal the tumor in half the cases, and weight loss, pain and a palpable mass in the others. Hypoglycemia can be revealed by a confusion episode, sometimes suggesting Alzheimer's disease, a fall in the elderly, focal neurological signs mimicking a stroke, or the consequences of metastasis [START_REF] Jannin | Non-islet-cell tumour hypoglycaemia (NICTH): About a series of 6 cases[END_REF]. Any behavioral or mental change in a patient with tumor indicates hypoglycemia screening if iatrogenic causes and brain metastasis are eliminated. In addition to its role in carbohydrate metabolism, IGF-2 can lead to seborrheic keratosis and rhinophyma acromegaloid syndrome.

Paraneoplastic secretion of Big-IGF2 is characterized by hypoglycemia with collapsed plasma concentrations of insulin, C-peptide and IGF-1. Diagnosis is confirmed by IGF2/IGF1 ratio >10

(normal value <3), since total plasma IGF2 level can be normal or high, while that of Big-IGF-2 is usually elevated. These assays, restricted to research laboratories, are not essential for diagnosis.

Management remains poorly codified. Tumor resection is the treatment of choice. When it is not possible, corticosteroid therapy (30-60 mg prednisone/day) appears to be the most effective alternative, sometimes in association with somatostatin analogs. Recombinant GH (rGH) is associated with a risk of stimulating tumor growth [START_REF] Bodnar | Management of non-islet-cell tumor hypoglycemia: a clinical review[END_REF]. Anti-IGF2 antibodies, IGF2 antagonists and trials to inhibit transcription of the IGF 2 gene are under evaluation.

B. Ectopic insulin secretion

Besides NICTH, exceptional cases of ectopic insulin secretion, mimicking insulinoma, have been reported as paraneoplastic causes of hypoglycemia [START_REF] Gogna | Insulin-mediated hypoglycaemia secondary to recurrent clear cell renal carcinoma[END_REF].

IV. Autoimmune causes

Autoimmune causes of hypoglycemia are related to the presence of anti-insulin and anti-insulin receptor antibodies [START_REF] Taylor | Syndromes of autoimmunity and hypoglycemia. Autoantibodies directed against insulin and its receptor[END_REF][START_REF] Lupsa | Autoimmune forms of hypoglycemia[END_REF]. Sometimes both are associated, and they can also be induced by drugs (proton pump inhibitors, clopidogrel, methimazole, etc.).

A. Anti-insulin antibodies

The presence of anti-insulin antibodies usually suggests exposure to exogenous insulin, except in case of insulin-treated diabetes or screening for type 1 prediabetes. In type 1 diabetes, high anti-insulin level may be associated with particular diabetes instability [START_REF] Hu | Fengling Chen Exogenous insulin antibody syndrome (EIAS): a clinical syndrome associated with insulin antibodies induced by exogenous insulin in diabetic patients[END_REF]. Spontaneous occurrence of these antibodies in a non-diabetic person (Hirata syndrome) is reported in some cases, leading to predominantly postprandial hyperinsulinemic hypoglycemia [START_REF] Sahni | Insulin Autoimmune Syndrome: a rare cause of postprandial hypoglycemia[END_REF]. Graves' disease is frequently present in Hirata syndrome, which appears to be particularly prevalent in Japan [START_REF] Paudyal | Spontaneous hypoglycaemia in a patient with Graves' disease[END_REF].

Onset of hypoglycemia is linked to the formation of autoantibody-insulin complexes and insulin release independently of blood sugar level. Immunoprecipitation with polyethylene glycol should be used with caution. Chromatography on filtration gel with the addition of exogenous insulin may enhance detection sensitivity [START_REF] Church | Diagnosis of insulin autoimmune syndrome using polyethylene glycol precipitation and gel filtration chromatography with ex vivo insulin exchange[END_REF].

B. Insulin-receptor antibodies

Hypoglycemia linked to anti-insulin receptor antibodies usually presents as fasting hypoglycemia in women in the context of other autoimmune diseases such as lupus, primary biliary cirrhosis, Hashimoto's thyroiditis or multiple sclerosis [START_REF] Taylor | Syndromes of autoimmunity and hypoglycemia. Autoantibodies directed against insulin and its receptor[END_REF]. However, hypoglycemia can also occur without any autoimmune context, including in insulin-resistant diabetes [START_REF] Maiza | Anti-insulin receptor antibodies related to hypoglycemia in a previously diabetic patient[END_REF]. This is type B insulin resistance If the manifestations are disabling, corticosteroid therapy, or even immunosuppressive regimen in case of life-threatening events, can be started [START_REF] Klubo-Gwiezdzinska | Combined Immunosuppressive Therapy Induces Remission in Patients With Severe Type B Insulin Resistance: A Prospective Cohort Study[END_REF]. Antibodies generally disappear within months or even years [START_REF] Arioglu | Clinical course of the syndrome of autoantibodies to the insulin receptor (type B insulin resistance): a 28-year perspective[END_REF].

V. Conclusion

The analysis of rare causes of hypoglycemia cannot be exhaustive. It should be borne in mind that certain conditions can, confusingly, lead to hyper-as well as hypoglycemia.

Diagnosis of IEMs most often requires a specialist opinion in an IEM reference center.

  syndrome, often accompanied by acanthosis nigricans. The usual tests are often difficult to interpret because these antibodies can be associated with inhibition of insulin binding, resulting in an apparent hyperinsulinism with low C-peptide, because hypoglycemia slows the secretion of beta cells. The differential diagnosis of this biological profile is artificial hypoglycemia by surreptitious insulin injection. Detection of anti-insulin receptor antibodies (PRISIS Reference Center -Pr. Vigouroux) is contributive. In some cases, insulinemia and pro-insulinemia are also low. The serum and purified immunoglobulin fraction are able to activate the proximal signaling pathway (tyrosine phosphorylation of the insulin receptor beta-subunit and IRS1 (Insulin Receptor Substrate-1)) and distal signaling (phosphorylation of Akt / PKB) in vitro dose-dependently, mimicking the action of insulin[START_REF] Maiza | Anti-insulin receptor antibodies related to hypoglycemia in a previously diabetic patient[END_REF].

Figure 1 .

 1 Figure 1. Main causes of hypoglycemia in adults.

Figure 1 .

 1 Figure 1. -Principales causes des hypoglycémies chez l'adulte.

Figure 2 .

 2 Figure 2. Procedure in case of fasting hypoglycemia with suspicion of IEM (inborn error of

Figure 2 .

 2 Figure 2. -Conduite à tenir devant une hypoglycémie de jeûne avec suspicion d'EIM (erreur innée du

Figure 3 .

 3 Figure 3. Main genetic beta cell abnormalities that can lead to hypoglycemia.

Figure 3 .

 3 Figure 3. Principales anomalies génétiques de la cellule bêta susceptibles d'entraîner des

Figure 4 .

 4 Figure 4. Interpretation of a metabolic fasting test. The examination should only be carried out in a

Figure 4 .

 4 Figure 4. Interprétation d'une épreuve de jeûne métabolique. L'examen ne doit être réalisé que dans

Figure 5 .

 5 Figure 5. Physiology of the main inborn errors of metabolism responsible for hypoglycemia in adults:

Figure 5 .

 5 Figure 5. Physiologie des principales erreurs innées du métabolisme responsables d'hypoglycémies

de l'adulte : glycogénoses de type 0, de type I ou maladie de Von Gierke et de type III ou maladie de Cori-Forbes (en haut à droite), troubles de la bêta-oxydation des acides gras (en bas à droite) et troubles de la néoglucogenèse (à gauche).Several of these figures were previously published in Douillard, "Correspondances en MétabolismesHormones Diabètes & Nutrition" (Vol XXII-n°9-10 Nov-Dec2018/ Vol.XXIII-n°1 Jan-Feb 2019).


Mohamed Ali Mami

Marzouk Lajili

Besma Khiari

Mejdi Jeguirim
email: mejdi.jeguirim@uha.fr

Combustion of raw and densified Tunisian oleic by-products in a fixed bed reactor

Keywords: Olive pits, Olive pomace, Sawdust, pellets, fixed bed reactor, Combustion characteristics, Gaseous emissions

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

Introduction

Olive oil is now knowing an ever more pronounced infatuation around the world for its countless virtues. To meet this increased demand, the oleic industry has flourished, especially in the Mediterranean basin region. But, these countries have to cope with huge amounts of generated by-products such as olive mill wastewaters (OMWW) and olive mill solid wastes (OMSW), mainly composed of olive pits (Pi) and olive pomace (Po). Indeed, these materials are extremely pollutant and cause nocive effects unless they are conveniently treated or better still, recovered. Fortunately, different valorization routes are being continuously investigated. For instance, OMSW were used in the petrochemical and refining industries as the main chemical feedstock for cosmetics abrasive, activated carbon, biosorbent, furfural production, etc. [START_REF] Azbar | A Review of Waste Management Options in Olive Oil Production[END_REF][START_REF] Khiari | Dynamics and kinetics of cupric ion removal from wastewaters by Tunisian solid crude olive-oilwaste[END_REF]. They could also be branded as renewable sources of energy [START_REF] Christoforou | A review of olive mill solid wastes to energy utilization techniques[END_REF][START_REF] Bartocci | Pyrolysis of Olive Stone for Energy Purposes[END_REF][START_REF] Mata-Sánchez | Development of olive stone quality system based on biofuel energetic parameters study[END_REF]. Indeed, it is well known that biomass energetic valorization is possible during different thermochemical conversion processes such as combustion [START_REF] Elorf | Excess air ratio effects on flow and combustion caracteristics of pulverized biomass (olive cake)[END_REF], pyrolysis [START_REF] García | Characterization and modeling of pyrolysis of the two-phase olive mill solid waste[END_REF][START_REF] Volpe | Assessment of olive wastes as energy source: pyrolysis, torrefaction and the key role of H loss in thermal breakdown[END_REF][START_REF] Gagliano | Effectiveness of thermodynamic adaptive equilibrium models for modeling the pyrolysis process[END_REF] and gasification [START_REF] Lajili | Fast pyrolysis and steam gasification of pellets prepared from olive oil mill residues[END_REF][START_REF] Ducom | Comparative analyses of three olive mill solid residues from different countries and processes for energy recovery by gasification[END_REF][START_REF] Gagliano | Evaluation of the performance of a small biomass gasifier and micro-CHP plant for agro-industrial firms[END_REF]. As such, OMSW were often reported as interesting biofuels, showing high combustion characteristics, thanks mainly to their high energetic content (17 MJ/kg). The olive pits and pomace were either used as such after natural solar drying to feed traditional furnaces and combustion chambers [START_REF] Stover | Direct use of biomass powder in internal combustion engines[END_REF] or densified in the form of pellets, briquettes or logs to be used in boilers, fixed beds, and stoves [START_REF] Lajili | Performance of a household boiler fed with agropellets blended from olive mill solid waste and pine sawdust[END_REF][START_REF] Khlifi | Investigation of the combustion characteristics of briquettes prepared from olive mill solid waste blended with and without a natural binder in a fixed bed reactor[END_REF].

In a recent work [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF], the combustion of pellets made of olive pits using a counter-current fixed bed reactor showed many advantages in comparison with wood pellets. The gaseous O2, CO2, CO, H2O (steam), Corg (organic carbon), SO2 and NOx (NO + N2O) were emitted in acceptable concentrations according to the European standards FprEN 303-5 [START_REF] Schmidl | European Wood-Heating Technology Survay: An Overview of Combustion Principles and the Energy and Emissions Performances Characteristics of Commercially Available Systems in Austria[END_REF][START_REF] Carvallo | Performance of a pellet boiler fired with agricultural fuels[END_REF].

To enhance combustion efficiency while being coherent with the context of environmentalfriendly approaches, olive pits and pomace could be blended with woody biomass [START_REF] Lajili | Physico-chemical properties and thermal degradation characteristics of agropellets from olive mill by-products/sawdust blends[END_REF][START_REF] Barbanera | Characterization of pellets from mixing olive pomace and olive tree pruning[END_REF][START_REF] Miranda | Characterization and combustion of olive pomace and forest residue pellets[END_REF].

Such mixture would achieve high-quality pellets with more suitable characteristics [START_REF] Khlifi | Investigation of the combustion characteristics of briquettes prepared from olive mill solid waste blended with and without a natural binder in a fixed bed reactor[END_REF][START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF][START_REF] Gil | Mechanical durability and combustion characteristics of pellets from biomass blends[END_REF][START_REF] Heschel | Abatement of emissions in small-scale combustors through utilisation of blended pellet fuels[END_REF], decrease the ash content [START_REF] Miranda | A Review of Pellets from Different Sources[END_REF], reduce notably CO, NOx and SO2 emissions [START_REF] Gil | Mechanical durability and combustion characteristics of pellets from biomass blends[END_REF] and in the wake slow down forest logging [START_REF] Gehrmann | Synergies Between Biomass and Solid Recovered Fuel in Energy Conversion Processes[END_REF].

In this context, many mixtures of oleic by-products with woody biomass such as pine sawdust were investigated to produce pellets used to feed small powered boilers (< 50 kW). Results showed that these alternative solid fuels are efficient and exhibit low gaseous emissions [START_REF] Lajili | Performance of a household boiler fed with agropellets blended from olive mill solid waste and pine sawdust[END_REF][START_REF] Maxwell | Emissions from the combustion of torrefied and raw biomass fuels in a domestic heating stove[END_REF]. Lajili et al. [START_REF] Lajili | Performance of a household boiler fed with agropellets blended from olive mill solid waste and pine sawdust[END_REF] have shown that the gaseous emissions of CO, CO2, CxHy, and NOx when using an 8-12 kW domestic boiler remained acceptable by comparison to the En 303-5 normalization. Miranda et al. [START_REF] Miranda | Characterization and combustion of olive pomace and forest residue pellets[END_REF] have analyzed the emissions during the forest residues-olive pomace pellets combustion in a pellet stove (5.8 kW). The authors then showed that the mixture pellets produce higher CO when the blending process exceeds 50% of olive pomace.

In this study, the behavior of four different samples, involving olive pits and olive pomace mixed or not with woodchips and sawdust, is examined during their combustion in a 40 kW counter-current fixed bed reactor. Data such as bed temperature, reaction front position, specific heat release, reaction front velocity, mass conversion rate, and ignition rate are determined and compared to previous works [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF][START_REF] Porteiro | Experimental analysis of the ignition front propagation of several biomass fuels in a fixed-bed combustor[END_REF]. Ultimately, the conclusions would serve to extend the study up to the industrial scale.

Materials and methods

Biofuels preparation and characterization

The olive pomace (Po) and olive pits (Pi) used in this study were provided by "Zouila Company", located in the vicinity of Mahdia city (Tunisia) while the wood chips (WC) and the sawdust (S) were collected from Germany (Karlsruhe city).

Four different samples were prepared: (i) pellets made of 50% Po and 50% S, indicated by (50Po-50S)pel (ii) pellets made of 50% Pi and 50% S, indicated by (50Pi-50S)pel (iii) raw grained material made of 50% Pi and 50% WC, indicated by (50Pi-50WC)raw (iv) raw powder of 100% Pi, indicated by (100Pi)raw Based on our previous works in which we tested different percentages blends of olive mill solid wastes and pine sawdust, we concluded that we can't exceed 50% of olive mill solid waste due to the ash content restriction [START_REF] Lajili | Physico-chemical properties and thermal degradation characteristics of agropellets from olive mill by-products/sawdust blends[END_REF]. Moreover, this type of blend (50-50%) decreases at maximum the pressure on woody biomass exploitation which yields to the decrease of forest uncontrolled harvesting worldwide.

The pelletization operating conditions are displayed in Table 1 and were carried out in agreement with the procedure of Serrano et al [START_REF] Serrano | Effect of moisture content, particle size and pine addition on quality parameters of barley straw pellets[END_REF]. For all samples, the moisture content is at a low level after solar drying. The moisture loss after the pelletizing step is mainly due to the evaporation process. More details can be found in our previous work [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF].

Table 1. Operating pelletization conditions

The characterization of these biofuels was carried out at the Chemical and Microbiological Institute UEG GmbH (Germany). Moisture content was determined according to the UNI EN 14774:2009 standard while the ash contents were determined by applying the UNI EN 14775:2010 analytical method, using a muffle furnace heated at 815 ºC. The ultimate analysis was carried out, following the UNI EN 15104:2011 standard. The gross calorific value also called of High Heating Value (HHV), from which it is possible to calculate the Low Heating Value (LHV) by subtraction the steam latent heat, and Energy Density (ED), were calculated using equations (1-3) [START_REF] Lajili | Performance of a household boiler fed with agropellets blended from olive mill solid waste and pine sawdust[END_REF][START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF]

: = 0.3536 + 0.1559 -0.0078 ℎ (1) = - % + % (2)
ED = " #$ . LHV (3)
where, " #$ is the bulk density, M is the moisture, H is Hydrogen content, FC is the Fixed Carbon content and VM is Volatile Matter.

Combustion tests

As it is illustrated in Figure 1, the used experimental plant is composed of three main components: (1) the combustion chamber containing the fixed bed, the freeboard zone and Its total volume is 10 liters. Thirteen K-type thermocouples were arranged all along the combustor axis at intervals of 20 mm for bed temperature measurements (Figure 2). A more detailed description of this experimental device was reported in our previous papers [START_REF] Lajili | Performance of a household boiler fed with agropellets blended from olive mill solid waste and pine sawdust[END_REF][START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF][START_REF] Kraiem | Energy recovery from Tunisian agri-food wastes: Evaluation of combustion performance and emissions characteristics of green pellets prepared from tomato residues and grape marc[END_REF][START_REF] Romero | Characterization of biomass ashes produced at different temperatures from olive-oil-industry and greenhouse vegetable wastes[END_REF].

The reactor was fed consecutively by 5.75 kg of (100Pi)raw, 2.99 kg of (50Pi-50WC)raw, 5.25 kg of (50Po-50S)pel and 4.98kg of (50Pi-50S)pel. The primary specific airflow was 310 kg/(h.m²) at ambient temperature (Tin = 25°C). This airflow is calculated on the surface area cross section of air input equal to about 0.032 m 2 .

Figure2. A schematic view of the used experimental device

During the combustion tests, characteristic numbers such as the reaction front velocity (uRF), the mass conversion rate (MCR), the ignition rate (IR) and the specific heat release rate (HR) were determined as follows [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF][START_REF] Gehrmann | Synergies Between Biomass and Solid Recovered Fuel in Energy Conversion Processes[END_REF]:

( )* = +, +- (mm/min) (4)
. = /0 12 314,5

(kg.m -2 .s -1 ) (5)

6. = ( )* × " # (kg.m -2 .s -1 ) (6)
. = . × (MW.s -1 ) (7)
In these equations, h is the bed height [mm], 80 is the mass loss rate [kg/s], A is fuel bed cross section [m²], " # is the bulk density of the fuel [kg/m 3 ].

These characteristic numbers are scale-independent and can, therefore, be useful when extending this type of study to the industry field.

When realizing energy recovery using biofuels, it is always necessary to calculate the combustion and the boiler efficiencies. In our case, the fixed bed reactor efficiency, after being equipped by a water recovery system, can be calculated as it has been done by Gehrmann et al. [START_REF] Gehrmann | Synergies Between Biomass and Solid Recovered Fuel in Energy Conversion Processes[END_REF], especially since it is the same used plant. Indeed, for assessing the effects of the addition of solid recovered fuels to biomass and the influence of a decreasing primary air temperature on the system efficiency Gehrmann et al. [START_REF] Gehrmann | Synergies Between Biomass and Solid Recovered Fuel in Energy Conversion Processes[END_REF] proposed a simple expression of efficiency like Carnot's formula in this case of a heat boiler generating steam:

9 *# ≈ 1 - ; <= ; >? (8)
Where @ *A is the boiler outlet temperature (K) and @ 1+ is the adiabatic combustion temperature (K). It is to be highlighted that in eq.8, @ 1+ should be determined from the energy balance by taking it as a function of; λ, fuel/air ratio (kg air/kg fuel) as minimum air demand, air temperature and inlet fuel temperature respectively [START_REF] Gehrmann | Synergies Between Biomass and Solid Recovered Fuel in Energy Conversion Processes[END_REF].

Results and discussion:

Samples characterization

Table 2 summarizes the ultimate and the proximate analyses of the different samples. The equivalent formula of each sample is then determined. The ash values for all samples are smaller than 5% d. b., which is under the EN 303-5

normalization. These values remain inferior to those reported in the literature [START_REF] Romero | Characterization of biomass ashes produced at different temperatures from olive-oil-industry and greenhouse vegetable wastes[END_REF]. Besides, except for (100Po)pel, nitrogen content, which is responsible for the nitrogen oxide (NOx) releasing during the combustion, is always less than 1%. Moreover, SOx emissions are expected to be negligible regarding the low contents of sulfur in the samples (< 0.1%). The low heating value LHV, the bulk density BD, the energy density and the durability Du values are presented in Table 3. The durability is the percentage of the material remaining in a fuel pellet before and after some specific mechanical tests. It affects directly the transport efficiency and storage operation [START_REF] Whittaker | Factors affecting wood, energy grass and straw pellet durability -A review[END_REF]. Durability tests should be conducted conformingly to CEN/TS 15210-1 (or 2) standards. This consists at exposing 500g pellets to controlled collisions against each other and versus walls of gyratory chamber turning at about 50 rpm during 500 rotations. The durability is estimated by measuring the mass of remaining pellets after elimination of abraded and fine released particles [START_REF] Carone | Influence of process parameters and biomass characteristics on the durability of pellets from the pruning residues of Olea europaea L[END_REF] Table 3 exhibits values of durability smaller than 97.5% which is the required standard norm. In order to resolve this shortcoming, it is recommended the use of a binder agent like corn starch during biofuel briquetting under a high pressure as it is realized in a recent work [START_REF] Khlifi | Briquettes Production from Olive Mill Waste under Optimal Temperature and Pressure Conditions: Physico-Chemical and Mechanical Characterization[END_REF].

The LHV values and the energy densities of the olive pomace and the olive pits when blended in pellets with sawdust are higher than those of 100% sawdust pellets but decreased in comparison with pellets made of 100% pomace and pits respectively because of the low bulk density of pine sawdust. However, their durability was enhanced, especially for pits. It is to be recalled that biomass used in previous work [START_REF] Lajili | Physico-chemical properties and thermal degradation characteristics of agropellets from olive mill by-products/sawdust blends[END_REF] was composed by a mixture of pits, pomace and about 4% of residual oil.

Bed temperature

The combustion process progresses in the bed with three distinct steps: the moisture evaporation, volatile gas release and combustion and, the char residual oxidation and gasification.

Figure 2 shows the temperature evolution all along the reactor axis at various heights during the combustion process for the different pellets. Since the reactor is counter-current, the propagation of the reaction front is observed from the upper to the bottom of the bed reactor [START_REF] Thunman | Co-current and counter-current fixed bed combustion of biofuel-a comparison☆[END_REF]. More precisely, the bed surface slowly heats up, as the bed temperature rises and gets drier. The surface bed ignites due to the incident radiation and, the reaction front reaches the bed bottom between 10 and 35 min. The residual char oxidation zone is marked by a maximum value of temperature [START_REF] Porteiro | Study of the reaction front thickness in a counter-current fixed-bed combustor of a pelletised biomass[END_REF], which varies from one sample to another (1075 °C for (50Po-50S)pel, 990 °C for (50Pi-50S)pel, 700 °C for (50Pi-50WC)raw and 800°C for (100Pi)raw). These values are lower than those reported in some previous works (1200 °C for olive pomace pellets, 1090 °C for olive pits pellets, 1050 °C for wood pellets and 1000°C for wood chips [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF][START_REF] Schmidl | European Wood-Heating Technology Survay: An Overview of Combustion Principles and the Energy and Emissions Performances Characteristics of Commercially Available Systems in Austria[END_REF]. The lowest temperatures may be due to the fast convective cooling phenomenon [START_REF] Porteiro | Study of the reaction front thickness in a counter-current fixed-bed combustor of a pelletised biomass[END_REF], but the discrepancies are more probably attributed to the differences in the relative HHV. Mixtures of samples containing sawdust or wood chips are subject to a notable decrease in the maximum combustion temperature. This assumption can be explained by the heat radiative absorption of some gases molecules characterized by high absorption coefficient and present in the medium with important concentrations; H2O, CO and CO2 respectively.

However, this crucial parameter is calculated in this study using a more refined expression including the gas concentrations species [START_REF] Kolb | Characterization of combustion behaviour of refuse derived fuel[END_REF]: It is worth noting that the O2 is the only gas detected at the beginning of the test due to the injection of the primary air and, tends to a fluctuating small percentage when the combustion in the second zone is triggered. In this stage, the fuel bed began drying. The water steam (H2O) was yielded and affected the medium by its radiative absorption property. This property complies with other factors that make the maximum temperature decrease as it was shown in Figure 2. Moreover, in the main combustion zone (i.e. between 2.5 and 48 min), the concentrations of CO, CO2, Corganic, and H2O increased up to a maximum value, before dropping and fluctuating. This period is also called the quasistationary combustion zone [START_REF] Porteiro | Study of the reaction front thickness in a counter-current fixed-bed combustor of a pelletised biomass[END_REF] and marks the combustion of the residual char. This observation may be attributed to the non-optimal distribution of the primary and secondary air, and to the low ash melting temperature which causes their agglomeration at the bottom of the bed [START_REF] Steer | Biomass co-firing trials on a down-fired utility boiler[END_REF]. Consequently, the injection of the primary air can be obstructed causing incomplete combustion which explains the increase in certain gaseous emissions [START_REF] Roy | An experimental study of combustion and emissions of biomass pellets in a prototype pellet furnace[END_REF]. More precisely, the CO2 concentration for the (50Pi-50WC)raw is higher than that of (100Pi)raw (19.1% versus 16.1%). Similarly, CO2 concentration is higher for (50Po-50S)pel than for (50Pi-50S)pel (17.75% versus 16.82%). All these values are greater than CO2 emitted by pomace pellets (13%) [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF], pits pellets (11%) [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF], wood pellets (13.5%) [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF] and wood chips (15.5%) [START_REF] Lajili | Performance of a household boiler fed with agropellets blended from olive mill solid waste and pine sawdust[END_REF]. In contrast, CO concentrations are all smaller compared to the samples of the above references. The fuel conversion process ended after about 25-70 min depending on the sample.

Q = RS T U VW RS T U VW 3RS T U XYZ [ R\SU T [ ]^T_ T [ R\ X`abWVc U T (10)

Gas emissions in the post-combustion zone

Figure 4 illustrates the evolution of gaseous emissions such as CO2, CO, and O2 as a function of the time in the post-combustion chamber (Zone II). Sulfur dioxide (SO2), nitrogen oxides (NOx) and nitrous oxide (N2O) were also followed and are expressed in mg/Nm 3 .

Figure 4. Gaseous emissions in the post-combustion chamber

One can see that NOx emissions are much higher in comparison with SO2 emissions. This result is predictable regarding the low content of sulfur in all samples. Moreover, the (100Pi)raw sample released the highest NOx with 820 mg/Nm 3 against 650 mg/Nm 3 for (50Po-50S)pel and 500 mg/Nm 3 for (50Pi-50S)pel. This result is expected, given the nitrogen content in each of these samples. In comparison with our previous work [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF], raw olive pits released more NOx than when densified in the form of pellets (820 versus 750 mg/Nm 3 ), but still less than the olive pomace (893 mg/Nm 3 ). In general, blending olive pomace and olive pits with pine sawdust and/or woodchips in a 50-50 mixture decreases NOx, which was also reported in dedicated literature [START_REF] Heschel | Abatement of emissions in small-scale combustors through utilisation of blended pellet fuels[END_REF]. The reduction in bed temperature, as shown in Figure 2, has also probably a positive effect on lowering NOx emissions [START_REF] Ren | Carbon, sulfur and nitrogen oxide emissions from combustion of pulverized raw and torrefied biomass[END_REF]. , it is possible to express all gas concentrations in mg/Nm 3 as it is consigned in Table 5.

Regarding the European standards (EN 303-5) obtained values of SO2 are acceptable for all samples and those for NOx are also acceptable except (100Pi)raw. Moreover, even though concentrations of CO are found higher than the standard value (500 ppm or 625 mg/Nm 3 ), they are lower than concentrations found by Limousy et al. [START_REF] Limousy | Gaseous products and particulate matter emissions of biomass residential boiler fired with spent coffee grounds pellets[END_REF] and of the same magnitude or lower than values found recently by Nosek et al. [START_REF] Nosek | Energy Utilization of Spent Coffee Grounds in the form of Pellets[END_REF].

Mass loss and characteristic parameters

Mass loss curves and combustion characteristic numbers are presented in Figure 5 and Table 6 respectively. The ignition rate (IR) describes the quantity of fuel ignited per unit surface area and time due to the movement of the reaction front. The IR can be used as a scale-up parameter to industrial facilities and to estimate when the reaction front reaches the bottom of the fuel bed indicating the end of the combustion process. In addition, the heat release rate HR is determined as the product of the calorific value and the mass conversion rate according to (eq.7). More precisely, the specific heat release in the main combustion zone I (HR I) is calculated using the MCR I and the higher heating value of the fuel. Also, in the char residual oxidation phase, the heat release rate named (HR II) is obtained when multiplying MCR II by the higher heating value of the residual char (coke).

It is to be highlighted that the HHV of the coke can be calculated according to the following correlation [START_REF] Ahmaruzzaman | Proximate analysis and predicting HHV of chars obtained from cocracking of petroleum vacuum residue with coal, plastics and biomass[END_REF][START_REF] Vargas-Moreno | A review of the mathematical models for predicting the heating value of Biomass materials[END_REF]:

HHVcoke= 19.6 ycoke + 14.119 [MJ/kg] ( 11
)
Where yCcoke is the mass fraction of carbon in the coke, which is approximately 0.85 for lignocellulosic biomass coke [START_REF] Vargas-Moreno | A review of the mathematical models for predicting the heating value of Biomass materials[END_REF][START_REF] González | Combustion optimisation of biomass residue pellets for domestic heating with a mural boiler[END_REF]. Then HHV coke is ≈ 30.8 MJ/kg. The results collected in Table 4 show also some benefits when pelletizing and when blending olive pomace and olive pits with pine sawdust, in terms of mass conversion rate, notably in the freeboard zone and in terms of ignition rate. The efficiency lFB also increased with the pelletized mixtures, the highest one being recorded for the (50Po-50S)pel sample (79.8%), which is in the same order than wood pellets (79%). It was proved in previous works when using the ökofen automatic boiler (8-12 kW) fed with some types of pellets that the combustion efficiency exceeds 90% [START_REF] Lajili | Physico-chemical properties and thermal degradation characteristics of agropellets from olive mill by-products/sawdust blends[END_REF][START_REF] Miranda | Characterization and combustion of olive pomace and forest residue pellets[END_REF][START_REF] Kraiem | Energy recovery from Tunisian agri-food wastes: Evaluation of combustion performance and emissions characteristics of green pellets prepared from tomato residues and grape marc[END_REF][START_REF] González | Combustion optimisation of biomass residue pellets for domestic heating with a mural boiler[END_REF].

It is to be concluded that the 50-50 mixture of olive pomace or pits with sawdust/woodchips leads to a more homogenous combustion and higher energy efficiency.

Besides, Figure 6 shows better the variation of the reaction front positions versus time. The curve could be fitted linearly with a good approximation. This confirms again that the flame shows a steady and uniform propagation. More precisely, the raw olive pits sample exhibited a reaction front velocity of approximately 4.87 mm/min while it was 5.06 mm/min for the pelletized pits. The 100% pomace pellet sample reaction front velocity was only 4 mm/min. For wood pellets [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF] and woodchips [START_REF] Lajili | Physico-chemical properties and thermal degradation characteristics of agropellets from olive mill by-products/sawdust blends[END_REF], this parameter was 5.33 mm/min and 18.5 mm/min respectively. All these values were calculated with a regression factor (R²) higher than 0.9.

Gas emissions in partial oxidation

Tests of partial oxidation (10% and 13% O2) of olive pomace and olive pits blended with sawdust in pellets were carried out in the combustor. Gaseous emissions of (50Po-50S)pel and (50Pi-50S)pel within the primary and the secondary combustion chamber are measured and compared to other pellets partial combustion reported in the literature and gathered in Table 7. Also, a graphical comparison was realized in Figure 7a and Figure 7b in order to show better emissions of tested samples in the primary combustion chamber and in the secondary combustion chamber at two oxygen references. Hence, in order to justify the comparison, all results were converted at 10% and 13% of O2 using the following expression:

m n o ,qrs = m n o ,/rt4 u 3Rn o U POv u 3Rn o U NOEJ (12)
where, X is the gas concentration at the fixed oxygen ratio (XO2,ref) and measured during the experiment (XO2,meas) .

It was observed that gaseous emissions obtained in this work are in the same order of magnitude as what was reported, especially for the lignocellulosic biomass. As an example, the concentrations of nitrogen oxides (N2O+NO) emitted out from our two pellets at the post-combustion chamber were 103 and 82 ppm at 10% of O2 respectively, which is close to the 113 ppm obtained with the standards wood pellets. Moreover, Limousy et al. [START_REF] Limousy | Gaseous products and particulate matter emissions of biomass residential boiler fired with spent coffee grounds pellets[END_REF] stated that NOx decreased during the combustion, in a 12 kW household boiler, of pellets made of 50% of grounded spent coffee and 50% pine sawdust (50SPG-50S)pel.

Furthermore, they observed that NOx values varied from 206 ppm for the 100% SCG to 201 ppm for (50SPG-50S)pel at 13%O2, which is higher compared to the same molecules emitted by (50Po-50S)pel and (50Pi-50S)pel for the same oxidizing atmosphere (75 and 59 ppm respectively). Nevertheless, our results are more comparable to pellets made of tomato wastes and grape marc blended with sawdust ((50TW-50S)pel and (50GM-50S)pel respectively) [START_REF] Kraiem | Energy recovery from Tunisian agri-food wastes: Evaluation of combustion performance and emissions characteristics of green pellets prepared from tomato residues and grape marc[END_REF], and for the olive pits and the olive pomace pellets ((100Po)pel and (100Pi)pel respectively) [START_REF] Mami | Investigation of the olive mill solid wastes pellets combustion in a counter-current fixed bed reactor[END_REF]. As for SO2, as it was expected, the emissions don't exceed 15 ppm whatever is the O2 ratio. These values remain much lower than the 82 ppm recorded for the (50TW-50S)pel and the 44 ppm for the (50GM-50S)pel at 10% of O2 [START_REF] Kraiem | Energy recovery from Tunisian agri-food wastes: Evaluation of combustion performance and emissions characteristics of green pellets prepared from tomato residues and grape marc[END_REF]. SO2 and NOx emissions are in all cases negligible compared to 0-4000ppm for NO, 0-1000ppm for N2O and 0-500 ppm for SO2) at 10% O2 [START_REF] Gehrmann | Synergies Between Biomass and Solid Recovered Fuel in Energy Conversion Processes[END_REF].

Conclusion

In this paper, four sample types involving olive pomace and olive pits, in their raw form or blended with pine sawdust or wood chips in the form of pellets, were degraded in a counter-current fixed bed reactor. Three distinct phases during the combustion process were observed: the ignition delay time, the quasi-stationary combustion zone and the remaining char combustion. Scale independent numbers characterizing the whole combustion process such as the mass conversion rate, the ignition rate, and the reaction front velocity were determined. Gaseous emissions were also analyzed and measured, both in the primary chamber and in the post-combustion chamber. In comparison with our previous papers as well as with other testes biofuels, obtained results in this study, show a notable improvement as the combustion process was more homogenous and, the ashes on one hand and NOx on the other hand decreased. In compliance with the highest European standards, this work poses the first steps to use the tested mixtures as alternative biofuels which can be exploited either for household activities or in industrial plants for clean energy production.

a heated furnace. ( 2 )

 2 The post combustion chamber. (3) The flue gas cleaning system incorporating a carbon absorber, a heat exchanger and a bag filter. The facility is suited to perform tests of several kilograms of solid fuel samples. The fuel bed has a volume of 10

  liters. The furnace and the secondary combustion chamber (post-combustion chamber) can be heated electrically up to a temperature of 1100 ºC.

Figure. 1 .

 1 Figure. 1. Sketch of the KLEAA fixed bed facility at ITC, Karlsruhe, Germany.

Figure 2 .Table 4 .Figure 3

 243 Figure 2. Fuel bed temperature at different heights Table4exhibits the recapitulation of the main bed temperature points and the ignition time for the 4 tested samples. It is to be highlighted that the highest reached maximum temperature and the highest ignition time were reached with the (50Pi-50WC)pel, whereas, the lowest maximum temperature and the lowest ignition time were exhibited by the (50Po-50S)pel. The ignition time can be determined be the inflection point when examining the temperature profile corresponding to each thermocouple. It can be concluded that after moisture evaporation the process can be divided into 2 phases: The first stage is marked by the release of volatiles and its combustion called main combustion zone, and then the second stage corresponding to the char residual combustion.

Figure 3 .

 3 Figure 3. Gaseous emissions evolution in the freeboard zone

Figure 5 .

 5 Figure 5. Mass loss evolution versus the time

Figure 6 .

 6 Figure 6.Reaction front positions

  Mill Solid Waste, EOMSW: Exhaust Olive Mill Solid Waste, S: pine sawdust, TW: Tomato Waste, GM: Grape Marc, n. d.: non-determined 1: In the primary combustion chamber 2: In the secondary combustion chamber

Figure 7a .

 7a Figure 7a.Gaseous emissions at the combustion chamber for the (50Po-50S)pel and (50Pi-50S)pel

Pellet (50Po-50S)pel (this work) (50Pi-50S)pel (this work) (100Po)pel [14] (100Pi)pel [14] Pelletizing temperature (°C)

		74-82	74-89	60	50
	Biomass moisture (%wb)	14.09	14.79	14.7	15.3
	Pellet moisture (%wb)	11.28	11	12.4	13.7
	%wb: wet basis

Table 2 .

 2 Ultimate

	Sample	Equivalent		Ultimate analysis		Proximate analysis
		formula	C	H	O	N	S	Ash	FC	VM
	(50Po-50S)pel	CH1.46O0.7N0.013	48.28 5.90 45.07 0.75 <0.1 1.70	21	77.3
	(50Pi-50S)pel	CH1.68O0.79N0.006	45.50 6.35 47.80 0.35 <0.1 1.20	20.50 78.8
	(100S)pel [19]	CH1.4O0.63N0.002	46.3 5.40 39.4 0.11 0.022 0.30	24.30 75.40
	(50OW-50S)pel [19] CH1.41O0.7N0.012	48.33 5.71 45.31 0.68 0.05 1.97	16.23 70.50
	(100Po)pel [16]	CH1.3O0.57N0.021	49.5 5.40 43.7 1.3	<0.1 2.90	17.50 79.40
	(100Pi)pel [16]	CH1.65O0.74N0.011	46.5 6.30 46.6 0.5	<0.1 1.90	15.70 82.40

and proximate analysis (%db) and Equivalent formula (db.af) db: dry basis; db.af: dry basis and ash-free; OW: olive waste (Pi+Po)

Table 3 .

 3 Energy contents of the pellet samples

	Sample	LHV (MJ.kg -1 )	BD (kg.m -3 )	ED (GJ.m -3 )	Du

Table 5

 5

	below summarizes the gaseous emissions in both freeboard zone (Zone I) and post-
	combustion chamber (Zone II). Knowing that: 1vol. %-wet = 10000 ppm, and

Table 5 . Recapitulation of gas emissions in Zone I and Zone II.

 5

	Samples	(50Po-50S)pel	(50Pi-50S)pel	(50Pi-50WC)raw (100Pi)raw	Standards value
	CO2 [mg/Nm 3 ]	3486.6	3303.9	3751.7	3162.5	(European Norms)
	CO [mg/Nm 3 ]	2756.3	2387.5	1263.7	2125.0	2500 (class1)-4500
						(class 2) and 12500
						(class 3) [42]
	SO2 [mg/Nm 3 ]	99.57	98.5	15	22.3	200 [43, 44]
	NOX [mg/Nm 3 ]	650	500	600	820	500-650 [43, 44]

Table 6 .

 6 Combustion characteristic numbers

	Sample	(100Pi)raw	(50Pi-50	(50Po-	(50Pi-	WC	(100Pi)pel	(100P)pel	(100S)pel
			WC)raw	50S)pel	50S)pel	[17]	[16]	[16]	[16]
	MCRI [kg/m²•s]	0.042	0.045	0.047	0.052	0.047	0.049	0.040	0.050
	MCRII [kg/m²•s]	0.004	0.006	0.004	0.006	0.007	0.006	0.005	0.003

Table 7 .

 7 Emissions values corrected at (a) 10% of O2 and (b) 13% of O2

	Samples	O2	CO2	H2O	CO 1	CO 2 (ppm) NOx	NO2	NO	SO2
		(%)	(%)	(%)	(%)		(ppm)	(pmm)	(ppm)	(ppm)
	(50Po-50S) pel	11	9.47 10.52	11.04 a) 34	a)103 n.d.	n.d.	a) 14
						b) 25	b) 75	n.d.	n.d.	b) 10
	(50Pi-50S)pel	10.53 8.65 10.8	10.26 a) 8	a) 82	n.d.	n.d.	a) 15
						b) 6	b) 59	n.d.	n.d.	b) 11

Acknowledgments

Mohamed Ali Mami would like to express deep thanks to colleagues Ghrmann, Hans-Joachim and as well as to technical team of Institute for Technical Chemistry (ITC) of Karlsruhe Institute of Technology (KIT) for test of combustion realization. Also, to Rainer Bolduan as well as to technical team of European Institute for Energy research (EIFER) for providing woody biomass and tests of pellets characterization.