Behavior of Phosphorus and Other Inorganics during the Gasification

ARTICLE pubs.acs.org/EF

Behavior of Phosphorus and Other Inorganics during the Gasification of Sewage Sludge Christine Bourgel,†,‡ Emmanuel Veron,†,‡ Jacques Poirier,*,†,‡ Franc-oise Defoort,§ Jean-Marie Seiler,§ and Carlos Peregrina# †

CNRS, UPR3079 CEMHTI, 1D Avenue de la Recherche Scientifique, 45071 Orleans Cedex 2, France Universite d’Orleans (Polytech), Avenue du Parc Floral, BP 6749, 45067 Orleans Cedex 2, France § CEA, DTN, 17 Rue des Martyrs, 38054 Grenoble, France, # CIRSEE, Suez Environnement, 38 Rue du President Wilson, 78230 Le Pecq, France ‡

ABSTRACT: The context of the present study is the gasification of dried sludge at high temperature. The aim of this work is to shed new light on the impacts of sludge ashes on gasification process. The purification sludge can contain up to 50% inorganic matter. The objective of this study is to understand the role of these inorganics, especially phosphorus, during the heat treatment. Several techniques are used to solve this problem. First, using thermodynamic calculations (FactSage), the evolution of the volatility of the inorganics is observed and the condensed phases formed during the heat treatment are determined. The simulations are done under atmospheric pressure, from 500 to 1500 °C. Second, to compare with the calculus, XRD and TGA experiments are carried out on model samples, with composition close to the real sludge, to determine which species volatilize and to compare the experimental results with the thermodynamic calculations.

1. INTRODUCTION This research deals with biomass thermochemical conversion by gasification to produce fuel synthetic gas or “syngas”. The type of biomass that was selected for this study is the dried sewage sludge. The high organic content of sludge (50 70 wt % of the total dry solids) constitutes a potentially valuable renewable energy resource without any impact on food cultures. Moreover, sewage sludge gasification is a technical challenge,1 as a result of the particularly high mineral content of the biomass (i.e., 30 50% of the dry solids). Such a highly mineral matter may cause sintering of the reacting sludge during gasification or promote the formation of inorganic vapors that must be removed from the syngas prior to utilization. Finally, from an environmental viewpoint, gasification may provide a solution for the waste volume reduction of sludge with a high yield of energy production (i.e., electricity and/or fuel). In Europe, for instance, the year round production of sludge is estimated at 10 million tons of dry solids (or 42 TWh). Concerning the mineral part, it could be considered as a final waste whose valorization could be in agronomics or ceramic fields. Indeed, thanks to the high temperature gasification processes as the EFR (entrained flux reactor),1 pollutant inorganic species are expected to be treated differently than by combustion, incineration, or methanization. Moreover, the presence of low melting point oxides (K2O, Na2O, and P2O5) in the sludge mineral matter involves the potential formation of liquid ashes in the gasification reactor, which influences the working temperature of the EFR reactor.2 However, these species are known to be volatile at high temperature so that they could leave the slag, thus increasing the work temperature of the reactor. However, it occurs at a very low rate because its kinetic of evaporation is rather slow. In any case, this phenomenon is not yet fully known r 2011 American Chemical Society

and would be worthwhile to understand the role of these inorganics during gasification in an entrained flow reactor. In this research, the behavior of P2O5 will be the focus because of its significant amount in the sludge. Moreover, this component is actually poorly understood, neither in simple nor in complex systems such as sludge.3 6 In this paper, several techniques are used to predict the volatilization of inorganics in sludge. First, thermodynamic calculations were performed to know the evolution of the volatility of the inorganics and the condensed phases formed during the heat treatment. This method is very often used for studying the volatilization behavior of trace metals (named 1000 ppm wt as N, S, Ca, Al, Fe, P, etc.) is also calculated for biomass gasification.11 14 However, to the best of our knowledge, volatilization of minor species for sludge gasification has only been partially calculated up to 1000 °C.10 Calculations were performed with two real sludge compositions containing 21 inorganic elements and with four model compositions of sludge containing the three main inorganic elements (i.e. Si Ca P) as oxides. Second, experimental work is performed to determine the behavior of the phosphorus during the heat treatment of these model samples of sludge. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) in situ measurements are used and the results obtained are compared with the thermodynamic calculations. Received: September 1, 2011 Revised: November 9, 2011 Published: November 14, 2011 5707

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0

1.4 10

0

1.8 10

3.6 10 9.8 10 5.07 9.67 sludge 2

19.09

14.09 45.93

0.69

1.65 2.72

0.49

0.25

0.09 0.02

0.13

2.1 10

5.2 10

3 10

2.1 10

3

1.9 10

2

1.4 10

3

6.3 10

2 2

2.1 10 2.4 10 23.85 10.12 sludge 1

18.72

19.93 21.51

1.50

1.87 1.23

0.68

0.27

0.05 0.01

0.14

3

4.8 10

NiO

2

ZnO

Pb

Cr2O3

2 3

CuO B Mn2O3 F Cl Fe2O3 Al2O3 P2O5 MgO K2O SO3 Na2O TiO2 CaO SiO2 oxides (wt %)

Table 1. Composition of the Inorganic Part for Two Types of Sludge Chosen for Calculations

2

3

3

BaO

2

Ag

3

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Figure 1. Ternary phase diagram of CaO SiO2 P2O5. The colored triangles represent the four compositions studied. The green triangle corresponds to the first composition (CS0P), the orange circle to the second one (CS30P), the pink circle to the third one (CS40P), and the blue circle to the fourth one (CS50P).

2. THEORETICAL STUDY OF THE VOLATILITY BEHAVIOR OF SLUDGE In this study, the aim is to understand the phenomenon of volatility occurring during the gasification of sludge by using thermodynamic calculations. Compositions of the studied sludge are presented in section 2.1. The tool used to perform the calculations is described in section 2.2, followed by the description of the main results. 2.1. Initial Conditions Taken for Calculations (Mass Balance, Temperature, and Pressure). 2.1.1. Real Sludge Composition. For the municipal sewage sludge inorganic fraction, Si, Al,

Fe, P, and Ca are the major inorganic elements15 (often called minor species in the literature, by opposition to traces). Two different samples of sludge mineral matter were chosen for the calculations. The first one represents the average composition of 34 different French types of sludge. The second one is an extreme case, rich in phosphorus. Their inorganic contents are presented in Table 1. Of course the major difference comes from the amount of phosphorus, which is much higher in the second sludge than in the first one. The initial mass balance taken for the calculation are thus the following: • 1 mol of biodry (C6H9O4), which represents the organic part of the sludge (i.e., 145 g) • 1.3 mol of ashes, which represents the inorganic part of the equivalent of 100 g of dry sludge • 2.5 mol of O2 and 0.5 mol of N, which represent the air added during gasification with partial oxygen enrichment. To simulate the gasification process in an entrained flow reactor, the calculations are done for temperatures varying between 500 and 1500 °C, under atmospheric pressure. 2.1.2. Model Sludge Composition. To understand the volatilization behavior of the phosphorus in the real case, some other calculations were performed with model sludge containing the oxides CaO SiO2 P2O5. The choice of CaO and SiO2 oxides (instead of Fe2O3 or Al2O3) is made because P2O5 forms mainly stable oxides with CaO, which can alternatively be bound in silicates or aluminosilicates. Thus, this quasi-ternary system is suitable to explain the behavior of the complex sludge. Four compositions have been studied in this ternary system. The first one is free from phosphorus and contains calcium and 5708

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Figure 2. Evolution of a fraction of elements present in gas phase against temperature. (a) Calculation on sludge 1. (b) Calculation on sludge 2. The dotted line represents the working temperature range of EFR.

silicon oxides in equal proportions. It is called CS0P and is represented by a green triangle in Figure 1. The second one is composed of 28 wt % P2O5, 35 wt % CaO, and 35 wt % SiO2 and is called CS30P (marked by an orange triangle in Figure 1). The CS40P is composed of 40 wt % P2O5, 30 wt % CaO, and 30 wt % SiO2 (marked by a pink triangle in Figure 1). The last sample contains 50 wt % P2O5, 25 wt % SiO2, and 25 wt % CaO and is called CS50P (marked by a blue triangle in Figure 1). 2.2. Thermodynamic Tool Used for Calculations. The calculations were done using version 6.0 of the FactSage software. The databases used are the following: • ELEM: database for elements • FACT 53: database for gases and condensed species

Some compounds were eliminated manually from the database to make calculations possible. • The gases, solids, and liquids for which the temperature Tmax of the specific heat (Cp) is less than 700 K (because the calculations are done for elevated temperatures >500 °C) • The liquids of FT53 base The number of compounds is thus 1457 for the sludge 1 calculation and 1344 for the sludge 2 calculation: • 591 (sludge 1) and 545 (sludge 2) gases • 46 (sludge 1) and 44 (sludge 2) liquids • 820 (sludge 1) and 755 (sludge 2) solids For the real sludge calculations, it is impossible to take the solutions into account because the calculations are too slow and, 5709


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Figure 3. Evolution of condensed phases against temperature. (a) Calculation on sludge 1. (b) Calculation on sludge 2.

sometimes, impossible. Thus, in this study, the calculations are done without solution. Solutions were not taken into account for the model sludge calculations either, mainly because there is no available database having the solution (liquid and solid) descriptions. The “Equilib” program of the software was used. The compositions of all the elements (Table 1) are entered, in the tab “reactants” of the “Equilib” menu. 2.3. Calculation Results. Results of the volatility of all inorganic elements taken into account in the calculations are presented in section 2.3.1 for the two real types of sludge during the gasification process. The effect of phosphorus will be particularly studied in section 2.3.2. Results concerning the model sludge are presented in section 2.3.3. 2.3.1. Volatility of Real Sludge Composition. The calculations will allow us to determine which elements evaporate and which ones remain as a solid phase during the heat treatment. Moreover, the software calculates the phases present at the thermodynamic equilibrium for each temperature of the heating. The results will thus distinguish the major gases (when the molar

fraction of gas exceeds 0.05), the minor gases (all the other gases), and the condensed phases. To know in which phase each element (gas or condensed phase) is, it is interesting to calculate the release fraction Fi of each element (quantity of element i in the gas on its initial quantity). Figure 2 presents the evolution of the release fraction of each element in gas form as a function of the temperature, for sludges 1 and 2. For sludge 1 (Figure 2a), elements C, B, F, N, Cl, S, Pb, Zn, Ag, Cu, and H are entirely in gas phase (Fi = 100%) in the 1300 1500 °C range of the working temperature of the EFR. On the contrary, the elements Si, P, Al, Ba, Ca, Mg, and Ti are completely in condensed phase (Fi = 0%). The Ni, Cr, Mn, K, Na, and Fe elements are partially in gas and condensed phases. For sludge 2, which is richer in phosphorus (Figure 2b), the same behavior is observed for all elements except the higher phosphorus volatility starting from 700 °C. Figure 3 presents the evolution of the condensed phases with the temperature, for sludge types 1 and 2. Carbon C(s) is not 5710


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Figure 4. Evolution of the molar fraction of the major gases against temperature. (a) Calculation on sludge 1. (b) Calculation on sludge 2.

drawn because of a scale problem (12 wt % at 500 °C and 8 wt % at 600 °C for the first sludge and 11 wt % at 500 °C and 8 wt % at 600 °C for the second sludge). The quantity of solid phases is equivalent in both cases. Indeed, in Figure 3a, 24.8% of the mass is in solid phase at 500 °C and that decreases to 12.5% at 1500 °C. In Figure 2b, 21.7% of the mass is in solid phase at 500 °C and decreases to 8.3% at 1500 °C. This study also shows that calcium is present in all major compounds for both cases. Moreover, for sludge 1 (Figure 3a), these calcium compounds contain Fe, Mg, P, and Al elements. For sludge 2, which is richer in phosphorus (Figure 3b), the calcium compounds contain Al and P. The elements Mg, Fe, and K are present in aluminosilicate compounds. Figure 4 shows the composition of the major gases as a function of the temperature, for sludges 1 and 2. In both cases, mainly CO, H2, H2O, CO2, and CH4 gases are formed. This result is in accordance with the release of most of the organic

matter of sewage sludge.16,17 Among these gases, the formation of H2 and CO exceeds the others from 650 °C. Besides, the 700 900 °C temperature range corresponds to both a major release of H2 and CO and, thus, a maximum LHV (lower heating value) of the syngas. However, the conversion is not complete and it can be expected that around 0.7 1.5% of the initial dried sewage sludge is converted into tars and CH 4 within this range of temperature.18 N2 remains almost constant around 2.5 mol %. Figure 5 presents the evolution of the minor gas compounds as a function of the temperature, for sludges 1 and 2. H2S, HCl, HF, COS, H3BO3, HBO2, HS, and NH3 are both present in sludge types 1 and 2. Their evolutions have the same behavior. In these calculations, H2S is the gas with the highest concentration. Moreover, its value, in Figure 5a, is around 6 10 2 mol % and is almost constant from 500 °C. In Figure 5b, H2S increases from 8 10 2 mol % to 0.1 mol % between 500 and 900 °C. After, it maintains this value up to 1500 °C. 5711


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Figure 5. Evolution of the molar fraction of the minor gases against temperature. (a) Calculation on sludge 1. (b) Calculation on sludge 2.

Table 2. Correspondence between the Variation of P Content in Moles and the Weight Fraction of P2O5 in the Case of the Second Sludge P (mol) P2O5 (wt %)

0 0

0.02 14.4

0.04 25.2

0.06 33.6

0.08 40.3

0.1 45.7

Then, the HCl and HF are also constant at, respectively, 2 10 2 mol % and 10 2 mol %, in Figure 5a and 4 10 2 mol % and 2 10 2 mol % in Figure 5b, between 500 and 1500 °C. During this calculation, some of the initial chlorine is found in form of KCl and NaCl. Indeed, these gases concentrations reach 5 10 3 mol % and 6 10 3 mol % respectively in the first and second sludge, at 1500 °C. Moreover, there is formation of NaOH and KOH. Their amounts are relatively high at 1500 °C: nearly 1 10 3 mol % in Figure 5a and nearly 5 10 4 mol % KOH and 6 10 4 mol % NaOH in Figure 5b.

In both sludges, the gasification of phosphorus is observed in form of (P2O3)2, PO2, and PO. Although they are present in the first sludge calculation (Figure 5a), their amounts do not exceed 5 10 4 mol %, which is negligible. On the other hand, in the second sludge, (P2O3)2 overtakes the HCl amount from 800 °C and almost reaches 0.1 mol % at 1500 °C. The other P rich gases (PO2 and PO) reach, respectively, 4 10 3 mol % and 3 10 3 mol % at 1500 °C. In both cases, all the other gases are found with compositions under 5.10 3 mol %. NH3 concentration decreases to less than 1.10 4 mol % at 1500 °C. 2.3.2. Effect of Phosphorus. The next sections focus on the behavior of the element phosphorus to understand its conditions of volatilization during the thermal treatment. Indeed, an evaporation of P is observed essentially in the P richer sludge calculation. That is the reason why a sensitivity analysis of the calculated volatilization of inorganics is performed, varying the P2O5 composition in sludge 2, ranging from 0 to 0.1 mol, corresponding to 0 to 45.7 wt % P2O5 (Table 2). The other constituents will remain constant (see Table 1). 5712


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Figure 6. Evolution of the fraction of P in gas phase as a function of the amount of P2O5 at different temperatures.

Table 2 shows the different amounts of P for the next calculation and the corresponding weight fraction of P2O5. Figure 6 represents the evolution of the fraction of P in the gas phase as a function of the amount of P2O5 for different temperatures. The volatilization of element P begins at 700 °C when the amount of P2O5 is above 33 wt %. It increases to 11.5% for 46 wt % P2O5. The higher the temperature is, the higher the fraction of P in the gas phase will be. At 1500 °C, the volatilization begins at 25 wt % P2O5 and reaches 55.5% volatilization. At work temperature (∼1300 °C), the evaporation begins for a P2O5 content up to 25 wt %. The composition containing 46 wt % P2O5 releases 45% of the initial phosphorus. The distribution of the gas and condensed phases for the expected operating temperature of the gasification reactor (i.e. 1300 °C) is shown in Figure 7. The gas phase containing phosphorus is mainly composed of (P2O3)2. The other gases such as PO, PO2, and PS constitute only 3 10 4% of the initial amount of P (Figure 7a). On the other hand, the condensed phases containing P are mainly composed of Ca3(PO4)2. This element is thus divided into (P2O3)2(g) and Ca3(PO4)2(s) at 1300 °C. The more the amount of P increases, the more the condensed phase will be replaced by the gas phase. No volatilization of P is thus expected for sludge 1, in which the amount of P is almost 17 wt % P2O5. Indeed, 100% of the initial P is in a condensed phase, in the form of Ca3(PO4)2(s). On the contrary, almost 40% of the initial P evaporates from sludge 2, in the form of (P2O3)2(g). The rest of P remains as a condensed phase, in the form of Ca3(PO4)2(s). 2.3.3. Results with Model Sludge. To understand the volatilization behavior of phosphorus in real sludge some other calculations were performed with a model sludge composed of the three major inorganic elements (Table 1), that is, the oxides CaO SiO2 P2O5. Four compositions, described in section 2.1.2 have been studied in this ternary system (cf Figure 1). Thanks to the abovementioned thermodynamic modeling code, the volatility of phosphorus is calculated as a function of the amount of P2O5 in the ternary system CaO SiO2 P2O5, under the same

atmospheric conditions as previously, that is, under air with partial oxygen enrichment (2.5 mol O2 and 0.5 mol N). The results are shown in Figure 8 for the 1300, 1400, and 1500 °C temperatures. The synthesized compositions are indicated in Figure 8 by colored circles, corresponding respectively to the first (CS0P, green circle), the second (CS30P, orange circle), the third (CS40P, pink circle), and the fourth (CS50P, blue circle) samples. The volatility of P is thus observed for an amount of phosphorus exceeding 30 wt %. Moreover, the evaporation of phosphorus is observed essentially for the highest temperature and reaches 36% of the P in the form of gas for both 40 and 50 wt % of the phosphorus content in the ternary system. Compared to the real sludge (Figure 6), the evaporation of phosphorus is slightly less in the ternary system, under the same conditions of calculation. That should indicate that other oxides, which are not taken into account in the model sludge, take part in the phosphorus volatilization. Note that, in this case, phosphorus volatilizes in the form of (P2O5)2 and PO2 in equivalent proportions, contrary to the more complex sample (section 2.3.2). However, it condenses mainly in the form of Ca3(PO4)2, as in the previous calculation (section 2.3.2) and in Ca2P2O7. Nevertheless, the calculations can determine which condensed phases are expected for these samples at the reactor temperature (1300 °C). According to the calculations, the CS0P sample should contain two different phases: pseudowollastonite (CaSiO3) and rankinite (Ca3Si2O7). As for the CS30P, the expected phases are calcium phosphate (Ca3(PO4)2), tridymite (SiO2), and pseudowollastonite (CaSiO3). The calcium phosphate phase (Ca2P2O7) and the tridymite (SiO2) are expected for the CS40P and for the CS50P. Moreover, phosphorus is expected to condense in another calcium phosphate phase (CaO6P2) in the CS50P sample.

3. VOLATILIZATION EXPERIMENTS WITH MODEL SAMPLES OF SLUDGE The next step of this study consists in comparing these calculations with high temperature X-ray diffraction (HTXRD) experiments and a thermogravimetric analysis (TGA). 5713


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Figure 7. (a) Evolution of the fraction of gas phases containing P as a function of the amount of P2O5 at 1300 °C. (b) Evolution of the fraction of condensed phases containing P as a function of the amount of P2O5 at 1300 °C.

All of the samples, CS0P, CS30P, CS40P, and CS50P, are synthesized by mixing CaCO3, SiO2, and NH4H2PO4 powders (Alfa Aesar) in stoichiometric proportions. A thermal treatment at 500 °C for 1 h is realized for all samples to eliminate NH3 and H2O from the P2O5 precursor. 3.1. HTXRD Results. X-ray diffraction measurements were performed on a D8 Advance Bruker Bragg Brentano diffractometer (Cu Kα radiation) equipped with a Vantec-1 linear detector. In-situ high-temperature diffraction data were collected on a HTK16 Anton Paar chamber. The sample was deposited on a platinum ribbon used as a heating stage. The diffraction patterns were collected during a heating of 10°/min from room temperature to 1350 °C under air. Each scan was obtained with a total acquisition time of 5 min (all 50 °C). The results are shown in Figure 9, corresponding to the determination of the crystalline phases at 1300 °C on samples CS0P, CS30P, CS40P, and CS50P.

At 1300 °C, the phases observed in the CS0P by XRD, are quartz (SiO2), cristoballite (SiO2), a silicone oxide phase (SiO2), and larnite (Ca2SiO4). As for the CS30P sample, the phases determined with the XRD at 1300 °C are as follows: a silicone oxide phase (SiO2), cristoballite (SiO2) and two phases of calcium phosphate (Ca3(PO4)2 and Ca2P2O7). Concerning the CS40P sample, the crystalline phases observed at 1300 °C are silicone oxide phase (SiO2), cristobalite (SiO2), and three phases of calcium phosphate (two phases of Ca2P2O7 and one of Ca3(PO4)2). Finally, the CS50P is composed at 1300 °C of two silicone oxide phase (SiO2), cristobalite (SiO2), and two phases of calcium phosphate (Ca2P2O7). 3.2. TGA Results. The thermogravimetric analysis consists in the observing weight variation of the sample during a heating treatment. The device used is a SETARAM 7.6 SETSYS 2400 5714


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Figure 8. Evolution of the fraction of P in gas phase as a function of the amount of P2O5 in the ternary system CaO SiO2 P2O5 at 1300, 1400, and 1500 °C. The green circle corresponds to the first synthesized composition (CS0P), the orange circle to the second one (CS30P), the pink circle to the third one (CS40P), and the blue circle to the fourth one (CS50P).

Figure 9. X-ray diffraction of samples CS0P, CS30P, CS40P, and CS50P at 1300 °C.

Evolution in the ATD 1600 version. Sample mass loss was logged from room temperature to 1250 °C with a heating rate of 10 °C/min. The aim of these measures is to compare the weight loss as a function of the phosphorus content. In our case, the volatility of the elements implies a decrease in the weight of the sample. Figure 10 represents the evolution of the weights of the samples as a function of the temperature. The first decrease corresponds to a loss of weight of 0.5% for the CS0P and the CS50P, 0.7% for the CS40P and 0.8% for the CS30P. It is explained by the loss of residual water and NH3 from the P2O5 precursor. It has been confirmed with the HTXRD results, which show the disappearance of NH4CaPO3O9 phase between room temperature and 700 °C (Figure 11).The second and major decreases in these curves correspond to a loss of

Figure 10. Evolution of the weight loss of samples against temperature by TGA.

sample weight of around 8.5% for the CS0P and CS30P, 6.5% for the CS40P, and 7.7% for the CS50P. This can be related to the decarbonatation of the samples. This phenomenon has also been observed in the XRD measurements with the disappearance of calcite (CaCO3) between 750 and 900 °C (Figure 11). There is no more loss of weight in both first samples at higher temperature. This confirms the calculations which predicted no volatilization of phosphorus in these samples. On the contrary, a major decrease in weight is observed for the samples containing 40 and 50 wt % P2O5. Indeed, at 1300 °C, the total weight loss is of 18.5% for the CS40P and 15.5% for the CS50P. This loss could be explained by the evaporation of the phosphorus, as it has been predicted by the calculations. 5715


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4. COMPARISON BETWEEN EXPERIMENTS AND THERMODYNAMIC CALCULATIONS WITH MODEL SAMPLES OF SLUDGE Table 3 summarizes the crystalline phases obtained by calculation and by HTXRD experiment for the four synthesized samples at 1300 °C. Comparison between XRD experiment and calculations indicate a relatively good agreement. For the samples CS0P and CS30P, the phases determined with the experiment are not exactly the same as those calculated. Only the stoichiometry of the phases obtained through measurements differs from the predictions. In fact, the calculations are done with the condition of the thermodynamic equilibrium, contrary to the experiment, where this equilibrium is not reached because no step time is realized. That could explain the differences. For the two last samples (CS40P and CS50P), the crystalline compositions are close to the ones previously calculated because two major phases are obtained: a silicone oxide phase and a calcium phosphate phase. The TGA curves confirm the calculations and the XRD measurements showing only the evaporation of the water and the CO2 from the samples CS0P and CS30P. No sign of volatility of phosphorus has appeared for the samples containing less than

Figure 11. X-ray diffraction against temperature in sample CS40P.

30 wt % P2O5. The loss of weight in the CS40P and CS50P samples above 800 °C could be due to a phosphorus volatilization. The answer could be obtained with a mass spectrometer, which would analyze the gas formed at 1300 °C in these samples. Moreover, CaO is generally bound in silicates and aluminosilicates. Their composition and distribution change with temperature. CaO in “excess” proportion can then form phosphates, which are very stable and not very volatile. As long as there is enough CaO for the formation of phosphates available, there will be no vaporisation of P-species. On the contrary, if there is a lack of CaO, P-species will be observed in the gas phase. Thus, if there was much more CaO in sludge 2 (e.g., instead of Fe2O3), there would be much less release of P. Another way to see this effect is to study a composition with both the silicate and the phosphate saturated with calcium (e.g., Ca2SiO4 Ca3(PO4)2).

5. CONCLUSION The behavior of the volatility of the inorganics during the gasification of sewage sludge has been studied in this paper by comparing thermodynamic calculations with experiments. Thermodynamic calculations were performed with two real sludge compositions (21 elements taken into account)—one richer in phosphorus than the second one—and with four model samples of sludge containing the major inorganic oxides, that is, CaO SiO2 P2O5. These results were compared with experiments. In both types of sludge, thermodynamic calculations performed in EFR (entrained flux reactor) gasification conditions (1300 1500 °C) showed that the major gases formed (CO, CO2, and H2), the minor gases (H2S, COS, HF, HS, HCl, and NH3), and Na and K vaporization (KCl, NaCl, KOH, and NaOH) are the same; they describe the same evolution with the temperature, in the same proportions. Calculations with the real sludge showed that the major difference in the evolution of the release fraction of each element in the gas is the phosphorus behavior. In fact, this element partially evaporates up to about 50% of the initial P in form of PO2, PO and (P2O3)2 only in the richer P sludge, whose composition contains almost 42 wt % P2O5. The gaseous species (P2O3)2 appears from 700 °C and overtakes all the others. Focusing the condensed phases, calcium is present in all major compounds. Phosphorus condenses mainly in the form of Ca3(PO4)2. Indeed, Mg rich calcium compounds are found in both

Table 3. Comparison of the crystalline phases obtained by calculation and by XRD experiments at 1300 °C CS0P

CS30P

calc

XRD

calc

XRD

quartz (SiO2)

tridymite (SiO2)

silicone oxide (SiO2)

cristoballite (SiO2)

pseudo-wollastonite (CaSiO3)

cristoballite (SiO2)

pseudo-wollastonite (CaSiO3)

Silicone oxide (SiO2)

calcium phosphate (Ca3(PO4)2)

calcium phosphate (Ca3(PO4)2)

rankinite (Ca3Si2O7)

larnite (Ca2SiO4)

calcium phosphate (Ca2P2O7)

CS40P calc tridymite (SiO2)

CS50P XRD

calc

XRD

silicone oxide (SiO2)

tridymite (SiO2)

cristobalite (SiO2) calcium phosphate Ca3(PO4)2)

two phases of silicone oxide (SiO2)

calcium phosphate phase (CaO6P2)

cristobalite (SiO2)

calcium phosphate phase (Ca2P2O7) two phases of calcium phosphate Ca2P2O7 calcium phosphate phase (Ca2P2O7) two phases of calcium phosphate Ca2P2O7 5716


Energy & Fuels types of sludge. The results of the calculations show the presence of Al and P elements in the calcium compounds. As for the elements Mg, Fe, and K, they are found either in calcium rich compounds or in aluminosilicate ones. Calculations for a simplified system formed by CaO SiO2 P2O5 predict the volatilization to start for samples with an amount of P2O5 of at least 30 wt %. However, within this ternary system, the evaporation of 36% of the initial amount of P at 1500 °C is observed, even for the samples with an initial content of 40 wt % P2O5. This is just a little less than the calculations for the real sludge rich in phosphorus. The results of the XRD experiment agree relatively well with the calculations. Only the stoichiometry of the phases obtained during the measurements differs from the predictions. In fact, the thermodynamic equilibrium that is assumed for the calculations is not reached during the experiment and could explain the different, noncoherent results. TGA curves are in agreement with the calculation and the XRD measurements, showing only evaporation of residual water and NH3 and then CO2 from the samples. No sign of volatility of phosphorus has appeared for samples containing less than 30 wt % P2O5. A mass spectrometer should be required for samples richer in phosphorus (i.e., 40 and 50 wt % P2O5) to determine the gas composition at 1300 °C and thus explain the weight loss observed above 800 °C. As noticed, the behavior of the phosphorus is slightly different for a complex matrix than for a simplified ternary system (model sludge). The other inorganic elements of the sludge seem to influence the reaction of the phosphorus during thermal treatment. For gasification purposes, where inorganic volatile matter and condensed phases may induce operating difficulties, this issue should be investigated to set a convenient operating temperature in the reactor, establishing a trade-off between the maximum conversion of organic matter into syngas, on the one hand, and preventing the inorganics volatilization, on the other hand. If deeper understanding is required, other samples need to be produced by adding the elements of the sludge one by one in order to observe their influence on the volatility of P. Moreover, a structural study could be set up, by nuclear magnetic resonance spectroscopy, to understand the role of the phosphorus in these samples.

ARTICLE

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank ANR (Agence National de la Recherche, France) and the PNRB (Programme National de Recherche sur les Bioenergies) for their helpful financial contribution in this research. ’ REFERENCES (1) Dupont, C.; Commandre, J. M.; Gauthier, P.; Boissonnet, G.; Salvador, S.; Schweich, D. Fuel 2008, 87, 1155. (2) Berjonneau, J; Colombel, L; Poirier, J; Pichavan, M; Defoort, F; Seiler, J. M. Energy Fuels 2009, 23, 6231. (3) Watson, E. B. Contrib. Mineral. Petrol. 1976, 56, 119. (4) Watson, E. B. Geochim. Cosmochim. Acta 1977, 41, 1363. (5) Hart, S. E.; Davis, K. E. Earth Planet. Sci. Lett. 1978, 40, 203. 5717


Behavior of Phosphorus and Other Inorganics during the Gasification

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