Influence of Phosphorus on Potassium during Co-Combustion of Wheat Straw with Municipal Sewage Sludge

Paper from the AFRC 2013 conference titled Influence of Phosphorus on Potassium during Co-Combustion of Wheat Straw with Municipal Sewage Sludge by Qiangqiang Ren

Alkali metal-related problems are the major barrier for large scale utilization of biomass, and municipal sewage sludge which is rich in sulfur, nitrogen and phosphorus is a potential choice to inhibit the alkali metal related problems during biomass combustion. The effect of phosphorus on the behavior of potassium and the reaction mechanism between potassium and phosphorus during co-combustion of wheat straw and municipal sewage sludge are investigated in a bench-scale reactor. The elemental composition of ash is analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES), and the morphology and crystalline phase are analyzed by scanning electron microscopy (SEM), and X-ray powder diffraction (XRD). The results indicate that the addition of sewage sludge to wheat straw enhances potassium retention in ash due to the formation of high melting point compounds such as potassium aluminosilicates (KAlSi2O6 and KAlSi3O8) and alkali phosphates Ca9MgK(PO4)7. Calcium phosphate (Ca3(PO4)2) is selected as a model compound of phosphorus in municipal sewage sludge ash to study the phosphorus chemistry. The addition of Ca3(PO4)2 is effective in preventing sintering and fusion of wheat straw ash. Ca3(PO4)2 not only dilutes the ash but also absorbs and reacts with potassium compounds to form Ca10K(PO4)7. Further reaction mechanism of phosphorus and potassium is investigated through the reaction between Ca3(PO4)2 and KCl, which produces Ca10K(PO4)7 and Ca5(PO4)3Cl at about 800ºC and higher temperatures, revealing the mechanism of potassium phosphate formation during the co-combustion of sewage sludge and wheat straw.

Influence of Phosphorus on Potassium during Co-Combustion of Wheat Straw with Municipal Sewage Sludge Qiangqiang Ren*, Linna Li, Shiyuan Li, Shaolin Bao,Qinggang Lu Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100190, China Corresponding Author: * Tel.:[phone]; E-mail: [e-mail] (Q. Ren) ABSTRACT Alkali metal-related problems are the major barrier for large scale utilization of biomass, and municipal sewage sludge which is rich in sulfur, nitrogen and phosphorus is a potential choice to inhibit the alkali metal related problems during biomass combustion. The effect of phosphorus on the behavior of potassium and the reaction mechanism between potassium and phosphorus during co-combustion of wheat straw and municipal sewage sludge are investigated in a bench-scale reactor. The elemental composition of ash is analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES), and the morphology and crystalline phase are analyzed by scanning electron microscopy (SEM), and X-ray powder diffraction (XRD). The results indicate that the addition of sewage sludge to wheat straw enhances potassium retention in ash due to the formation of high melting point compounds such as potassium aluminosilicates (KAlSi2O6 and KAlSi3O8) and alkali phosphates Ca9MgK(PO4)7. Calcium phosphate (Ca3(PO4)2) is selected as a model compound of phosphorus in municipal sewage sludge ash to study the phosphorus chemistry. The addition of Ca3(PO4)2 is effective in preventing sintering and fusion of wheat straw ash. Ca3(PO4)2 not only dilutes the ash but also absorbs and reacts with potassium compounds to form Ca10K(PO4)7. Further reaction mechanism of phosphorus and potassium is investigated through the reaction between Ca3(PO4)2 and KCl, which produces Ca10K(PO4)7 and Ca5(PO4)3Cl at about 800ºC and higher temperatures, revealing the mechanism of potassium phosphate formation during the co-combustion of sewage sludge and wheat straw. KEYWORDS: Municipal sewage sludge, wheat straw, co-combustion, potassium, phosphorus 1 INTRODUCTION Biomass combustion for heat and power generation has gained worldwide interest due to the renewable and carbon dioxide neutral feature of biomass . However, some biomass fuels such as straw with high amount of alkali metals have shown a tendency to cause operational problem during combustion, such as fouling, deposition, corrosion and bed agglomeration1-3. For fluidized bed combustion, bed agglomeration is the major reason for defluidization and unscheduled shut down in power plant 2. Potassium is a critical type of alkali metal, which will form troublesome potassium compounds through different routes 4-6, including: (i) low melting point potassium salts (i.e. KCl) or the eutectics involved potassium salts and other compounds (i.e. KCl and K2SO4, KCl and CaSO4) during fuel particle devolatilization and char burnout 7. (ii) potassium silicates through reactions between the potassium oxides or salts and silicon in the fuels or bed materials 8-11. (iii) melts with chemical composition in ternary K2O-CaO-SiO2 with eutectic temperature lower than 740 oC 12. (iv) low melting point potassium phosphates (i.e. KPO3) during the combustion of biomass fuels with high content of phosphorus 13. Therefore, it is important to control and monitor the transformation of potassium, and the basic method is to transfer potassium into less problematic compounds with higher melting point. Co-combustion of municipal sewage sludge and biomass with high content of potassium is an alternative method to mitigate the alkali metal problems, such as decreasing KCl concentration in the fuel gas and reducing superheater corrosion 14-17. Municipal sewage sludge is the byproduct of wastewater disposal. Besides the traditionally major ash forming elements (Ca, K, Na, Si, Al, Fe), there is significant amount of phosphorous in municipal sewage sludge6, 18. Åmand 14 and Aho 16 indicated the importance of aluminum silicate reaction in alkali capture during the co-combustion of municipal sewage sludge and wood. Elled 17 found that the addition of sewage sludge was beneficial due to the existence of aluminum, silica, phosphorus, iron, sulfur and calcium, all of which were involved in potassium capture. A detailed reaction mechanism for the sulfation of alkali metals was suggested by Glarborg 19 and Jiménez 20, and the mechanism between aluminum, silica and potassium was investigated by Steenari 21. However, the effect of phosphorus in the municipal sewage sludge on the behavior of potassium during the co-combustion of municipal sewage sludge and biomass, especially agricultural straw with high content of alkali metals, remains unclear. Co-combustion of phosphorus-rich fuels and additives has been studied and it is found that phosphorus may solve the problems related with alkali metals by converting the alkali species into high-temperature melting alkali phosphates in a ternary phase K2O-CaO-P2O5 system. Kekish 22 and Sørensen 23 reported methods of raising the fusion point of slag for reducing agglomeration, sintering and deposit in gasification / combustion of biomass by phosphate compounds. Zeuthen 24 found the addition of mono calcium phosphate can bind alkali metal as phosphates and reduce the content of chlorine in the deposits during straw combustion. Additionally, Boström 25 and Grimm 26 found that the potassium-capturing ability of phosphorus could decrease the concentration of potassium-vapor left for reacting with the quartz bed grains to comparatively low levels during fluidized bed combustion of phosphorus-rich fuels / fuel mixtures. Grimm 27 also reported that the addition of phosphoric acid to logging residues and wheat straw changed the alkali distribution to a system dominated by crystalline coarse ash of K-Ca/Mg-phosphates and K2SO4. So far, little has been reported about the formation mechanisms of the alkali phosphates during combustion. Considering the abundance of phosphorus in municipal sewage sludge, it is essential to get more thorough understanding on the effect of phosphorus on the behavior of potassium and the phosphorus-potassium reaction mechanism during co-combustion of municipal sewage sludge and biomass with high content of alkali metals, which is also the objective of this work. 2. EXPERIMENTAL SECTION 2.1. Fuel Characteristics Wheat straw (WS) is a typical type of biomass in China with high potassium content, thus selected as the biomass sample. Dry municipal sewage sludge (MSS) was obtained from Beijing Qinghe wastewater treatment plant. Both WS and MSS were crushed, dried at 105oC to get constant weight and sieved into150~200 μm. The proximate and ultimate analyses are listed in Table 1. The chemical elements of the raw materials are shown in Table 2. It can be seen that the content of potassium and silicon is high in wheat straw, while in sewage sludge, the contents of potassium, silicon, aluminum, calcium, iron and phosphorus are high. The content of sodium is significantly lower than that of potassium in the samples. Thus, it is especially paid attention to the behavior of potassium in this study. Table 1 Ultimate and proximate analyses of samples Fuel Proximate analysis(wt %) Ultimate analysis (wt %) LHVar（MJ/kg） Mad Aad FCad Vad Cad Had Nad Oad Sad WS 1.80 6.84 19.38 71.98 44.29 5.50 0.56 40.81 0.20 16.46 MSS 6.39 31.82 8.51 53.28 32.62 4.61 5.61 18.08 0.87 13.67 Table 2 The chemical compositions of dry samples (%) Fuel Al Ca Fe K Mg Na P Si Cl WS 0.032 0.338 0.073 1.796 0.177 0.200 0.11 2.374 0.670 MSS 1.760 1.767 1.582 1.059 0.946 0.3308 2.479 5.271 0.027 2.2. Apparatus and Method The schematic illustration of the experimental system is shown in Figure 1. The laboratory reactor consists of a quartz tube in an electrically vertical tube furnace and is equipped with a temperature controller and a flow meter, and a cooling system. There is a preheating section for the quartz reactor and a reaction tube with a quartz filter plate is arranged for supporting sample and distributing air. The typical experiment procedure is as follows: (i) The furnace is heated to the set temperature. (ii) The quartz tube loaded with sample is rapidly put into the furnace. The experiment lasts one hour every time, and air is introduced from the bottom of quartz tube at a continuous flow of 320 ml/min, to ensure burnout of the volatile and sufficient time for mineral phase transformation. (iii) After the test, the quartz tube is taken out, and cooled to the room temperature under ambient conditions. Thereafter, the residual ash or reaction product is collected, weighed and stored in a desiccator for analysis. 2 3 1 5 6 4 7 1-air cylinder, 2-control value, 3-flow meter, 4-temperature controller, 5-vertical tube furnace, 6-quartz tube reactor, 7-cooling system. Figure 1 Schematic illustration of the experimental system The combustion experiments are carried out according to the following steps: (i) To investigate the behavior of potassium during co-combustion of municipal sewage sludge with wheat straw.Wheat straw was pre-mixed with sewage sludge in the weight proportion of 30% and 50%. Approximate 3.00 g sample was selected. The experiments ran at constant temperatures of 700, 800 and 900 oC, respectively, which are typical temperatures in biomass boilers. The combustion experiments are repeated five times to ensure the reproducibility, and the results are satisfactory by comparing the weight of residual ash. (ii) To study the effect of phosphorus in sewage sludge on the behavior of potassium in wheat straw during co-combustion, a laboratory reagent phosphorus compounds tricalcium phosphate (Ca3(PO4)2) was selected as phosphorus-containing model compound in municipal sewage sludge. Ca3(PO4)2 was sieved for the particle size less than 125 μm. Approximate 3.00 g of the wheat straw was prepared. The phosphorus compound was pre-mixed with wheat straw with the molar ratio of P/(K+Na) equal to 1. (iii) Finally, to obtain the phosphorus-potassium reaction mechanism and conditions during co-combustion, a series of experiments were conducted with model compounds Ca3(PO4)2 and KCl. Before the experiments, the samples were well mixed in an agate mortar. 2.3 Analysis Methods Inductively coupled plasma optical emission spectrometer (ICP-OES, Thermo Fisher IRIS Intrepid II XSP) is used to analyze the main chemical composition of the raw materials (Al, Ca, Fe, K, Na, Mg, P, Si) and the content of potassium in residual ash. Ion chromatography (IC, Dionex ICS-3000) is chosen to analyze chlorine in the raw materials. X-ray powder diffraction (XRD, D/max2550HB+/PC) is selelcted to identify the crystallized phases (qualitative analyses). The XRD system is equipped with CuKa radiation and runs at a scan speed of 3.32 °/min with a sampling interval of 0.02°/step over the range of 10~90.02°. Scanning electron microscopy (SEM, HitachiS-4800) is used to observe the morphology of the samples. 3. RESULTS AND DISCUSSION 3. 1 Effect of municipal sewage sludge on the behavior of potassium 3.1.1 Potassium in Residual Ash Residual ash was collected to characterize the behavior of potassium, and the gas-phase release was quantified by a sample mass balance on the system 28 .To determine the behavior of potassium for the samples, the retention ratio β (wt %) is used to illustrate the release of potassium, which is defined as the ratio of potassium retained in residual ash to the total content in the initial sample listed in equation (1) 4 Kt K0 100% m m β = × (1) K0 m and K t m represent the content of potassium in the initial sample and residual ash respectively. Thus, the release percentage of potassium can be given by: K-release (wt %) =100-β (wt %). (2) The results of potassium retention ratio from experiments with different sewage sludge mixing ratios are presented in Figure 2. Figure 2 Retention ratio of potassium for mixture with different ratios For wheat straw, release of potassium is enhanced by increasing temperature. The potassium retention ratio deceases to about 35 % at 900 oC. It has been reported that potassium release during combustion is closely correlated with chlorine through sublimation of KCl 25 and as the fuel reaches complete chlorine-release (above 800 oC), further release of potassium would be expected to be inhibited and retained in the ash 28. Johansen 29 gave the correlation between the K-release and temperature based on seven types of different annual crop fuels during combustion in bench-scale fixed-bed reactor, as the equation (3). The results were approximately 10 % higher than that in the prediction of potassium-release by the equation (3). K-release (wt %)=0.105T - 48.2, for T is from 800 oC to 1150 oC (3) For the sewage sludge, as the temperature increases, there is little potassium released, which may be partly caused by little content of chlorine in sewage sludge, and partly by high contents of sulfur, silicon, and aluminum that can capture potassium in the residual ashes. For the mixture, regardless of the temperature, the potassium retained increases with the increase in the sewage sludge addition, confirming the potassium capture ability of the sewage sludge. This may be related to the difference in chemical composition and modes of element combination in the reagent mixtures, which will be discussed in part 3.1.2. 3.1.2 Crystallized Phases in Residual Ash Figure 3 shows XRD patterns of the residual ash samples at different temperatures. SiO2 is the main phase in all the ash samples. It can be seen from Figure 3(a) that there is KCl and K2SO4 in wheat straw ash at 700 oC and 800 oC. The peak intensity for KCl decreases evidently at 800 oC, while that for K2SO4 changes little. At 900 oC, the peaks for KCl disappear and those for K2SO4 still remain, and moreover, a trace amount of Ca (Mg, Fe)Si2O6 and CaAlSi2O8 have been formed. Similar silicate products, such as Ca2SiO4, CaMg(SiO3)2, Ca3Si2O7 and CaSiO3 have been reported earlier in wood ash and wheat / barley straw ash 30. KCl and K2SO4 are the main potassium compounds in the wheat straw ash. The intensity of KCl peaks decreases at higher temperatures, which confirms the influential impact of temperature on potassium release in accordance with the results in part 3.1.1. As is shown Figure 3(b), mineralogical composition of sewage sludge is different from that of wheat straw. Ash from sewage sludge contains ferrite Fe2O3, aluminum silicates KAlSi3O8 and phosphates Ca9(Fe, Al)(PO4)7, Ca4(Mg, Fe)5(PO4)6. KAlSi3O8 is the main potassium compound in sewage sludge ash, which is hard to volatilize. Phosphorus in the municipal sewage sludge exists as phosphorous compounds in the form of Ca9(Fe, Al)(PO4)7, and Ca4(Mg, Fe)5(PO4)6. These phosphorous compounds may derive from Ca3(PO4)2, where there are five metal positions (M1-M5) and three phosphorous positions (P1-P3) in the structure and the metal positions M4 and M5 can either be occupied, unoccupied or partly occupied by other elements than Ca, such as Fe, Al or Mg31. (a) wheat straw (b) sewage sludge (c) 70 wt% wheat straw / 30 wt% sewage sludge (d) 50 wt% wheat straw / 50 wt% sewage sludge 1-SiO2; 2-KCl; 3-K2SO4; 4-Ca(Mg, Fe)Si2O6; 5-CaAl2Si2O8; 6-Fe2O3; 7-KAlSi3O8; 8-Ca9(Fe, Al)(PO4)7; 9-Ca4(Mg, Fe)5(PO4)6; 10-KAlSi2O6; 11-Ca9MgK(PO4)7 Figure 3 XRD Patterns of the residual ash samples Ca3(PO4)2 is a typical phosphate found during sewage sludge combustion. Elled 32 found that phosphorus was condensed in either Ca3(PO4)2 or Ca5(PO4)3OH during sewage sludge combustion. Elias 33 also revealed Ca3(PO4)2 in sludge ash. The formation mechanism of Ca3(PO4)2 during combustion of sewage sludge has been described in literature 34. As is demonstrated in Figure 3(c) and Figure 3(d), the crystalline phases of the mixture ash are different from those of individual sample ash because of the complicated interactions among the volatile elements (such as K) from straw and major ash-forming elements of Al, Si, Ca and P. For the mixture ash, the results indicate that the mixtures exhibit similar composition. The major minerals are Fe2O3, KAlSi2O6, KAlSi3O8, Ca9(Fe, Al)(PO4)7 and Ca9MgK(PO4)7. The peaks for KCl and K2SO4 could not be distinguished. With the addtion of municipal sewage sludge, the potassium content in the residual ash will increase, because potassium is bound as aluminosilicate minerals (KAlSi2O6 and KAlSi3O8) and phosphates Ca9MgK(PO4)7, which mainly consist in the ternary systems of K-Al-Si and K-Ca-P 35. In the ash of the mixture, Ca9MgK(PO4)7 is observed in the mixture, related to the reactions involved phosphorus in the sewage sludge and potassium in the wheat straw. Elled 17 also found Ca9MgK(PO4)7 in the ash during co-combustion of municipal sewage sludge with biomass containing high amounts of potassium and chlorine. Queiroz reported Ca9MgK(PO4)7 owns the stable phase crystallized at 850~900 ºC and is developed from earlier crystalline phosphate phase precursors Ca5(PO4)3O, which seems to evolve from Ca3(PO4)2 36. Malin 31 presented that the ability of the Ca3(PO4)2 to form solid solutions with alkali metals, alkaline earth metal and transition metals must be considered when identifying phosphate phases in ash from the combustion of phosphorous rich fuels. However, the reliable reaction mechanism between Ca3(PO4)2 and alkali metals is not yet established. Therefore, Ca3(PO4)2 is selected as phosphorus-containing model compound in municipal sewage sludge to identify the reaction mechanism between phosphorus in sewage sludge and the potassium in wheat straw. 3.1.3 Morphology of Residual Ash Figure 4 shows SEM micrographs of wheat straw ash from different temperatures. Significant effect of the temperature on the microcosmic structure could be noticed. At 700 oC and 800 oC, wheat straw ash is sintered but that does not undergo any great fusion yet. Part of the surface appears to be rough, and part smooth. The sintered ash may contribute to the eutectic phenomenon between KCl and K2SO4 occuring at 700 oC ~800 oC 7. Many zones show a great degree of fusion and re-solidified at 900 oC. (a) 700oC (b) 800 oC (c) 900 oC Figure 4 SEM micrographs of wheat straw ash from different temperatures Figure 5 shows SEM micrographs of sewage sludge ash at different temperatures. The surface of the sample appears rough and compact at 700oC, but does not present any significant fusion. A slight melting layer covers on the surface of sewage sludge ash at 800oC, then the tendency of fusion aggravates with higher temperature, which may be caused by the reaction of Fe2O3 and quartz to form eutectic with low melting point leading to ash melting and fouling during sludge combustion 37. In addition, it can be noticed that the microcosmic structure of sewage sludge ashes is different from that of wheat straw ashes. (a) 700ºC (b) 800 ºC (c) 900 ºC Figure 5 SEM micrographs of municipal sewage sludge ash According to Figure 6, the surface of the blend ashes presents loose and granular structure. It is worthy of noticing that the addition of sewage sludge in wheat straw has distinct positive impact on reducing the sintering tendency of wheat straw ash, which may be partly because potassium phosphates and aluminosilicate minerals can increase the ash melting point and prevent the potassium volatilization 17, 21, partly because Fe2O3 may react with the alkali compounds (K2O and K2CO3) and form eutectic mixture K2Fe2O4 with melting temperatures exceeding 1135oC 9. (a) 70% wheat straw/30% sewage sludge (b) 50% wheat straw/50% sewage sludge Figure 6 SEM micrographs of mixed ash at 800 oC 3.2 Phosphorus-potassium reaction mechanism during co-combustion 3.2.1 Co-combustion of wheat straw and Ca3(PO4)2 Figure 7 presents the retention ratio of potassium for co-combustion of wheat straw with Ca3(PO4)2 addition. The potassium retained increases with Ca3(PO4)2 addition, confirming the potassium capture ability of the Ca3(PO4)2 at the temperatures from 700ºC to 900ºC. Figure 7 Retention ratio of potassium for co-combustion wheat straw and Ca3(PO4)2 addition According to Figure 8, the main products formed during co-combustion of wheat straw and Ca3(PO4)2 from 700ºC to 900ºC are Ca10K(PO4)7 and Ca5(PO4)3Cl. Though KCl still remains at 700 ºC and 800 ºC, the intensity is obviously weaker than that in wheat straw ash, which also indicates the interaction between Ca3(PO4)2 and KCl. At 900ºC, the peaks for Ca5(PO4)3Cl become rather faint. Moreover, Ca9MgK(PO4)7 is not found in the ash of co-combustion of wheat straw and Ca3(PO4)2. The formation of Ca9MgK(PO4)7 in the mixture ash during co-combustion is less known and deserves further research. 1-SiO2; 2-KCl; 3-Ca10K(PO4)7; 4-Ca5(PO4)3Cl Figure 8 XRD patterns of the residual ash from co-combustion of Ca3(PO4)2 and wheat straw SEM analysis in Figure 9 shows that the surface of the residual ash from different temperatures also has loose and granular structure as that of the mixture ash. Compared with the residual ash of wheat straw (as is shown in Figure 4), it can be concluded that the addition of Ca3(PO4)2 is effective in preventing sintering and fusion of wheat straw ash, may be because Ca3(PO4)2 absorbs and reacts with potassium compounds presenting to form Ca10K(PO4)7. (a) 700ºC (b) 800 ºC (c)900 ºC Figure 9 SEM micrographs of residual ash of co-combustion of Ca3(PO4)2 with wheat straw 3.2.2 Reaction between Ca3(PO4)2 and KCl The material for the experiments was Ca3(PO4)2 with KCl, and the molar ratios of Ca3(PO4)2 to KCl were 0.5/1, 2/1, 5/1 and 8/1, respectively. The mixtures were exposed to temperature at 800ºC for one hour. Figure 10 illustrates the results of the produces under different molar ratios. 1-KCl; 2-Ca5(PO4)3Cl; 3-Ca10K(PO4)7 Figure 10 XRD patterns of the products from Ca3(PO4)2 and KCl calcined at different molar ratios at 800 ºC When the molar ratio of Ca3(PO4)2 to KCl is 0.5/1, the XRD pattern consists of peaks corresponding to KCl and Ca5(PO4)3Cl, implying that KCl is surplus. As the molar ratio increases to 2/1, the pattern consists of strong intensity Ca5(PO4)3Cl peaks and faint Ca10K(PO4)7 peaks, and moreover, no KCl peaks are detected. When the molar ratio is 5/1, due to the formation of Ca10K(PO4)7, there is obvious change showing the same XRD pattern reported in literature 36. The formation mechanism of Ca10K(PO4)7 could be described as a solid solution based on the Ca3(PO4)2 structure, where Ca has been substituted by K. When the amount of Ca3(PO4)2 increases continuously, the peaks for Ca10K(PO4)7 and Ca5(PO4)3Cl still exists but the intensity becomes a little weaker. The experimental results also indicate that for the Ca3(PO4)2 and KCl mixture, if KCl is excessive, Ca5(PO4)3Cl forms crystallized phase first. According to the products and element conservation, the reaction between Ca3(PO4)2 and KCl can be assumed as (4). 5Ca3(PO4)2+ KCl→Ca10K(PO4)7+ Ca5(PO4)3Cl (4) The mixtures of Ca3(PO4)2 with KCl with molar ratio of 5/1 according to the equation (4) were exposed to the temperatures in the range of 700~1000℃ for one hour, respectively. XRD diffractions of the products are shown in Figure 11, showing that only Ca5(PO4)3Cl peaks exist at 700 ºC. The reaction between KCl and Ca3(PO4)2 at 800~1000 ºC results in the formation of Ca10K(PO4)7 and Ca5(PO4)3Cl. The higher temperature promotes the intensity of Ca10K(PO4)7 peaks, whereas the intensity of Ca5(PO4)3Cl increases first then decreases. It can be seen that Ca10K(PO4)7 can form the stable crystallized phase at 800ºC, instead of 850ºC in literature 36. The products demonstrate the chemical equation (4) reasonable at temperature around 800 ºC and higher temperatures. 1-Ca5(PO4)3Cl; 2-Ca10K(PO4)7 Figure 11 XRD patterns of the reaction products from Ca3(PO4)2 and KCl calcined at different temperature In order to verify the effect of reaction time on the produces, the reaction time ranging from 15 to 120 minutes was studied. Figure 12 reveals that Ca10K(PO4)7 and Ca5(PO4)3Cl develop in 15 minutes, but the intensity of Ca10K(PO4)7 are very faint. Then keep increasing the reaction time, and the intensity of the peaks of Ca10K(PO4)7 and Ca5(PO4)3Cl simultaneously increases in one hour. After that, the intensity of the peaks of Ca10K(PO4)7 and Ca5(PO4)3Cl becomes a little weaker. The experimental results indicate that the reaction time for one hour is enough for generating well crystallized phases of Ca10K(PO4)7 and Ca5(PO4)3Cl. 1-Ca5(PO4)3Cl; 2-Ca10K(PO4)7 Figure 12 XRD patterns of the reaction products from Ca3(PO4)2 and KCl calcined in different time 4. CONCLUSION Co-combustion of wheat straw and municipal sewage sludge has been performed in a bench-scale reactor. The effect of phosphorus on the behavior of potassium and the phosphorus-potassium reaction mechanism during co-combustion are investigated. The addition of municipal sewage sludge to wheat straw enhances potassium retention in residual ash. Phosphorus, silicon and aluminum in municipal sewage sludge can react with potassium in wheat straw to form alkali phosphates Ca9MgK(PO4)7 and potassium aluminosilicates (KAlSi2O6 and KAlSi3O8) with high melting points. The addition of Ca3(PO4)2 as the phosphorus-containing modal compound in sewage sludge to wheat straw is effective in preventing sintering and melting of wheat straw ash and enhances potassium retention in residual ash due to the formation of Ca10K(PO4)7. The high-temperature solid state reaction between Ca3(PO4)2 and KCl produces Ca10K(PO4)7 and Ca5(PO4)3Cl. The reaction can be assumed as 5Ca3(PO4)2+KCl→Ca10K(PO4)7+Ca5(PO4)3Cl. The reaction occurs at around 800ºC and higher temperatures. For the Ca3(PO4)2 and KCl mixture, if KCl is excessive, crystallized phase of Ca5(PO4)3Cl forms first. The reaction may suggest the mechanism between phosphorus in municipal sewage sludge and the potassium in wheat straw. ACKNOWLEDGEMENT This work is funded by National Natural Science Foundation of China (No. 51106157). REFERENCES [1] Van CJ, Brems A, Lievens P, Block C, Billen P, Vermeulen I, Dewil R, Baeyens J, Vandecasteele C. 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