Study On The Mechanism Of Municipal Sewage Sludge To Inhibit The Agglomeration During Straw Combustion In Fluidized Bed

Due to the advantages of wide fuel flexibility, high combustion efficiency, and low pollutant emission, fluidized bed combustion technology has been one of the most promising technologies for efficient and large-scale utilization of biomass. Straws play an important role in biomass resources in China, but the content of alkali metals is relatively high, which will cause several problems such as slagging, bed agglomeration, deposition and corrosion during fluidized-bed combustion. Among these problems, bed agglomeration is more severe, because it can lead to defluidization. It is practically valuable for the energy-oriented utilization of straws to solve the agglomeration initiated by alkali metals during straw fluidized bed combustion.In this thesis, a new method is proposed to deal with the agglomeration by co-combusting straw and municipal sewage sludge. The co-combustion characteristics of straw/municipal sewage sludge blends are researched. The influence of municipal sewage sludge addition to straw on agglomeration characteristics during fluidized bed combustion is investigated. The effect of phosphorus in sewage sludge on the behavior of alkali metals in straw and the phosphorus-potassium reaction mechanism are explored. The ultimate objective of this thesis is to master the method of inhibiting agglomeration during straw fluidized bed combustion with municipal sewage sludge addition.The combustion characteristics and gas products distribution during co-combustion of straw and municipal sewage sludge (sludge for short) were investigated by thermogravimetry-mass spectroscopy(TG-MS). The results show that there are two weight loss steps for combusting straws and municipal sewage sludge separately, and their co-combusting:the volatile releasing and burning, and the fixed carbon burning. For the blends, the stage of the volatile releasing and burning is controlled by the straw, while the stage of the fixed carbon burning is controlled by the sewage sludge. Adding sewage sludge to the straw decreases the combustion performance of straw, but is helpful to lower the emission peaks of CO2, NH3, HCN, and NO. The behavior of alkali metals during co-combustion of municipal sewage sludge with straw was explored in a vertical tubular furnace reactor. The results indicate that adding sewage sludge to the straws enhances the retention of alkali metals retention in ash due to the formation of high melting point compounds such as potassium aluminosilicates (KA1Si2O6and KAlSi3O8) and alkali phosphates Ca9MgK(PO4)7. Chemical reaction mechanisms between alkali metals and three kinds of phosphorous-rich additives are studied to investigate the effect of phosphorus on the behavior of alkali metals during wheat straw combustion. The selected phosphrous-rich additives are ammonium dihydrogen phosphate (NH4H2PO4), monocalcium phosphate (Ca(H2PO4)2·H2O) and calcium phosphate (Ca3(PO4)2). The results results demonstrate that a certain amount of phosphorous-rich additives is positive for capturing alkali metals, restraining the volatilization of alkali metals and inhibiting the sintering of wheat straw ash. The reaction between NH4H2PO4or Ca(H2PO4)2·H2O and potassium metals in wheat straw forms CaK2P2O7, and that between Ca3(PO4)2and potassium metals produces Ca10K(P04)7,Ca10Na(P04)7and Ca5(PO4)3Cl. Calcium phosphate (Ca3(PO4)2) is selected as a model compound of phosphorus in municipal sewage sludge ash to study the reaction mechanism between phosphorus and alkali metals during co-combustion of wheat straw and municipal sewage sludge. The reaction between Ca3(PO4)2and KC1produces Ca10K(PO4)7and Ca5(PO4)3Cl at about800℃and higher temperature, revealing the mechanism of potassium phosphate formation during the co-combustion of sewage sludge and wheat straw.The influence of sewage sludge addition on the agglomeration characteristics during wheat straw combustion was investigated in a5kW bubble fluidized bed. The results reveal that the sewage sludge addition decreases the agglomeration tendency and can extend the defluidization time during wheat straw combustion. Adding sewage sludge to wheat straw leads to the formation of potassium aluminosilicates (KAlSi2O6and KAlSi3O8) and alkali phosphates Ca9MgK(PO4)7in the bottom ash, which will reduces the amount of K available for the reaction with the quartz bed material grains, and effectively restrains the reaction between the alkali and quartz sands, thus preventing the agglomeration. The influence of phosphorus in sewage sludge on the agglomeration characteristics during straw combustion in fluidized bed was explored using the model compound of phosphorus Ca3(PO4)2. The results proves that the phosphorus in sewage sludge can decrease the agglomeration tendency. The higher the temperature is, the more Ca3(PO4)2will be needed.Based on Gibbs free energy minimization, the equilibrium analysis software FactSage6.1was used to simulate thermodynamics state of the transformation behavior of alkali during co-combustion of straws and municipal sewage sludge. The results show that the distribution of K and Na in straws is in different chemical forms. As the temperature increases, K exists in the form of molten and gaseous. Compared with corn stalk, the alkali metals in wheat straw are mainly in the molten and gaseous state. Compared with straw, the alkali metals in sewage sludge are mainly in the solid and molten state. The addition of sewage sludge to straws will change the distribution of alkali metals, and the blending mass ratio has greater effect on the distribution of K than Na. The addition of sewage sludge would help improve the distribution of the alkali metals in straw, due to the generation of a refractory aluminosilicate KAlSi2O6and reduction in the release of gaseous KC1.