Experimental Study On Alkali Emission During Straw Pyrolysis

Rice straw and wheat straw are the most common agricultural byproducts in the rural area of China. Its widely distribution and huge quantity have brought a lot of handling problems, such as resource waste, environmental pollution. How to utilize this kind of biomass effectively is of great importance. While the commonly high content of alkali mineral always bring kinds of matters during thermochemical utilization. Agglomeration in the fluidized bed, deposition on the convention surfaces and metal erosion in the high temperature in the furnace, are usual problems, which limit the usage of straw in large-scale utilization.To better understanding the alkali transformation behaviors during the thermochemical convention of agricultural residua, and benefit the solving of alkali problems, a set of method for determination of the form and quality of inorganic elements in straw was explored. The optimized conditions for the measurement were determined to achieve better accuracy. The strong acid resolution method and AAS are adopted to detect the content of potassium and sodium; While high temperature hydrolyzing and chlorine ion colorimetric analysis are adopted to detect the chlorine content. The measurement method provides a solid base for the quantitative analysis of alkali problems. Using this method, the rice straw with different store period was analyzed. The results form fresh rice straw shows most of (about 90%) alkali elements and chlorine in the raw material are in water soluble form, which is the most loosely bounded form and tended to release during pyrolysis or other thermochemical conversions. It indicates the possible severe alkali problems and also the effective way of rinsing to decrease the risk. The result from the timeworn straw shows the store period has obvious affection on the inorganic elements distribution in the straw. One-year's store seems transfer some of the water-soluble alkali into more strictly bounded form. On the other hand, the decreasing of alkali and chlorine with the same proportion maybe caused by the crystalline KC1 and NaCl falling off physically during the store. Anyway, the decreasing of alkali and chlorine content will help the solving of concerned alkali problems.Because pyrolysis is a primary stage of all biomass thermochemical conversion, the way of alkali transformation during pyrolysis is of great importance to the alkali problems research. To better understand the process, work has been done on the specially designed experimental setup. The releasing of relative inorganic elements with different form in straw during pyrolysis was explored based on the chemical fractionation of solid residues from every pyrolysis stage. The results show that alkali release is inevitable during pyrolysis, even at the temperature as low as 200 centigrade. Alkali is released from the pyrolytic rice straw in two temperature range: (1) in connection with the pyrolysis process, the quick releasing taking place at 200-500C, and (2) in connection with high temperature surrounding, the inorganic material (most of salt) physical loss gradually at temperature above 600C. But, there are still about 35 per of alkali left in the solid phase even the temperature reach as high as 1100C. The analyzing of residues from chemical fractionation show that the curve of water-soluble alkali releasing is almost parallel with the oneof overall alkali releasing, this indicated that the emission of alkali is mostly caused by those in the form of water-soluble. The influence of chlorine was also studied. The possibility of chlorine releasing in the form of alkali chloride during high temperature pyrolysis and in the form of HC1 under lower temperature stage was also proposed. The releasing tendency of water-soluble alkali is favorable to decrease the risk of alkali problems by rinsing.Based on the research done about the alkali measurement and the understanding of its behaviors during thermochemical conversion, the evaluation of the alkali problems of an industrial combustion project was also launched. The...