Experimental Study And Fuzzy Evaluation On Comprehensive Characteristics Of Biomass And Coal Co-Combustion

The utilization of biomass becomes one of the hot researches in the world today. In the use of biomass technology, incorporation of biomass with coal in power generation, this approach has a lot of advantages. For the characteristics of combustion and pollutant emission in co-combustion process of biomass and coal, the domestic and foreign scholars have lauched a series of theoretical and experimental studies, but the integrated study of combustion, pollutant emission, ash fusion are limited. In this paper, the comprehensive characteristics and the additive effects to the process of biomass blended with coal were studied, and then a further fuzzy evaluation was made, while the best mixing ratio could be determined.First of all, Mettler-Toledo TGA/SDTA851e analyzer was used to study the co-combustion characteristics of biomass and coal. The experiments showed that as the increase of the temperature, there was a sharp decline in peak on DTG curves of biomass, and the peak was higher. The combustion of coal alone varied widely compared to the biomass, the weight loss peak appearred in the late 500～600℃. Straw's accession could reduce the ignition temperature of lean coal effectively, and improve the ignition performance of the lean coal; the larger incorporation amount of biomass made that the samples shorter late combustion time, faster burning and better ignition performance. Therefore, the accession of the biomass improved the mixed fuel combustion characteristics obviously.Secondly, tube-type furnace was used to study the pollutant emission characteristics of biomass blended with coal. NO and SO2 emission characteristics was quite different between biomass and coal, while the peak of lean coal was significantly higher than the bituminous coal and other four kinds of biomass. Overall, NO, SO2 emissions increased as the increase of the nitrogen and sulfur content in the fuel. As the increase of the biomass amount, SO2 emissions curve peak width became bigger, and NO, SO2 emission peak decreased obviously.The alkaline and alkaline earth metal content in biomass ash was high, which made low ash melting point, while the (SiO2+Al2O3) content in coal ash was high, then the slagging tendency of ash was smaller. The experiments showed that with the increment of the biomass proportions, there would be a higher ash slagging tendency in the co-combustion. During combustion of biomass blended with coal, the reduction capacities of different biomass component were different, as soybean straw> wheat straw> pine> peanut shell. We learned from the coal in discriminant index for ash deposition evaluation during combustion of biomass and coal.Through additives effects to the co-combustion of the biomass and the coal, it was found that adding CaO, Fe2O3, SiO2 three additives was unfavorable to the chemical reaction in the combustion process. With the increase of the ratio of added CaO, SO2 emission peak and emission amount decreased obviously, and desulfurization efficiency could achieve more than 90%, While NO emission trends contrasted to the SO2. Kaolin had a role in sulfur fixing when joining the small amount, but while the quantity was high, kaolin had a catalytic effect on the conversion of sulfur. MgO also had a certain role in sulfur fixing, and NO emission increased as the MgO mixing decreased, but to a certain amount, NO emissions increased. Adding a substance or making a pretreatment to the biomass and coal mixing fuel would affect the properties of fuel ash. ST value increased with the mixing of 6%Al2O3 and kaolin, while decreased obviously with the mixing of K2CO3, CaO, MgO and Fe2O3. Kaolin did not act as a matter which increased the ash melting point only, that was different from the coal ash.Finally, the fuzzy comprehensive evaluation model was selected to judge the co-combustion characteristics of the biomass and coal. With the experimented and then calculated data, different mixing ratios of the biomass and coal was classified as three grades, such as "good, normal and worse" with the spirit of safety, environmental protection and effective weight distribution. Then, a score vector was introduced to measure the integrated scores of the biomass and coal in different mixing ratios, and the best mixing ratio was determined.