Study On The Co-Pyrolysis And Sulfur Pollutant Emission Characterictics Of Biomass And Coal Blending

It is generally known that coal is the primary energy sources in our country. Moreover, coal combustion has always been a dominant contributor to the environmental pollution, and emission controlling of the sulfur oxides is an important item to cure the increasing pollution. Hence, using coal in clean style and controlling of sulfur pollutant are the emphasis of utilizing energy and controlling the pollutant. As a kind of green energy source, the biomass has many advantages, such as more volatile element, active carbon, low nitrogen and sulfur content, low ash content. Utilizing the biomass has a crucial meaning not only in lessening pollutant but also in relaxing the crisis of energy. Compared to the coal, biomass has shorter growing time and looser structure. In addition, biomass has its own characteristic in combustion and pyrolysis. The co-combustion of coal and biomass results in pollutant reduction. Most notable (noticeable) is the impact on the emission of Sulfur Oxide (SO_x) and Nitrogen Oxide(NO_x). The pyrolysis process is not only the initial course of the combustion and gasification but also an independent technology centralizing great usable power. Coal pyrolysis is the important preliminary phases of coal combustion, which is the primary evolution phase of sulfur contamination, so investigation on the characteristics of co-pyrolysis of coal and biomass and the effect of biomass on the release of sulfur pollutant of coal is of great signification in finding and evaluating new Desulphurization technology and coal conversion techniques.The pyrolysis characteristics of the natural biomass (including wheat stalks, corn stalks, cotton stalks and poplar scrap) and different metamorphic grade coals have been studied using the thermo-gravimetric analysis (TGA) method respectively.The pyrolysis behaviors of the blendings of coal and biomass and the synergy of biomass and coal have been experimented with thermo-gravimetric analysis (TGA) method. In addition, a range of pyrolysis experiments have been performed using thermo gravimetric analysis (TGA), Gas Chromatogram (GC) and Mass Spectrum (MS) to study the characteristics of H2S and COS that released from the co-pyrolysis of corn stalks and coals which are of different metamorphic grade.The pyrolysis characteristics of the natural biomass (including wheat stalks, corn stalks, cotton stalks and poplar scrap) have been studied using the thermo-gravimetric analysis (TGA) method respectively. The result shows that: the process of natural biomasses has three periods, which are heating period (water smoking), rapid agravic period (main pyrolysis period) and slow agravic period (charring period). The first period is during the temperature of room-temperature and 230℃, in this phase, surface layer water is evaporated so that there is a little drop in weight. The second is in the range of 230 400℃,which is the main period of releasing fugitive constituent. The last phase is in the range of 400～850℃. In this phase, decomposition is not obvious and is deemed as the result of degradation of C-C and C-H. In the end, coke and ash are remained; the differential value is changed weakly.The characteristics of pyrolysis of three different metamorphic grade coals are studied using the method of TGA. The result shows that there are four periods in the process of pyrolysis of different coals. The first phase which is before 100℃is called dehydration phase. In this phase, surface bound moisture and a small quantity of CO₂ and CH₄ sealed in the aperture gap of the coal. The second phase is holding phase, in which the temperature of coal sample steps up. The third period during 300～550℃is the main phase of the whole process. The coals tested started to devolatilize at a high devolatilization rate. The TG curves decline obviously and there are distinct peaks in DTG curves. In this phase, primal molecular constitution begins with depolymerization and decomposition, as the result, a large quantity of volatile matter escape. The forth phase during 550～850℃is called quadratic degasification. In this phase, there is mainly bunching reaction in molecules and then semi-coke turns into coke. As the degree of coalification rises, the whole TG curves remove to higher temperature, which means pyrogenation temperature of coal is higher with higher degree of coalification. In this experiment, the weight losses of the anthracite coal, the soft coal and the lignitic coal at the final temperature are 17%,30.07% and 33.4% , so we can get the conclusion that the final weight loss is less with higher degree of coalification.Four different biomass samples above have been mixed with the different metamorphic grade coals in different proportions .The pyrolysis behaviors of the blendings have been experimented with thermo-gravimetric analysis (TGA) method at the same heating rate. The differences between the pyrolysises of the biomass samples and coal have been studied. Besides, the influence of biomass samples on the coal has been studied when they were mixed together. The results show that: the maximum decomposition rate is high and its corresponding temperature is low enough for the biomass, but the coal is by contrary. When the biomass and coal are mixed and pyrolyzed together, the general characteristics are basically the superposition of every component, and agree with the individual pyrolysis characteristics of the biomass or the coal at different periods; the differential quotient thermo-gravimetric curves( DTG )and calculated curves of biomass samples and coal pyrolysis in different proportions are basically accordant before 400℃.while there are deviation between two curves after 400℃to some extent. As a result, the biomass samples have no obvious influence on the pyrolysis of coal before 400℃, however, obvious synergy was observed during co-pyrolysis of coal and biomass after 400℃.Thermal-gravimetric analysis (TGA), gas chromatogram (GC) and mass spectrum (MS) method have been used to study the characteristics of hydrogen sulphide that released from the co-pyrolysis of corn stalks and coals which are of different metamorphic grade. The effects of biomass on the hydrogen sulphide gas released from different coal pyrolysis have been studied. The results show that: when the biomass is blended with coals of different metamorphic grade, the quantity of hydrogen sulphide is influenced obviously. Hydrogen sulfide released from biomass and coal blending is earlier than coal,and its quantity is more with larger proportion of biomass before 380℃.This rule results from that the biomass is pyrolyzed earlier than the coal, abundant active methyl radical and hydrogen are released from the process of biomass pyrolysis, which provide more hydrogen to combine with sulfur. Furthermore, the influence above becomes more obvious when the content of hydrogen in the coal is less. In addition, the quantity of hydrogen sulfide is decreased with biomass after 380℃, which is owed to the corporate function of the accelerate effect of hydrogenation and the desulphurization effect of alkali metals minerals in biomass.When coal is blended with biomass, the quantity of released sulphide pollutant gases is influenced obviously. COS coming out of blends is earlier than that of coal. The COS is increased with more biomass.The influence above becomes more obvious when the content of oxygen in the coal is less.