Pyrolysis Processes Of Coal And Biomass At Low Temperature

It is very significant to save coal resources, protect the environment and promote the coal pyrolysis reaction after biomass is added to coal pyrolysis. This paper studies coal pyrolysis processes, biomass pyrolysis processes and coal/biomass blends co-pyrolysis processes by thermal gravimetric analysis, discussing the factors affecting coal/biomass pyrolysis. Kinetic parameters of coal/biomass blends are all obtained by Coats-Redlfern method. In addition, gases and solid residues produced from coal/biomass blends co-pyrolysis by atmospheric pressure tubular reactor are analyzed. The main researches are as follows:(1) Coal pyrolysis processes, biomass pyrolysis process and coal/biomass blends co-pyrolysis process were studied by thermal gravimetric analysis, discussing the factors such as coal types, biomass types and biomass ratios affecting coal/biomass pyrolysis There are three stages in coal pyrolysis process and biomass pyrolysis process. The first stage is dring desorption(<200 ℃), The second stage is devolatilization(biomass 200~400 ℃, coal 400~650 ℃), The third stage is carbonization(biomass> 400 ℃, coal> 650 ℃). Compare with coal pyrolysis, the rate of biomass devolatilization increases rapidly and total weight loss of biomass was higher than coal, these differences are due to their different chemical structure. Biomass components combine together through a relatively weak ether bond, while the coal components is mainly connected through a carbon atom, it only can be broken at higher temperature, thus temperature of coal main pyrolysis stage is higher than that of biomass. There are three peaks in co-pyrolysis DTG curves corresponding to three distinct weight loss regions. The first peak shows evaporation of water, the second peak reflects the pyrolysis of biomass in co-pyrolysis, and the third peak suggests the pyrolysis of coal in co-pyrolysis. After biomass is added to coal, the temperature of biomass maximum devolatilization rate increases, while the temperature of coal maximum devolatilization rate decreases. Experiment values of coal maximum devolatilization rate are generally greater than the theoretical values. Volatiles yields of coal/biomass blends co-pyrolysis in Stage Ⅲ are more than theoretical values too. So it infers that biomass can significantly increase volatiles yields of coal main pyrolysis stage, and the greater the biomass ratio, the greater the volatiles yields.(2) Kinetic parameters of coal/biomass blends are all obtained by Coats-Redlfern method. Coal pyrolysis processes are third-order reaction and biomass pyrolysis processes are first-order reaction. The main pyrolysis stage activation energies of HG, HL, BD coal are 77.5 kJ·mol-1, 77.6 kJ·mol-1, 64.3 kJ·mol-1 respectively, and are all greater than those of the two biomasses. The main pyrolysis stage activation energies of coal is the smallest. After biomass is added to coal, the activation energy of biomass main pyrolysis stage increases, while the activation energy of coal main pyrolysis stage decreases, and the greater the biomass ratio, the lower the apparent activation energy. It infers that biomass can significantly catalyze coal pyrolysis at the main stage, and the greater the blending ratio, the more obvious catalysis. For DN coal, the activation energy of coal pyrolysis of coal/biomass blends with low biomass ratio in Stage Ⅲ is larger than that of coal pyrolysis, while in larger biomass raio, it can be lower. Thus, it infers that biomass with larger ratio can significantly catalyze DN coal pyrolysis at the main stage, and biomass with low ratio could hinder coal pyrolysis.(3) Gases and solid residues produced from coal/biomass blends co-pyrolysis by atmospheric pressure tubular reactor are analyzed. The yields of gas produced such as CO2, CO, C2-C4, CH4 is gradually increasing as the temperature increases, the maximum value occurs at about 600 ℃. And after adding corn straw, CO2, CO gas yields are significantly larger than those of DN coal, total yields increase by 2 and 1 times. This is mainly due to-OH groups of biomass react with organic matter. Besides, the C2-C4 yield is twice more than that of DN coal pyrolysis and the CH4 production from blends pyrolysis is just three fifth of that from DN coal pyrolysis. The H2 yield is gradually increasing as the temperature increases, the maximum value occurs at about 800 ℃. The H2 can be acquired effectively as its yield is almost 204.72 dm3 more than that of DN coal pyrolysis. The carbon content, the hydrogen content, nitrogen content from residue are 59 %, 17 %, 65.6 % of those from the blends respectively, so the carbon mainly concentrates in the residue, hydrogen decreases a lot and nitrogen is partly removed.