Research On Co-pyrolysis Of Biomass And Coal In A Free Fall Reactor

An experimental study on co-pyrolysis of biomass and coal was performed in a free fall reactor under atmospheric pressure with nitrogen as balance gas. The coal sample selected are Dayan lignite (D Y) and Tiefa bituminous (TF), while the biomass used are legume straw (LS) and pine sawdust (SD). The study is focused on possible synergetic effects between biomass and coal. The effects of temperature (500⁷00℃) and blending ratio of biomass in mixture (BR, 0～100 wt.%) on the yields of the major pyrolysis products (e.g. gas, liquid, and char), the gaseous components, the char reactivities and the tar compositions were studied in the experiment. Moreover, the investigations into the pyrolytic behaviours of coal and biomass were also conducted using thermogravimetric analysis (TGA). The effect of heating rate (10-40 ℃/min) was analyzed and the kinetic parameters were also obtained.The results from TGA showed that the DTG peaks of devolatilization move to a higher temperature range and the peak becomes wider with the increase of the heating rate. In the case of the pyrolysis processes with different devolatilization temperatures at low heating rate, the two processes could happen simultaneously at high heating rate. Therefore, it can be deduced that there might exist synergetic effects if co-pyrolysis of biomass and coal occurs in a free fall reactor.The studies on co-pyrolysis of biomass and coal in the free fall reactor indicated that there exist some synergies between biomass and coal indeed. Compared with the theoretical values calculated on pyrolysis of each individual fuel, the yields of chars are lower, and consequently the yields of liquid and tar are higher. Moreover, the experimental results showed that even the compositions of the gaseous products (e.g. H₂, CH₄, CO, etc.) from blended samples are not all in accordance with those of their parent fuels.BR has a different effect on the synergies. The noticeable synergies always occur under the higher BR conditions: for the co-pyrolysis of biomass with Dayan lignite, the obvious synergies occur under the BR of 70 wt.%;for the co-pyrolysis of biomass with Tiefa bituminous, the obvious synergies occur under the BR of 50 wt.%. In addition, temperature also has an obvious effect on the synergies. Compared with 500 and 700℃, 600℃ may be a more suitable temperature for noticeable synergies in the co-pyrolysis of LS/DY and LS/TF, but for that of SD/DY and SD/TF, the more suitable temperatures for synergies are 500 and 600℃, respectively. So it can be deduced that the relatively lower temperature (500 and 600 ℃) is more in favor of obvious synergies in the co-pyrolysis. Finally, the fuel type also affectsthe synergies. Compared with pine sawdust, the co-pyrolysis of legume straw with coal shows more obvious synergy. Compared with Dayan lignite, Tiefa bituminous used for co-pyrolysis is more in favor of synergy, considering the synergies occur under the BR of 50 wt.%.CO2 reactivities of the chars obtained from individual pyrolysis and co-pyrolysis in the free fall reactor were determined in a fixed-bed reactor. The results showed that the reactivities of the chars obtained from the co-pyrolysis under the higher BR (70 wt.%) conditions are about twice as high as those of coal char alone, even higher than those of biomass alone. This also identified the synergies during the co-pyrolysis of biomass and coal in the free fall reactor.One set of composition analysis method was introduced for pyrolysis tar, in which tar samples are separated into Aspheltenes, Phenols, Aliphatics, Aromatics and Polar fractions of different polarity, then FTIR is used for the characterization of Aspheltenes and GC is used for the other fractions. It is found that the quality of tar from co-pyrolysis is improved and the light compositions in the tar increase. Compared with the calculated values, the yield of aspheltenes fraction from co-pyrolysis decreases, and the quantity and quality of phenols fraction is improved, and the yield of hydrogenation aromatic hydrocarbon increases, and the yield of aliphatic fraction decreases. Therefore, the improvement of the tar quality also confirmed the synergies in the co-pyrolysis to a deeper degree.