| Due to widespread, abundant, carbon neutral, and renewable, biomass is considered as one of the most promising energy sources to replace the fossil fuels. The utilization of biomass as fuel is one of the most effective methods to solve the growing global energy crisis. Among the thermal utilization technologies of biomass, gasification is well known as the most promising one, because it sufficiently expands the advantages of biomass, including low ash content, low sulfur content, and low nitrogen content, etc. Biomass gasification produces not only useful products, including fuel gases and chemicals, but also some byproducts, including tar, alkali metals, particulates, NOx, sulfur contaminants, and chlorine contaminants, etc. With the development of biomass gasification, tar is regarded as one of the most urgent problem, because it is harmful to the gasification system and cause energy loss. So, it is very important to develop effective and low-cost tar removal technology. Most tar removal technologies can be divided into primary methods and secondary methods, by which the tar is removed inside or after the gasifier. In this paper, pressurized torrefaction-gasification technology was proposed to improve the fuel property of biomass and suppress the tar formation during gasification at the same time. The effects of atmospheric pressure (AP) torrefaction, gas pressure (GP) torrefaction, and mechanical pressure (MP) torrefaction pretreatments on biomass upgrading, fast pyrolysis, CO2 gasification characteristics, and reaction kinetics were detailedly studied, respectively.The influence of torrefaction temperature (200? (LT),250? (MT), and 300? (ST)) on product distributions, carbon distributions, oxygen distributions, characteristics of semi-char, and energy loss during biomass AP torrefaction, GP torrefaction, and MP torrefaction was investigated. It was found that the semi-char yield of GP torrefaction was the lowest but the biomass upgrading effect was the best during the three torrefaction methods. Because the gas pressure accelerated the decomposition reactions during biomass torrefaction. Besides, mild liquefaction and gasification reactions of torrefied biomass probably occurred with the GP torrefaction. It was found that MT-GP torrefaction (gas pressure torrefaction at 250?) was the the optimal biomass pretreatment method. When the rice straw and pine sawdust was torrefied under 5 MPa of gas pressure at 250?, the carbon yield of semi-char was only 60%-65%. but the carbon content of the semi-char was as high as 65%-68%, the oxygen content was as low as 25%-29%, and the O/C value and O/C value were decreased from 0.65-0.69 to 0.27-0.33 and from 1.28-1.42 to 0.92-1.00, respectively. Furthermore, the higher heating value of the MT-GP semi-char was increased from 15.9-17.7 MJ/kg to 24.7-26.2 MJ/kg, the fuel property of semi-char was very close to lignite. Besides, the energy loss during the MT-GP torrefaction was as low as 29%-30%.The AP torrefaction-fast pyrolysis, GP torrefaction-fast pyrolysis, and MP torrefaction-fast pyrolysis were studied systematically. The liquid yield of the torrefied biomass fast pyrolysis at 600? was increased with the volatile matter content of the semi-char. During AP and GP torrefaction,12%-70% and 63%-83% of volatile matter was removed by generating liquid and gaseous products, respectively, so the liquid and gaseous products yields of semi-char fast pyrolysis were reduced correspondingly. Compared with AP torrefaction, much more liquid products (mainly H2O) were removed during the GP torrefaction, which resulted in lower water content of liquid products of GP semi-chars fast pyrolysis. The pyrolysis kinetics of MT-AP, MT-GP, and MT-MP semi-char were investigated using thermogravimetric analyzer, and the kinetic parameters were calculated by the distributed activation energy model (DAEM). The activation energy ranges of MT-AP, MT-GP, and MT-MP semi-char pyrolysis were 169-249 kJ/mol?145-287 kJ/mol? 152-395 kJ/mol, respectively, which were higher than that of raw rice straw (143-181 kJ/mol).The characteristics of AP torrefaction-CO2 gasification, GP torrefaction-CO2 gasification, and MP torrefaction-CO2 gasification were investigated systematically. The results showed that AP, GP, and MP torrefaction pretreatments could enhance the CO and H2 yields and suppress the formation of tar of biomass gasification. The CO and H2 yields of the semi-char CO2 gasification at 900? were increased with the fixed carbon content of the semi-char, and the tar content in producer gas was increased with the volatile matter content of the semi-char. Therefore, the gasification efficiency of GP semi-char was the highest, and the tar content of GP semi-char gasification was the lowest. There are three main reasons for these results. Firstly, the effect of the volatile matter removal of the GP torrefaction was the best. Secondly, the concentration of mineral matters (such as K and Ca) in the GP semi-char, which could catalyze the tar cracking reactions, was the highest. Thirdly, part of volatile matter in the biomass was converted into fixed carbon by cross-linking reactions, which were accelerated by GP torrefaction. The CO and H2 yields of rice straw gasification at 900? were increased by 120% and 64% by MT-GP torrefaction, respectively. The tar content tar content in producer gas was decreased by 73% at the same time. If they were converted to raw biomass basis, the CO and H2 yields of MT-GP semi-char gasification were 754.2 Nml per gram raw rice straw (Nml/g) and 80.0 Nml/g, just slightly smaller to those of the raw rice straw gasification which were 797.9 Nml/g for H2 and 113.3 Nml/g for CO. This can effectively compensate the low semi-char yield of the MT-GP torrefaction, which was only 43.0%. The CO2 gasification kinetics of raw rice straw and AP, GP, and MP semi-chars were investigated using thermogravimetric analyzer. The results showed that the activation energy of raw rice straw CO2 gasification was 143.3 kJ/mol. The influence of AP torrefaction on the biomass gasification activation energy was not significantly. The gasification activation energies of GP and MP semi-chars were lower and higher than that of raw rice straw, respectively. The most important reason was that the concentrations of inherent minerals in the GP semi-chars, which had catalytic effect on the gasification reactions of biomass, were higher than that of raw rice straw.In summary, gas pressure torrefaction pretreatment at around 250? could not only suppress the tar formation during biomass gasification effectively, but also increase the yields of gas products and improve the gasification characteristics of biomass. Therefore, biomass gas pressure torrefaction & gasification technology might be promising. |
PDF Full Text Request