Biofuels 2015 - Upgrading liquids from fast pyrolysis of biomass

摘要

Fast pyrolysis for production of high yields of liquids(bio-oil) has now reached commercial reality, andthere continues to be considerably increasingactivities at the RD level to develop processes andimprove the quality of the liquid. Biomass pyrolysisis the thermal decomposition of biomass occurring inthe absence of oxygen. It is the fundamentalchemical reaction that is the precursor of both thecombustion and gasification processes and occursnaturally in the first two seconds. The products ofbiomass pyrolysis include biochar, bio-oil and gasesincluding methane, hydrogen, carbon monoxide, andcarbon dioxide. Depending on the thermalenvironment and the final temperature, pyrolysis willyield mainly biochar at low temperatures, less than450 0C, when the heating rate is quite slow, andmainly gases at high temperatures, greater than 8000C, with rapid heating rates. At an intermediatetemperature and under relatively high heating rates,the main product is bio-oil. Pyrolysis can beperformed at relatively small scale and at remotelocations which enhance energy density of thebiomass resource and reduce transport and handlingcosts. Pyrolysis offers a flexible and attractive wayof converting solid biomass into an easily stored andtransported liquid, which can be successfully usedfor the production of heat, power and chemicals. Awide range of biomass feedstocks can be used inpyrolysis processes. Virtually any form of biomasscan be considered for fast pyrolysis. Most work hasbeen performed on wood, because of its consistencyand comparability between tests. However, nearly100 types of biomass have been tested, ranging fromagricultural wastes such as straw, olive pits, and nutshells to energy crops such as miscanthus andsorghum. Forestry wastes such as bark and thinningsand other solid wastes, including sewage sludge andleather wastes, have also been studied. In this review,the main (although not exclusive) emphasis has beengiven to wood. The literature on wood/biomasspyrolysis, both fast and slow, is surveyed and boththe physical and chemical aspects of the resultingbio-oils are reviewed. The effect of the woodcomposition and structure, heating rate, andresidence time during pyrolysis on the overallreaction rate and the yield of the volatiles are alsodiscussed. Although very fast and very slowpyrolyses of biomass produce markedly differentproductsThe pyrolysis process is very dependent onthe moisture content of the feedstock, which shouldbe around 10%. The technology of fast pyrolysis isdescribed followed by a comprehensiveexaminationof the characteristics and qualityrequirements of biooil. This considers all aspects ofthe special characteristics of bio-oil – how they arecreated and the solutions available to help meetrequirements for utilisation. Particular attention ispaid to chemical and catalytic upgradingincluding,for example, incorporation into an oilrefinery, production of hydrocarbons, chemicals,synthesis gas and hydrogen production which haveseen a wide range of new research activities. Anappreciation of the potential for bio-oil to meet abroad spectrum of applications in renewable energyhas led to a significantly increased RD activity thathas focused on addressing liquid quality issues bothfor direct use for heat and power and indirect use forbiofuels and green chemicals. This increased activityis evident in North America, Europe and Asia withmany new entrants as well as expansion of existingactivities. The only disappointment is the morelimited industrial development and also deploymentof fast pyrolysis processes that are necessary toprovide the basic bio-oil raw material.