Behaviors Of Radicals In The Processes Of Pvrolvsis Of Coals And Biomass And Solvent Extraction Of Coals

Pyrolysis is commonly regarded as a simple and an effective method to produce tars, consequently fuels, and chemicals. Since the liquid yield increases with the heating rate, fast pyrolysis has been studied worldwide, but few were reported to be successful on commercial scale due to problems including poor tar quality and high solid content. The pyrolysis (slow or fast pyrolysis) tars need to be upgraded, including refinery or hydrotreatment, to produce fuels and chemicals, but the coke formation in the process of heat treatment of the the tar always block the system or lead to inactivation of the catalyst. In the pyrolysis system, the high temperature dust removal devises or the high temperature pipe lines also blocked by the solid deposition, which will influence the normal operation of the system. To solve the above problems, extensive technical efforts have been made in past, including further increases in heating rate of coal and in cooling rate of volatiles, pretreatment of coals, introduction of catalysts, and incorporation of multiple high temperature dust removal devises. These technical efforts, however, did not show sufficient improvement in solving the problems. It seems to suggest that new insight should be gained to solve the problems especially on the fundamental chemistry involved.Pyrolysis follows a radical mechanism. Take the pyrolysis of coal for example, the first step of the pyrolysis is the breakage of weak covalent bond in coal and formation volatile radical fragments; then the volatile radical fragments will continue react and produce tars, gas, and coke. The process of pyrolysis of biomass is similar with that of coals. Change of pyrolysis conditions will influence the radical reaction during the pyrolysis and finally affect the properties and radical concentration of the tars.In this paper, we have studied the influence of pyrolysis conditions on radical concentration and properties of the tars, and the radical reaction of the tar maintained at various temperatures in a point of view of pyrolysis mechanism--free radicals. We found that the high environment temperature which the volatiles will experience after releasing from the coal surface is the main reason for the poor quality of the tars pyrolyzed from fast pyrolysis; The tars produced at high temperature still contain some weak covalent bond and will continue covalent bond cracking when experience temperatures higher than 300℃, which result in radical increase and coke formation in the tars; In addition, based on preliminary understanding of free radicals and radical reactions, we extend our study to the radical reaction in the solvent extraction and found that the radical reaction indeed exist during the solvent extraction. The followings are the main conclusions of our work:(1) The radical concentration of the tars is about 1017 spins/g, which lower than that of coal (1018-1019 spins/g) and pyrolysis char (1019-1020 spins/g). The radical concentrations of the tars, coals, and chars all increase with the carbon content of coals. The higher the heating rate, the higher the radical concentration, and the more the number of condensed aromatic rings of the coal tars. The main reason for these phenomena is that the higher the heating rate, the higher the environment temperature which the volatile experienced after leaving the coal surface. Radicals in tars include those reactive but low in mobility due to the high viscosity of the tars and those un-reactive because the unpaired electrons are confined in macromolecules. An increase in coal pyrolysis heating rate increases the amounts of both radicals in the tars. A large carrier gas flow rate will results in a short residence time of the volatile in the furnace, and the radical concentrations of the tars produced at this condition are relatively low.(2) When the tars experienced temperature higher than 300℃, the weak covalent bond in the tars will continue cracking and generate more radicals, and the radical concentrations of the tars increase with temperature and time. We also found coke formation when the tar maintained at high temperature. The coke formed at high temperature contains a lot of radicals and is main reason for radical concentration increase of the coal tar at high temperature. Tars produced from pyrolysis of high heating rate are more easily to form coke and have a high radical concentration increasing rate at high temperature, probably because the high heavy component content in these tars. The radical concentration of the tars produced from different carrier gas flow rates are almost the same.(3) The radical concentrations of the biomass tars produced at the same pyrolysis conditions are lower than that of coal tars, about 1016 spins/g. The radical concentration and coke content of the bio-tars both increase with temperature and time when experienced temperatures higher than 300℃. The coke content of the bio-tar is proportional to the radical concentration of the bio-tar. The stability of the bio-tars is worse than that of coal tars. The bio-tars are more difficult to be upgrading, because the radical concentration and coke formation of the bio-tars are higher than that of coal tars for several times when maintained at same high temperature.(4) When the coal extracted by hexane, tetrahydrofuran (THF), toluene, and tetrahydronaphthalene (THN) at room temperature, the radical concentrations of the coals decrease with the increasing of time, but in N-methylpyrrolidone (NMP) solvent, the radical concentration of the coals increase with time. This also means that radical reactions indeed occured during the solvent extraction. Increasing extraction temperature and using ultrasonic extraction both have affect on radical reactions during solvent extraction. Compared the relationship of radical concentration and extraction yield with that of different extraction conditions (room temperature, room temperature+ultrasound, and 65℃), we found that the higher the extraction yield, the more the descend of radical concentration when the coals were extracted by hexane, THF, Toluene, and THN, but in NMP solvent, the extraction yield have no relation with the radical concentration.