Experimental Investigation Of Homogeneous Conversion And Char Heterogeneous Decomposition Of Pyrolytic Biomass Tar

There is a large number of agriculture residue as crop straw resources in China, alarge parts of them are abandoned without effectively utilizing, on the other side, witheconomic development, rural energy requirement growth continuous. Developing asmall-scale biomass gasification power generation based on distributed energysupplying system is an effective way to resolve this contradiction due to its flexibilityand efficiency. However, high tar content in the syngas is the bottle neck in restrictingthe commercial application of biomass gasification power generation technology. Incurrently, two-stage downdraft gasifier is one of the best gasifier which with higherefficiency, lower tar content than others. However, tar conversion mechanism underhomogeneous and heterogeneous catalytic decomposition remains unclear in the gasifier,which limits its further improvement and utilization.Tar decomposition processes in the gasifier are interrelated, tar removal is not onlyaffected by the tar formation and conversion properties, but also related to theconversion conditions such as atmosphere and the catalytics. Therefore, the aim of thispaper is to investigate tar conversion and removal mechanism in two-stage downdraftgasifier, by means of constructing the probable road map of tar formation, homogeneousconversion and heterogeneous decomposition, and reveal the catalytic mechanism ofbiomass char decompose tar, provide a theoretical guideline for optimizing tarconversion and removal processes, and optimize the design of the two-stage gasifier.Pyrolysis tar formation and homogeneous conversion properties wereexperimentally investigated in an improved two-stage fixed bed reactor, includingpyrolysis tar formation factors as pyrolysis temperature, biomass species, particle size,as well as carrying gas flow rate, combined with the feasibility study of throat injectingreforming gas for the tar removal process, and explored the different atmosphere withreforming agents such as H₂O, CO₂and O₂on tar homogeneous conversion properties.There are three kinds of tar analysis methods including qualitative analysis, GC/MScomponent analysis and typical tar component of the GC/MS quantitative analysis. Tarcompounds vary in the formation, conversion process have been studied intensively, and results show that the pyrolysis temperature and the biomass species are the main factorsinfluence of pyrolysis tar composition,400-500℃is a reasonable pyrolysis temperaturerange for tar release. Phenols, aldehydes, ketones and furans consist a major proportionof the pyrolysis tar. In pyrolysis tar homogeneous conversion process, temperature playa significant role, O₂can promote tar cracking by forming active free radicals, thepresence of H₂O can contribute a significant reduction both in tar species and in total.Meanwhile, high concentration of CO₂atmosphere is almost completely transformedpyrolysis tar. Organic mass spectrometry analytical method can effectively analyze thetar thermal cracking pathes. By synthesis analyzing of experimental results and currenttheories, a possible roadmap of pyrolysis tar formation and conversion was constructed.Three main aspects effect the char reactivity have been investigated in this paper,including micro-physical structure, micro-chemical structure, as well as metallic speciesand content. Evolution properties of micro-physical structure in biomass char vary insurface area, pore size and pore volume with different conditions. The result shows thattemperature has a strong effect in promoting the formation of char porosity, but highertemperature will induce char shrinking and reduce pore content. In porous biomass char,micro pore accounts for the main component, moreover, the fractal dimension values ofchar can be more effective in reflect the complexity of matrix structure than surface area,and the char activity. Micro-chemical structure of oxygen, nitrogen, sugar and aliphaticfunctional groups gradually transformed with preparation temperature increasing, andthe aromatic hydrocarbon structure-containing C=C gradually increased, formingdifferent kinds of aromatic, char chemical reactivity decreased. Alkali metal and alkaliearth metal（AAEM） content depends on biomass species and preparation temperature,the alkali metal concentration in the char has a first increase and then drop, with amaximum value at500℃. Along with the pyrolysis temperature increasing, the totalalkali metal monotonically decrease.In order to reveal the mechanism and conversion property of biomass char catalyticremoval of tar, an intensive study have been made with biomass char reduce the tarmodel compound as naphthalene in a self-designed bench scale fixed bed micro-reactor.The results show that naphthalene began to yield coke at800℃, and carbon depositionsignificantly improved at900℃. Rice straw char on naphthalene conversion rategradually decreased with time on stream. Increase reaction temperature or naphthaleneconcentration will promote the carbon deposition process on the char active site whichmeans catalyst surface. Low naphthalene concentration8.25g/Nm³and with charpyrolysis temperature in the600-900℃, rice straw char in the deactivation process can removal naphthalene rate of0.036g/grice straw. Biomass char coke formation occursmainly in the microspore region, less obvious on the macropore region. Coke formedchar with CO₂gasification will be more active than those without coke formed char, butgasification reactivity will be rapidly decreased or even inactivated for char continuescoking exceeds a certain range. Char catalytic mechanism in decomposition tar wasrevealed based on experimental results and exiting theories.Biomass char catalytic removal of homogeneous converted raw tar compoundshave been concerned. The results indicate that tar species was selectively decomposedin biomass char, such as PAHs species almost completely converted, but parts of onering aromatic tar removal property is less effective. Biomass char can removal the rawtar and the composition of yields tar dominated by toluene, xylene, ethylbenzene andnaphthalene, few other components at900℃. With the same temperature conditions,biomass char can be7¹2%higher than thermal cracking in gravimetric tar removalefficiency. Tar removal efficiency will be further improved with higher alkali and alkaliearth metal content in char, smaller particle and longer residence time. Temperature isone of the most significant factors in promoting biomass char removal tar. Toluene is animportant converted product between tar and biomass char interaction. Oxidation orreforming atmosphere exist, biomass char catalyst can also promote the carbongasification reaction, and thus inhibit the coking process. The results reveal the primarycause of low tar contents of syngas in a two-stage downdraft gasifier is the effectivelysegregation and coupling of tar homogeneous conversion process and heterogeneousdecomposition process, which are converted the tar into aromatic one and selectivelydecomposed the aromatic tar, especially PAHs（Polycyclic Aromatic Hydrocarbons）.Finally, some suggestions in optimizing process for two-stage downdraft gasifier havebeen proposed.