Research On Microbial Depolymerization Of Lignite

The amount of lignite in our country is abundant, accounting for13%of totalcoal reserves. However, it is viewed as an inferior fuel due to low coalification,calorific heating value, and high moisture, ash content as well as the low combustionefficiency and serious environmental pollution. Therefore, advanced technologies arein great demand for high efficient and value-added utilization of lignite. Now, thelignite conversion is achieved for the value-added product by chemical gasification orliquefaction, which pollutes environment, requires high energy and large cost, and isoperated under strict conditions. In addressing these issues, bio-conversion offers anew pathway for coal process and clean utilization, considering its mild conditions,low energy costs, environmental friendly property and economical effectiveness. But,the bio-conversion also suffers from low efficiency and unclear reaction mechanisms.The thesis, under such background, is trying to systemically research on the theoxidation process of lignite, lignite solubilization in solid media, lignitedepolymerization in liquid media, as well as separation and analysis of product andlignite, providing a scientific foundation for the microbial application of lignite.First of all, a strain of fungus F8was selected from rotten wood around the mines.It was identified as Ascomycota Hypocrea lixii on the basis of its18S rRNA genesequence. The fungus’ ability to produce lignin peroxidase, laccase and Manganeseperoxidase and its capability to degrade lignin were confirmed by qualitativeexperiments. Different methods were also utilized to oxidize the lignite for improvingthe bioconversion of lignite. The results showed that the optimal rate and efficiency ofbio-liquefaction in solid media was found in the oxidized lignite after the treatment of8M HNO3(1g lignite:5ml HNO3) for24h at room temperature. Upon this, a study oflignite solubilization was conducted using fungus F8. The liquefaction of oxidizedlignite in different solid media was investigated, and the enzyme production of fungusand the degradation of oxidized lignite were determined in the liquefaction process.The result showed that the ability of SMA to solubilize lignite was the best among theused solid media. Meanwhile, the fungal liquefaction product in the SMA medium,oxidized lignite and residual lignite after liquefaction were analyzed. The conclusionwas drawn that oxidized lignite underwent redox reaction in the presence of fungus,which led to aromatic products of smaller molecular weight, lower carbon and oxygencontent. Secondly, the bio-depolymerization of oxidized lignite in liquid media wasconducted using fungus F8. Optimized condition for depolymerization was achievedbased upon the single-variable experiments, which was: depolymerization temperature30.5℃, shaker speed154.0rpm, fungal amount3.0aperture/150ml anddepolymerization time10.5d. Under these conditions, the depolymerization rate wasup to43.85%. Then, the residual lignite and the oxidized lignite were extractedsequentially with seven organic solvents and each extract was analyzed by GC/MS. Inaddition, depolymerization product, residual lignite and oxidized lignite wereanalyzed by FTIR. The result showed that some new substances of smaller molecularweight appeared in the each extract from residual lignite and oxygen-containingcompounds increased, in comparison with the oxidized lignite; simpler aromaticcompounds increased with a decreasing of ether linkages and side-chain groups ofaromatics, indicating that ether linkage and side-chain groups of aromatics werebroken in the presence of microbes.Finally, in order not to introducing unfavorable functional groups and in the hopeof advancing bio-depolymerization, fractional depolymerization of lignite wasconducted using fungus F8in liquid media. Very complex organic matter of lignite israther resistive to direct depolymerization actions of microorganisms. Therefore, thelignite was seperated into several simpler fractions with different major structure andgroups by the fractional extraction. Then, each fraction was depolymerized by fungusF8under the same condition, and the optimal depolymerization fraction was obtainedwith depolymerization percentage as the parameter. Finally, the optimaldepolymerization fraction before and after depolymerization was analyzed. The resultshowed that lignite was sequentially extracted with five solvents to separate intosimpler extracts (E1-E5). Based on GC/MS analysis, E1-E5were humic acids, saturatedaliphatic hydrocarbons, alkyl aromatics and aromatic esters, CAs and aromatic estersand ketones, respectively. E1was the optimal component of lignite at a highdepolymerisation percentage of35.59%. From analysis of E1before and afterdepolymerization, the fungus broke the side-chain functional groups of aromatic ringcompounds and the chemical bonds in the internal structure of humic acid, with theproduction of simpler aromatic compounds and smaller substances.