Experimental Study On The Pyrolysis Characteristics Of Lignocellulosic Biomass Impregnated With Ammonia Source And Potassium Salts

As one of the most promising clean energy technologies, biomass pyrolysis has been received increasing attention. The liquid product of biomass pyrolysis has great potential to be alternative of traditional fossil energy in fuel and chemical field. However, the poor physicochemical properties, including high oxygen content, low pH, high water content, and various compounds with low content, increases the cost of its application. Recent researches demonstrate that most of these characteristics of bio-oil are related to the oxygen containing reactive functional groups represented by carbonyl group. During the storage of bio-oil, the carbonyl group can react a series of chemical reaction with other functional group, which have negative impacts on the stability of bio-oil, and is also conducive to the coke of catalyst during the upgrading process of bio-oil. Therefore, reducing the carbonyl content in bio-oil and increasing the enrichment of high-valued compounds is one of the urgent issues to solve for promoting the large-scale application of bio-oil. Based on these, the pyrolysis characteristics of different varieties of biomass were researched in this study, and then the new method was proposed to reduce the content of carbonyl compounds by introducing ammonia sources during the pyrolysis of lignocellulosic biomass based on the Maillard reaction between carbonyl and amino. Afterwards, the appropriate ammonia sources and catalytic were selected to achieve the enrichment of phenols and nitrogenous heterocyclic compounds. Besides, the pyrolysis of lignocellulosic biomass impregnated with potassium salts were carried out.(1)Study on the pyrolysis characteristics of biomassThe thermogravimetric characteristics of two kinds of biomass were studied with a thermo gravimetric analyzer, demonstrating that similar biomass has similar thermogravimetric characteristics, and the TG curves were mainly depends on the content of cellulose, hemicelluloses and lignin. The effect of pyrolysis temperature and pyrolysis time on biomass pyrolysis characteristics were studied by Py-GC/MS. While the pyrolysis temperature is low, the kind and yield of production changes can be seen the trend with temperature changes. While the pyrolysis temperature is high, temperature only has great effects on the yield of products. When the pyrolysis temperature is 550℃, the highest yield of phenols can be obtained. The less species was obtained when pyrolysis temperature is low, and increasing the pyrolyisis time can increased the enrichment of compounds, which is good for the isolation and purification of compounds. Under high temperature, There was secondary thermal cracking of phenols with long pyrolysis time. The optimizing pyrolysis time will decrease with the increase of temperature.The chlorella and cornstalk were selected as the representative of lignocellulosic and algal biomass, respectively. The pyrolysis experiments were carried out in a pyroprobe pyrolyzer, and the pyrolytic vapors were identified by gas chromatography/mass spectrometry (GC/MS). Subsequently, the compositionally difference of the two kinds of pyrolytic products was presented, and relevant pyrolysis mechanism was analyzed systematically. There exist great differences in products distribution of this two kinds of pyrolytic vapors. The main pyrolysis products in cornstalk pyrolytic vapor are ranked in the order of phenolics, ketones, linear acids, anhydrosugars, and furans, while for chlorella, the order becomes linear acids, alcohols, hydrocarbons, nitrogen-containing compounds, phenolics, furans, and esters. The nitrogen-containing compounds is characteristic products in chlorella pyrolytic vapor, while the content of aldehydes is slight. This is attributable to Maillard reaction combining the carbonyl group from carbohydrates and amino group from proteins to produce nitrogen-containing compound, which is benefit for the stability of bio-oil. The species and origin of the compounds in the same group are also different. The major acids in pyrolysis vapor of cornstalk are acetic acid derived from the hemicelluloses, while the acids for chlorella are long-chain acids cracked from lipid. The phenols in pyrolysis vapor of cornstalk are main originated from lignin, while for chlorella are from the crack of amino acids with low molecular weight.(2) Pyrolysis characteristics of lignocellulosic biomass impregnated with ammonia sourceFour kinds of rice husk impregnated with ammonium acetate, urea, ammonium sulfate and ammonium dihydrogen phosphate were prepared, and the pyrolysis tests were carried out in a fixed bed reactor, then the properties of liquid, gas and solid products were researched. The carbonyl compounds were converted into nitrogenous heterocyclic compounds when the ammonia sources were introduced into pyrolysis. The lower impregnation of ammonium acetate have finite effects on the pyrolysis. While the impregnation of urea can increased the content of nitrogenous heterocyclic compounds and phenols to 8.35% and 30.4%, respectively. The species of compounds reduced remarkably in liquid products for ammonium sulfate and ammonium dihydrogen phosphate-impregnated biomass, and CO and H2 were enriched in gas products. The N content of solid products increased for the ammonia source-impregnated biomass, which is benefit for their application in biochar-based fertilizer and activated carbonBiomass samples with different loading of urea were prepared by dipping rice husk in different concentration of urea solution. Through pyrolysis experiments of samples, it shows that the loading of urea onto biomass increased with the concentration of urea solution linearly, and then slowly with higher concentration of urea solution. The yield of liquid products is also related to the concentration of urea solution, while the yield of gas and solid products showed the reverse change trend. Increased concentration of urea solution can promote the content of phenols and nitrogen heterocyclic compounds in liquid product. When the concentration of urea solution is 8 wt%, the highest CO2 content was obtained in the product gas, and CO content is the lowest. The average pore diameter of solid product increases with increasing concentration of urea solution.The effects of HZSM-5 and CuO/ZSM-5 catalysts on the mixture of rice husk and urea were also studied in this paper. The results shows that the material mixed with rice husk/urea can achieve the conversation of carbonyl functional groups into nitrogen heterocyclic compounds. The water content, C and O content decreased and the N content increased remarkably. The two kinds of catalysts can improved the yield of bio-oil and increased the content of nitrogen heterocyclic compounds and phenols, especially for the CuO/ZSM-5 catalyst.(3) Influence of potassium salts on pyrolysis characteristic of biomassThe pyrolysis experiments of biomass samples with potassium acetate, potassium carbonate, potassium chloride, potassium sulfate and potassium phosphate were investigated by thermo gravimetric analyzer and Py-GC/MS, and the effects of different potassium salts on the pyrolysis of biomass were compared. The significant difference exist in thermo-gravimetric curves and distribution of pyrolysis products shows that both potassium ions and acid radical ion can impact on biomass pyrolysis. The TG curves and maximum weight loss peak of DTG curves showed a trend to lower temperature area, and the value of maximum weight loss peak decreased. The content of phenols, acids in the pyrolysis products were increased, while the contents of aldehydes decreased.