Effects of HZSM-5 and MCM-41mixing ratios on rape straw catalytic pyrolysis

Abstract: In recent years, the number of fossil fuels is decreasing sharply, and the harmful gases produced by fossil fuel combustion endanger human health and pollute the environment, people are devoted to looking for the alternative fuels of fossil fuels. Biomass energy is a good choice, it is renewable and clean. But the biomass was always burned directly, it is a great waste of biomass sources. In order to reduce the waste of biomass resources and protect the environment, we studied the rapid pyrolysis and catalytic cracking technology to convert the biomass to liquid fuel, which we called the liquid bio-oil. Choosing rape straw as biomass raw materials, HZSM-5 and MCM-41 were used to catalyze the rape straw pyrolysis with different mixing ratios. The experimental instruments include electronic controller, catalytic reactor, pyrolysis reactor, vacuum pump, cold trap, cooling tower, bio-oil collector and connecting pipe. The rape straws were placed in the pyrolysis reactor, the catalysts were replaced in the catalytic reactor, the vacuum pump pressure was controlled at 5kPa and the temperature of the cooling tower was set to ?20 ℃. According to the physical and chemical properties, FT-IR and GC-MS analysis results of organic oil and the BET analysis results of catalysts, we studied the effect of HZSM-5 and MCM-41 mixing ratios on the quality of bio-oil. The results showed that with the increase of MCM-41 content, the yield of bio-oil organic phase decreased first and then increased. The density, oxygen content and kinematic viscosity of refined organic oil decreased first and then increased，while carbon content and calorific value increased first and then decreased, when HZSM-5 and MCM-41 were mixed with 1:1, the higher calorific value was 34.31 MJ/kg. The organic phase of refined oil showed six obvious absorption peaks, respectively, they were the absorption peaks of alcohols or phenols, aromatic compounds, acids and aldehydes, ketones, carboxylates, esters and substituted aromatic compounds. The mass fraction of aromatic hydrocarbon increased first and then decreased, while the mass fraction of carbonyl compounds showed the opposite trend, the mass fraction of acids was continuously reduced. With the increase of MCM-41 content, the catalytic activity and absorption capacity of mixed catalysts improved and tended to be stable when the mass fraction of MCM-41 was 50%. From these analysis results, we can speculate the catalytic cracking reaction mechanism of rape straw on HZSM-5 and MCM-41. Because of the large specific surface area and pore size of mesoporous molecular sieve MCM-41, most of the macromolecules in its pore channels can be decomposed into small molecules such as aldehydes, ketones and other small molecules by decarboxylation, dehydroxylation and ring-opening polymerization in the pore channels. These small molecules re-entered the pores of the microporous molecular sieve HZSM-5, and generated aromatic hydrocarbons, H2O, CO2 and CO through the decarbonylation, aromatization and deoxidation, and then the oxygen content in organic phase decreased, aromatic substances content increased, the acidity reduced, so that the quality of bio-oil was improved. This study could provide the experimental basis and the theoretical basis for the efficient utilization of biomass.