Physicochemical properties of nitrogen rich in oil and char during biomass nitrogen-rich pyrolysis

Abstract: Biomass is one of the most important renewable resources. Pyrolysis for producing high value added products provides additional value for biomass energy utilization. Through the introduction of exogenous nitrogen in biomass pyrolysis in nitrogen-rich conditions, it can get high value of nitrogen-containing products, i.e. nitrogen-rich char and oil. In this study, wood chips were used as raw materials. The experiment was carried out in a fixed bed reactor at 350 to 850℃, and the effect of temperature and impregnation ratio(5%, 10%, 15%, 20%) on products yields, compositions and characteristics were investigated. Firstly, wood chips were immersed in different mass fractions of urea solution, stirred for 12 hours with a magnetic mixer at room temperature. The woodchips was then separated from solution by filtration and was dried after for 24 hours in an oven. The dried woodchips was then gone through pyrolysis and the bio-oil obtained at different temperatures was analyzed by gas chromatography-mass spectrometry. Derived bio-oil and char were also analyzed for their compositions to trace nitrogen mass transfer. The surface physicochemical property of the char under nitrogen-rich conditions was characterized using a diffuse reflectance infrared spectroscopy and X-ray photoelectron spectroscopy. With the rise of impregnation ratio, the yield of char and bio-oil had little change. Temperature had a remarkable effect on the yield and nitrogen content of boichar and bio oil. Change of product yields was mainly due to the three components (cellulose, ligin and semicellulose) decomposing at different temperatures, and to the volatilization of the secondary cracking at high temperature. The GC-MS results indicated that after treated with urea solution, biomass pyrolysis oil contains large amounts of nitrogen-containing chemicals that can be used to refine high value-added chemical products. The nitrogen-containing chemicals in the bio-oil mainly include aromatic amine, nitrogenous heterocyclic compounds and amide substances. As the temperature increased, the nitrogen content in pyrolysis bio-oil decreased. The larger of urea solution mass fraction, the more nitrogen-containing chemicals in the pyrolysis oil. Pyrolysis char surface contains large amounts of aromatic structure and abundant nitrogen-containing functional groups. Diffuse reflectance IR analysis showed that at the low temperature char surface was rich in C-N, and -NH groups caused by reactions of amino and bio-char. It was confirmed that introducing the exogenous nitrogen into the biomass pyrolysis process can produce nitrogen rich char with a large number of nitrogen-containing functional groups. With temperature increasing, various functional groups decomposed gradually. At 750 and 850℃ only some hydroxyl groups and carbonyl groups were left on the char surface. The urea solution mass fraction had less effect on the types of surface functional groups on the pyrolysis char, but impacted functional group intensity. XPS analysis revealed that the char surface mainly contained C=N, C-N and N-COO functional groups and at 850℃, the char surface only contained pyridine-N and pyrrole-N. This phenomenon could be explained by that thermal stability of pyridine-N and pyrrole-N was better than amide substances. At the pyrolysis temperature of 550℃, the carbon in pyrolysis char was mainly amorphous carbon. With temperature increasing, some aliphatic chain C-H bond, carbonyl and some oligomeric C-C bonds disappeared, while when the temperature reached 850℃, the carbon in char was dominated by graphite carbon.