Effect of temperature on preparation and characteristics of corn straw pellets torrefaction biochar

Abstract: China is an agricultural country with huge corn planting area. According to data of the 2015 Statistical Yearbook of China, corn planting area was 37 million hectares in 2014, which produced about 225 million tons of corn stalk wastes. The large amount of corn stalk wastes are treated by incineration or landfill, resulting in serious environmental pollution and waste of resources. In order to solve these problems, curing molding, thermo-chemical conversion and biochemical conversion are used to prepare all kinds of biofuel (e.g. gas, bio-oil, and biochar). Preparation of biochar from corn straw pellets is an effective method by thermal conversion. The torrefaction is a mild pyrolysis mode between 200 and 350 ℃ which results in partial devolatilization of the solid biomass. Biochar pellets from the torrefaction technology have a considerable density, desirable hardness, and good hydrophobicity. Using biochar pellets reduces transport and investment costs for fuel storage and process feeding to greater extent than using biochar powder. Therefore, there are obvious advantages of lower equipment cost, simpler equipment system, and higher production efficiency when biomass pellets are pyrolyzed. In this paper, biochar pellets of corn straw were produced by the fixed bed pyrolysis at different temperature (250-400 ℃) at the heating rate of 10 ℃/min for a residence time of 30 min, with the N2 flow rate of 100 mL/min. Biochars were collected after the pyrolysis reaction, and their characteristics were analyzed. Characteristics of biochar pellets were analyzed by elemental analysis, proximate analysis, energy yield, mass yield, mechanical properties, hydrophobicity, FTIR, SEM and K contents. With the increase of torrefaction temperature, the O content decreased, and the C content increased. The results caused the higher heating value of biochar increased and the mass yield and energy yield decreased. At 400 ℃, the higher heating value and energy yield of biochar pellets were 21.86 MJ/kg, 50.17%, respectively. The surface cracks of biochar pellets increased and the mechanical properties were reduced. The mechanical properties of biaochar at 350 ℃(CSP350) were better than that of biaochar at 400℃(CSP400), which was lower than that the raw corn straw pellets. But the mechanical properties of biochar pellets could meet the requirement for the use of storage and transportation. Biochar pellets showed good hydrophobicity, which benefitted their storage in outsides. Moreover, biochar pellets will not decay and disperse during storage. Analysis of FTIR indicated that dehydration, decarbonylation and demethylation occurred during torrefaction process of corn straw pellets, the cellulose and hemicellulose should be basically finished, and lignin began to pyrolyze. Analysis of SEM and BET showed that the pore size decreased and the surface area increased with the increase of temperature. The K content in biochar increased from 1.68% of raw pellets to 3.12% of CSP400. This phenomenon has an adverse effect on the combustion of biochar pellets, which is easy to slag in combustion furnace. In conclusion, the results showed that biochar pellets from corn straw could be used as biofuel, and the optimum preparation temperature was 300-350 ℃.