Effect of torrefaction pretreatment on the properties of gasified products from maize straw

Lignocellulosic biomass is a clean and renewable energy source due to its abundance, wide distribution, and CO₂ neutrality. Biomass gasification technology can convert the biomass into producer gas which can be used as clean fuels in boiler or engine for heating and electricity supply. It is of great important to achieve the goal of “carbon neutrality” and “emission peak” in China. However, the lignocellulosic biomass has the major drawback of high content of water, high content of oxygen element, and low heating value, resulting in a lower gasification efficiency and higher content of tar. Torrefaction pretreatment can effectively upgrade biomass into high-grade fuels with a lower content of oxygen element and higher energy density. First, the torrefaction pretreatment was employed to improve the quality of maize straw at different torrefaction temperatures (200, 220, 240, 260, 280 and 300 ℃). The properties of the raw and torrefied maize straw was analyzed by the ultimate analysis, proximate analysis, X-Ray diffractometer, Fourier transform infrared spectrometer, and thermo gravimetric analyzer. Then, the torrefied maize straw was gasified in a home-made small-scale fixed-bed gasification reactor. The effect of torrefaction temperature on the properties of gasified products (producer gas, char, and tar) was investigated. Results showed that the contents of oxygen element and volatiles gradually decreased with the increase of torrefaction temperature from 200 to 300 ℃, while the contents of carbon element and fixed carbon increased, resulting in an increase of the calorific value from 17.26 MJ/kg to 25.50 MJ/kg. With the increase of torrefaction temperature, the mass yield of producer gas decreased, while the H₂ content in producer gas increased from 13.85% to 22.56%. The highest lower-heating-value (LHV) of producer gas was 9.36 MJ/Nm³, achieving at the torrefaction temperature of 220 ℃. Higher torrefaction temperature resulted in a higher mass yield of bio-char. In addition, the contents of carbon element and fixed carbon increased with the increase of torrefaction temperature, while the volatile fraction and ash decreased, and the calorific value increased from 22.74 MJ/kg to 24.52 MJ/kg. Higher torrefaction temperature resulted in a lower mass yield of tar. The tar was mainly composed of aromatics, phenols, and aliphatic hydrocarbons, among which the relative content of aromatics and phenols is much higher than that of aliphatic hydrocarbons. With the increase in torrefaction temperature, the content of aromatics first increased, and then decreased, while the content of phenols first decreased and then increased. Therefore, the torrefaction pretreatment is a well-established approach to improve the quality of biomass by removing oxygen element. In addition, the gasification performance of biomass is also improved after torrefaction pretreatment, resulting in an increase of the LHV of producer gas and the reduction of the yield of tar.