烘焙预处理对玉米秸秆气化产物特性的影响

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

我国生物质资源丰富,是一种清洁可再生的原料,通过生物质气化技术将其转化为可燃气,用于替代燃煤和天然气供热或发电,对我国实现“碳达峰”和“碳中和”的“双碳”目标具有重要意义。然而,生物质原料存在含水率高、含氧量高及热值低等缺点,导致气化可燃气存在热值低和焦油含量高的缺陷,使得利用农林废弃生物质“变废为宝”面临较大挑战。选取玉米秸秆为原料,首先采用程序控温管式炉对其进行烘焙预处理,并用元素分析、工业分析、X射线衍射仪、傅里叶红外光谱仪、热重分析仪等仪器对其基本物理化学特性进行分析;其次,采用自制的小型固定床气化炉装置,研究烘焙预处理温度对玉米秸秆气化可燃气、炭和焦油产物产率和特性的影响。研究结果表明,随着烘焙温度的升高,玉米秸秆的O质量分数及挥发分明显降低,C质量分数与固定碳显著增加,使得热值从17.26 MJ/kg增加至25.50 MJ/kg;烘焙温度对气化产物的质量产率和特性也有显著的影响。随着烘焙温度的升高,可燃气的质量产率逐渐下降,H2的体积分数从13.85%大幅增加到22.56%,可燃气的低位热值在烘焙温度为220℃时达到最高值,为9.36 MJ/Nm^(3)。较高的烘焙温度导致气化残炭的质量产率上升,并且气化残炭中的C元素与固定碳增加,而挥发分与灰分减少,热值从22.74 MJ/kg增加至24.52 MJ/kg。较高的烘焙温度使得焦油的质量产率从21.07%大幅下降至12.67%,焦油组分主要由芳烃类、酚类及脂肪烃类组分构成,其中芳烃类物质与酚类物质的质量分数较高,随烘焙温度升高,芳烃类物质的质量分数先增加后减少,而酚类物质质量分数先减少后增加,其他脂肪烃物质质量分数变化规律不明显。因此,烘焙预处理可显著改善玉米秸秆的品质,进而提升气化可燃气的热值,降低焦油含量。

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 ele-ment 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 spectro-meter, 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 temperat-ure, 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/Nm^(3), 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 in-creased. Therefore, the torrefaction pretreatment is a well-established approach to improve the quality of biomass by re-moving oxygen element. In addition, the gasification performance of biomass is also improved after torrefaction pretreat-ment, resulting in an increase of the LHV of producer gas and the reduction of the yield of tar.