秸秆类生物质气炭联产全生命周期评价

摘要

为探究秸秆类生物质热解转化生物炭及热解气过程的能源转化过程的效率、经济性及温室气体排放，该文依据全生命周期评价分析原理，建立秸秆类生物质气炭联产全生命周期3E（economic，energy and environment）模型，对以玉米秸秆为原料的生物质气炭联产过程进行全生命周期分析，评价范围从作物种植到生物气炭产物的利用，系统分为玉米作物种植阶段、秸秆从田间到转化工厂的收储运阶段、生物质气炭转化阶段、生物质气炭应用阶段等4个阶段，并对比分析了横流移动床生物质气炭联产和竖流移动床生物质气炭联产2种工艺技术优劣。结果表明，横流移动床生物质气炭联产的净能量6542.2 MJ/t，能量产出投入比为4.5，其中，居能源消费的前三位的是种植氮肥、种植农机油耗、热解电耗，分别占总能耗的30.8%、20.4%、17.2%；气炭联产转化的总成本319.4元/t，其中热解气炭转化阶段成本最高，约占总成本34.0%，产品收入567.6元/t，纯利润248.2元/t；能源消耗过程的温室气体CO2当量排放量18.05 g/MJ，经生物炭还田固碳，CO2当量减排量约为40 g/MJ。竖流移动床生物质气炭联产技术能源效益较横流略低，但经济效益较高，2种生物质热解气炭联产技术各具优势，可根据产品应用特点选择最适宜的转化工艺方案。2种气炭联产技术具有一定的经济效益，而且均有较大的节能、减少温室气体排放的效益，具有一定的推广应用价值。%Chinese straw biomass gas-carbon cogeneration conversion technology is in the pilot phase, including the cross flow moving bed and vertical flow moving bed technologies. The effects of a variety of product utilization modes after straw pyrolysis are highlighted. For example, the better energy utilization efficiency can be seen:the gas product can be converted to power or biogas and so on, and the carbon product can be turned into different products of energy and carbon-base fertilizer, which can replace part of the fossil energy, reduce CO2 emissions and pollutants. However, there is still not quantitative evaluation about life cycle economy and energy consumption of current biomass cogeneration technology, and the effects of products and byproducts to the environment are not clear. In order to explore the efficiency, economy and greenhouse gas emissions of energy conversion about biomass forming biochar and fuel gas through pyrolysis process, the life cycle assessment principle was applied, and then the 3E (economy, energy and environment) model for the cogeneration of biochar and fuel gas was established. To be more precise, corn straw was selected as the material, and the analysis range was from crop planting phase to the utilization of fuel gas and biochar. There were 4 stages: corn planting, collection, storage and transportation to the factory, conversion to fuel gas and biochar, and product application. In the meanwhile, the advantages and disadvantages of 2 technologies i.e. cross flow moving bed and vertical flow moving bed were discussed. The results showed that the net energy of applying cross flow moving bed technology was 6 542.2 MJ/t, and the energy output-input ratio reached 4.5. Among them, the top 3 energy consumption items were nitrogen fertilizer for planting, fuel consumption for agricultural machines and power consumption for pyrolysis, of which the energy consumption proportions were 30.8%, 20.4%and 17.2%, respectively. The total cost of the fuel gas-biochar cogeneration was 319.4 yuan/t, of which the cost at pyrolysis conversion phase accounted for about 34.0%, and the product revenue and the net profit were 567.6 and 248.2 yuan/t, respectively. The CO2 equivalent emission was 18.05 g/MJ through the energy consumption process, and the amount of reducing CO2 equivalent was 40 g/MJ after biochar was returned to the field to fix carbon. Two biomass pyrolysis conversion technologies have their own features. Therefore, based on the product application characteristics, applying appropriate conversion process is vital. Both technologies have some benefits, such as economic advantages, energy saving and low greenhouse emissions, which have certain application value.