秸秆直燃发电供应链气体及颗粒污染物排放的生命周期评价

Emission analysis of air and PM pollution on straw power plant supply chain based on life cycle assessment

秸秆发电以其极好的环保效用和可再生能源利用引起各界关注,但是由于秸秆轻、薄、散的特性,导致在秸秆收集和运输的效率十分低下,从而会消耗大量石化燃料,排放出大量的气体及颗粒污染物,加之秸秆发电供应链的终端直燃发电也会排放一定污染物,必然会对其环保效用带来影响。该文依据生命周期评价方法,通过建立4个秸秆发电供应链场景,包含秸秆收集、打捆、运输存储、解包破碎以及最终发电等阶段,对其中机械及车辆石化燃料的消耗以及污染物的排放进行定量计算,发现4个秸秆发电供应链的污染物排放有较大差距,其中CO2相差1.2~2倍,CO相差1.4~5.4倍,NOX相差1.5~4.2倍,PM相差1.4~6.5倍。秸秆发电供应链环保效果与秸秆发电供应链中收集、预处理及运输存储模式有较大关系,预处理压缩环节、短距离农用车运输及压缩后道路运输距离是关键,这些因素的合理配置直接关系到秸秆发电供应链环保效用的高低。对秸秆发电供应链的整体排放以处理单位质量秸秆排放的污染气体和可吸入颗粒排放指数为标准,通过与秸秆露天燃烧和燃煤发电供应链对比分析,发现相对秸秆露天燃烧PM排放指数12.95 g/kg,秸秆发电供应链的PM排放指数为0.12 g/kg;相对燃煤发电供应链CO2排放指数1 010.1 g/kg,秸秆发电供应链的CO2排放指数为43.44 g/kg,在减少由露天焚烧秸秆造成的应急性雾霾天气和减少碳排放方面有积极作用。

Crop straw power plant is environmental-friendly, but crop straw collection and transportation will cause pollution. Crop straw has low density, so the efficiency of the collection and transportation is very low, and per unit of weight crop straw reutilization will make more pollution. In this paper, based on LCA （life cycle assessment）, the data of pollution in the crop straw power plant supply chain are analyzed through quantitative calculation of exhaust pollution of per unit weight and per unit power generation. Straw power plant supply chain contains straw collection, compression and baling, transportation, uncompression and crash and straw power generation. The compression of straw is divided into 3 types, i.e. not baled, the compression in the field and compression in the warehouse; the transportation is also divided into 2 types, the transportation by farm vehicle on the narrow road and heavy-duty vehicle on the highway. There are 4 scenarios and each one is composed of 3 or more parts in straw power plant supply chain, as transport of straw uncompressed, or transport of straw compressed. Because of the difference from agricultural machinery and vehicle, we use the limits for exhaust pollution from diesel engines of non-road mobile machinery （China III） and the limits for exhaust pollutants from compression ignition and gas fuelled positive ignition engines of vehicles （III, IV, V） as emission factors of agricultural machinery and vehicle, and by survey data, calculate the exhaust pollutants that include CO2, CO, NOx, SO2 and PM （particulate matter） in each section. Sections in the straw power plant supply chain are crop straw pretreatment in the field, crop straw pretreatment in the stockhouse, carrying uncompress bale by farm machinery, carrying by farm machinery after pretreatment in the field, carrying uncompress bale by heavy-duty vehicle, carrying by heavy-duty vehicle after pretreating straw in the field, carrying by heavy-duty vehicle after pretreating straw in the stockhouse, transfering in the stockhouse, pretreatment in the straw power plant and crop straw direct-fired generation. After that, through key factor analysis for exhaust pollutants from each section, the density of the straw bale and distance of transportation are very important for exhaust pollutants in the straw power plant supply chain, and reasonable allocation model of straw power plant supply chain can decline the exhaust pollutants. Then comparison to exhaust pollutants from burning in the field and coal-fired power generagtion supply chain that includes coal mining and processing, coal transportation, stock and coal-fired power generation. We find PM pollutant of straw power plant is 100 times less than burning in the field, and carbon emission is about 23 times less than coal-fired power generation supply chain. As desulfurization and denitrification equipment are not installed, NOx and SO2 pollutions are just over the limits of the newest emission standard of air pollutants for thermal power plants. On the other hand we conclude that the effect of environment protection from straw power plant supply chain depends on the model of collection and transportation in the supply chain, and keys are the pretreatment of crop straw compression, the distance of transportation by farm vehicles and the distance of transport trucks. Reasonable deployment of these factors may improve the effect of environmental protection from straw power plant supply chain.