Evaluation of energy-oriented utilization potential of main Chinese crop residues based on soil protection functions

The return of crop residue to soils can prevent soil and water erosion, maintain soil organic matter and plant nutrient balance, etc. Meanwhile, the utilization of crop residue energy can relieve energy stress and improve energy structure in China. Therefore, it is important for the development of ecological agriculture and the sustainable use of agricultural biomass to evaluate ecological potential of crop residues in terms of energy utilization and protection of soil functions. To this end, this study first advanced a concept of Ecological Straw Returning Amount with the consideration of soil and water conservation, soil organic matter maintaining and crop yield increase. Then a scenario analysis method was used to design optimal ecological straw return of different crops. Three scenarios (low, medium and high return) were designed for each crop residue return. The ecological potential of straw energy use was not only affected by ecological straw return, but influenced by crop planting area, per-unit crop yield, crop planting structure and other crop residue uses such as industrial and agricultural uses. Therefore a bottom up dynamic analysis model that was coupled with Gray Neural Network and linear regression analysis was built to calculate ecological potential of straw energy utilization in different regions of China. Ecological potential of energy-oriented utilization of crop residues of different regions was evaluated from three aspects-spatial distribution, resource density and residual resource components. Then based on the direct straw-fired power generation and cellulosic ethanol project, some recommendations were put forward for the development of crop residue energy utilization. The study revealed that:1) In low, medium and high scenario conditions, ecological potentials of straw in terms of energy utilization were respectively 2.28×108 tons, 1.37×108 tons and 7.76×107 tons, with crop residual densities of 172 t·km^-2, 103 t·km^-2 and 58 t·km^-2. Available ecological straw resource for the production of bio-energy comprised mainly of paddy straw, potato straw and wheat straw under low and medium scenario conditions, and mainly of potato straw and sugar straw under high scenario condition. The resource was mainly distributed in Henan, Shandong, Heilongjiang and Sichuan Provinces under the three scenario conditions. 2) Under low scenario condition, residue resource density and total amount of straw in the provinces in mainland China (with the exception of Beijing, Tianjin, Shanghai and Tibet) met the requirement for 6 MW direct straw-fired power generation that was the equivalent of an annual output of 10 000 tons of cellulosic ethanol. Under the medium scenario, only the crop straw resources of Fujian, Guangdong, Guangxi, Hainan and Chongqing were suitable for an annual output of 50 000 tons of cellulosic ethanol. A 25 MW direct straw-fired power generation or an annual output of 50 000 tons of cellulosic ethanol was possible for Fujian, Guangdong, Guangxi, Hainan and Chongqing under the high scenario condition.