Effect of crushing degree and returning method on straw combustion

Rice straw burning is a major source of pollutants emissions in China. Its contribution to the emission of various pollutants is much higher than that of corn and wheat straws, which has resulted in tremendous pressure on the surrounding urban environment and residents' health. Simultaneously, rice straw will become the most important source of organic fertilizer for rice production in the future. To fundamentally forbid rice straw burning and promote rice straw return to the field, in this study, we set the current conventional mode of rice harvest, including rice straw stripped to the field without crush (T1) and with moderate crush (T2) as controls, and crushed rice straw stripped to the field (T3) and evenly returned to the field (T4) as the treatments. The treatments relied on a self-invented device that crushed and homogeneously scattered the rice straw in field tests. The field simulation tests aimed to study the influence of different crushing degrees and returning methods on the rice straw combustion characteristics. The results showed that the scattering homogeneity and returning density of rice straw increased significantly with increased crushing degree, but the returning thickness of rice straw significantly decreased. The rice straw average length of T4 was 5.3 cm, which was only 13.6% and 36.8% of T1 and T2, respectively. The scattering homogeneity of T4 was 87.4%, which was 49.7% and 42.0% higher than that of T1 and T2, respectively. The thickness of rice straw returned to the field of T4 was 2.7 cm, which was only 22.1% and 27.8% of T1 and T2, respectively. The density of rice straw returned to the field of T4 was 17.6 kg·m^-3, which was 88.3% and 17.3% higher than that of T1 and T2, respectively. Under the rice straw strip-returning modes (T1, T2, T3), the higher the degree of rice straw crushing, the slower the decline in moisture content, the longer the combustion time, the lower the combustion rate, the slower the combustion speed, the higher the ash content, and the less sufficient the combustion. Although T4 can accelerate the decrease in rice straw moisture content by homogeneous scattering, the combustion time, combustion speed, and ash content were only 0.3%, 6.0%, and 1.7%, respectively. This was significantly lower than those in the other treatments, indicating that the rice straw was almost unburned. These results indicate that the rice straw could not be burned when crushed and evenly thrown to the field, which was beneficial for achieving a ban on rice straw combustion. Therefore, local government functional departments need only to compulsorily install the devices for crushing and homogeneously scattering rice straw with the combine harvester, which will completely prohibit the burning of rice straw. This will promote the fertilization of paddy soil and greatly reduce the difficulty and costs of agricultural law agencies enforcing rice straw burning bans.