Effects of acid treatment conditions on pyrolysis characteristics of corn stovers

Biomass pyrolysis is an important way for converting biomass to high-value products. It is significant to improve the quality of biomass pyrolysis products through pretreatment. Acid washing is one of the key technologies to effectively remove alkali metals and alkaline earth metals (AAEMs) from biomass. In this paper, acetic acid was used as the washing solution, K+ was used as research objects, and pyrolysis experiments of corn straw were carried out by thermogravimetric and pyrolysis-gas chromatography/mass spectrometry analyses. The study of the effects of different acid washing concentration (3%, 5%, 7%), acid washing temperature (25, 50, 75 ℃)and acid washing time (1, 2, 3 h) on the pyrolysis characteristics of corn straw were conducted. The results showed that the K+ can be effectively removed by acid washing, and as the acid washing concentration, acid washing temperature and acid washing time increase, the removal efficiency is higher, at the acid washing time is three hours, the K+ content reaches a minimum of 39.49 mg/kg. The TG and DTG curves of pretreated corn straw moved to a high temperature range, and the peaks caused by the degradation of hemicellulose in the DTG curve of pretreated corn straw reduced. Meanwhile, the maximum pyrolysis weight loss rate increases in parallel with the increase of washing concentration and washing temperature, but the trend of increase goes first and then it decreases with the growing of washing time, finally it reaches a maximum value of 15.49 %/min when the acid washing temperature was 75 ℃. At the same times, the pyrolysis kinetics results show that, after acid washing under different conditions, the pyrolysis activation energy and frequency factor of the samples were increased compared with the original sample. With the increase of acid washing concentration and temperature, the sample pyrolysis activation energy and frequency factor gradually increased, but too long acid washing time had a negative effect on the sample activation energy and frequency factor. What is more, the main products of pyrolysis of corn straw were phenols, ketones and furan compounds. After corn straw was pretreated by acid washing, the yield of phenol substances increased significantly and gradually increased with the increase of acid washing concentration. When acid washing concentration was 7%, the maximum yield reached 16.75%. However, the yields of ketones and furan compounds decreases, and gradually decreased with the increase of acid washing concentration, reaching the minimum values of 0.10% and 7.13% at an acid washing time of one hour and an acid washing concentration of 7%, respectively. For the carbon yield, when the temperature is lower than 361 ℃，the yield of acid-washed carbon is higher than the original samples. When the temperature is higher than 361 ℃，the yield of the acid-washed carbon is decreased, and reached the minimum value when the acid washing time is two hours, but increasing the acid washing time had little effect on the yield of carbon. To study the effects of different treatment conditions of organic acid on the pyrolysis characteristics of corn straw is useful to find a good way for the selection of acid solution in biomass pretreatment.