Abstract
To enhance the hydrothermal carbonization (HTC) process on biomass waste and improve the quality of biomass solid fuel. Corn straw was pretreated with acid washing and subsequently hydrothermally carbonized at 180–270 °C. The solid product obtained (hydrochars) was compared with the solid product produced from untreated hydrothermally carbonized straw. The results show that the acid pretreatment removed 7.9% of the ash from the straw. ICP and XRD analysis show that most of the alkali and alkaline earth metals have been removed. This addresses the defect of high ash content as the HTC temperature increases. The HHV of hydrochars produced by HTC after acid washing can reach 27.7 MJ/kg, which is nearly 10% higher than that of hydrochars prepared without acid washing pretreatment, and nearly 70% higher than that of straw raw materials. Elemental analysis and FTIR analysis show that the acid washing pretreatment changed the content and structure of the biomass components in the straw, resulting in a more complete HTC reaction and higher carbon sequestration. The decrease of H/C and O/C deepened the degree of coal-like transformation of hydrochars, with the lowest approaching the bituminous coal zone. The combustion characteristics of the hydrochars prepared after acid washing were significantly upgraded, the comprehensive combustion index and thermal stability of hydrochars both increased. Therefore, HTC after acid washing pretreatment is beneficial to further improve the high heating value and combustion characteristics of hydrochar.
Highlights
As a major agricultural country, China has produced more than 900 million tons of waste straw every year
The main objective of this paper is to reduce the alkaline earth metals (AAEM) content in the straw by acid washing pretreatment and to change the internal composition of the straw, followed by hydrothermal carbonation experiments at different final temperatures
It can be seen that inorganic elements have been reduced to varying degrees
Summary
As a major agricultural country, China has produced more than 900 million tons of waste straw every year. Corn straw (CS) has great potential as a solid fuel. The high moisture content of straw (about 26%), high transport and storage costs (about $47.3/tonne), low energy density, high alkali and alkaline earth metal content, and environmental pollution caused by direct combustion limit the use of straw as a solid fuel [4]. Research on the use of HTC to convert waste biomass into solid fuels, land sorbents, and other applications has received much attention in recent years [6,7]. HTC has a unique advantage over treatments such as pyrolysis as it can be carried out without drying the feedstock, saving energy, producing a more cohesive carbon structure and being less harmful to the environment [8,9]
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