Thermogravimetric characteristics of corn straw and bituminous coal co-pyrolysis based the ilmenite oxygen carriers

Abstract

Herein, the co-pyrolysis reaction characteristics of corn straw (CS) and bituminous coal in the presence of ilmenite oxygen carriers (OCs) are investigated via thermogravimetry coupled with mass spectrometry. The results reveal that the participation of OCs weakens the devolatilization intensity of co-pyrolysis. When the CS blending ratio is <50%, the mixed fuel exhibits positive synergistic effects. The fitting results according to the Coats-Redfern integral method show that the solid–solid interaction between OCs and coke changes the reaction kinetics, enhancing the co-pyrolysis reactivity at the high-temperature zone (750–950 °C). The synergistic effect is most prominent at a 30% CS blending ratio, with co-pyrolysis activation energy in the range of 26.35–40.57 kJ·mol⁻¹.

1. Introduction

Carbon dioxide (CO2) emissions from coal and other fossil fuels majorly contribute to global warming. Efficient and clean conversion of energy depends on the development of green energy and innovation in process design. Biomass, as a potential gasification feedstock, is a renewable carbon source with the highest potential to replace fossil fuels [1]. The advantages of biomass, such as high alkali metal content, high char reactivity, and low pollutant emission, are favorable for increasing the reaction intensity and product yield of thermochemical conversion processes [2]. Unfortunately, the associated low volume density, high transport costs, and seasonal distribution limit the sustainable and stable supply of biomass energy [3]. Co-pyrolysis of coal and biomass can be directly performed to produce solid, liquid, and gas three-phase products, and it is an effective way to improve the quality of energy utilization [4]. Co-pyrolysis is advantageous for improving the conversion mode of coal, increasing carbon conversion efficiency, and reducing environmental pollution. In addition, it helps achieve resource utilization of biomass and sustainable utilization of coal while alleviating the current situation of the energy supply crisis.

Currently, the conventional technology of the co-pyrolysis of coal and biomass has been widely studied. Li et al. [5] investigated the co-pyrolysis behavior of Shenfu coal and rice straw via thermogravimetry coupled with mass spectrometry (TG-MS). Their results indicated that products released during the co-pyrolysis of some fuels were similar to those released during the individual pyrolysis of the fuels. Vuthaluru [6] explored the co-pyrolysis characteristics of bituminous coal (BC) and biomass (wood chips and wheat straw) via TG. The optimal mixing ratio of coal and biomass was reported as 5:5; however, no interaction was observed between the fuels. Chen et al. [7] analyzed the kinetic reaction behavior of the co-pyrolysis of coal and walnut shells using the distributed activation energy model. The results revealed some interaction between coal and biomass. Krerkkaiwan et al. [8] studied the co-pyrolysis characteristics of BC and rice straw in a fixed-bed reactor. The results showed a synergistic effect between coal and biomass at a mass ratio of 1:1, exhibiting high gas production and low tar yield. Huang et al. [9] investigated the co-pyrolysis of BC and biomass (wheat straw, walnut shells, and wood chips) in a high-pressure fluidized bed. The results revealed a synergistic effect at a coal-to-biomass mixture ratio of 7:3, with low tar and coke yields. Overall, the differences in co-pyrolysis synergistic effects may be attributed to various factors, such as feedstock types, reactor types, and experimental parameters (such as heating rate, temperature, and biomass blending ratio). Notably, the co-pyrolysis of coal and biomass has a positive role in increasing gas yield and reducing tar yield.