Copyrolysis of Biomass and Coal: A Review of Effects of Copyrolysis Parameters, Product Properties, and Synergistic Mechanisms

Abstract

Concerns in the last few decades regarding the environmental and socioeconomic impacts of the dependence on fossil fuels have resulted in calls for more renewable and alternative energy sources. This has led to recent interest in copyrolysis of biomass and coal. Numerous reviews have been found related to individual pyrolysis of coal and biomass. This review deals mainly with the copyrolysis of coal and biomass and then compares their results with those obtained using coal and biomass pyrolysis in detail. It is controversial whether there are synergistic or additive behaviours when coal and biomass are blended during copyrolysis. In this review, the effects of reaction parameters such as feedstock types, blending ratio, heating rate, temperature, and reactor types on the occurrence of synergy are discussed. Also, the main properties of the copyrolytic products are pointed out. Some possible synergistic mechanisms are also suggested. Additionally, several outlooks based on studies in the literature are also presented in this paper.

Effect of the Amount of Sludge on Physicochemical Properties and Chemical Structure of Low‐rank Coal under Hydrothermal Conditions

“…This result suggested that sludge was decomposed more easily than low‐rank coal during the HT process. The weak covalent bonds in sludge may be easily broken under the thermal hydrolysis condition, and some organic compounds and mineral salt are dissolved in the liquid products during the HT process . As shown in Fig.…”

“…Besides, the H/C ratio of HT solid products reduced quickly as the amount of sludge increased. Sludge linked mainly by the weak ether R–O–R bonds can be easily decomposed under the heat, but the cross‐linked structure of coal is mostly composed of stronger C=C bonds, which may be difficult to break during the HT process . Figure shows the gas yield under different HT conditions.…”

“…As the amount of SS increased after 50 wt%, more carbon‐containing gases such as CO 2 were available because of the HT of the increased volatile matter from SS. Thus, as SS had high ash content, some metals could act as catalyst for a water–gas shift reaction, which could increase the yield of H 2 and CO 2 . The experimental and theoretical values of gas yield are also shown in Fig.…”

“…For the HT of both low‐rank coal and sludge, sludge contains higher hydrogen content than coal, causing more free hydrogen groups around coal particles in the HT process. The hydrogen bonds are rearranged, and hydrogen transfer reactions occur between sludge and coal, which promotes the polycondensation and repolymerization in the HT process and produces synergistic positive effects . In addition, many macromolecules in the sludge break down into highly active small molecules during HT process.…”

“…found that synergistic effect promotes the further coalification of solid products during the HT of both low‐rank coal and biomass. Quan et al . found that there were synergistic positive effects on the fuel properties and air pollutant emissions in the thermal conversion process of both biomass and low‐rank coal because of the similar chemical molecular structure.…”