A review on gasification of municipal solid waste (MSW): Syngas production, tar formation, mineral transformation and industrial challenges

Abstract

This review provides an overview of the research progress related to syngas quality, tar formation, and minerals transformation. In addition, this paper gives a detailed description of the current technology under construction and commercial applications. On this basis, the challenges and prospects for the commercial operation are outlined. In general, different gasification agents have critical temperatures and attention should be paid to the temperature window of 700–850 °C, between which free energy of Boudouard reaction is lower than that of water-gas reaction. Above the critical temperature, the reverse water-gas shift reaction will be promoted to inhibit H2 release. Oxidant/municipal solid waste (MSW) ratio has an optimal value, which is especially obvious for air atmosphere. The optimal equivalence ratio is usually in the range of 0.1–0.4, and the LHV of the synthetic gas is in 4–10 MJ/m3. Steam to MSW ratio usually varies in a wide range and excess steam may have little impact on syngas quality. Simulation evaluation of parameter weight shows steam to MSW ratio in steam atmosphere is the most important influencing factor, while temperature in air atmosphere plays a decisive role in the conversion of syngas. High temperature in the air atmosphere and low temperature in the steam atmosphere are the best choice for MSW gasification. The existence form of tar in different atmospheres is also quite different, such as PAHs in CO2 atmosphere and chain hydrocarbons in steam atmosphere. The development of efficient catalyst gradually becomes a technical bottleneck. Non-volatile minerals usually retain in bottom slag during MSW gasification. Elements with high boiling points are existed in the form of metal, while heavy metals with low boiling points are captured in the fly ash. The steam atmosphere can promote the volatilization of heavy metals. Most of the toxic heavy metals enter into the fly ash, so the leaching ability of the bottom slag is significantly weakened. Apart from technical bottlenecks such as unstable syngas quality, catalyst deactivation, fouling and slagging, and MSW treatment capacity, great effort should be given to lower its investment cost and operation costs such as syngas purification and reforming. At the same time, energy price regulatory and policy barriers that affect the market should be addressed.