MSW pyro-gasification using high-temperature CO2 as gasifying agent: Influence of contact mode between CO2, char and volatiles on final products

Abstract

The volatiles and char derived from municipal solid waste (MSW) pyrolysis can be catalytically reformed and gasified using high-temperature CO2 (HT-CO2) as gasifying agent and char as a catalyst simultaneously to obtain high quality synthesis gas, but the reactor's design for this purpose is still a question. In this research, the contact configuration between the HT-CO2, the volatile compounds, and the char from MSW pyrolysis were studied to understand the relevant reaction behaviors and to establish guidelines for the reactor’s design. Three contact modes were designed, including: M1, where volatiles and HT-CO2 contact first, then contact the char; M2, where volatiles, CO2, and char contact simultaneously at the bottom of the char layer; and M3, where CO2 contacts with the char first, then the volatiles contact in the middle of the char layer. The temperature evolution in the char layer, the yields and properties of the resultant combustible gases, used char, and tar were investigated. Experimental results revealed that the contact mode significantly affected the levels of char gasification and volatiles' reforming. For M1, intense thermal cracking of volatiles occurred and 65.41% of the input heat of HT-CO2 was consumed for thermal cracking, resulting in substantial carbon deposition and limited energy transfer from char to the synthesis gas. While, the char contacting HT-CO2 firstly in M3 improved its catalytic activity, causing 73.33 % of the input heat utilized for gasification and reforming; as a result, the maximum synthesis gas yield of 0.71 Nm3/kgMSW and gas energy ratio of 76.3 % were obtained respectively in M3 with the lowest tar yield of 5.45 %; additionally, the used char corresponded to the highest specific surface area of 10.12 m2/g. Ultimately, M3 is constructive and recommended, and the findings of this study offer helpful guidance for the design of pyro-gasification reactors.