Dual fluidized bed gasification of biomass with selective carbon dioxide removal and limestone as bed material: A review

Dual fluidized bed steam gasification is a well-known technology for the thermochemical conversion of biogenic fuels into a valuable product gas comprising hydrogen, carbon monoxide, carbon dioxide, and methane. Based on the dual fluidized bed gasification principle, further benefits can be obtained by considering the bed material: Limestone allows the removal of carbon dioxide from the gasification reactor and its release in the combustion reactor. Therefore, the gasification and combustion reactors must be operated at 650 degrees C and 830 degrees C, respectively. Moreover, this sorption enhanced reforming process affords hydrogen contents of similar to 70 vol.-%(db) and allows an adjustment of the hydrogen-to-carbon monoxide ratio in the range of 2-10. The circulating bed material undergoes carbonation and calcination cycles, that lead to a decrease in the carbon dioxide sorption capacity with time. This paper discusses the main reasons for this decrease as well as the positive or negative effects of steam and carbon dioxide. The major disadvantage of using limestone in the fluidized beds is its low attrition resistance. Thus, gentle separation units for the limestone as bed material are recommended to overcome this problem. Several process configurations have been published by different institutions, mainly by IFK Stuttgart and TU Wien. The most important experimental and theoretical results, including the dependence of the product gas composition on the gasification temperature and the bed material cycle rate, tar content in the product gas, and test runs in the industrial scale are summarized and discussed. Sorption enhanced reforming of different fuel types, sorption enhanced reforming with oxyfuel combustion, and performance indicating key figures of the process are also investigated.