CO2 capture from direct combustion of solid fuels with Chemical-Looping Combustion
Conference contribution, 2008
Chemical-looping combustion (CLC) is a combustion technology where an oxygen carrier is used to transfer oxygen from the combustion air to the fuel, thus avoiding direct contact between air and fuel. Thus, the CO2 is inherently separated from the flue gases with a considerable lower energy penalty and cost compared to other techniques for CO2 separation.
The oxygen carrier is circulated between two reactors, a fuel and an air reactor, where the flue gas from the air reactor contains only N2 and some unreacted O2 and the flue gas from the fuel reactor contains only CO2 and H2O. The water can easily be condensed and the remaining CO2 can be transported for underground storage.
Most of the prior work with CLC has focused on using natural gas and syngas as fuel. However, recent work on adapting the CLC process for solid fuels shows promising results. Two main strategies for achieving this are: 1) using syngas from coal gasification in the fuel reactor and 2) introduction of the coal directly to the fuel reactor where the gasification of the coal and subsequent reactions with the metal oxide particles will occur simultaneously.
This paper will focus on this second route, and present results from reactivity investigations in a laboratory fluidized-bed reactor system of a number of different solid fuels, including three types of bituminous coal, petroleum coke, lignite and char from bio fuel. As oxygen carrier the previously investigated natural mineral ilmenite is used. The experiments were conducted at 970°C with 92% steam in the fluidizing gas. The reaction rates were considerably higher compared to investigations using lower steam fractions. The fraction of volatiles in the fuel was found to be important for the conversion rate of the fuel. Furthermore, the presence of an oxygen-carrier was shown to enhance the conversion rate of the intermediate gasification reaction compared with normal gasification performed without the presence of an oxygen carrier.
Chemical-looping combustion (CLC) has been introduced as a technique where the greenhouse gas CO2 is inherently separated during combustion. The CLC-process is composed of two fluidized bed reactors, an air and a fuel reactor. The fuel is introduced to the fuel reactor where it reacts with an oxygen carrier to CO2 and H2O, reaction (1). The reduced oxygen carrier is transported to the air reactor where it is oxidized back to its original state by air, reaction (2). In this paper, when oxidation and reduction is mentioned, it refers to oxidation and reduction of the oxygen carrier.