Chemical-looping combustion in a 300 W continuously operating reactor system using a manganese-based oxygen carrier

Abstract

Chemical-looping combustion (CLC) is a method for the combustion of fuel gas with inherent separation of carbon dioxide. This technique involves the use of two interconnected reactors. A solid oxygen carrier reacts with the oxygen in air in the air reactor and is then transferred to the fuel reactor, where the fuel gas is oxidized to carbon dioxide and water by the oxygen carrier. Fuel gas and air are never mixed and pure CO₂ can easily be obtained from the flue gas exit. The oxygen carrier is recycled between both reactors in a regenerative process. This paper presents the results from a continuously operating laboratory CLC unit, consisting of two interconnected fluidized beds. The feasibility of the use of a manganese-based oxygen carrier supported on magnesium stabilized zirconia was tested in this work. Natural gas or syngas was used as fuel in the fuel reactor. Fuel flow and air flow was varied, the thermal power was between 100 and 300 W, and the air ratio was between 1.1 and 5.0. Tests were performed at four temperatures: 1073, 1123, 1173 and 1223 K. The prototype was successfully operated at all conditions with no signs of agglomeration or deactivation of the oxygen carrier. The same particles were used during 70 h of combustion and the mass loss was 0.038% per hour, although the main quantity was lost in the first hour of operation. In the combustion tests with natural gas, methane was detected in the exit flue gases, while CO and H₂ were maintained at low concentrations. Higher temperature or lower fuel flows increases the combustion efficiency, which ranged from 0.88 to 0.99. On the other hand, the combustion of syngas was complete for all experimental conditions, with no CO or H₂ present in the gas from the fuel reactor.

Introduction

Carbon dioxide emissions derived from human activities have increased the concentration of greenhouse gases in the atmosphere, contributing to global climate change. While the effects on global climate are uncertain, many scientists agree that there could be serious environmental consequences [1]. About a third of the global CO₂ emissions comes from the burning of fossil fuels in power production. One option to reduce the emissions of CO₂ and still use fossil fuels is the possibility to capture and store CO₂ from fossil fuel combustion. Chemical-looping combustion (CLC) has been suggested as an energetically efficient method for the capture of carbon dioxide from fuel gas.

CLC is a combustion technology with inherent separation of carbon dioxide. A chemical-looping combustion system consists of two reactors, a fuel and an air reactor, and an oxygen-carrier transferring the oxygen from the air to the fuel, see Fig. 1. This means that the combustion air and the fuel are never mixed, and the flue gas from the fuel reactor consists of carbon dioxide and water, and is not diluted with nitrogen. The water can be condensed and the remaining carbon dioxide compressed for storage.

The fuel could be syngas from coal gasification or natural gas. The main reaction in the fuel reactor is:

$$(2n+m){\text{Me}}_x{\text{O}}_y+{\text{C}}_n{\text{H}}_{2m}\to (2n+m){\text{Me}}_x{\text{O}}_{y-1}+n{\text{CO}}_2+m{\text{H}}_2\text{O}$$

Me_xO_y denotes a metal oxide and Me_xO_y−1 the reduced compound.

The oxygen-carrier is transported to the air reactor where the oxygen-carrier is oxidized by the oxygen in the incoming air:

$${\text{Me}}_x{\text{O}}_{y-1}+\frac{1}{2}{\text{O}}_2\to {\text{Me}}_x{\text{O}}_y$$

The oxygen-carrier is returned to the fuel reactor for a new cycle. The oxidation of the oxygen-carrier is exothermic, while the reaction in the fuel reactor could be either endothermic or exothermic, depending on the oxygen-carrier and the fuel.

There are some important criteria for the selection of oxygen-carrier particles. First, the thermodynamic equilibrium for the reaction with the fuel has to be favourable in order to achieve high fuel conversion to CO₂ and H₂O. Metal oxide systems which are feasible for use as oxygen carriers in CLC are Mn₃O₄/MnO, Fe₂O₃/Fe₃O₄, NiO/Ni, CuO/Cu and CoO/Co [2], [3], [4]. Also the rate of oxidation and reduction has to be sufficiently fast. Otherwise the amount of oxygen-carrier needed in the reactors would be too large. Moreover the oxygen transfer capacity needs to be sufficient. This is to avoid the circulation of particles between the reactors being very large. Since the proposed reactor system consists of fluidized beds, the particles need to have a low tendency for fragmentation and attrition. It is also vital that they do not agglomerate under real reaction conditions. The possibility of deactivation of the oxygen carriers is another factor to be analyzed. For example, sulphur compounds in the fuel gases could react with the oxygen carrier resulting in deactivation of the particles because of the formation of metal sulphides or sulphates. Jerndal et al. [4] made a thermodynamic analysis for the reaction of sulphur species in the fuel with several metals used in CLC. They concluded that sulphur should not be a problem with Mn-, Cu- and Fe-based oxygen carriers. However, with Ni- and Co-based oxygen carriers there are risks for formation of sulphides at conditions which may be encountered in the fuel reactor. There are also other aspects than chemical and physical, such as the cost of the oxygen-carrier, and environmental and health aspects. With respect to the last, the use of Ni-based oxygen carriers may require safety measures because of its toxicity.

In general, particles tested have active oxides of Fe, Ni, Cu or Mn as active part combined with some type of inert carriers, such as Al₂O₃, ZrO₂, TiO₂ or MgO. This inert material acts as a porous support, providing a higher surface area for reaction, and may also act as a binder to increase the mechanical strength and improve the resistance to attrition. Different oxygen-carrier particles have been tested, and a large number of publications related to the development of oxygen-carrier particles have been issued by different research groups at Tokyo Institute of Technology [5], [6], Chalmers University of Technology [7], [8], CSIC-ICB in Zaragoza [9], [10], Korea Institute of Energy Research [11], TDA Inc. [12], National Institute for Resources and Environment in Japan [13] and Politecnico di Milano [14]. An overview of the work which has been performed on different oxygen-carriers can be found in theses by Cho [15] and Brandvoll [16].

In general, the reactivity of potential oxygen-carrier particles has been investigated in laboratory reactors in a batch-wise fashion, for instance in thermogravimetric analysers (TGA) and laboratory fixed-bed or fluidized-bed reactors. In those tests the particles are exposed alternately to air and fuel. However, it is only possible to simulate a chemical-looping system to a certain extent. In order to gain a more adequate understanding of the behaviour and usefulness of the particles in this process, tests are needed in a more real system where the particles are continuously circulated between an air and a fuel reactor. Ishida et al. [6] investigated the hydrogen combustion with a Ni-based oxygen carrier in an experimental system with circulation of solid particles. Preliminary designs of full-scale systems have been proposed [17], [18], [19]. Furthermore, a 10 kW chemical-looping combustor prototype has been built and successfully demonstrated [20], [21]. This system was designed for the use of fuel gas of high CH₄ content, like refinery gas or natural gas. Therefore, the fuel reactor was made with an increasing cross-section to accommodate for the volume expansion when one mole of CH₄ reacts with one mol of CO₂ and two moles of H₂O. The prototype has been used both to test oxygen-carriers and to show a possible design of an industrial chemical-looping combustor. However, because of the efforts and costs associated with this system, e.g. the large amount of oxygen-carriers needed, it is not feasible to test a large number of different oxygen-carriers in this type of reactor. Thus, there is a need for a laboratory size continuously operated reactor system where different types of oxygen-carriers can be tested in a way which closely simulates conditions of a CLC. Such a unit, i.e. a 300 W laboratory chemical-looping combustor has been designed [22] and constructed [23]. This unit has here been used to test the feasibility of using a manganese based oxygen carrier.

From a thermodynamic analysis, manganese-based oxygen carriers are feasible for use as oxygen carriers in a CLC [2], [3], [4]. However, only limited work has been reported for the system Mn₃O₄/MnO. A manganese oxygen carrier supported on alumina was tested by Mattisson et al. [24]. This oxygen carrier reacts to low extent, because of the chemical reaction between the alumina and metal oxide to nonreactive compounds. Adánez et al. [9] found that when zirconia was used as inert the utilization of manganese was complete and reaction times less than 1 min were obtained for complete conversion of the metal oxide at 1223 K using methane as fuel. In the same study, Al₂O₃, SiO₂, TiO₂ or sepiolite were found to be unsuitable as inerts together with manganese oxide. A new Mn-based oxygen carrier supported on magnesium-stabilized zirconia has been tested in a laboratory fluidized bed reactor [15], [25]. This oxygen-carrier appears to have excellent reactivity and only limited tendency to agglomerate. To test the ability of this Mn-based oxygen carrier in a continuous chemical-looping reactor, combustion experiments were performed in the 300 W unit using both syngas and natural gas as fuel.