A fluidized-bed combustion process with inherent CO₂ separation; application of chemical-looping combustion

Abstract

For combustion with CO₂ capture, chemical-looping combustion has the advantage that no energy is lost for the separation of CO₂. In chemical-looping combustion oxygen is transferred from the combustion air to the gaseous fuel by means of an oxygen carrier. The fuel and the combustion air are never mixed, and the gases from the oxidation of the fuel, CO₂ and H₂O, leave the system as a separate stream. The H₂O can easily be removed by condensation and pure CO₂ is obtained without any loss of energy for separation. This makes chemical-looping combustion a most interesting alternative to other CO₂ separation schemes, which have the drawback of a large energy consumption. A design of a boiler with chemical-looping combustion is proposed. The system involves two interconnected fluidized beds, a high-velocity riser and a low-velocity bed. Metal oxide particles are used as oxygen carrier. The reactivities needed for oxygen carriers to be suitable for such a process are estimated and compared to available experimental data for particles of Fe₂O₃ and NiO. The data available on oxygen carriers, although limited, indicate that the process outlined should be feasible.

Introduction

It has been known for more than a 100 years that CO₂ is a greenhouse gas and that the release of CO₂ from fossil fuel combustion may affect the climate of the earth (Arrhenius, 1896). In the last years the concern over the effects of an increased release of greenhouse gases has been growing. The release of CO₂ from fossil fuel combustion is the most important of these emissions. In the developing countries, the economic growth results in a rapid increase in the demand for energy supplied by fossil fuels, while the developed countries have not yet found the means for substantially decreasing their use of these fuels. In the future it is not unlikely that radical measures to decrease CO₂ emissions will be demanded. Therefore, various options need to be investigated.

One strategy to decrease CO₂ emissions is to separate the CO₂ from the fuel gas and to store it. Several possibilities for such sequestration have been proposed:

Storage in used oil and gas fields: On an average, about twice as much CO₂ can be stored in depleted gas fields as the CO₂ obtained from burning the gas extracted from the field (Blok, Williams, Katofsky, & Hendriks, 1997). In the case of oil fields, the CO₂ can be used to extend the production, the so-called enhanced oil recovery.

Storage in deep coal beds: The CO₂ injected in a coal seam is absorbed in the coal and stored in its pore matrix, releasing methane trapped, which can be collected and sold (Anon, 1999). The process is called enhanced gas recovery.

Storage in aquifers: The potential storage capacity is large (Riemer, 1998). This possibility is already in use in Norway, where natural gas containing CO₂ is cleaned, with the resulting CO₂ injected into an aquifer 800–$\text{1000}\text{m}$ below the bottom of the sea (Lindeberg & Holloway, 1998). In fact, one million tons of CO₂ per year or 3% of Norway's total CO₂ emissions are disposed in this way (Herzog, Eliasson, & Kaarstad, 2000).

Deep sea storage: At pressures above approximately $\text{50}\text{bar}$, corresponding to a depth of $\text{500}\text{m}\text{,}\text{CO}{}_2$ becomes a liquid. The density of this liquid is somewhat lower than that of the surrounding seawater and in its pure form the liquid will rise slowly and ultimately reach the level where gas bubbles are formed. However, if proper measures are taken to allow the CO₂ liquid to mix with the seawater, the resulting mixture obtains a higher density than the seawater and instead sinks. The storage capacity is enormous, but local effects on the environment need to be investigated.

Deep sea bottom storage: At high pressures, i.e. $\text{3000}\text{m}$ below the sea level, the density of pure CO₂ is higher than that of seawater. Thus, CO₂ released at these depths will form “lakes” on the ocean floor. The potential storage capacity is vast considering the fact that the average depth of the sea is $\text{3700}\text{m}$ and that more than 50% of the earth's surface is found at a depth of more than $\text{3000}\text{m}$. However, local effects on the sea floor environment are obviously inevitable.

The estimated cost of CO₂ disposal, e.g. 4–8 US $/ton C (Riemer, 1998), is small compared to the costs for separation of CO₂, which is typically in the range 100–200 US $/ton C (Freund, 1998). Consequently, the major problem seems to be to extract the CO₂ from the fuel conversion process. The available or proposed technologies all have the disadvantage of consuming large amounts of energy. The major loss in efficiency arises from the energy needed for separation of CO₂ and for compression to liquid form. Typically, the energy needed for compression is about one-fourth of the total energy needed for separation and compression. For a coal-fired power plant, roughly one-fifth of the electricity produced will be lost for CO₂ separation and compression (Lyngfelt & Leckner, 1999). This decrease in efficiency alone increases the cost for electricity production with one-fourth, and in addition there are costs related to separation and handling of CO₂. If natural gas is used as a fuel, the relative loss in efficiency is somewhat smaller, but still substantial.

Today, power production contributes with one-third of the CO₂ released from fossil fuel combustion world-wide (Herzog et al., 2000). Although the cost for separation of CO₂ is substantial, power plants using fossil fuel and CO₂ capture may well be the least costly alternative for CO₂-free power production (Keith & Parson, 2000).

Chemical-looping combustion (CLC) offers a solution where no energy is needed for the separation. The process uses a solid oxygen carrier to transfer the oxygen from the air to the fuel. The oxygen carrier is recycled between a fuel reactor, where it is reduced by the fuel, and an air reactor, where it is oxidized by the air. Thus, the air is never mixed with the fuel, and the CO₂ does not become diluted by the nitrogen of the flue gas. The outgoing gas from the reduction step will contain water vapour and CO₂. The water vapour can easily be separated by condensation, and the CO₂ is delivered without an energy penalty for the separation. Energy is still needed, however, to compress CO₂ into a liquid suitable for sequestration.

The purpose of the present paper is to assess the potential of chemical-looping combustion — is it technically and economically realistic? In order to do this, a tentative design of the process is made. Aspects of importance for judging the realism of the process are discussed.