•A review of chemical-looping combustion of solid fuels has been made.•Technology is similar to combustion of coal in circulating fluidized-bed.•Unique potential for dramatic reduction in cost and ...energy penalty for CO2 capture.•Operational experience involves more than 500h in five units of sizes 0.5–100kW.•Low-cost oxygen carriers work well but performance can be improved further.
Chemical-looping combustion (CLC) of solid fuels is a technology with the potential of reducing the costs and energy penalty dramatically for CO2 capture. The potential for low costs is based on the similarity to coal combustion in fluidized beds. However, this assumes reaching high performance with respect to fuel and gas conversion, or that inadequate performance can be readily mitigated by downstream options. There are uncertainties with respect to the performance that can be reached in large-scale units, as well as with the extra costs needed to compensate for inadequate performance. Performance will be dependent on both reactor design and oxygen carrier properties. The status of chemical-looping combustion of solid fuels is discussed with respect to performance and experiences from pilot operation.
More than 2000 h of solid-fuel CLC operation in a number of smaller pilot units clearly indicate that the concept works. A scale-up of the technology to 1000 MWth is investigated in terms of mass and ...heat balances, flows, solids inventories, boiler dimensions and the major differences between a full-scale Circulating Fluidized-Bed (CFB) boiler and a Chemical-Looping Combustion CFB (CLC-CFB). Furthermore, the additional cost of CLC-CFB relative to CFB technology is analysed and found to be 20 (sic)/tonne CO2. The largest cost is made up of compression of CO2, which is common to all capture technologies. Although the need for oxygen to manage incomplete conversion is estimated to be only a tenth of that of oxy-fuel combustion, oxygen production is nonetheless the second largest cost. Other significant costs include oxygen-carrier material, increased boiler cost and steam for fluidization of the fuel reactor.
Because the CO2 capture is inherent in chemical looping combustion (CLC), thus ideally avoiding costly gas separation, this process has potential for uniquely low costs of CO2 capture. The review ...reports on operational experiences with different oxygen carriers in CLC pilot operation. Further, the application to solid fuels is discussed in terms of technology challenges, routes for upscaling to commercial size, downstream gas treatment, options for achieving adequate circulation, and the use of biofuels in CLC to reach negative emissions. It is concluded that the necessary elements for a scale-up are at hand. Oxygen carrier materials of low cost have been tested in extended operation and found to have reasonable performance with respect to reactivity and lifetime. Designs for large-scale units have been performed, indicating that the process is technically realistic and should have a low cost of CO2 capture. A scale-up strategy to minimize risk and costs has been suggested.
Chemical-Looping Combustion of solid fuels has been studied for ten years and significant progress has been made. The paper discusses operational experiences and various aspects of up-scaling, ...including similarities to fluidized-bed combustion, key challenges, cost structure and strategies for reducing costs for demonstration. Based on more than 9000h of CLC operation in 34 pilots, of which >3000h with solid fuels, it is concluded that there are oxygen carrier materials suitable for solid fuels, and that the technology should be ready for scale-up.
Chemical-looping combustion, CLC, is a combustion concept with inherent separation of CO
2. The fuel and combustion air are kept apart by using an oxygen carrier consisting of metal oxide. The oxygen ...carriers used in this study were prepared from commercially available raw materials by spray-drying. The aim of the study was to subject the particles to long-term operation (>1000
h) with fuel and study changes in particles, with respect to reactivity and physical characteristics. The experiments were carried out in a 10-kW chemical-looping combustor operating with natural gas as fuel. 1016
h of fuel operation were achieved. The first 405
h were accomplished using a single batch of NiO/NiAl
2O
4-particles. The last 611
h were achieved using a 50/50
mass-mixture of (i) particles used for 405
h, and (ii) a second batch of particles similar in composition to the first batch, but with an MgO additive. Thus, at the conclusion of the test series, approximately half of the particles in the reactor system had been subjected to >1000
h of chemical-looping combustion. The reason for mixing the two batches was to improve the fuel conversion. Fuel conversion was better with the mixture of the two oxygen carriers than it was using only the batch of NiO/NiAl
2O
4-particles. The CO fraction was slightly above the equilibrium fraction at all temperatures. Using the oxygen carrier mixture, the methane fraction was typically 0.4–1% and the combustion efficiency was around 98%. The loss of fines decreased slowly throughout the test period, although the largest decrease was seen during the first 100
h. An estimated particle lifetime of 33 000 h was calculated from the loss of fines. No decrease in reactivity was seen during the test period.
Chemical-looping combustion is a novel technique used for CO
2 separation that previously has been demonstrated for gaseous fuel. This work demonstrates the feasibility of using solid fuel (petroleum ...coke) in chemical-looping combustion (CLC). Here, the reaction between the oxygen carrier and solid fuel occurs via the gasification intermediates, primarily CO and H
2. A laboratory fluidized-bed reactor system for solid fuel, simulating a CLC-system by exposing oxygen-carrying particles to alternating reducing and oxidizing conditions, has been developed. In each reducing period, 0.2
g of petroleum coke was added to 20
g of oxygen carrier composed of 60% active material of Fe
2O
3 and 40% inert MgAl
2O
4. The effect of steam and SO
2 concentration in the fluidizing gas was investigated as well as effect of temperature. The rate of reaction was found to be highly dependent on the steam and SO
2 concentration as well as the temperature. Also shown was that the presence of a metal oxide enhances the gasification of petroleum coke. A preliminary estimation of the oxygen carrier inventory needed in a real CLC system showed that it would be below 2000
kg/MW
th.
This work presents the design and experimental evaluation of a cold-flow model, built to simulate a 100kW chemical-looping combustor for solid fuel. A theoretical background is provided, as well as ...some initial results using air as fluidization medium. The cold-flow model has been operated for about 10h and shows no indication of imbalances in the bed inventories. In the fuel and air reactors, the mass fluxes were found to be linear in the riser pressure drop and the corresponding measured mass flows were approximately proportional to the mass flows calculated from the riser pressure drop. From the study of mass flows, residence times in both the fuel and air reactor were obtained. From pressure profile investigations, it was found that the system remained stable to changes in the fluidization velocity. Thus, both the internal circulation in the fuel reactor, and the circulation between air and fuel reactor, could be varied in a large range with only minor impact on the solids inventories of the air and fuel reactors.
The system is a 3m high, 58% scale, cold-flow model of a 100kW unit. It is made from acrylic glass, with eight inlets for fluidization of small sand particles. When fluidizing, the sand particles circulate in the system, simulating the chemical-looping combustion process of the 100kW unit. Display omitted
► The mass fluxes in the fuel and air reactors are linear in the riser pressure drop. ► The mass flows in the fuel and air reactors are close to linear in the riser flow. ► The residence time in the fuel reactor lie between 2 and 11min. ► The residence time in the air reactor lie between 3 and 21min. ► The solids inventory is stable to changes in the fluidization velocity.
Chemical-looping combustion (CLC) is a combustion process with inherent separation of carbon dioxide (CO
2
), which is achieved by oxidizing the fuel with a solid oxygen carrier rather than with air. ...As fuel and combustion air are never mixed, no gas separation is necessary and, consequently, there is no direct cost or energy penalty for the separation of gases. The most common form of design of chemical-looping combustion systems uses circulating fluidized beds, which is an established and widely spread technology. Experiments were conducted in two different laboratory-scale CLC reactors with continuous fuel feeding and nominal fuel inputs of 300 W
th
and 10 kW
th
, respectively. As an oxygen carrier material, ground steel converter slag from the Linz–Donawitz process was used. This material is the second largest flow in an integrated steel mill and it is available in huge quantities, for which there is currently limited demand. Steel converter slag consists mainly of oxides of calcium (Ca), magnesium (Mg), iron (Fe), silicon (Si), and manganese (Mn). In the 300 W unit, chemical-looping combustion experiments were conducted with model fuels syngas (50 vol% hydrogen (H
2
) in carbon monoxide (CO)) and methane (CH
4
) at varied reactor temperature, fuel input, and oxygen-carrier circulation. Further, the ability of the oxygen-carrier material to release oxygen to the gas phase was investigated. In the 10 kW unit, the fuels used for combustion tests were steam-exploded pellets and wood char. The purpose of these experiments was to study more realistic biomass fuels and to assess the lifetime of the slag when employed as oxygen carrier. In addition, chemical-looping gasification was investigated in the 10 kW unit using both steam-exploded pellets and regular wood pellets as fuels. In the 300 W unit, up to 99.9% of syngas conversion was achieved at 280 kg/MW
th
and 900 °C, while the highest conversion achieved with methane was 60% at 280 kg/MW
th
and 950 °C. The material’s ability to release oxygen to the gas phase, i.e., CLOU property, was developed during the initial hours with fuel operation and the activated material released 1–2 vol% of O
2
into a flow of argon between 850 and 950 °C. The material’s initial low density decreased somewhat during CLC operation. In the 10 kW, CO
2
yields of 75–82% were achieved with all three fuels tested in CLC conditions, while carbon leakage was very low in most cases, i.e., below 1%. With wood char as fuel, at a fuel input of 1.8 kW
th
, a CO
2
yield of 92% could be achieved. The carbon fraction of C
2
-species was usually below 2.5% and no C
3
-species were detected. During chemical-looping gasification investigation a raw gas was produced that contained mostly H
2
. The oxygen carrier lifetime was estimated to be about 110–170 h. However, due to its high availability and potentially low cost, this type of slag could be suitable for large-scale operation. The study also includes a discussion on the potential advantages of this technology over other technologies available for Bio-Energy Carbon Capture and Storage, BECCS. Furthermore, the paper calls for the use of adequate policy instruments to foster the development of this kind of technologies, with great potential for cost reduction but presently without commercial application because of lack of incentives.
For combustion with CO
2 capture, chemical-looping combustion (CLC) with inherent separation of CO
2 is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel ...reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g. metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. The feasibility of using oxygen carrier based on oxides of iron, nickel, copper and manganese was investigated. Oxygen carrier particles were produced by freeze granulation. They were sintered at 1300 °C for 4 h and sieved to a size range of 125–180 μm. The reactivity of the oxygen carriers was evaluated in a laboratory fluidized bed reactor, simulating a CLC system by exposing the sample to alternating reducing and oxidizing conditions at 950 °C for all carriers except copper, which was tested at 850 °C. Oxygen carriers based on nickel, copper and iron showed high reactivity, enough to be feasible for a suggested CLC system. However, copper oxide particles agglomerated and may not be suitable as an oxygen carrier. Samples of the iron oxide with aluminium oxide showed signs of agglomeration. Nickel oxide showed the highest reduction rate, but displayed limited strength. The reactivity indicates a needed bed mass in the fuel reactor of about 80–330 kg/MW
th and a needed recirculation flow of oxygen carrier of 4–8 kg/s, MW
th.
A screening of Fe- and Mn-based ores and industrial products was made in order to identify suitable low-cost materials that could be used as oxygen carriers in chemical-looping combustion (CLC). A ...laboratory fluidized bed reactor system, simulating chemical-looping combustion by exposing the sample to alternating reducing and oxidizing conditions, was used. Fifteen grams of each material with a particle size of 125−180 μm was exposed to a flow of 450 mLn/min of either methane or syngas (50% CO, 50% H2) during reduction. During the oxidizing phase to a flow of 1000 mLn/min, 5% O2 in nitrogen was used. All materials had a high reactivity with syngas. Some materials such as the Mn-based Colormax and the Fe-based Glödskal had also a high reactivity with methane making them possible candidates for CLC with gaseous fuel. Some of the materials, especially the Mn-based ones, showed poor mechanical stability and poor fluidizing properties. Roughly half of the Fe-based materials, but only one of the Mn-based materials, had properties that could make them suitable as oxygen carriers in a CLC system for solid fuels.
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