Anders Lyngfelt

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

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.

The use of iron oxide as an oxygen carrier in chemical-looping combustion of methane with inherent separation of CO2

Abstract Chemical-looping combustion (CLC) has been suggested as an energy efficient method for capture of carbon dioxide from combustion. The technique involves the use of a metal oxide as an oxygen carrier which transfers oxygen from the combustion air to the fuel, and the direct contact between fuel and combustion air is avoided. Thus, the products of combustion, i.e. carbon dioxide and water, are kept separate from the rest of the flue gas. After condensation of the water almost pure CO 2 is obtained, without any energy lost for the separation. In this paper, the feasibility of using Fe 2 O 3 as an oxygen carrier has been investigated in a fixed bed quartz reactor. Iron oxide was exposed to repeated cycles of air and methane at 950 ° C, with the outlet gas concentrations measured. The time under reducing conditions and the amount of bed material were varied in a wide range. The reduction rate of Fe 2 O 3 to Fe (d X /d t ) with 100% methane was between 1–8%/min and was a function of both the conversion range of the solid material, Δ X , as well as the yield of methane to carbon dioxide, γ red . The rate of oxidation was also a function of Δ X and the gas conversion, γ ox , and was considerably faster than the reduction, with rates up to 90%/min. The parameters d X /d t , Δ X , and γ are closely related and can be used to establish design criteria of a CLC system based on interconnected fluidized beds. The rates of both reduction and oxidation found should be sufficient to be employed in a CLC system based on two interconnected fluidized beds.

Solid fuels in chemical-looping combustion

The feasibility of using a number of different solid fuels in chemical-looping combustion (CLC) has been investigated. A laboratory fluidized bed reactor system for solid fuel, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions, was used. In each reducing phase 0.2 g of fuel in the size range 180–250 μm was added to the reactor containing 40 g oxygen carrier of size 125–180 μm. Two different oxygen carriers were tested, a synthetic particle of 60% active material of Fe2O3 and 40% MgAl2O4 and a particle consisting of the natural mineral ilmenite. Effect of steam content in the fluidizing gas of the reactor was investigated as well as effect of temperature. A number of experiments were also made to investigate the rate of conversion of the different fuels in a CLC system. A high dependency on steam content in the fluidizing gas as well as temperature was shown. The fraction of volatiles in the fuel was also found to be important. Furthermore the presence of an oxygen carrier was shown to enhance the conversion rate of the intermediate gasification reaction. At 950 °C and with 50% steam the time needed to achieve 95% conversion of fuel particles with a diameter of 0.125–0.18 mm ranged between 4 and 15 min depending on the fuel, while 80% conversion was reached within 2–10 min. In almost all cases the synthetic Fe2O3 particle with 40% MgAl2O4 and the mineral ilmenite showed similar results with the different fuels.

Construction and 100 h of operational experience of a 10-kW chemical looping combustor

Publisher Summary Chemical-looping combustion (CLC) is a new technology for burning gaseous fuels, with inherent separation of CO2. Metal oxide particles are used for the transfer of oxygen from the combustion air to the fuel, thus the combustion products CO2 and H2O are obtained in a separate stream. This chapter presents a 10-kW prototype for CLC that is run with nickel-based oxygen-carrier particles. A total operation time of more than 100 h is accomplished with the same batch of particles that is without adding fresh, unused material. A high conversion of the fuel is reached, with approximately 0.5% CO, 1% H2 and 0.1% methane in the exit stream, corresponding to a fuel conversion efficiency of 99.5% based on fuel heating value. There is no detectable leakage between the two reactor systems. 100% of the CO2 is captured in the process. No decrease in reactivity or particle strength is seen during the test period. The loss of fines is small and decreased continuously during the test period.

The grace project: Development of oxygen carrier particles for chemical-looping combustion. Design and operation of a 10 kW chemical-looping combustor

Publisher Summary A comprehensive research program was undertaken to develop chemical-looping combustion technology. First, a large number of possible oxygen-carrier materials were produced and tested with the best ones selected for further testing and development. Second, fluidization conditions and recirculation flows were studied in cold-flow models indicating the feasibility of both the full-scale design and a small 10 kW prototype unit. Thirdly, a 10 kW prototype unit was constructed and operated, demonstrating both this new combustion process as well as the durability of the oxygen-carrier particles under realistic process conditions. Lastly, a preliminary costing of a 200 MWth chemical-looping boiler unit for refinery gas combustion has been performed, indicating that chemical-looping combustion (CLC), should feature strongly among the best options for reducing the cost of CO2 capture.

SO2 capture fluidised-bed boilers: re-emission of SO2 due to reduction of CaSO4

Abstract Sulphur capture by lime was studied in a 16-MW fluidised-bed boiler. The desulphurization process, i.e. the addition of limestone which is calcined and sulphated, results in an accumulation of CaSO 4 in the bed. The CaSO 4 is shown to be decomposed to CaO with subsequent release of SO 2 at temperatures above 880–890°C, and at an excess air ratio of 1.4. At 930°C the amount of sulphur leaving the boiler as SO 2 was more than double the amount of sulphur added to the boiler in the form of fuel sulphur. The decomposition of CaSO 4 can be explained by the reaction of CaSO 4 with combustion intermediate such as CO and H 2 in the dense (particle) phase of the bed. The results explain the decrease in sulphur capture performance with increased temperature observed in fluidised-bed boilers and indicate the important effect or reducing conditions. Increased understanding of these phenomena may provide solutions, with respect to boiler design and operation, that make the use of limestone for desulphurization more efficient.

Gas leakage measurements in a cold model of an interconnected fluidized bed for chemical-looping combustion

Abstract In chemical-looping combustion (CLC) a gaseous fuel is burnt with inherent separation of the greenhouse gas carbon dioxide. The oxygen is transported from the combustion air to the fuel by means of metal oxide particles acting as oxygen carriers. A CLC system can be designed similar to a circulating fluidized bed, but with the addition of a bubbling fluidized bed on the return side. Thus, the system consists of a riser (fast fluidized bed) acting as the air reactor. This is connected to a cyclone, where the particles and the gas from the air reactor are separated. The particles fall down into a second fluidized bed, the fuel reactor, and are via a fluidized pot-seal transported back into the riser. The gas leaving the air reactor consists of nitrogen and unreacted oxygen, while the reaction products, carbon dioxide and water, come out from the fuel reactor. The water can easily be condensed and removed, and the remaining carbon dioxide can be liquefied for subsequent sequestration. The gas leakage between the reactors must be minimized to prevent the carbon dioxide from being diluted with nitrogen, or to prevent carbon dioxide from leaking to the air reactor decreasing the efficiency of carbon dioxide capture. In this system, the possible gas leakages are: (i) from the fuel reactor to the cyclone and to the pot-seal, (ii) from the cyclone down to the fuel reactor, (iii) from the pot-seal to the fuel reactor. These gas leakages were investigated in a scaled cold model. A typical leakage from the fuel reactor was 2%, i.e. a CO 2 capture efficiency of 98%. No leakage was detected from the cyclone to the fuel reactor. Thus, all product gas from the air reactor leaves the system from the cyclone. A typical leakage from the pot-seal into the fuel reactor was 6%, which corresponds to 0.3% of the total air added to the system, and would give a dilution of the CO 2 produced by approximately 6% air. However, this gas leakage can be avoided by using steam, instead of air, to fluidize the whole, or part of, the pot-seal. The disadvantages of diluting the CO 2 are likely to motivate the use of steam.

Ash behaviour in a CFB boiler during combustion of coal, peat or wood

This paper presents selected results from an extensive on-site measurement campaign where the ash behaviour in a 12 MW CFB boiler was studied during firing of coal, peat and wood. Samples were taken from all in-going (bed material, fuel) and out-going solid material streams (secondary cyclone and bag filter) as well as from the bed and the return leg. Deposit samples were further collected from the cyclone inlet and from two different locations in the convective path. In addition, the boiler operation was monitored, including collection of operational data, flue gas temperature profiles and emissions. The paper discusses the differences in the ash chemistry that were detected between the three different combustion cases and draws conclusions on the impact of the chemistry on the bed agglomeration and fouling tendency for each fuel.

SO2 capture and N2O reduction in a circulating fluidized-bed boiler: influence of temperature and air staging

Abstract The effects of air staging and bed temperature on sulfur capture performance in a circulating fluidized-bed boiler fired with bituminous coal were studied. Normal air staging, no staging and intensified staging were compared at normal (850 °C) and high (930 °C) temperatures. The results clearly show that high temperatures and staged combustion are detrimental to sulfur capture performance. Sulfur capture was reduced by more than half with intensified staging at 850 °C. At the high temperature, sulfur capture was still possible without staging, but at greatly reduced efficiency. In the two cases with staging, the sulfur capture was negative in the period 2–3 h after a temperature increase, as a result of reductive decomposition of calcium sulfate. The temperature-dependence without staging indicates that reducing conditions affect sulfur capture in this case as well. The increase in temperature from 850 to 930 °C halved N2O emissions, from ~ 100 to 50 ppm (at 6% O2).

Sulphur capture in fluidized bed boilers: The effect of reductive decomposition of CaSO4

Abstract Sulphur capture by lime was studied in a 16 MW stationary fluidized bed boiler (FBB). A marked fall-off in sulphur capture was noted at temperatures above about 880 °C. The proposed explanation is that the combustion produces reducing conditions in the particle phase, and thus allows for a reductive decomposition of CaSO 4 . This explanation is supported by (i) thermodynamics showing the instability of CaSO 4 under reducing conditions; (ii) in-bed oxygen measurements indicating reducing conditions in the particle phase; (iii) the observed fall-off in sulphur capture with temperature, which is not seen in laboratory tests under oxidizing conditions or in a circulating FBB, where the sorbent particles experience oxidizing conditions to a greater extent; (iv) the observation that the temperature dependence of the sulphur emission is very strong even when the net sulphur capture is zero provided always that CaSO 4 is present and (v) literature data indicating the rate of the proposed reaction.