Post-combustion Calcium Looping Research Articles

Impact of fuel selection on the environmental performance of post-combustion calcium looping applied to a cement plant

Calcium looping CO2 capture is a promising technology to reduce CO2 emissions from cement production. Coal has been seen as a logical choice of fuel to drive the calcium looping process as coal is already the primary fuel used to produce cement. This study assesses the impact of using different fuels, namely coal, natural gas, woody biomass and a fuel mix (50% coal, 25% biomass and 25% animal meal), on the environmental performance of tail-end calcium looping applied to the clinker production at a cement plant in North-western Europe. Process modelling was applied to determine the impact of the different fuels on the mass and energy balance of the process which were subsequently used to carry out a life cycle assessment to evaluate the environmental performance of the different systems. Using natural gas, biomass or a fuel mix instead of coal in a tail-end calcium looping process can improve the efficiency of the process, as it decreases fuel, limestone and electricity consumption. Consequently, while coal-fired calcium looping can reduce the global warming potential (life cycle CO2 emissions) of clinker production by 75%, the use of natural gas further decreases these emissions (reduction of 86%) and biomass use could results in an almost carbon neutral (reduction of 95% in the fuel mix case) or net negative process (−104% reduction in the biomass case). Furthermore, replacing coal with natural gas or biomass reduces most other environmental impact categories as well, mostly due to avoided impacts from coal production. The level of improvement strongly depends on whether spent sorbent can be utilized in clinker production, and to what extent sequestered biogenic CO2 can reduce global warming potential. Overall, the results illustrate the potential of using alternative fuels to improve the environmental performance of tail-end calcium looping in the cement industry.

Impact of Fuel Selection on Techno-environmental Performance of Post-combustion Calcium Looping Process Applied to a Cement Plant

Calcium looping CO2 capture is a promising technology to reduce CO2 emissions from cement production. Coal is generally considered the fuel used to drive the calcium looping process as coal is already used as feedstock for cement production. This study assesses the impact of different fuels (coal, natural gas and woody biomass) on the technological and environmental performance of post-combustion calcium looping at a cement plant in North-western Europe. Process modelling is used to determine the impact of the different fuels on the mass and energy balance of the process. Life cycle assessment is carried out to evaluate the environmental performance of the different systems. Results indicate that firing natural gas or biomass instead of coal in an add-on calcium looping process can improve the efficiency of the process, as it decreases the fuel, limestone and electricity consumption. Consequently, while coal fired calcium looping can reduce life cycle climate change potential by 92%, the use of natural gas or biomass can make the process carbon neutral (reduction of 100%) or negative (-169%), respectively. Further research is required to complete the environmental perspective of using alternative fuels, but these results already illustrate a potential low-hanging fruit to improve the environmental performance of post combustion calcium looping in the cement industry.

Evolution of the CO2 carrying capacity of CaO particles in a large calcium looping pilot plant

Post-combustion calcium looping is an emerging capture technology that uses CaO particles as CO2 sorbent. This work analyses the average maximum CO2 carrying capacity of the sorbent (Xave) in a continuous large-scale calcium looping (CaL) pilot plant, which is believed to adequately represent the lifetime of CaO particles in larger CaL systems based on the regeneration of CaO by oxyfuel combustion in the calciner. The CO2 carrying capacity is a key variable in designing the CO2 capture carbonator reactor and for optimizing the performance of the system. Several experimental campaigns have been carried out in La Pereda 1.7MWth pilot plant both under oxy-fired and air-fired combustion conditions, in which the evolution of Xave with time has been measured under a wide range of conditions in the calciner. A methodology based on a closure of mass and particle population balances has been used to estimate Xave. Closure of the sulphur balance was found to be very accurate as an internal calibration tool to quantify the make-up flow of limestone that effectively participates in the CO2 capture process in any given set of conditions. When the calciner reactor was operated in conditions far from the equilibrium of CO2 on CaO, a good agreement was found between the experimental Xave values and those calculated from population mass balances which account for the number of carbonation-calcination cycles of each particle. However, a CO2 carrying capacity lower than expected was observed in some tests in oxy-combustion mode when the calciner was operating close to equilibrium. Departure from ideal sorbent behavior and more intense deactivation was found to increase with longer residence times in the calciner (∼3min). This deactivating effect may be due to the effective increase in the number of carbonation-calcination cycles of the particles, caused by switching between carbonating and calcining conditions inside the calciner when the reactor operates close to equilibrium. The results obtained in this study indicate that optimum operation of the calciner for sorbent activity is achieved in conditions sufficiently far from equilibrium and with low bed inventories (short particle residence time in the calciner).

Effect of SO2 and steam on CO2 capture performance of biomass-templated calcium aluminate pellets.

Four types of synthetic sorbents were developed for high-temperature post-combustion calcium looping CO2 capture using Longcal limestone. Pellets were prepared with: lime and cement (LC); lime and flour (LF); lime, cement and flour (LCF); and lime, cement and flour doped with seawater (LCFSW). Flour was used as a templating material. All samples underwent 20 cycles in a TGA under two different calcination conditions. Moreover, the prepared sorbents were tested for 10 carbonation/calcination cycles in a 68 mm-internal-diameter bubbling fluidized bed (BFB) in three environments: with no sulphur and no steam; in the presence of sulphur; and with steam. When compared to limestone, all the synthetic sorbents exhibited enhanced CO2 capture performance in the BFB experiments, with the exception of the sample doped with seawater. In the BFB tests, the addition of cement binder during the pelletisation process resulted in the increase of CO2 capture capacity from 0.08 g CO2 per g sorbent (LF) to 0.15 g CO2 per g sorbent (LCF) by the 10th cycle. The CO2 uptake in the presence of SO2 dramatically declined by the 10th cycle; for example, from 0.22 g CO2 per g sorbent to 0.05 g CO2 per g sorbent in the case of the untemplated material (LC). However, as expected all samples showed improved performance in the presence of steam, and the decay of reactivity during the cycles was less pronounced. Nevertheless, in the BFB environment, the templated pellets showed poorer CO2 capture performance. This is presumably because of material loss due to attrition under the FB conditions. By contrast, the templated materials performed better than untemplated materials under TGA conditions. This indicates that the reduction of attrition is critical when employing templated materials in realistic systems with FB reactors.

Economic evaluations of coal-based combustion and gasification power plants with post-combustion CO2 capture using calcium looping cycle

Coal-based power generation sector is facing important changes to implement energy efficient carbon capture technologies to comply with emission reduction targets for transition to low carbon economy. This paper assesses CaL (Calcium Looping) as one of the innovative carbon capture options able to deliver low energy and cost penalties. The work evaluates how the integration of post-combustion calcium looping influences the economics of power plants providing up-dated techno-economic indicators. Coal-based combustion plants operated in both sub- and super-critical steam conditions were evaluated, as well as coal gasification plant using an oxygen-blown entrained-flow gasifier. As benchmark options used to quantify the carbon capture energy and cost penalties, the same power generation technologies were evaluated without CCS (Carbon capture and storage). The power plant concepts investigated in the paper generates around 545–560 MW net power with at least 90% carbon capture rate. Introduction of CaL technology for CO2 capture results in a 24–42% increase of specific capital investment, the O&M costs are increasing with 24–30% and the electricity cost with 39–48% (all compared to non-CCS cases). As the techno-economic results suggest, CaL has good application potential in combustion-based power generation.

Biomass combustion with in situ CO2 capture by CaO in a 300 kWth circulating fluidized bed facility

This paper reports experimental results from a new 300kWth calcium looping pilot plant designed to capture CO2 “in situ” during the combustion of biomass in a fluidized bed. This novel concept relies on the high reactivity of biomass as a fuel, which allows for effective combustion around 700°C in air at atmospheric pressure. In these conditions, CaO particles fed into the fluidized bed combustor react with the CO2 generated during biomass combustion, allowing for an effective CO2 capture. A subsequent step of regeneration of CaCO3 in an oxy-fired calciner is also needed to release a concentrated stream of CO2. This regeneration step is assumed to be integrated in a large scale oxyfired power plant and/or a larger scale post-combustion calcium looping system.The combustor-carbonator is the key reactor in this novel concept, and this work presents experimental results from a 300kWth pilot to test such a reactor. The pilot involves two 12m height interconnected circulating fluidized bed reactors. Several series of experiments to investigate the combustor-carbonator reactor have been carried out achieving combustion efficiencies close to 100% and CO2 capture efficiencies between 70 and 95% in dynamic and stationary state conditions, using wood pellets as a fuel. Different superficial gas velocities, excess air ratios above stoichiometric requirements, and solid circulating rates between combustor-carbonator and combustor-calciner have been tested during the experiments. Closure of the carbon and oxygen balances during the combustion and carbonation trials has been successful. A simple reactor model for combustion and CO2 capture in the combustor-carbonator has been applied to aid in the interpretation of results, which should facilitate the future scaling up of this process concept.

Economic implications of pre- and post-combustion calcium looping configurations applied to gasification power plants

Abstract Gasification is a promising conversion technology to deliver high energy efficiency simultaneously with low energy and cost penalties for carbon capture. This paper is devoted to in-depth economic evaluations of pre- and post-combustion Calcium Looping (CaL) configurations for Integrated Gasification Combined Cycle (IGCC) power plants. The poly-generation capability, e.g. hydrogen and power co-generation, is also discussed. The post-combustion CaL option is a gasification power plant in which the flue gases from the gas turbine are treated for CO2 capture in a carbonation–calcination cycle. In pre-combustion CaL option, the Sorbent Enhanced Water Gas Shift (SEWGS) feature is used to produce hydrogen which is used for power generation. As benchmark case, a conventional gasification power plant without carbon capture was considered. Net power output of evaluated cases is in the range of 550–600 MW with more than 95% carbon capture rate. The pre-combustion capture configuration was evaluated also in hydrogen and power co-generation scenario. The evaluations are concentrated for estimation of capital costs, specific investment cost, operational & maintenance (O&M) costs, CO2 removal and avoidance costs, electricity costs, sensitivity analysis of technical and economic assumptions on key economic indicators etc.

The impact of calcium sulfate and inert solids accumulation in post-combustion calcium looping systems

Postcombustion CO2 capture by calcium looping (CaL) is being rapidly developed for coal combustion applications. This work discusses the impact of the accumulation of CaSO4 and other inert solids on CO2 capture efficiency and the overall CaL process performance. Several process configurations are considered, and the mass and energy balances and an updated carbonator reactor model are solved for each configuration. The minimum fresh sorbent requirements for sustaining a certain level of CO2 capture efficiency are quantified as well as the effects of an increase in the make-up flow. It was found that the main effect on the CaL process is produced by the sulfur present in the coal fed to the calciner and in the flue gas entering the carbonator. For a typical set of operating conditions it was calculated that the deactivating effect caused by an increase of 0.5% in the sulfur content with respect to a reference coal (low ash content) fed to the calciner is similar to the effect caused by the accumulation of inerts when using a coal with 15% more ash.

Modeling of the oxy-combustion calciner in the post-combustion calcium looping process

The calcium looping process is a fast-developing post-combustion CO2 capture technology in which combustion flue gases are treated in two interconnected fluidized beds. CO2 is absorbed from the flue gases with calcium oxide in the carbonator operating at 650°C. The resulting CaCO3 product is regenerated into CaO and CO2 in the calciner producing a pure stream of CO2. In order to produce a suitable gas stream for CO2 compression, oxy-combustion of a fuel, such as coal, is required to keep the temperature of the calciner within the optimal operation range of 880–920°C. Studies have shown that the calcium looping process CO2 capture efficiencies are between 70% and 97%. The calciner reactor is a critical component in the calcium looping process. The operation of the calciner determines the purity of gases entering the CO2 compression. The optimal design of the calciner will lower the expenses of the calcium looping process significantly. Achieving full calcination at the lowest possible temperature reduces the cost of oxygen and fuel consumption. In this work, a 1.7MW pilot plant calciner was studied with two modeling approaches: 3-D calciner model and 1-D process model. The 3-D model solves fundamental balance equations for a fluidized bed reactor operating under steady-state condition by applying the control volume method. In addition to the balance equations, semi-empirical models are used to describe chemical reactions, solid entrainment and heat transfer to reduce computation effort. The input values of the 3-D-model were adjusted based on the 1-D-model results, in order to model the behavior of the carbonator reactor realistically. Both models indicated that the calcination is very fast in oxy-fuel conditions when the appropriate temperature conditions are met. The 3-D model was used to study the sulfur capture mechanisms in the oxy-fired calciner. As expected, very high sulfur capture efficiency was achieved. After confirming that the 1-D model with simplified descriptions for the sorbent reactions produces similar results to the more detailed 3-D model, the 1-D model was used to simulate calcium looping process with different recirculation ratios to find an optimal area where the fuel consumption is low and the capture efficiency is sufficiently high. It was confirmed that a large fraction of the solids can be recirculated to both reactors to achieve savings in fuel and oxygen consumption before the capture efficiency is affected in the pilot unit. With low recirculation ratios the temperature difference between the reactors becomes too low for the cyclic carbonation and calcination. As a general observation, the small particle size creates high solid fluxes in the calcium looping process that should be taken into account in the design of the system.

Life cycle greenhouse gas assessment of a coal-fired power station with calcium looping CO2 capture and offshore geological storage

Carbon Capture and Storage (CCS) is an essential technology for reducing global CO2 emissions in the context of continued fossil fuel use in the power sector. To evaluate the emission reduction potential of any low-carbon generation technology it is necessary to consider emissions over the entire lifetime of the plant. This work examines the lifecycle greenhouse gas emissions of a 500 MWe pulverised coal-fired power plant with post-combustion Calcium Looping (CaL) and off-shore geological storage. CaL uses solid CO2-sorbent derived from abundant and non-toxic limestone (CaCO3) and is currently being piloted at the 1–2 MWth scale in Europe (Spain and Germany). This technology promises to be very competitive with the more mature chemical absorption processes, with the potential to reduce the efficiency and cost penalties of CO2 capture. We demonstrate that the emission intensity of a coal-fired power plant with CaL is at least comparable with one using MEA-solvent technology (i.e., ∼ 229 gCO2e/kWh vs. 225 gCO2e/kWh). However, there is significant potential for additional emissions reduction when considering the recarbonation of exhausted sorbent in landfill. Furthermore, a coal-fired power plant with CaL could be carbon-neutral – or even achieve a net removal of CO2 from the atmosphere. That is, if the exhausted sorbent is used in the cement industry substituting the input of fresh-limestone; or if the exhausted sorbent is disposed in the ocean forming bicarbonate.

Post-combustion calcium looping process with a highly stable sorbent activity by recarbonation

This paper presents a novel sorbent regeneration technique for post-combustion calcium looping CO2 capture systems. The advantage of this technique is that it can drastically reduce the consumption of limestone in the plant without affecting its efficiency and without the need for additional reagents. The method is based on the re-carbonation of carbonated particles circulating from the carbonator using pure CO2 obtained from the gas stream generated in the calciner. The aim is to maintain the CO2 carrying capacity of the sorbent close to optimum values for CaL post-combustion systems (around 0.2). This is achieved by placing a small regeneration reactor between the carbonator and the calciner. This reactor increases slightly the conversion of CaO to carbonate so that it exceeds the so-called maximum CO2 carrying capacity of the sorbent. This increase compensates for the loss of CO2 carrying capacity that the solids undergo in the next calcination–carbonation cycle. Two series of experiments carried out in a thermogravimetric analyzer over 100 cycles of carbonation–recarbonation–calcination show that the inclusion of this recarbonation step is responsible for an increase in the residual CO2 carrying capacity from 0.07 to 0.16. A conceptual design of the resulting capture system shows that a limestone make-up flow designed specifically for a CO2 capture system can approach zero, when the solid sorbents purged from the CaL system are re-used to desulfurize the flue gas in the existing power plant.