CO2 capture. Chemical Looping Combustion (CLC)
Example of a CLC system (click to enlarge)
The Chemical Looping Combustion (CLC) concept is based on the transfer of oxygen from the combustion air to the fuel by means of an oxygen carrier in the form of a metal oxide, avoiding the direct contact between fuel and air.
The CLC system is made of two interconnected reactors, designated as air and fuel reactors. In the fuel reactor, the fuel gas is oxidised by a metal oxide through the chemical reaction:
CH4 (CO, H2) + MeO â CO2 + H2O (CO2, H2O)+ Me
The exit gas stream from the fuel reactor contains CO2 and H2O. After water condensation, almost pure CO2 can be obtained with little energy lost for component separation.
The metal or reduced oxide, Me, is further transferred into the air reactor in which it is regenerated by taking up oxygen from the air.
Me + Â½O2 â MeO
The flue gas leaving the air reactor contains N2 and unused O2. The total amount of heat evolved over the two reactors in CLC process is the same as for normal combustion, where the oxygen is in direct contact with the fuel.
The significant advantage compared to normal combustion is that the CO2 is not diluted with N2. As opposite to other technologies proposed for CO2 separation, this process has no significant energy penalty for the capture process, and external capture devices are avoided. Thus, the process is expected to be less costly than available technologies for CO2 separation. A conceptual process scheme is shown in the figure below.
Different metal oxides have been proposed as possible candidates for CLC process: CuO, CdO, NiO, Mn2O3, Fe2O3, and CoO. In general, these metal oxides are combined with an inert which acts as a porous support providing a higher surface area for reaction, as a binder for increasing the mechanical strength and attrition resistance, and, additionally, as an ion conductor enhancing the ion permeability in the solid.
The only drawback of the overall CLC process is that the oxygen carriers are subjected to strong chemical and thermal stresses in every cycle and the performance could be poor after enough number of cycles in use.
Research on chemical-looping combustion at ICB-CSIC
The work includes different projects funded by ECSC, Capture of CO2 in Coal Combustion (CCCC), and by EU. In these projects we co-operate with Chalmers University of Technology in Sweden, Technical University of Vienna, BP (UK) and Alstom (France). The work is also supported by the Spanish Ministry of Education and Science.
The objectives of this research line are:
- To develop an oxygen carrier with appropriate reduction and oxidation rates, resistant to attrition and with high durability, maintaining its chemical, structural and mechanical properties after a high number of reduction-oxidation cycles.
- To investigate the possible designs with respect to the fluidization conditions.
- To demonstrate and evaluate this new combustion technology in a laboratory-scale chemical-looping combustor.
- Main achievements of this research line are:
- To develop a Cu-based oxygen carrier without agglomeration problems and excellent properties for the CLC process.
- To demonstrate the CLC technology a prototype of 10 kWth has been designed and built in the ICB-CSIC. This pilot plant was satisfactorily run during 200 hours burning methane and using a Cu-based oxygen carrier.
500 Wth (for CLC) and 900 Wth (for CLR) Pilot plant for gaseous fuels
500 Wth (for CLC) and 1.5 kWth (for CLOU) Pilot plant for solid fuels
CO2 Capture. Oxy-fuel combustion
Diagram of an Oxy-fuel plant (click to enlarge)
Carbon dioxide (CO2) and sulphur dioxide (SO2) emissions are a major concern in combustion processes using fossil fuels, as for example coal; the former is implicated in global climate change and the latter produces acid rain. CO2 is one of the major contributors to the build-up of greenhouse gases in the atmosphere. At the same time, fuels containing sulphur produce SO2 pollution during combustion.
The capture of CO2, emitted in large quantities from power stations, is considered an option to be explored in the medium term for reducing CO2 levels released to the atmosphere. Oxy-fuel combustion is one of the possibilities under investigation within the different options for CO2 capture. This technology uses for combustion O2 mixed with recirculated flue gases, instead of air used in conventional combustion, to produce a flue gas stream with high concentration of CO2.
Until now, most of research has been directed towards the development of oxy-fuel systems in pulverised fuel boilers. However, it is believed that a circulating fluidised bed combustor (CFBC) will be an important candidate for new coal fired power plants because solids recirculation can help to an effective control of the temperature. In-situ desulphurisation is another of the best known advantages of fluidised bed combustion. However, sulphur capture by calcium compounds is a process highly dependent on the temperature and CO2 concentration. In oxy-fuel combustion, CO2concentration in the bed may be enriched between 40 and 90%. Under so high CO2 concentration, different from that in conventional coal combustion atmosphere (0-15% CO2), the calcination and sulphation behaviour of the sorbent must be defined previously to decide the optimum operating temperature in the combustor.
For oxy-fuel operating conditions, with high CO2 concentration in the combustor, the SO2 retention could occur depending on the operating temperature by different processes: direct sulphation, simultaneous calcination/sulphation or sulphation of calcines. The objectives of this research line are to study these ways of sulphur capture and to optimize the operating temperature for sulphur retention in oxy-fuel operating conditions during fluidised bed coal combustion. In addition, because the characteristics of the calcium sorbent (as specific surface area, pore size, composition, etc.) have a very strong influence on the rate of sulphation, it will be analysed the behaviour of different limestones and dolomites for application in oxy-fuel coal combustion in fluidised beds.
Test runs will be carried out in a continuous oxy-fuel fluidised bed combustor with different coals, sorbents and test conditions. Special attention will be paid to the SO2 retention and emission.
Finally, a mathematical model of a circulating fluidised bed combustor will be developed to simulate and to optimise the sulphur retention in this type of combustors working in oxy-fuel conditions. The model will be a useful tool for the design and scale-up of this type of combustors.
Specific objectives may be summarised as follows:
1. To model the calcination and sulphation processes happening in the calcium based sorbents (limestones and dolomites) and to determine the kinetic parameters of these processes during oxy-fuel fluidised bed combustion.
2. To find the optimum operating temperature of the combustor to maximise the sulphur retention during oxy-fuel fluidised bed combustion.
3. To analyse the effect of the main operating conditions of the oxy-fuel fluidised bed combustors (coal type, O2/CO2 ratio fed to the combustor, sorbent type, Ca/S molar ratio, O2 excess, etc.) on the behaviour of the calcium based sorbents and on the SO2 retention in this kind of reactors.
3 kWth Oxy-fuel Pilot plant for solid fuels
Combustion in bubbling and circulating fluidised beds using solid fuels as coal, biomass, and wastes
The group has a tradition of research on bubbling and circulating fluidised bed combustion and gasification. Combustion in bubbling fluidized beds is a widely developed technology for a great variety of fuels and installation scales. Its application to high reactivity fuels such as Spanish lignites, characterized by high sulphur and ash contents has shown their feasibility, from the point of view of combustion efficiencies and SO2 retention. Circulating fluidised beds (CFB) as a well-known modern reactor of two-phases, gas and solids, is receiving wide research attention in view of its potential as an economic and environmentally acceptable technology for burning low-grade coals, biomass and organic wastes, which are becoming interesting for energy production. In forest rich countries, CFB combustion has increased its market share of biomass combustion during recent years. The main reasons are high burnout of the fuel, wide fuel span, high thermal efficiency, low emissions of harmful gases and a competitive price. The same hold true also for biomass gasification but in an earlier stage in market expanding.
The group investigation has been concentrated on the analysis different aspect of the process, such as fluid-dynamics of gas-solids flow, heat transfer, devolatilization and char combustion/gasification kinetics, SO2 sorbents characterization, etc. In addition, because mathematic modelling has been proven as an efficient method to advance our know-how in a systematic way, and together with experimental data, create efficient tools for R&D and engineering work, an important emphasis has been done on the modelling and simulation of carbon/biomass combustion efficiencies and SO2 retentions.
Batch fluidized bed reactor for combustion/devolatilization of solid fuels studies
10kW Bubbling Fluidised bed combustion/gasification pilot plant with sulphur retention by calcium sorbents
Hot gas cleaning of sulphur compounds (SO2, H2S, etc.) in combustion/gasification processes
Integrated gasification combined cycle (IGCC) is a promising technology which is expected to contribute a considerable share of electricity in the next years. In theses systems, fuel gas is generated in a gasifier, purified in a gas cleanup system and sent to a gas turbine where it is burned. An important loss of energy is produced with the present cold desulfurisation systems which reduce the temperature below 300 Â°C with wet techniques or below 700 Â°C with regenerable sorbents. In situ desulfurization by direct injection of calcium-based sorbents at high temperatures (800-1100 Â°C) would make a substantial contribution to the improvement of the thermal efficiency of electrical power generation in IGCC systems.
The aims of group investigation are to analyse the sulphur capture effectiveness of different natural Ca-based sorbents (limestones, dolomites, calcium hydroxides) under atmospheric or pressurised gasification conditions, and to study the kinetics of sulphidation of Ca-based sorbents under calcining or non-calcining conditions. To improve the retention process, modified calcium hydroxides and novel sorbents as calcium acetate and calcium magnesium acetate, are also used.
The different Ca-based sorbents for H2S removal are tested in thermogravimetric analysers, fluidised bed reactor, moving bed reactors and in a drop tube reactor.
The direct combustion of coals with high sulphur contents, such as Spanish lignites, is problematic due to the high SO2 emissions. These SO2 emissions can be reduced (lowered) by addition of a Ca-based sorbent, like limestone or dolomite, into the boiler. So, the use of calcium based sorbents in pulverised coal boilers and fluidised bed combustors has been studied for our group.
The injection of calcium based sorbents into the post-flame zone of a boiler for the purpose of controlling SO2 emissions has been recently selected as the lowest cost option to retrofit old power plants burning coals of low sulphur content. When a Ca-based sorbent is introduced in a pulverised coal boiler decomposes and the result is a porous solid which reacts with SO2 and O2 to form CaSO4. The sulphation of CaO is a thoroughly studied heterogeneous reaction due to its technological importance in connection with desulphurisation. In our group, the sulfation process is studied in a drop-tube reactor system at temperatures from 950 to 1250 Â°C, by using similar conditions to those existing in the sorbent injection in pulverized coal boilers. The main variables affecting the sulfation process are analysed: Ca/S molar ratio, SO2 concentration, temperature, sorbent particle size and reaction time.
Fluidized bed coal combustion allows in-bed sulphur retention by sorbent addition. The Ca-based sorbent feedrate needed to reach a fixed sulphur retention varies greatly depending on the characteristics of the sorbent used. So, it is necessary to know the sorbent behaviour before using it in a combustor. The effect of the principal variables affecting the sulphur retention (calcination conditions, sorbent particle size, temperature and SO2 concentration) are analysed in a thermogravimetric analyser and in a fluidized bed. In addition, mathematical models for the simulation and optimization of the sulphur retention in bubbling and circulating fluidised bed combustor have been developed.
Drop-tube reactor facility up to 1200ÂºC for calcium based sorbent injection studies
Pressurized moving bed reactor facility for H2S removal by injection of Ca-based sorbents
IEA Greenhouse Gas R&D: http://www.ieagreen.org.uk/
Chalmers University : http://www.entek.chalmers.se/~anly
CO2 Capture Project : http://www.co2captureproject.org/index.htm
Department of Energy (DOE), Carbon sequestration: http://www.fe.doe.gov/programs/sequestration/
CO2NET, CO2 Sequestration European Technology Network Development Programme: http://www.ieagreen.org.uk/co2net.htm
University of Regina, CO2 separation research: http://www.uregina.ca/engg/co2/resrchac.htm
ZECA - Zero Emission Coal Alliance: http://www.zeca.org/
Howard J. Herzog - Research into Carbon Management (MIT) : http://web.mit.edu/energylab/www/hjherzog/carbonmgmt.html
IPCC Homepage: http://www.ipcc.ch/