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Energy Procedía
ELSEVIER
Energy Procedía 1 (22009) 133-140
www.elsevier.com/locate/procedia
GHGT-9
C02 Capture Technologies for Cement Industry
Adina Bosoaga a'b*, Ondrej Masekb, John E. Oakeyb
"Mott MacDonald, Victory House, Trafalgar Place, Brighton, BN1 4FY, UK. bEnergy Technology Centre, Cranfield University, Cranfield, MK43 OAL, UK
Abstract
The effect of the increasing concentration of CO 2 in the atmosphere on climate change is a major driving force for the development of advanced energy cycles incorporating CO 2 mana gement options. Growing interest in the technical and economic feasibility ofC02 capture from large coal -based power plants has led to increased efforts worldwide to develop new concepts for greater C02 reductions in the future. Greenhouse gas emissions, especially C02, have to be reduced by 50-80% by 2050, according to the IPCC [1].
The type of fuel used in cement manufacture directly impacts on CO 2 emissions, with coal accounting for around 60 -70% of CO 2 emissions from cement installations. Therefore, the large amount of carbon dioxide emitted during cement manufacturing process - 5% of the total emissions of C02 from stationary sources worldwide -is a cause of great concern and has to be tack led in order to comply with current legislation.
Several technologies are available and have been proposed for the separation of C02 from the flue gases from new and existing plants with retrofit capture units. Few studies have been undertaken on C 02 captu re in cement plants to assess the suitable technologies, with oxy-combustion and amine scrubbing as the possible options (pre-combustion capture not being viable). This paper summarises the different CO 2 capture technologies suitable for cement industry and assesses the potential of the calcium looping cycle [2, 3] as a new route for C02 capture in the cement industry. The potential advantage of this system is the very low efficiency penalty expected (<6%) compared with other capture technologies as the heat required for calcination is balanced by heat released during the carbonation (C02 capture) step and can be utilized efficiently at high temperature in the plant's steam cycle. Since limestone is already used for cement manufacture, and because it is a cheap material with good geographical distribution, it allows the use of local limestone resources with minimal limestone -related infrastructure investment. Another envisaged benefit of this new technology is that the lime purged from the cycle could be used as a raw material for the production of cement clinker. Therefore, the calcium looping cycle can potentially have an important impact in reducing C02 emissions from the cement industry, and may also be applicable in othersecto rs.
© 2009 Elsevier Ltd. All rights reserved.
PACS: Type pacs here, separated by semicolons ;
Keywords: Cement industry; 002 capture; post combustion; oxyfiring; calcium looping
* Corresponding author. Tel.: +44-127-336-5405; Fax: +44-127-336-5197. E-mail address : adina.bosoaga@mottmac.com. doi:10.1016/j.egypro.2009.01.020
1. Introduction
Climate change is one of the greatest and probably most challenging environmental threats facing the world this century. Therefore, mitigation measures to reduce the extent of global warming are crucial. The Kyoto Protocol — the first international agreement on tackling climate change — stipulates that industriali sed countries must act first to curb emissions, giving time for developing countries to grow their economies. At the end of the summit held this year in Hokkaido, Japan, G8 countries' leaders reaffirmed their commitment to the UN goal of achieving at least a 50 per cent reduction of global emissions by 2050.
With carbon dioxide representing the largest share of greenhouse gas emissions, C02 capture and storage (CCS) technology is increasingly being seen as a critically important element needed to tackle climate change, offering important potential for further utilisation of fossil fuels particularly for power generation and energy intensive industrial applications.
The cement industry contributes to about 5% of the global anthropogenic C02 emissions, making the cement industry an important sector for C02 emission mitigation strategies. As Figure 1 shows, worldwide cement production has grown consistently over the last few years, with cement production figures rising to 2.77 billion tonnes for 2007 [4].
-CEMBUREAU
Figure 1. World cement production by region evolution 2000 -2007 [4]
Of the C02 emitted by the cement industry 50% result from the calcination process of limestone, 40% from combustion of fuels (coal/pet coke/tyres/waste oil/solvents/sewage sludge etc.) in the kiln, 5% from transportation and the remaining 5% from the electricit y used in manufacturing operations. The type of fuel used in cement manufacture directly affects C02 emissions, with coal accounting for around 60 -70% of C02 emissions. Therefore, the large amount of C02 emitted during cement manufacturing is causing a great concern and has to be tackled in order to comply with current legislation.
Cement plants are large industrial sources of C02 emissions, with a high C02 concentration in their flue gases o f about 14-33%, compared to 12-14 % C02 for coal -fired power plants and 4% C02 for gas -fired power plants, and therefore represent a good oppo rtunity for implementing CCS.
Several technologies are available and have been proposed for the separation of C02 from the flue gases from new and existing power plants with retrofit capture units. This paper reviews the use of oxy-combustion and amine scrubbing technologi es as possible options for CCS in cement plants, and focuses on the calcium looping cycle [2, 3] as a new route for carbon capture which could be developed for use in cement plants in the near future. Pre combustion was not addressed as not being a viable option for the cement industry as it would only be able to capture the C02 produced from combustion of the fuel used and not the higher quantity of C02 emitted during the limestone calcination process.
2. The Cement Plant Process
At present, approximately 5 % of global anthropogenic carbon dioxide emissions result from the manufacture of cement, with nearly 0.7-1.1 tone of C02 being emitted for every tone of cement produced.
A cement plant comprises the following steps: raw material preparation: crushing and milling, pre-heating, pre-calcin ing, kiln firing, clinker and additive mixing and cooling, cement milling and finally storage/packing. The raw materials are crushed and milled into a fine powder before entering a preheater and being fed into a rotating kiln [5]. Fuels are burned at the lower end of the kiln so that it reaches about 2000°C, allowing the materials to be heated to around 1 500°C, where they become partially molten. When the limestone (CaC03) reaches about 900°C, it undergoes the chemical reaction known as calcination, whereby C02 is released and calcium oxide formed, before this converts to clinker.
The main chemical reaction is:
CaC03 ^ CaO + C02
The clinker or kiln product is then cooled and the excess heat is typically routed back to the preheater units. Prior to packaging, appropriate additives or clinker substitutes are added to the clinker to form the end-product known as Portl and cement.
600 kg C02 A 1566 kg N2
0.94 kg air
Figure 2. Schematic of process flow of cement plant without CO 2 captu re
Figure 2 provides a process flow diagram of the general cement manufacturing process with an example of a mass balance for the production of one tonne of cement [6].
Cement production is either "wet" or "dry", depending o n the water content of the raw material feedstock. The wet process allows for easier control of the chemistry and is better when moist raw feedstocks are available. However, it has higher energy requirements due to the need to evaporate the 30%.. slurry watebefore heating the raw materials to the necessary temperature for calcination. The dry process avoids the need for water evaporation and is, as a consequence, much less energy intensive.
3. Convention al Options for Reducing CO^ Emissions
Many opportunities exist for C02 emission reduction in the cement industry, with three preferred measures by which C02 emissions have been mitigated over the last years:
• Energy efficiency improvement
• Fuel switching by use of waste as alternative fuel
• Blended cements by reduction of clinker/cement ratio using industrial by-products
3.1. Energy Efficiency Improvement
Through energy efficiency improvement, the C02 emissions from fuels and the costs of cement manufacturing can be reduced. Using energy-efficient equipment and replacing old installations represent another way to improve the energy efficiency of the process. As the heat used in the large rotary kiln represents the largest proportion of energy consumed in cement manufacture, improving fuel efficiency will reduce the energy input. Since the dry process with pre -heaters and pre-calcination is more energy efficient than the wet process, converting from the wet to the dry process represents an opportunity to further improve the energy efficiency of the kiln, and therefore the efficiency of the entire process. Converting the from wet to the semi-wet process could also lower the energy intensity for clinker production with only a modest increase in power consumed. Optimization of the clinker cooler, improvement of preheating efficiency, improved burners and process control and management systems are also part of process upgrading.
3.2. Fuel Switching
Fuel switching to lower carbon fuels and fuels qualifying for emissions offsets represent another potential route for C02 mitigation. The use of w aste-derived alternative fuels in the cement industry has increased over recent years, to become today 's current practice. Using certain wastes is seen as an important opportunity to reduce the long-cycle carbon emissions, diminishing their disposal requirements and reducing the use of fossil fuels. However, waste material might have adverse effects on the cement quality and increased emissions of harmful highly volatile elements like mercury and thallium. As waste used can be regarded as C02 neutral, some governments have already credited the cement industry for thereductiono f equivalent C02 emissions.
3.3. Blended Cements
The production of clinker is the most energy -intensive step in the cement manufacturing process resulting in large process emissions of C02. In blended cement, the clinker/cement ratio is reduced by replacing a part of the clinker with coal fly ash or blast furnace slag. These industrial by -products are mixed with the ground clinker to give a blended cement product. The global potential for C02 emissionreductions through producing blended cement is estimated to be at least 5% of total C02 emissions from cement m aking, but may be as high as 20% [7].
4. CCS Potential in the Cement Industry
The effects on climate change of the increasing C02 emissions in the atmosphere represent a major driving force for the development of advanced energy cycles incorporating C02 management options. Therefore, CO 2 capture and storage is seen as another opportunity, apart from the conventional ones presented above, to further mitigate the C02 emissions generated during cement process. C02 capture can be performed using either pre-combustion or postcombustion technologi es and there are a number of potential storage destinations: saline aquifers, porous geologic formations, depleted oil and gas reservoirs and coal seams.
As described above, during the cement manufacturing process, C02 is generated from three different sources [4]
• De-carbon ation of limestone in the kiln - about 525 kg C02 per tonne of clinker
• Fuel combustion in the kiln - about 335 kg C02 per tonne of cement
• Use of ele ctricity - about 50 kg C02 per tonne of cement.
The typical exhaust gases from cement process are shown in Table 1 below.
Table 1
Component Concentration
CO, 14-33% (w/w)
N02 5-10% of N0X
NOx <200-3000 mg/Nm3
S02 <10-3500 mg/Nm3
o2 8-14 % (v/v)
4.1. Amine scrubbing
Post combustion capture by amine scrubbing (e.g. using Monoethanolamine (MEA )) is a commercially mature technology commonly used in chemical industry for separation of C02. The C02 is stripped from the amine solution, dried, compressed and transported to the storage site. The main challenge, however, lies in the scale up of this process and its implem entation for scrubbing of C02 from flue gases that contain a number of contaminants potentially detrimental to the operation of the scrubber unit.
Several other methods can in principle be also considered for the large -scale separation: scrubbing of gases solid sorbents, separation with membranes an d cryogenics.
Post-combustion capture has been widely studied for application in power generation plants, but there are few studies that have examined its application in cement plants [8].
The process flow of a cement plant allows the capture equipment to be fitted without major modifications to the existing plant. As NOx and SOx react with amines to form heat stable salts which result in solvent degradation, a NOx maximum level of 20 ppmv is imposed and SOx is restricted to approximately 10 ppmv [@ 6% 02]
The required equipment in order to capture the C02 emitted is as follows:
• SCR fitted between the preheater and the raw mill to reduce NOx in order to comply with MEA process requirements
• FGD fitted to remove SOx from the flue gas stream
• C02 capture pi ant based on amine s (absorber, stripper, and the auxiliary equipment)
• CHP plant or independent steam generators with grid connection for electricity requirements to generate the steam needed for MEA stripping and to provide the electrical power for the am ine scrubber operation and CO 2 compression plant.
• C02 compression plant, where the C02 captured stream is cleaned, compressed and dried, prior to transport by pipeline at a typical pressure of about 110 bar.
A considerable advantage of cement plants over power generation plants is the high concentration of C02 in the flue gas which has a direct impact on absorber unit size, and the power requirements for C02 compression will be much lower when compared with the power demand for coal and gas power plants.
4.2. Oxy-firing
In oxy-firing technology, the combustion air is replaced by reasonably pure oxygen from an air separation unit (ASU), with the C02-rich flue gas being recycled to moderate the flame temperature. Because of the high percentage of C02 in flue gases originating from the calcination process, combustion in a C02/02 atmosphere looks like one of the best options for C02 reduction in a cement plant.
The equipment required for oxy-firing is as following:
• Air S eparation Unit (ASU) - to separate the oxygen from air prior to feeding into the pre-calciner
• Ducting for recirculation of C02-rich exhaust gas back to the pre-calciner burners (around 50% of the total exhaust gases produced in the pre -calciner)
• C02 treatment and compression plant where the captured C02 stream is cleaned, compressed and dried, before transport by pipeline.
Control of air leakage into the kiln, cooling of the cement after the kiln, the consequence of the higher C02 partial pressure on the calcination process and the control of C02 emissions during start/stops of the cement plant need further exploration. If cement kilns could be successfully operated with a high C02 atmosphere (which will increase the temperature required for calcination) and in -leakage could be greatly reduced, oxy -combustion of the kiln could be a feasible option.
The main advantage of oxy-firing for cement plants is the low oxygen consumption with only 1/3 of the amount of 0^ needed per tonne of C02 captured compared to a coal -fired boiler [8].
4.3. Calciumlooping
As the cost of the separation of C02 from flue gases introduces a large economic penalty, a range of emerging approaches to separate C02 with more cost-effective processes are being explored. Intense, worldwide research activity exists to develop lower cost processes to separate C02.
The lime carbonation/calcination cycle is based on the separation of C02 from combustion gases by the use of lime as an effective C02 sorbent to form CaCO 3. The separation of CO 2 is carried out at high temperature u sing C aO as regenerable sorbent - Figure 3. The reverse, calcination reaction produces a gas stream rich in C02 and rqgenrat es the sorbent - CaO - forsubsequent carbonation cycles .
Figure3. Lime Carbonation/Calcination Cycle [9]
CaO particles will react with C02 from combustion flue gases, at atmospheric pressure and temperature around 650°C, to produce CaC03. The carbonation reaction takes place in a reactor similar to a circulating fluidised bed (FB) combustor. The CaC03 particles are then separated from the flue gas and sent to a different vessel for regeneration - calcination to produce pure C02 suitable for storage and CaO for further use . The newly formed CaO is recycled to the capture reactor. The main option considered at present for calcination is coal oxy-firing in a fluidised bed calciner at temperature over 900 °C, with the calcined solids cycled continuously to the carbonator, establishing a C02 chemical loop between the carbonation and calcination reactors.
An upper limit to the C02 capture efficiency is given by the equilibrium of the carbonation reaction at the temperature and pressure of the carbonator which imposes a limit on the operating conditions. The equilibrium calcination temperature is dependent on the C02 pressure as presented in Figure 4, followingthe equation:
log|Q Peq (atm)=7.079-8308/T (K) (1)
It is assumed that the calcination temperature should be at least 50 °C above the equilibrium condition.
Figure 4. The equilibrium pressure of C02 over CaO
The other limitation placed on C02 capt ure in this process is that of the CaO capacity itself. It is known that sorbent reactivity and durability decrease considerably with the number of cycles and that the decrease is promoted by presence of SOx in the flue gas [3]. Figure 5 compiles data obtained by several authors in very different carbonation-calcination conditions. It shows that all the results follow a similar trend, reflecting a strong deactivation mechanism irrespective of the limestone type and conditions.
Figure 5. Decay in carbonation conversion with number of cycles
As a result of the sorbent deactivation, a small fraction of fresh sorbent has to be continuously added to the system to maintain the overall sorbent activity. The deactivated sorbent is then purged from the system and could be used as a precalcined feedstock for the clinker kiln. However, t he utili s ation of the solid purge in the cement process places a restriction on the purge composition, especially with respect to ash and sulphur contents. The composition of the purge depends on the non-volatile content of coal ash, the sulphur content of coal and the specific composition of limestone.
There are four different options for using this technology for cement industry as follows:
(i) FB calcium looping for supplying cement kiln lime. This option offers maximum savings of raw
material, fuel and C02 emissions. However, additional investmentrequired.
(ii) Adding the purge coming from the calcium cycle applied to a coal power station to the cement
plant's raw material mixture. For this option only little investment is needed. Clinker chemistry, alkali/S03 ratio in the cement kiln and S03 content in the cement clinker represent limiting factors that need further assessment. In addition, transport distance for the solid material will play an important role in the economic assessment of this option.
(iii) Adding of sintered purge to cement clinker. Sintering of the purge materi al at temperatures < 1,250°C
without decomposition of C aS04, grinding of sintered purge together with ordinary Portland cement clinker; only a little thermal energy is needed because of the exothermic reactions during sintering. Limiting lactors are S03-content in the cement clinker chemistry.
(iv) Modification of clinkering for high sulphur raw material. Sintering of the purge materi al at
temperatures > 1,400°C in order to achieve decomposition of CaS04, reformation of gypsum by flue gas desulfurization with limestone; the thermal energy demand is much higher than in previous option and additives are required to achieve the desired clinker chemistry To make the spent lime purged from the calcium looping cycle, suitable as an input for cement production, the concentrations of ash and CaSO 4 in the purge stream need to be limited to Ashmax = 30 wt.% and CaSO 4mac = 10 wt.% [10]
5. Discussion
Limestone is the most suitable source of CaCO 3 for cement production. A large percentage of cement plants are located close to their source of limestone, thus limiting raw material transport costs. This is an essential requirement since limestone represents about two -thirds of the clinker composition by mass.
The cement industry will continue to play a strong part in the global effort to fight climate change. The potential advantage of the calcium looping system is the very low efficiency penalty expected (<6%) compared with other
capture technologies as the heat required for calcination is offset by the heat released during carbonation which can be utilised efficiently at high temperature in the plant's steam cycle.
The utilization of the lime purge mat erials from the calcium looping process as raw material substitute in cement production allows remarkable savings of fuel and C02-emissions because of it is a pre-calcined raw material. The amount of savings is qualified by the required quantity of additives to be mixed with the purge of given a chemical composition to achieve the required clinkerchemistry.
The substantial difficulties in operation of a cement kiln using purge as a mai n raw material component are re -carbonation of the raw meal by CO 2 contained in the kiln gas and the impact of increased sul phur input, which might cause higher S02 emissions.
Further work in close collaboration with the cement industry is needed in orde r to assess the maximum purge quantity that can be utilized in cement production, to evaluate the requirements for a suitable pre -calcined cement feedstock, investigate the limiting factors and the implication of the different options listed above as well as assessing the behavior of SO 3 during sintering.
6. Conclusion
This paper summarises the different C02 capture technologies suitable for cement industry proving that oxy-combustion and amine scrubbing are possible options, with pre-combustion capture not being a viable route.
The paper assesses the potential of a new technology - the calcium looping cycle - as a new route for C02 capture in the cement industry. The potential advantage of this system is the very low efficiency penalty expected (<6%) compared with other capture technologies. Since limestone is already used for cement manufacture, and because it is a cheap material with good geographical distribution, it allows the use of local limestone resources with minimal limestone-related infrastructure investment.
Another envisaged benefit of this new technology is that the lime purged from the cycle could be used as a raw material for the production of cement clinker. Therefore, the calcium looping cycle can potentially have an important impact in reducing C02 emissions from the cement industry, and may also be applicable in other sectors.
7. Acknowledgments
The authors wish to thank the European Community (SES6 - 019914) for their funding of this study and our partners in this project: Instituto Nacional del Carbon Spain, ENDESA, CEMEX and University of Stuttgart.
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