Flue Gas CO2 Emission Reduction Technologies and Applications

SEETecholbgiesand ApplicationsBylHurp Liu Jianmin,, Zhu Fahua Ji Junfeng, Xue anming) Zhao Liang1. State Rower Environmemtal Pretection Research lnstituteDepartmen of Earth Scienees, Nanjing UniversityAbstract: The current status and trend of Co, emission from coal-fired power plants in China are introduced.Main flue gas decarbonization technologies and their prospective of applications in China are discussed intwo separate parts capture and sequestration. It is stated that the selection of CO, capture and sequestrationechnologies relates closely with the geographical location of power plants, with the destination of Co, beingthe key. Further, it is suggested that industrialized test centers or test platforms of national or industrial levelshould be set upKeywords: coal-fired power plant; Aue gas decarbonization; CO, capture and sequestration; technical analysisReducing carbon dioxide(Co)emissions to slow down the electric power industry should take the responsibilities ofhe global warming is a challenging issue of this century. CO 2 emission reduction. Therefore, it is important to selectCountries around the world are making positive efforts to appropriate Co, emission reduction technologies that aresolve this problem. Various emission reduction theories and suitable for the coal-fired power plants in Chinatechnologies have been developed and put into applicationSignificant progress and breakthrough in technicalco capture technologiesdevelopment have been achieved.Coal combustion is the main source of CO, emissionsChina has now become the worlds top one in terms of in thermal power plants. Based on China's emission data inCO, emission. The CO, emissions are mainly from power the past 40 years, CO2 generated from power generationgeneration,heat supply, and other energy production fields. has been increasing year by year according to the statisticsWith the rapid economic development, the emissions of Co, of International Energy Agency(IEA), Chinas coal-firedfrom the power industry will continue to increasepower generation(including heat generation) contributedabout 46.4% of total CO. emissions (l. With the continuousIn the Copenhagen Conference in December 2009, the development of economy and the adjustment of energyChinese government committed that China would achieve structure there is a trend that the ratio will be further40%0-45% reduction of carbon emissions per unit of GDP increasedcompared with the emissions in 2005. This has been中国煤化工included in the national economic development guidance as aCN MH Ghrough separationmandatory objective. As a major source of CO, emissions, and capture of Cm the tlue gas for recovery andELECTRICITY 2012.1LECTRICITYpermanent sequestration, is considered to be the most packed fluidized beds by using the intermolecular forceeffective approach for Co, reduction. The co, capture is a between the gas molecules and the active sites on thekey part of the treatment and requires in-depth research. At adsorbent surface. The commonly used adsorbents in thepresent, there are many mature theories and techniques for fluidized beds for CO, capture include zeolites, activatedCO, capture, such as chemical absorption, physical adsorption, carbon, molechular sieve. aluminamembrane separation and cryogenic separation. Along material Physical adsorption systems are operated inwith the in-depth study of this field, the capture techniques pressure swing adsorption(PSA)and temperature swingkeep improving. The study is now focused on simplifying the adsorption(TSA)modes. In PSA, gas is adsorbed at atreatment processes and reducing the capture costshigher pressure. Then pressure is reduced to desorb the gas.In TSA, the gas is adsorbed at a lower temperature. Then,Chemical absorptionthe temperature is raised to desorb the gas. PSA and TSAChemical absorption technique achieved CO, capture modes can be combined as a PtSa mode. researchersthrough chemical scrubbing of flue gas. It uses solvents to in the East China University of Science and Technologyreact with CO,. The solvents will be regenerated through developed an Electric Swing Adsorption(ESA)technologyinverse reactions, and the released CO, will be captured. The principle of this technology is to use the conductivityThough the high purification of Co, being captured, the of the new adsorbent. The regeneration of adsorbents takestechnique requires high investment and high operating place by passing a low-voltage electric current through thecosts,also has equipment corrosion problem during adsorbent. The energy consumption is reduced comparedoperationwith traditional steam heating methodFlue gas scrubbing using amine-based absorbents has Zhang Hui et al [) of China applied cheap industrial silicabeen widely used in CO, reduction in coal-fired power gel as an adsorbent, and used a three towers eight-stepsplants, which has been one of the most mature CO, reduction PSA process in a pilot study. They studied the adsorption oftechnologies. By spaying amine compounds solutions in flue co, with middle to low CO, concentration in flue gas. Thisgas, CO, is combined with amine compounds and absorbed method can increase the captured CO, product concentrationby the solvent. The CO, absorbed in the solutions will be above 74%, thus the Co, capture cost is reduced. Zhang Junreleased by heating the solution and amine compounds are and Paul a t9) abroad improved the zeolite adsorbent PSaregenerated. To date, China has put into operation three process. The CO, adsorption process takes place withincarbon capture demonstration projects in coal-fired power the fluidized bed with pressure only slightly higher thanplants, all of which use amine solvent scrubbing method. the atmospheric pressure. The desorption process happensThe key of this method is the absorbent selection. Current under the vacuum condition. This method is called vacuumresearches have been focused on the development of pressure swing adsorption(VSA). The nine-step method withhigh efficient and low consumption composite absorbent purification process and six-step method without purificationand the optimization of the composite combination ratio.The commonly used amine-based absorbents includemonoethanolamine(MEA), diethanolamine(DEA), andmethyldiethanolamine(MDEA),etcJacob N, Knudsen et al (6) performed a study on a pilotsystem with I th of Co, emission rate. The performanceof the traditional absorbents and the new absorbents theyhave developed were compared under different flue gasconditions. The results showed that, the Co, captureefficiency can reach 90%, and the cost of CO, capture canbe further reduced. Researchers in China also have madesignificant progresses in developing new adsorbents[. If thehigh energy consumption and equipment corrosion problemscan be solved, the technology will be widely used in coal-fired power plantsPhysical adsorption中国煤化工The method of physical adsorption is applied on theCNMH2012.1 ELECTRICITYSEEprocess were compared. The results showed that the Co, storage, ocean storage and mineral storageproduct from the nine-step method has a purity up to 90%to 95% and the capture cost is $18-32/t co In contrastEcologicalthe product from the six-step method has a purity up toEcological storage of Co, is to use the capacity of82%0-83%, and the capture cost is $12-25/t CO Compared ecosystem to absorb CO,, which requires effectivelwith other CO, capture technology, this technique has the preventing deforestation and avoiding degradation ofadvantage of low costgrassland. In addition, great efforts must be taken to developmore green areas, including planting trees, expanding theMembrane separationpasture area, using aquatic organisms such as algae toMembrane separation includes two types: gas separation increase carbon sequestration capacity and maximize themembrane and gas absorption membrane. the gas absorption of coseparation membrane uses the selectivity of the membranematerial to separate the CO The gas absorption membrane Either by direct emission into atmosphere or through theuses the microporous membrane as a contact reactor, industrial use of recovered Co, (i. e used for refrigeration,and the liquid absorbent and flue gas flow around both food packaging, welding, beverage, fire-fighting materialssides of the membrane where the adsorbent selectively production of urea, and co, gas fertilizers, etc. ) CO, willadsorb co, in flue gas. The gas separation membrane eventually enter the environment. Ecological storage is anmethod has the advantages of low investment, compact ideal and the cheapest method for CO, storage. However itequipment arangement, less space requirement, low energy requires significant green areas. A 500-MW coal-fired powerconsumption, simple operation and easy maintenance, which plant requires approximately 2 000 square kilometers(3000can be linearly scaled up and applied to power plants with thousand acres)of forest to capture its CO, emission 29.large flue gas flowTherefore, this method is not practical to be used as a mainCO2 storage mode for power plantsThe characteristics of the separation membrane andabsorbents determine the performance of separation. Geological sequestrationThe currently ongoing studies are mainly focused Geological storage is to pressurize co, and fill it intoon the improvements of these two aspects. The high the appropriate ground level, and use the pore space to storemolecular microporous polymeric membranes, such as CO,. This includes the use of depleted oil or gas wells, non-polytetrafluoroethylene(PTFE), polypropylene(PP) and minable coal mines, and deep brine layerpolyethylene(Pe)can be combined with some absorbentssuch as organic amines and amino acidic salts to improve Geological storage of co, has great applicationseparation efficiently. Lu Jiangang etc. o-l used poly- potentials. It is estimated that the depleted oil and gas wellspropylene(PP) hollow fiber membrane as a contactor, and globally have a total capacity up to 920 Gt. Some studiesstudied the effects of adding different organic and inorganic have shown that storage of co, in oil wells will helpsalts to the absorbent methyl diethanolamine ( MDEAimprove the oil well re-production rate and extend the lifeof oil well. Till 2007, there had been more than 70 oil wellsLow temperature separationinjected with CO, to enhance oil recovery rate. Injectingc Low-temperature gas separation is to repeatedly co, into the non-minable coal mines can also increase theompress and cool down the flue gas to liquefy CO, and re-producing rate, and is considered as a"value added"cOseparate it from flue gas. This includes direct distillation storage method. This type of "value added"storage greatlyseparation, double-column distillation, use of additives, and reduces the cost of Co, reduction. The storage costs alonefreezing controlling. These processes with simple ecan be lower than S5/t co. due to these advvantages, thsystems are technically reliable. It was reported that this method is one of the most active investigated methods.Themethod was able to achieve a co, recovery rate of 90% and China Shenhua Erdos coal liquefaction project will capturea purity of 97%. However, this method requires huge energy 3 000 000 t (92% purity) of CO,, of which 100 000 tonsconsumption,which has limited its applicationswill be used for geological storage. The U.S. SouthlandCorporation has performed three years pilot study withcO, sequestration technologiesa total injection of 7 500 tons CO,. Starting from 2011the company will annually inject 150 000 tons of cOAccording to the forms of the final destinations, CO, captured fromsequestration is divided into four types: eco-seal, geological oil fieldHa中国煤化工 o the citronelleCNMHGELECTRICITY 2012.1LECTRICITYGeological storage still has some technical risks. Forxample, the stored CO, may leak during the earthquake,causing a quick increase of co, concentration in atmosphereIt was also reported that cO, stored in deeper undergroundlevels may damage the surrounding mineral materialstructure. But so far. no conclusions have been made onwhether or not such damages can cause leak of CO,Ocean storageOcean storage is to use pipelines to pump highpressure CO, or drop dry ice form Co, to over 1 000 mdeep ocean for natural dissolution. Co, can also beinjected below 3 000 m depth of ocean. Since the densityof co, is bigger than the density of water, it will form asolid CO, hydrate or liquid CO,Lake, "and solidify at to 180% to store co, 4]. Countries around the world arethe storage point. This will delay the decomposition of actively investigating economically feasible technologiesCO, into the environment. Ocean storage has the largest to reduce the cost of Co, emission. Currently the U.SCO2 storage potential and is a very efficient storage Finland, Netherlands and other countries have performedmethodin-depth studies. In recent years, China has also started theresearch in this areaXu Yongfu et al [2 performed numerical study onthe effectiveness of ocean storage of CO2 varied with CO, mineral carbonation process is usually divideddepth. The results showed that, when injected into the into two methods: direct and indirect methods. In thedepth over 1 500 m, the remaining CO2 storage after 50 direct method, Co, reacts with the ore particles to formyears will be up to 95% of the amount injected. With carbonate salts. In the indirect method, the extractionthe increase of the injection depth, the storage efficiency agent is used to extract calcium and magnesium ions fronincreases. When the depth reaches 3 000 m, more than the ore, then the carbonation reactions are proceeded to99% of CO, will remain in ocean after 100 years storage. form carbonate salts and media. The media is thenHowever, another study showed that the injection of recycled. However both the direct and indirect methodshigh concentration of dissolved CO, will lower the pH are not mature yet for industrial applications. The costs ofof seawater to 4.5. This will affect the marine biological these methods are significantly higher than other storagesystems at different levels, including the zooplankton, technologies[isIbacteria and benthic animals(3). And the stored co. stillhas possibility to return to the atmospheric environment, Wouter J. J. Huijgen etc6. performed cost estimateso that the injection depth has to be over 3 000 m. This on the mineralization storage of CO, which was foundgreatly increases the technical difficulty and storage costs. that 50% of the total cost is used to purchase the oreTo reduce the cost, solid wastes such as waste steelMineralization storageslagi7, construction materials(s), waste incinerationMineralization storage of CO2 is to use natural bottom ashil and coal fired ash(20) etc which have highminerals for carbonation reactions. The formed stabilized alkaline level and rich Ca, Mg compositions, can be usedcarbonate salts are fixed in minerals. The suitable natural instead of ore. Researchers at the Jinaneversminerals include Brucite, Serpentine, Wollastonite and studied the use of co, to carbonize steel slag to produceother magnesium-rich or calcium-rich silicate minerals. manufacture products. The pilot study has been startedThe reaction products are stable magnesium carbonate If the produced manufacture products can meet the releva(MgCO,) and calcium carbonate( Caco,, limestone).The nt standards, this process will have very broad applicationreaction is as followsprospect as it uses one waste product to treat anotherwaste product2.(Mg,Ca)SiO→2+xCO2→+x(Mg,Ca)CO+ysioSelection of flue gas co. capture andBecause the natural carbonation reactions are verystorage te中国煤化工slow, pre-processing of minerals is required. Due to itsCNMHGhigh energy consumption, power plants will require 60% The CO, caput prullsscs ll clUer the existing512012.1 ELECTRICITSEEpower plants or the new power plants require high this storage method is that it has special geologicalinvestment and the project sizes are big. The key requirements. Only the power plants that can meetfactors that need to be considered include: the choice the geological storage conditions are suitable for thisof capture technology, site space, infrastructures, and methodeconomic analysis, etc. The fate of CO, is especiallyneeded to be considered, as it is closely related to the Geological storage method does not have Co, qualitysite locations of the power plants. For the existing requirements. It also has huge storage capacity. Thereforepower plants, the site location determines the fate of this method is suitable for power plants with largeCO,, and the fate of Co, determines the choice of the capacity, and it will allow the power plants to use low costCO2 capture technology. For the new power plants, the and large capacity flue gas CO, capture methodsfate of Co, capture technology affects the site selectionof the power plant and the selection of the CO, capture Ocean storage of Coprocess. On the other hand the selection of the coPower plants in coastal areas can consider ocean storagecapture process can in turn affect the choice of Co, of CO,. Although oceans have huge Co, storage capacity,storage options.this storage method does not provide the power plantswith any compensation for the cost on CO, capture.TheChina has a vast territory, complex geological plants need to pay additional storage cost over timecharacteristics, and unbalanced industrial development. Therefore, economic analysis must be performed inCoal-fired power plants are distributed throughout the selecting an appropriate storage methodcountry. Therefore, to determine the site locations of newpower plants and Co, capture programs, the fate of theCO. mineralizationcO. should be considered firstBased on the current status of the tudies. mineralizationis the most expensive method of CO, sequestrationCO, for industrial applicationsHowever this method has several advantages: 1)Coal-fired power plants have large amount of the natural minerals for CO, mineralization are richO, emissions. In order to use the captured CO, for in storage and easy for mining: 2) this method hasindustrial applications, the CO, requirements in the simple principle. The carbonation of the minerals issurrounding areas and the transportation capacity should an exothermic reaction which theoretically is feasiblebe considered. The power plants that are close to the 3)the product is stable carbonate, and can be permanentlyareas with chemical plants, food plants, and other plants stored. There is no need for monitoring of the potentialrequire large amount of CO, resource, and the power leakage. Therefore this method has the best environmentalplants near the coasts that can take advantage of lower benefits; 4)Compared with other methods, this methodtransport cost, may consider to use the captured CO, for is less limited by the site locations of the power plants; 5ndustrial applicationsTheoretically it is possible to achieve direct mineralizationof CO, in flue gas. Although it is difficult to reduce theDifferent industrial applications may have different cost of the COMehod mineralization to the same level as that ofuality requirements for CO,. To produce the carbonated other methods in a short time, this method should havedrinks, the cO, needs to reach food grade purity. great application potentials in areas with no oil, gas, coalThus, high purity capture technologies should be mines and chemical industriesused, for example, amine quenching capture, andmembrane separation method. Other applications, suchConclusions and recommendationsas manufacturing the carbon steel slag constructionmaterials, may accept low quality CO,, and may use 1)Since the 1980s, China's power industry has beenlower cost CO, capture technologies, such as physical developing very rapidly. The power generating capacityadsorption separation methodand electricity generation have maintained a high growthrate. The installed capacity of thermal power keeps atco, for geological storageabout 3/4 of the total installed capacity, and thermal powerGeological storage of CO, has several options, including generation accounts for more than 4/5 of total electricityinjecting into the oil ficlds to improve oil recovery generation. It is expected that coal-fired based electricityrate(EOR), injecting into the gas field, non-minable generation wilcoal mines and deep brine layers. The main constraint of of time中国煤化工 or a long periodCNMHG52ELECTRICITY 2012.1LECTRICITY ass2) Thermal power plants are the largest source of co, focus on theoretical studies, leading-edge technologyemissions, and their contributions to the total emission development, and industrial application studies. Specialhas a trend to be further increased. With more and focus should be put on the comparison of various similarmore pressure from the requirements of reducing CO, processes, and maximally save resourcesemissions, more in-depth researches have been done.Varies CO, capture and storage technologies are more and References:more mature. The coal-fired power plants should analyzethe application potentials of each option before making [1] OECD, IEA Co, Emissions from Fuel Combustionthe selection(2010 Edition)[M]. Paris: OECD Publishing, 2010[2] Zheng Y, Chi B H, Wang B W, et al. CO, emission3)Currently amine leaching is the most mature fluecontrol technology for coal combustion []. Electricgas Co, capture method. It has advantages of high capturePower,2006,39(10):91-94efficiency and can produce high quality CO,. However, [3] Wolsky A M, Daniels E J, Jody B J Recovering COthe disadvantages of this method include its high technicalfrom large- and medium-sizecombustorsdifficulties and high cost. It is suitable to be used for [] Air Waste Manage 1991 (41): 449-454providing the Co, resources to high value-added Co, [4] Mao Y R, Luo Z Y, Yu sr, et al. A study on COapplications; physical adsorption separation produces lowemission decreasing and controlling techniques forpurity CO,, but the cost is low. Its CO, product is mainlythermal power plants [] Thermal Power Generation,used for the applications requiring lower Co, quality,2003,(1):39-41such as for the geological storage or other purposes; If [5] Hong D J, Zhang D H. Ways to reduce the emissionmore suitable membrane material and absorbents can beof Co, []. Electric Power Environmental Protection,developed, membrane separation will be a promising CO2006,22(6):5-8recovery method due to its low initial investment and low [6] Jacob N K, Jogen N J, Poul J V. Experience with CO,operating costscapture from coal flue gas in pilot-scale testing ofdifferent amine solvents [J]. 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The national total consumption of the problems faced by the countryfossil fuels declined slightly in 201l, but the country stillChina plans to add 20 Gw of installed hydropowerfaces great challenges in optimizing its energy structure. capacity in 2012, while prioritizing ecological protectionAccording to a report by China Daily, non-fossil fuels and the relocation of residents in areas designated foraccounted for 9. 4 percent of Chinas overall primary major hydropower projects. According to China Electricityenergy consumption in 2011, compared with 8.7 percent Council, China's hydroelectric energy generation decreasedin 2010. The figure means the country needs to increase by 3.5 percent year-on-year in 2011, which resulted in athe proportion of non-fossil fuels in the mix by 0.5 percent great increase in the use of coal. The Council also reportedannually in the coming four years to realize a govemment that nuclear power output increased by 16.9 percent,target of 11. 4 percent by 2015. However, China is facing wind power increased by 48.2 percent and thermal powergreat pressure to reach the target because the efforts to production increased by 14. 1 percent over the same yearbuild the nation's nuclear capacity have slowed.China will adopt more policies aimed at stimulatingThe newly added capacity for coal output reached 95 the development of renewable energy. The government ismillion tons in 2011, and the country plans to add another also expected to launch the Renewable Portfolio Standard,200 million tons in 2012, according to the National Energy a scheme that will require electricity suppliers to provideConference in Januarya minimum level of electricity generated from renewableThe country is caught between the urgent need to sources. The mechanism, which is still at the design stagesave energy and reduce greenhouse gas emissions and the but is expected to be unveiled soon, will focus on electricdesire to increase energy generation to sustain economic energy generated from sources such as wind, solar anddevelopment. Reducing dependency on coal by promoting biomass.中国煤化工the use of cleaner energy sources, including wind, solarCNMHG54ELECTRICITY 2012.1