Flue gas desulphurization for hot recycle Oxyfuel combustion: Experiences from the 30 MWth Oxyfuel pilot plant in Schwarze Pumpe

Significant differences exist in the flue gas composition in hot recycle Oxyfuel conditions as e.g. the high CO₂ partial pressure (>90 vol%, dry), the very high SO₂ concentration and the high water content (approx. 30 vol%). Therefore certain design and operation criteria have to be observed for the flue gas desulphurization with forced oxidation under Oxyfuel combustion conditions. Several performance tests have been executed at the 30 MW_th Oxyfuel pilot plant in Schwarze Pumpe to evaluate the main performance parameters and to assess the influence of the major operation parameters. The results show that there are no fundamental problems for the operation of the flue gas desulphurization unit under Oxyfuel combustion conditions. High removal rates could be reached and no negative impact of the high CO₂ partial pressure was observed under the tested operating conditions. No major differences in the gypsum quality have been observed between air firing and Oxyfuel conditions.     

Ash and deposit formation from oxy-coal combustion in a 100 kW test furnace

Ash deposition is still an unresolved problem when retrofitting existing air-fired coal power plants to oxy-fuel combustion. Experimental data are quite necessary for mechanism validation and model development. This work was designed to obtain laboratory combustor data on ash and deposits from oxy-coal combustion, and to explore the effects of oxy-firing on their formation. Two bituminous coals (Utah coal and Illinois coal) and one sub-bituminous coal (PRB coal) were burned on a down-fired combustor under both oxy- and air-firing. Two oxy-fired cases, i.e., 27 vol% O₂/73 vol% CO₂ and 32 vol% O₂/68 vol% CO₂, were selected to match the radiation flux and the adiabatic flame temperature of air combustion, respectively. Once-through CO₂ was used to simulate fully cleaned recycled flue gas. The flue gas excess oxygen was fixed at 3 vol%. For each case, both size-segregated fly ash and bulk fly ash samples were obtained. Simultaneously, ash deposits were collected on an especially designed un-cooled deposition probe. Ash particle size distributions and chemical composition of all samples were characterized. Data showed that oxy-firing had insignificant impacts on the tri-modal ash particle size distributions and composition size distributions in the size range studied. Bulk ash compositions also showed no significant differences between oxy- and air-firing, except for slightly higher sulfur contents in some oxy-fired ashes. The oxy-fired deposits were thicker than those from air-firing, suggesting enhanced ash deposition rates in oxy-firing. Oxy-firing also had apparent impacts on the deposit composition, especially for those components (e.g., CaO, Fe₂O₃, SO₃, etc.) that could contribute significantly to ash deposition. Based on these results, aerodynamic changes in gas flow and changes in combustion temperature seemed more important than chemical changes of ash particles in determining deposit behavior during oxy-coal combustion.     

Biogas combustion in a chemical looping fluidized bed reactor

Chemical looping combustion (CLC) is a process in which oxygen required for combustion of a fuel is supplied by the metal oxide. Metal oxide plays the role of an oxygen carrier by providing oxygen for combustion when being reduced and is then re-oxidized by air in a separate reactor. Combustion is thus without any direct contact between air and fuel: as a consequence flue gas does not contain nitrogen of air which simplifies flue gas treatment prior to sequestration. In the present study, biogas combustion was analyzed in a chemical looping combustion fluidized bed reactor. NiAl_0.44O_1.67 and Cu_0.95Fe_1.05AlO₄ metal oxide particles were used as oxygen carriers. The experiments have shown the feasibility of biogas combustion in chemical looping combustion: CH₄ of the biogas was completely converted to CO₂ and H₂O with a small fraction of CO and H₂. The outlet flue gas distribution profile was not affected by ageing during the cycles of reduction and oxidation, indicating the chemical stability of the oxygen carriers. There was limited formation of carbon on the oxygen carriers during reduction.     

Study of Hg and SO3 behavior in flue gas of oxy-fuel combustion system

Oxy-fuel combustion systems have been under development to reduce CO₂ emissions from coal-fired power plants. In oxy-fuel combustion system, Hg in the flue gas causes corrosion in CO₂ purification and compression units. Also, SO₃ in the flue gas corrodes the equipment and ducts of oxy-fuel combustion system. Therefore, Hg and SO₃ need to be removed.Babcock-Hitachi conducted tests using a 1.5 MWth Combustion & Air Quality Control System (AQCS) test facility which consists of oxygen supply unit, furnace, Selective Catalytic Reduction (SCR) catalyst, Clean Energy Recuperator (CER), Dry Electrostatic Precipitator (DESP), flue gas recirculation system, Wet Flue Gas Desulfurization (WFGD), and CO₂ Compression and Purification Unit (CPU). In both cases of air and oxy-fuel combustion, the Hg removal across the DESP could be improved, and SO₃ concentration at the DESP outlet could be reduced to less than 1 ppm by installing a CER upstream of the DESP and reducing the gas temperature at the DESP inlet. Hg was not dissolved in the drain recovered from CO₂ compressor, and may be adsorbed at an inner part of CO₂ compressor. This indicated that Hg needs to be removed at a location upstream of the CO₂ compressor to prevent corrosion of the compressor.     

Oxy-fuel coal combustion—A review of the current state-of-the-art

Carbon dioxide emissions will continue being a major environmental concern due to the fact that coal will remain a major fossil-fuel energy resource for the next few decades. To meet future targets for the reduction of greenhouse gas (GHG) emissions, capture and storage of CO₂ is required. Carbon capture and storage technologies that are currently the focus of research centres and industry include: pre-combustion capture, post-combustion capture, and oxy-fuel combustion. This review deals with the oxy-fuel coal combustion process, primarily focusing on pulverised coal (PC) combustion, and its related research and development topics. In addition, research results related to oxy-fuel combustion in a circulating fluidised bed (CFB) will be briefly dealt with.During oxy-fuel combustion, a combination of oxygen, with a purity of more than 95 vol.%, and recycled flue gas (RFG) referred to as oxidant is used for combusting the fuel producing a gas consisting of mainly CO₂ and water vapour, which after purification and compression, is ready for storage. The high oxygen demand is supplied by a cryogenic air separation process, which is the only commercially available mature technology. The separation of oxygen from air as well as the purification and liquefaction of the CO₂-enriched flue gas consumes significant auxiliary power. Therefore, the overall net efficiency is expected to be decreased by 8–12% points, corresponding to a 21–35% increase in fuel consumption. Alternatively, ion transport membranes (ITMs) are proposed for oxygen separation, which might be more energy efficient. However, since ITMs are far away from becoming a mature technology, it is widely expected that cryogenic air separation will be the selected technology in the near future. Oxygen combustion is associated with higher temperatures compared with conventional air combustion. Both fuel properties as well as limitations of steam and metal temperatures of the various heat exchanger sections of the boiler require a moderation of the temperatures in the combustion zone and in the heat-transfer sections. This moderation in temperature is accomplished by means of recycled flue gas. The interdependencies between the fuel properties, the amount and temperature of the recycled flue gas, and the resulting oxygen concentration in the combustion atmosphere are reviewed.The different gas atmosphere resulting from oxy-fuel combustion gives rise to various questions related to firing, in particular, with respect to the combustion mechanism, pollutant reduction, the risk of corrosion, and the properties of the fly ash or its resulting deposits. In this review, detailed nitrogen and sulphur chemistry was investigated in a laboratory-scale facility under oxy-fuel combustion conditions. Oxidant staging succeeded in reducing NO formation with effectiveness comparable to that typically observed in conventional air combustion. With regard to sulphur, a considerable increase in the SO₂ concentration was measured, as expected. However, the H₂S concentration in the combustion atmosphere in the near-flame zone increased as well. Further results were obtained in a pilot-scale test facility, whereby acid dew points were measured and deposition probes were exposed to the combustion environment. Slagging, fouling and corrosion issues have so far been addressed via short-term exposure and require further investigation.Modelling of PC combustion processes by computational fluid dynamics (CFD) has become state-of-the-art for conventional air combustion. Nevertheless, the application of these models for oxy-fuel combustion conditions needs adaptation since the combustion chemistry and radiative heat transfer is altered due to the different combustion gas atmosphere.CFB technology can be considered mature for conventional air combustion. In addition to its inherent advantages like good environmental performance and fuel flexibility, it offers the possibility of additional heat exchanger arrangements in the solid recirculation system, i.e. the ability to control combustion temperatures despite relatively low flue gas recycle ratios even when combusting in the presence of high oxygen concentrations.     

Mercury measurement and control in a CO2-enriched flue gas

Oxycombustion is being considered as a promising solution to carbon capture and sequestration. Standard sampling and measurement methods may or may not be valid under oxycombustion conditions because the flue gas differs significantly from that of conventional air-blown coal combustion.Bench-scale tests were conducted to evaluate the measurement validity of continuous mercury monitors (CMMs), with and without a flue gas preconditioning unit, in a simulated oxycombustion flue gas with varied CO₂ concentrations. Tests also included mercury capture with activated carbon in typical oxyfuel combustion flue gas. Research data indicated that highly concentrated CO₂ streams affect the accuracy of the mass flow rate and the subsequent gaseous mercury measurement, although this is specific to the type of CMM. Concentrated CO₂ streams also induced solid precipitation in the wet-chemistry conversion unit and resulted in a biased measurement of the gas-phase mercury. Flue gas dilution appeared to provide accurate measurement of total gas-phase mercury and be applicable to mercury measurement in highly concentrated CO₂ streams, although mercury speciation appeared to be problematic and will require additional modification and validation. Mercury capture with activated carbon under CO₂-enriched conditions showed similar performance to typical high-acid coal combustion flue gas.     

双回路热解炉垃圾焚烧NOx排放特性研究

本文利用双回路立式热解焚烧炉对某地区的垃圾进行试验研究,通过调节过量空气系数,一、二次风机的风量等参数,研究双回路热解炉垃圾焚烧NOx排放特性.结果表明：焚烧炉的二燃室温度能够达到850℃,烟气停留时间超过2秒,燃烧灰的热灼减率小于3％.随着引风机风量系数的增加,烟气中NOx的含量先增加后减少;二燃室的温度先升高后降低.     

Validation of spectral gas radiation models under oxyfuel conditions—Part B: Natural gas flame experiments

Combustion of hydrocarbon fuels with pure oxygen results in a different flue gas composition as combustion with air. Standard CFD spectral gas radiation models for air blown combustion are out of their validity range. The series of three articles provides a common spectral basis for the validation of new developed models. Part B of the series presents spectral measurements in the spectral range of 2.4–5.4 μm of a 70 kW turbulent natural gas flame in air blown combustion and in wet and dry oxyfuel combustion. The experimentally measured intensity spectra were compared with simulated spectra based on measured gas atmospheres. The line-by-line database HITEMP2010 and the two statistical-narrow-band models EM2C and RADCAL were used for the numerical simulation. The measured spectra showed large fluctuations due to turbulence. The up to 75% increased averaged experimental intensity compared to the simulated intensity pointed out the importance of the effect of turbulence-radiation-interaction in combustion simulations.     

Assessment of the different parameters affecting the CO2 purity from coal fired oxyfuel process

The oxyfuel process is one of the most promising options to capture CO₂ from coal fired power plants. The combustion takes place in an atmosphere of almost pure oxygen, delivered from an air separation unit (ASU), and recirculated flue gas. This provides a flue gas containing 80–90 vol% CO₂ on a dry basis. Impurities are caused by the purity of the oxygen from the ASU, the combustion process and air ingress. Via liquefaction a CO₂ stream with purity in the range from 85 to 99.5 vol% can be separated and stored geologically. Impurities like O₂, NO_X, SO_X, and CO may negatively influence the transport infrastructure or the geological storage site by causing geochemical reactions. Therefore the maximum acceptable concentrations of the impurities in the separated CO₂ stream must be defined regarding the requirements from transportation and storage. The main objective of the research project COORAL therefore is to define the required CO₂ purity for capture and storage.     

双回路立式垃圾热解炉试验研究

利用双回路立式热解焚烧炉对某地区的城市生活垃圾进行试验研究,结果表明,焚烧炉的二燃室温度能够达到850℃,烟气停留时间超过2s,燃烧灰的热灼减率小于3%。在废气排放口测到的数据表明：常规烟气的各种有害气体成分含量均能满足标准的要求;二噁英的含量不仅满足我国标准的要求,也远远低于欧盟的标准限值。     

Validation of spectral gas radiation models under oxyfuel conditions. Part A: Gas cell experiments

Combustion of hydrocarbon fuels with pure oxygen results in a different flue gas composition as combustion with air. Standard CFD spectral gas radiation models for air combustion are out of their validity range. The series of three articles provides a common spectral basis for the validation of new developed models. In part A of the series gas cell transmissivity spectra in the spectral range of 2.4–5.4 μm of water vapor and carbon dioxide in the temperature range from 727 to 1500 ° C and at different concentrations were compared at a nominal resolution of 32 cm^?1 to line-by-line models from different databases, two statistical-narrow-band models and the exponential wide band model. The two statistical-narrow-band models EM2C and RADCAL showed a good agreement with a maximal band transmissivity deviation of 3%. The exponential-wide-band model showed a deviation of 6%. The new line-by-line database HITEMP2010 had the lowest band transmissivity deviation of 2.2% and was recommended as a reference model for the validation of simplified CFD models.     

The thief process for mercury removal from flue gas

The Thief Process is a cost-effective variation to activated carbon injection (ACI) for removal of mercury from flue gas. In this scheme, partially combusted coal from the furnace of a pulverized coal power generation plant is extracted by a lance and then re-injected into the ductwork downstream of the air preheater. Recent results on a 500-lb/h pilot-scale combustion facility show similar removals of mercury for both the Thief Process and ACI. The tests conducted to date at laboratory, bench, and pilot-scales demonstrate that the Thief sorbents exhibit capacities for mercury from flue gas streams that are comparable to those exhibited by commercially available activated carbons. A patent for the process was issued in February 2003. The Thief sorbents are cheaper than commercially-available activated carbons; exhibit excellent capacities for mercury; and the overall process holds great potential for reducing the cost of mercury removal from flue gas. The Thief Process was licensed to Mobotec USA, Inc. in May of 2005.     

The modelling of a hybrid combined cycle with pressurised fluidised bed combustion and CO2 capture

This study investigates the possibility of capturing CO₂ from flue gas under pressurised conditions, which could prove to be beneficial in comparison to working under atmospheric conditions. Simulations of two hybrid combined cycles with pressurised fluidised bed combustion and CO₂ capture are presented. CO₂ is captured from pressurised flue gas by means of chemical absorption after the boiler but before expansion. The results show a CO₂ capture penalty of approximately 8 percentage points (including 90% CO₂ capture rate and compression to 110 bar), which makes the efficiency for the best performing cycle 43.9%. It is 5.2 percentage points higher than the most probable alternative, i.e. using a natural gas fired combined cycle and a pulverised coal fired condensing plant separately with the same fuel split ratio. The largest part of the penalty is associated with the lower mass flow of flue gas after CO₂ capture, which leads to a decrease in work output in the expander and potential for feed water heating. The penalty caused by the regeneration of absorbent is quite low, since the high pressure permits the use of potassium carbonate, which requires less regeneration heat than for example the more commonly proposed monoethanolamine. Although the efficiencies of the cycles look promising it will be important to perform a cost estimate to be able to make a fair comparison with other systems. Such a cost estimate has not been done in this study. A significant drawback of these hybrid cycles in that respect is the complex nature of the systems that will have a negative effect on the economy.     

旋转喷雾半干法烟气净化技术研究进展

sox是垃圾焚烧发电的主要大气污染物之一。要控制s02的排放量,就必须采取有效的烟气净化技术,如干法烟气净化技术、旋转喷雾半干法、流化床半干法和石灰石一石膏湿法烟气净化技术等。旋转喷雾半干法是有效的烟气净化技术之一,雾化器作为旋转喷雾半千法的关键设备,其雾化效果直接影响到吸收剂的利用率及硫化物的去除率。喷雾液滴粒径大小是衡量雾化器雾化效果的重要指标,是影响尾气中硫化物脱除效果的关键因素。     

Chemical-looping combustion using syngas as fuel

Chemical-looping combustion (CLC) is a combustion technology where an oxygen carrier is used to transfer oxygen from the combustion air to the fuel, avoiding direct contact between air and fuel. Thus, CO₂ and H₂O are inherently separated from the rest of the flue gases and the carbon dioxide can be obtained in a pure form without the use of an energy intensive air separation unit. The paper presents results from a 3-year project devoted to developing the CLC technology for use with syngas from coal gasification. The project has focused on: (i) the development of oxygen carrier particles, (ii) establishing a reactor design and feasible operating conditions and (iii) construction and operation of a continuously working hot reactor. Approximately, 300 different oxygen carriers based on oxides of the metals Ni, Fe, Mn and Cu were investigated with respect to parameters, which are important in a CLC system, and from these investigations, several particles were found to possess suitable qualities as oxygen carriers. Several cold-model prototypes of CLC based on interconnected fluidized bed reactors were tested, and from these tests a hot prototype CLC reactor system was constructed and operated successfully using three carriers based on Ni, Fe and Mn developed within the project. The particles were used for 30–70 h with combustion, but were circulated under hot conditions for 60–150 h.     

Effect of gas composition in Chemical-Looping Combustion with copper-based oxygen carriers: Fate of light hydrocarbons

Chemical-Looping Combustion (CLC) is an emerging technology for CO₂ capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. Natural or refinery gas can be used as gaseous fuels and they may contain different amounts of light hydrocarbons. This paper presents the combustion results obtained with a Cu-based oxygen carrier using mixtures of CH₄ and light hydrocarbons (LHC) (C₂H₆ and C₃H₈) as fuel. The effect on combustion efficiency of the fuel reactor temperature, solid circulation flow rate and gas composition was studied in a continuous CLC plant (500 W_th). Full combustions were reached at 1073 and 1153 K working at oxygen to fuel ratios, ? higher than 1.5 and 1.2 respectively. Unburnt hydrocarbons were never detected at any experimental conditions at the fuel reactor outlet. Carbon formation can be avoided working at 1153 K or at ? values higher than 1.5 at 1073 K. After 30 h of continuous operation, the oxygen carrier exhibited an adequate behavior regarding attrition and agglomeration. It can be concluded that no special measures should be taken in a CLC process with Cu-based OC with respect to the presence of LHC in the fuel gas.     

Effect of gas composition in Chemical-Looping Combustion with copper-based oxygen carriers: Fate of sulphur

Chemical-Looping Combustion (CLC) is an emerging technology for CO₂ capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. Natural or refinery gas can be used as gaseous fuels and they may contain different amounts of sulphur compounds, such as H₂S and COS. This paper presents the combustion results obtained with a Cu-based oxygen carrier using mixtures of CH₄ and H₂S as fuel. The influence of H₂S concentration on the gas product distribution and combustion efficiency, sulphur splitting between the fuel reactor (FR) and the air reactor (AR), oxygen carrier deactivation and material agglomeration was investigated in a continuous CLC plant (500 W_th). The oxygen carrier to fuel ratio, ?, was the main operating parameter affecting the CLC system. Complete fuel combustion were reached at 1073 K working at ? values ≥1.5. The presence of H₂S did not produce a decrease in the combustion efficiency even when working with a fuel containing 1300 vppm H₂S. At these conditions, the great majority of the sulphur fed into the system was released in the gas outlet of the FR as SO₂, affecting to the quality of the CO₂ produced. Formation of copper sulphide, Cu₂S, and the subsequent reactivity loss was only detected working at low values of ? ≤ 1.5, although this fact did not produce any agglomeration problem in the fluidized beds. In addition, the oxygen carrier was fully regenerated in a H₂S-free environment. It can be concluded that Cu-based oxygen carriers are adequate materials to be used in a CLC process using fuels containing H₂S although quality of the CO₂ produced is affected.     

Polycyclic aromatic hydrocarbons (PAHs) and estrogenic compounds in experimental flue gas streams

The importance of combustion processes as a source of substances with estrogenic activity in the environment was investigated. Wood (nontreated and treated with wood preservatives), barbecue charcoal, meat, and kitchen waste were combusted in a laboratory-scale incinerator. Flue gas emissions (particulates and gaseous pollutants) were trapped in polyurethane foam cartridges. The cartridges were subjected to Soxhlet extraction and part of the extracts redissolved in dimethylsulfoxide (DMSO) for analyses of estrogenic activity by means of the yeast-based human estrogen receptor (hER) bioassay. A synthetic estrogen, 17-alpha-ethinylestradiol (EE2), was used as the reference estrogenic compound. Part of the extracts was analyzed for the 16 USEPA priority polycyclic aromatic hydrocarbons (PAHs). Estrogenic compounds in the flue gas (wood) were as high as 234 +/- 25 ng m(-3) EE2 equivalent compared with 27 to 81 ng m(-3) EE2 equivalent in flue gas from combustion of barbecue charcoal. Concentrations of polycyclic aromatic hydrocarbons in both flue gas streams were in the range of 21,000 +/- 2000 and 240 +/- 110 ng m(-3), respectively. In general, the concentrations of EE2 equivalent in the flue gas samples were at least a factor of 1000 lower than total PAH concentration. The EE2 levels were not related to the concentration of PAHs in any flue gas sample.     

Purification of oxyfuel-derived CO2

Oxyfuel combustion in a pulverised fuel coal-fired power station produces a raw CO₂ product containing contaminants such as water vapour plus oxygen, nitrogen and argon derived from the excess oxygen for combustion, impurities in the oxygen used, and any air leakage into the system. There are also acid gases present, such as SO₃, SO₂, HCl and NO_x produced as byproducts of combustion. At GHGT8 (White and Allam, 2006) we presented reactions that gave a path-way for SO₂ to be removed as H₂SO₄ and NO and NO₂ to be removed as HNO₃. In this paper we present initial results from the OxyCoal-UK project in which these reactions are being studied experimentally to provide the important reaction kinetic information that is so far missing from the literature. This experimental work is being carried out at Imperial College London with synthetic flue gas and then using actual flue gas via a sidestream at Doosan Babcock's 160 kW coal-fired oxyfuel rig. The results produced support the theory that SO_x and NO_x components can be removed during compression of raw oxyfuel-derived CO₂ and therefore, for emissions control and CO₂ product purity, traditional FGD and deNO_x systems should not be required in an oxyfuel-fired coal power plant.     

A novel process integration,optimization and design approach for large-scale implementation of oxy-fired coal power plants with CO2 capture

The widespread use of fossil fuels within the current energy infrastructure is considered as the largest source of anthropogenic emissions of carbon dioxide, which is largely blamed for global warming and climate change. At the current state of development, the risks and costs of non-fossil energy alternatives, such as nuclear, biomass, solar, and wind energy, are so high that they cannot replace the entire share of fossil fuels in the near future timeframe. Additionally, any rapid change towards non-fossil energy sources, even if possible, would result in large disruptions to the existing energy supply infrastructure. As an alternative, the existing and new fossil fuel-based plants can be modified or designed to be either “capture” or “capture-ready” plants in order to reduce their emission intensity through the capture and permanent storage of carbon dioxide in geological formations. This would give the coal-fired power generation units the option to sustain their operations for longer time, while meeting the stringent environmental regulations on air pollutants and carbon emissions in years to come.Currently, there are three main approaches to capturing CO₂ from the combustion of fossil fuels, namely, pre-combustion capture, post-combustion capture, and oxy-fuel combustion. Among these technology options, oxy-fuel combustion provides an elegant approach to CO₂ capture. In this approach, by replacing air with oxygen in the combustion process, a CO₂-rich flue gas stream is produced that can be readily compressed for pipeline transport and storage. In this paper, we propose a new approach that allows air to be partially used in the oxy-fired coal power plants. In this novel approach, the air can be used to carry the coal from the mills to the boiler (similar to the conventional air-fired coal power plants), while O₂ is added to the secondary recycle flow as well as directly to the combustion zone (if needed). From a practical point of view, this approach eliminates problems with the primary recycle and also lessens concerns about the air leakage into the system. At the same time, it allows the boiler and its back-end piping to operate under slight suction; this avoids the potential danger to the plant operators and equipment due to possible exposure to hot combustion gases, CO₂ and particulates. As well, by integrating oxy-fuel system components and optimizing the overall process over a wide range of operating conditions, an optimum or near-optimum design can be achieved that is both cost-effective and practical for large-scale implementation of oxy-fired coal power plants.