2018 Volume 35 Pages 139-149
Coal is an important energy resource for meeting future demands for electricity, as coal reserves are much more abundant than those of other fossil fuels. However, coal utilization technologies exhaust carbon dioxide more than other fossil fuels, because of the higher carbon content of coal. To control of global warming, development of new technologies for the reduction of carbon dioxide emission from a coal-utilized power station are thus required. For reduction of carbon dioxide emission, it is very important to utilize low carbon content fuel such as subbituminous coal and lignite, as well as carbon neutral biomass, and develop new technologies needed for a high efficiency power generation system. For the further reduction of carbon dioxide, effective removal and storage technologies are also necessary. In this paper, the utilization technology of low carbonized sub-bituminous coal and carbon neutral biomass in pulverized coal combustion is at first introduced. The development situation of a high efficiency coal-fired power generation technique including an integrated coal gasification combined cycle system (IGCC) and an integrated coal gasification fuel cell combined cycle system (IGFC) are overviewed, and carbon dioxide removal technologies in a thermal power station are investigated. Finally, the high efficiency power generation oxy-fuel IGCC system with CO2 removal is presented, and the characteristics of this system are examined.
Coal is an important energy resource for meeting the future demands for electricity. Coal reserves are much more abundant than those of other fossil fuels. However, the higher carbon content of coal exhausts carbon dioxide in the coal utilization process more than in other fossil fuel utilization processes. To prevent global warming, it is required to develop new technologies for the reduction of carbon dioxide emission from coal-fired power stations.
To reduce carbon dioxide emission, it is essential to develop a high efficiency power generation system, with the technology for utilization of low carbonized coal or carbon neutral biomass. Carbon dioxide removal technology is one option but it involves high power consumption and cost.
In this paper, the utilization technology of low rank coal, characterized by a low carbon content, and the utilization technology of biomass based fuels in pulverized coal power plants are firstly presented. Furthermore, the development situation of a high efficiency power generation system including an Integrated Coal Gasification Combined Cycle and an Integrated Coal Gasification Fuel Cell Combined Cycle, as well as power generation systems combined with carbon dioxide removal technology are introduced. Finally, a new high-efficiency power generation system for CO2 removal using an O2-CO2 blown gasifier is introduced.
As low carbonized coal such as sub-bituminous coal or lignite is lower in carbon content than bituminous coal, the utilization of low carbonized coal has the possibility of reducing CO2 emission. Especially, the mineable reserves of sub-bituminous coal are more than 30 % that of bituminous coal, so the utilization of sub-bituminous coal in a coal fired power plant has progressed in Japan. In a coal-fired power plant, coal is pulverized to particles of 40 μm median diameter size and then the pulverized coal is fired in a boiler. The properties of sub-bituminous coal are different from those of bituminous coal. For the reduction of CO2 emission by using sub-bituminous coal, it is important to retain the same power generation efficiency as produced with bituminous coal.
Fig. 1 indicates the difference in the oxygen concentration distribution in the test furnace (Ikeda, et al., 2002). Fig. 1(a) shows the oxygen concentration distribution on bituminous coal combustion and Fig. 1(b) shows that on sub-bituminous coal combustion. Oxygen consumption in the furnace is affected by the combustion situation, so this contour indicates the profile of the combustion flame. The oxygen consumption rate near the burner exit in sub-bituminous coal combustion is lower than that on bituminous coal combustion, as an evaporation period for moisture is necessary in sub-bituminous coal before combustion. So, the ignition of sub-bituminous coal takes place later as compared with bituminous coal. Also, after ignition, sub-bituminous coal combustion flame is diffused rapidly to the outer side of the furnace. This phenomenon is caused by the diffusion of pulverized coal particles. As sub-bituminous coal contains a high moisture content, the particles of sub-bituminous coal become porous or burst into fine particles by the rapid evaporation of moisture from the pore. Small coal particles or porous coal particles possess weak inertia, so these particles can be easily diffused by the swirl force of combustion air.
Comparison of oxygen concentration distribution in the furnace. Reprinted with permission from Ikeda et al. 2002. Copyright: (2002) The Japan Society of Mechanical Engineers.
As a solution to this problem, it is effective to reduce the flow rate of primary air and the swirl force of secondary air. The reduction of primary air velocity can move the ignition point closer to the burner exit and the reduction of swirl force can reduce the diffusion of coal particles to the outer side of the furnace. Fig. 2 indicates the oxygen concentration of the flame on sub-bituminous coal combustion after this modification. The flame shape in this condition is similar to the flame shape for bituminous coal combustion, as shown in Fig. 1(a). The power generation efficiency using sub-bituminous coal in a pulverized coal combustion plant can maintain almost the same as the power generation efficiency using bituminous coal by optimizing the burner operation condition.
Oxygen concentration distribution for sub-bituminous coal under the modified condition. Reprinted with permission from Ikeda et al. 2002. Copyright: (2002) The Japan Society of Mechanical Engineers.
Biomass is defined as carbon neutral since biomass is renewable. However, as there are many kinds of biomass, such as woodchips, wood pellets, and sewage sludge, the pulverizing and combustion characteristics of these species are different from each other. For utilization of biomass, the main problem is the high moisture content and the difficulty in pulverizing. Although many kinds of technology for biomass upgrading have been developed, the drying and carbonizing technologies are the most important. For high performance removal of moisture, the methods using evaporation are effective. However, these methods require higher power consumption because of increase in coal temperature. In CRIEPI, the extraction method using DME (di-methyl-ether) has been investigated. Fig. 3 shows the concept of the method (Kanda, et al., 2008). Although DME is in the gas phase in ambient pressure and temperature, it becomes liquid after pressurization in ambient temperature. The liquefied DME can include moisture. In this system, the liquefied DME is mixed with the biomass, and DME extracts the moisture. After separation of the liquefied DME including moisture extracted from the law biomass, DME is converted to gas phase by decompression. The gaseous DME is then separated with liquid water. The gaseous DME can be converted to liquid phase by pressurization, and the DME is used again by recycling. This method can reduce the power consumption required for the drying of biomass and be applied to biomass drying.
Concept of the moisture extraction method using DME.
For utilization of biomass (especially woody biomass), mixing with coal, as a blended fuel, in pulverized coal combustion power plants is a major technique. The pulverizing characteristics of biomass are important topic in blend combustion. The power consumption of a typical mill for coal is increased in the case of mixing biomass. To control the pulverizing characteristics of woody biomass, the carbonization of biomass is an effective technology. A typical carbonizing system is shown in Fig. 4. The woody biomass is carbonized in high temperature condition (600 K–1,000 K). The grindability and heat value of the carbonized biomass is increased which consequently enables the utilization of the biomass in a higher blend ratio. Tar produced in a carbonizing process is combusted and the generated heat is used as a heat source of carbonization.
Schematic of a typical carbonizing system.
A high efficiency power generation system can reduce CO2 emission while producing the required electric power. In Japan, the ultra-super critical (USC) boiler has been introduced into operation and the steam condition has been gradually improved (Fig. 5). For the greater improvement of power generation efficiency, the development of the integrated coal gasification combined cycle (IGCC) and the integrated coal gasification and fuel cell combined cycle (IGFC) are being promoted.
Improvement of power generation efficiency in Japan.
Fig. 6 indicates the system flow of the IGCC process. In Japan, the demonstration plant of IGCC has been operated since 2007. This plant has succeeded in a continuous run of 5,000 hours and is now used commercially. In the IGCC system, coal is pulverized to particles of about 40 μm median diameter size and the pulverized coal is then converted to gaseous phase fuel. The coal derived gas is purified by gas cleaning equipment and fired in a gas turbine combustor. Usually, IGCC plants use a wet gas cleaning system. For more power generation efficiency, the development of hot gas cleaning system and utilization of a high temperature gas turbine are important. In the future, the power generation efficiency of IGCC may be improved by the application of a hot gas cleaning system and high temperature gas turbine system.
System flow of IGCC process.
The high temperature fuel cell can maintain high power generation efficiency. IGFC can reduce CO2 emission more than IGCC. As a high temperature fuel cell, the molten carbonate fuel cell (MCFC) and the solid oxide fuel cell (SOFC) have been investigated.
Fig. 7 shows the system flow of IGFC. Pulverized coal is converted to gaseous fuel as in a usual IGCC system. Part of the gaseous fuel is introduced into a fuel cell and injected into a gas turbine combustor after generating electricity in the fuel cell, whereas in usual IGCC all of the gaseous fuel is introduced into a gas turbine combustor. Consequently, thermal power generation efficiency of IGFC surpasses that of IGCC. The major issues in the utilization of fuel cell are improvement in the durability of cell performance and the cost.
System flow of IGFC using MCFC.
Fig. 8 shows the power generation efficiency and CO2 emission rate of these high efficiency power generation systems. Consistent with the improvement in power generation efficiency, the CO2 emission rate is reduced.
Comparisons of power generation efficiency and CO2 emission rate.
To reduce CO2 emission to a great extent, the CO2 capture and storage (CCS) system is one of the foremost techniques. However, emission of CO2 from a thermal power plant is of a large amount, therefore the CO2 removal method requires a high consumption of power and thus a reduction in power generation efficiency. As a consequence, this method causes an extreme increase in power generation cost. Fig. 9 shows some examples of a power generation system with CO2 removal (Minchener et al., 2007). Post combustion and oxy-fuel combustion processes are for conventional coal-fired boilers, and pre-combustion process is for IGCC. The post combustion process captures CO2 in the exhaust gases after the coal has been fully burned out. The oxy-fuel process consists of oxygen combustion with recycled exhaust gases, where the exhaust gasses are mainly composed of CO2 and moisture. The generated as particles are collected in an electrostatic precipitator in the post combustion and oxyfuel combustion system. The pre-combustion process captures CO2 in a synthesis gas after conversion of CO into CO2 by using the shift reactor. Generated ash particles are collected by a ceramic filter in the pre combustion system.
System flow of thermal power plant with CO2 removal.
There are some ways of capturing CO2 from flue gas such as separation with chemical/physical solvents (absorption), adsorption, separation with membranes and cryogenic separation. At any rate, capturing CO2 from flue gas needs a huge amount of energy and decreases power generation efficiency. Fig. 10 indicates the comparison of estimation result of power generation efficiency with and without CO2 removal (Makino and Noda, 2011). In general, the power generation efficiency of a thermal power plant with CO2 removal is about 30 % lower as compared to that of a power generation plant without CO2 removal. In these CO2 removal systems, the CO2 removal technology in the IGCC system is better than the CO2 removal technology in the pulverized coal combustion power plant. For greater improvement of power generation efficiency of IGCC system with CO2 removal, the development of high performance CO2 separation technology is important.
Power generation efficiency with and without CO2 removal. (Makino and Noda, 2011)
As mentioned in a previous chapter, the consumption of a huge amount of energy has been a severe problem in installing the CO2 capturing system in thermal power plants.
The pre-combustion system (Fig. 9(c)) converts syngas to a mixed gas of CO2 and H2 using a shift reactor and then separates CO2 using a capture unit. The remaining H2 is injected into gas turbine combustors as fuel. However, a shift reactor requires large amount of steam which is extracted from the steam turbine system; as a result, thermal efficiency becomes lower than in a conventional IGCC without CO2 capturing.
Recently, a new concept of IGCC, namely the oxy-fuel IGCC system, which retains high efficiency while capturing CO2, has been proposed. The system applies the concept of the oxy-fuel combustion system (Fig. 9(b)) to IGCC. Fig. 11 shows the system flow of the oxy-fuel IGCC system. Pulverized coal is fed into a gasifier with recirculated exhaust gas (mostly composed of CO2) and added oxygen, and converted to syngas in a gasifier. Syngas is injected into gas turbine combustors as fuel in the downstream of dry type gas clean-up unit. The exhaust gas from the gas turbine is supplied to a regenerative heat exchanger and the heat exchanger heats up the gas at the combustor inlet using the heat of GT exhaust gas. As specific heat of CO2 is 1.6 times larger than that of N2, it is hard to utilize the heat in the gas turbine. Therefore, the regenerative heat exchanger plays an important role in the system. In contrast to a pre-combustion system, this system does not require a shift converter and a CO2 capture unit because of the high CO2 concentration in the flue gas. Therefore, the system can retain high efficiency (more than 42 % HHV) while capturing CO2.
System flow of the oxy-fuel IGCC system.
The other merit of the system is derived from the recycled CO2 injection into a gasifier. A conventional gasifier works with a mixed gas of O2 and N2. Although N2 is neutral in a coal gasification reaction, CO2 can work as a gasifying agent in coal gasification. Therefore, the performance of a coal gasifier is expected to increase under the oxy-fuel IGCC condition.
From 2008, the authors have developed the proposed system with the support of New Energy and Industrial Technology Development Organization (NEDO) in Japan.
3.2 Development status of O2-CO2 blown coal gasifierThe O2-CO2 blown coal gasifier is a key component of the oxy-fuel IGCC system; hence it is of importance in evaluating the effect of CO2 concentration on coalgasification reaction. The CO2 effect in a coal gasifier and validity of the O2-CO2 blown coal gasifier have been experimentally investigated (Hamada et al., 2016). Fig. 12 shows the schematic of coal gasifier used in their work. The lower chamber is called a “combustor” and the upper one is called a “reductor”. Two coal burner and two char burners were installed in the combustor, and two coal burners were installed in the reductor. The coal gasifier was a two-stage entrained flow type. The operating pressure was 2.0 MPa and the coal feeding rate was about 100 kg/h. Char discharged from the gasifier was collected by cyclone separator installed in the downstream of gasifier and recycled into the combustor as fuel. The coal ash was melted in the combustor and the melted ash, called “slag”, was dropped into an ash hopper installed under the gasifier. Coal gasification experiments were performed under the CO2 enriched condition. Fig. 13 shows the relationship between CO2 concentration and gasifier carbon conversion efficiency (CCE) (Oki et al., 2014). Here, CCE is defined as (carbon in product gas)/(carbon in coal and char fed into gasifier). For almost all types of coal, CCE increases with CO2 enrichment. The results show that CO2 enrichment promotes the char gasification reaction and increases the coal gasifier performance. Present results were under the condition where CO2 concentration was less than 30 % on account of the restrictions of the gas supplying system. However, CO2 concentration in the actual oxy-fuel IGCC condition is expected to be much higher. For our next step, it is planned to modify the gas supplying system to increase CO2 concentration to as high as in the actual oxy-fuel IGCC condition.
Schematic of coal gasifier.
Relationship between CO2 concentration and coal gasifier performance. (Oki et al., 2014)
In a previous chapter, current experimental results obtained in an O2-CO2 blown condition were introduced. As mentioned, CO2 concentration was much higher than in the current experimental condition. Also, the size of the coal gasifier was much larger. In the early stages of system development, experimental studies using actual size equipment are quite difficult. Therefore, the authors have been developing numerical tools by which to estimate the validity of the system and performance of the O2-CO2 blown coal gasifier. The numerical method is based on CFD code coupled with chemical reactions such as the coal gasification reaction. The details of the numerical method are addressed in the author’s previous paper (Watanabe et al., 2015).
The char particle reaction is the most important point in the modeling of the coal gasifier. Therefore, an appropriate particle reaction model was employed in the study (Umemoto et al., 2013).
The numerical simulation for the 100 kg/h class coal gasifier that Hamada et al. used was first validated comparing with experimental results. Table 1 shows the calculation condition of the 100 kg/h class coal gasifier. Case 1 is the typical condition for air-blown IGCC system whereas in cases 2 and 3, 15 % and 25 % N2 have been replaced by CO2, respectively.
Numerical condition of 100 kg/h scale coal gasifier.
| Items | Case 1 | Case 2 | Case 3 | |
|---|---|---|---|---|
| Coal feeding rate | kg/h | 100 | 100 | 100 |
| Oxygen ratio | — | 0.52 | 0.55 | 0.54 |
| Composition of gasifying agent | ||||
| O2 | vol% | 25 | 25 | 25 |
| CO2 | vol% | 0 | 15 | 25 |
| N2 | vol% | 75 | 60 | 50 |
Fig. 14 shows the comparisons of gaseous temperature distribution (Tanno et al., 2015). Axial location is normalized by gasifier diameter. Numerical results are in quite good agreement with experimental data for all cases. The burner in the combustor is located around z/D = 0.5 and the burner in the reductor is located z/D = 3.0. Gaseous temperature has a peak around the coal burner and rapidly decreases when getting closer to the reductor by reason of the endothermic coal gasification reaction. Gaseous temperature gradually decreases as a result of the heat release form the wall in the downstream of the reductor burner. Fig. 15 shows the comparisons of product gas composition (Tanno et al., 2015). The major product gases are H2, CO, CO2 and H2O. CO and CO2 concentrations increase and H2 decreases with increasing injected CO2 concentration. This is due to the coal gasification reaction and water gas shift reaction. Numerical results also reflect the change in product gas composition.
Comparison of gaseous temperature distribution. Reprinted with permission from Tanno et al. 2015. Copyright: (2015) The Japan Institute of Energy.
Comparison of product gas composition. Reprinted with permission from Tanno et al. 2015. Copyright: (2015) The Japan Institute of Energy.
From these results, the accuracy of the numerical results was validated. Subsequently, the numerical simulation for an actual size coal gasifier was performed. Fig. 16 shows the schematic of the actual scale coal gasifier. The gasifier is two-stage entrained flow type. Four coal burners and four char burners are installed in the combustor and four coal burners are installed in the reductor. The operating and the coal feeding rate is 70 ton/h. Table 2 shows the numerical conditions. A-25 case shows the typical conditions in an air-blown IGCC system whereas C25, C35 and C45 are the oxy-fuel IGCC conditions. 25, 35, and 45 denote oxygen concentration.
Schematic of the actual scale coal gasifier.
Numerical condition of actual scale coal gasifier.
| Items | A25 | C25 | C35 | C45 | |
|---|---|---|---|---|---|
| Coal feeding rate | t/h | 70 | 70 | 70 | 70 |
| Oxygen ratio | — | 0.45 | 0.45 | 0.45 | 0.45 |
| Composition of gasifying agent | |||||
| O2 | vol% | 25 | 25 | 35 | 45 |
| CO2 | vol% | 0 | 75 | 65 | 55 |
| N2 | vol% | 75 | 0 | 0 | 0 |
Fig. 17 shows the comparison of velocity vectors on horizontal planes. A25, the air-blown condition, shows an almost axisymmetric distribution and the strong swirling flow is formed by the jet from the four burners. Almost the same flow behavior is found in C25. The magnitude of the swirling flow decreases with increasing O2 concentration (C35 and C45). This is due to the fact that the flow rate is reduced to maintain the same oxygen amount with increasing O2 concentration. Fig. 18 shows the gaseous temperature distribution around the combustor. For all cases, the high temperature region is located near the coal burners, and gaseous temperature rapidly decreases as the gases pass through the throat section to the reductor. This is due to the fact that the exothermic reaction (combustion) dominant in the combustor changes to the endothermic reaction (gasification) in the reductor, namely, a chemical quenching. The same tendency is also observed in C25, C35 and C45. It is also seen that gaseous temperature significantly decreases when N2 in the gasifying agents is replaced by CO2 with the same O2 concentration (A25 to C25).
Velocity vector on horizontal plane. Reprinted with permission from Tanno et al. 2015. Copyright: (2015) The Japan Institute of Energy.
Gaseous temperature distributions.
This is due to the difference in specific heat between N2 and CO2. On the other hand, gaseous temperature recovers with increasing O2 concentration in the gasifying agent and almost the same level and distribution of temperature as with A25 is observed, when the O2 concentration is 35 % (C35). For a much higher O2 concentration condition (C45), the gaseous temperature increases to that of A25. The temperature trend can be clearly seen in Fig. 19 which shows the axial distributions of gaseous temperature. From the results, it is seen that even though gaseous temperature decreases as a result of replacing N2 by CO2 on account of the difference in specific heat, it can be recovered by optimizing the oxygen concentration.
Axial distributions of gaseous temperature. Reprinted with permission from Tanno et al. 2015. Copyright: (2015) The Japan Institute of Energy.
Fig. 20 shows the axial distributions of the concentration of the major product gas. Just by replacing N2 by CO2 (A25 to C25) does not increase CO concentration very much, although the CO2 concentration is very high. This is due to decrease in gaseous temperature in C25 suppressing CO production in the coal gasification. When O2 concentration increases and consequently gaseous temperature increases for C35 and C45, CO concentration significantly increases. From these results, maintaining a high temperature in the combustor is found to be important in order to take advantage of the oxy-fuel IGCC condition.
Axial distributions of major product gas. Reprinted with permission from Tanno et al. 2015. Copyright: (2015) The Japan Institute of Energy.
Fig. 21 shows the comparisons of carbon conversion efficiency (CCE). Here, combustor CCE is the carbon conversion efficiency of the coal fed into the combustor, reductor CCE is that fed into the reductor and total CCE is that introduced into the gasifier. Combustor CCE reaches almost 100 % in all cases; the difference in total CCE is thus due to the reductor CCE. Replacing N2 with CO2 (between A25 and C25) slightly suppresses total CCE. This is due to the decrease in gaseous temperature seen in Figs. 18 and 19. However, total CCE for C35 exceeds that for A25 in spite of the same level of gaseous temperature. This is due to the high CO2 concentration; the advantages of the oxy-fuel IGCC are therefore numerically indicated.
Comparisons of carbon conversion efficiency. Reprinted with permission from Tanno et al. 2015. Copyright: (2015) The Japan Institute of Energy.
Coal is an important energy resource for meeting the future demands for electricity, as coal reserves are much more abundant than those of other fossil fuels. For future coal utilization, it is necessary to reduce CO2 emission. It is thus very important to utilize low carbonized coal and carbon neutral biomass and develop new technologies for a high efficiency power generation system. For the further reduction of CO2 emission, CO2 removal technology is also effective, but requires high power consumption and cost. Recently, a new type of thermal power generation system which maintains high efficiency while capturing CO2 has been proposed. The O2-CO2 blown coal gasifier is a key component of the system, therefore feasibility of the O2-CO2 blown gasifier and gasification characteristics were experimentally and numerically investigated in this paper.
This work was in part supported financially by the New Energy and Industrial Technology Development Organization (NEDO) program “Innovative Zero-emission Coal Gasification Power Generation Project”, P08020.
Carbon conversion efficiency
DDiameter of gasifier
XiSpecies mass fraction
zAxial location of gasifier
Kenji Tanno
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Dr. Kenji Tanno is a senior research scientist of CRIEPI (Central Research Institute of Electric Power Industry) in Japan. He received his doctoral degree in 2007 in Mechanical Engineering at Kyoto University. He was employed at CRIEPI in 2007 as a research scientist in coal combustion group. His research covers numerical simulation of coal combustion and gasification, flue gas treatment technology and non-contact laser applied measurement for multiphase combustion.
Hisao Makino
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Dr. Hisao Makino is an executive research scientist of CRIEPI (Central Research Institute of Electric Power Industry) in Japan. He graduated from Kyoto University in 1977 and received M.E. degree in Chemical Engineering from Kyoto University in 1979 and a Ph.D. in Chemical Engineering from Kyoto University in 1995. He was employed at CRIEPI in 1979 as a Research Associate in combustion research section. His research interests are powder technology, coal utilization technology for the energy supply and environmental protection. Current main subjects are an advanced combustion technology for pulverized coal, upgrading technology for low rank coal and flue gas treatment technology.
https://ror.org/041jswc25 
