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CPH systems for cogeneration of power and hydrogen from coal
International Journal of Hydrogen Energy 31 (2006) 693 – 700 www.elsevier.com/locate/ijhydene
CPH systems for cogeneration of power and hydrogen from coal S.P. Cicconardi, A. Perna, G. Spazzafumo∗ , F. Tunzio Industrial Engineering Department, University of Cassino, Via G. di Biasio 43-03043 Cassino (FR), Italy Available online 8 September 2005
Abstract Three layouts with an integrated coal gasifier hydrogen production and a small powerplant section have been modelled using a computer code (ASPEN PLUSTM ). The integration allows to eliminate or to reduce the losses at the condenser of the powerplant: the steam is reheated and fed to the gasifier. The resulting counterpressure operation of the powerplant is justified like in a co-generator of heat and power (CHP). In this case we have a co-generation of power and hydrogen (CPH). Therefore the efficiency of the power plant is not high, but it shows an “apparent” efficiency very high. Even if the concept has been demonstrated, further work is required because power generation is very small with respect to the hydrogen production. 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Cogeneration; Hydrogen; Power; Coal
1. Introduction Within the next 20 years peaks in production of oil and natural gas are expected to be reached. After that time their cost will increase continuously. Renewable energy sources will not be yet able to cover the total energy demand in all countries: some countries will replace oil and natural gas with nuclear energy, some others with coal. Therefore it is important to develop coal technologies which will be clean and efficient. Coal is the fossil fuel with the highest content of carbon and therefore the conversion efficiency is important also to reduce the carbon dioxide emissions, waiting for effective sequestration systems. Electric energy and hydrogen will be the energy carriers of the future. And hydrogen will become fundamental for transport applications. Technologies to produce both these carriers from coal are already well known. The integration of clean coal technologies and coal to hydrogen production holds the promise of an improvement of the global
∗ Corresponding author. Tel.: +39 0585 51917; fax: +39 0775 881021. E-mail address: [email protected] (G. Spazzafumo).
conversion efficiency and a decrease of greenhouse gas emissions for those countries in which the exploitable renewable energy sources flows are largely offset by their exploitable coal resource endowment or where coal already plays a substantial role in the electricity generation mix. This idea arises from the observation that a steam power plant has the main energy loss at the condenser. At the same time a coal gasifier requires heat and steam. And it needs a condenser too, to dry the gas produced: an excess of steam is used to increase the chemical conversion to hydrogen. A general analysis of this concept was recently carried out [1], but the variety of layouts and components makes impossible to evaluate the real advantages. In order to know the improvement of efficiency and the reduction of carbon dioxide emissions, it is necessary to identify a reference system and to modify it following the concept introduced. In this paper a first attempt to translate this concept into a system layout has been made. A system based on a coal gasifier and devoted to the production of hydrogen has been chosen as a reference system. This first work on the subject is only a preliminary evaluation of the cogeneration of power and hydrogen (CPH) concept in the case of coal. No
0360-3199/$30.00 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.07.004
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Nomenclature APP apparent powerplant ASU air separation unit CC combustion chamber Ech chemical energy Eel electrical energy HEX heat exchanger HHV high heating value HP high pressure IGCC integrated gasifier combined cycle IGHP integrated gasifier hydrogen production investigations were carried out concerning the system optimisation. Some components of the systems examined have been taken from scientific literature, while other components and the whole plants have been simulated using ASPEN PLUSTM .
2. The reference layout The production of hydrogen from coal requires: • • • • • •
the the the the the the
production of steam; separation of oxygen from air; coal gasification; cleaning of the gas from impurity; conversion of carbon monoxide; separation of carbon dioxide and water.
Each of the above listed process steps can be performed via different technology options therefore yielding to several plant layouts. To narrow the focus of the analysis the gas clean-up process is first selected, given its critical role in determining the suitability of the produced hydrogen to various different energy end-use services and technologies. Three layouts are here considered with the complex of operations and components designed around a membrane-type reactor. The ultimate analysis of coal considered and its high heating value are reported in Table 1, and a simplified schematic of such a plant is shown in Fig. 1. Coal, water and air are the inputs to the system. The air goes into the air separation unit (ASU) where the oxygen is separated and compressed. Then it is fed to the coal gasifier. The nitrogen is released to the atmosphere after having been heated and expanded to recover energy. The oxygen is produced with a purity of 95% [2]. The water is divided in two streams: to the heat recovery steam generator (HEX1) and to the water gas shift reactor (WGSR). The first is used to produce steam to be fed to the coal gasifier, while the latter is used to convert most of the residual CO.
IP intermediate pressure LP low pressure p pressure SCOHS selective catalytic oxidation of hydrogen sulphide SEP separation of water and sequestration of carbon dioxide T temperature WGSR water gas shift reactor App apparent efficiency Table 2 shows common assumptions for the integrated gasifier hydrogen production (IGHP) and the different CPH layouts proposed later. Table 3 shows the main characteristics of IGHP system. The coal gasifier has been supposed to be an entrained bed type. The operating pressure is 16 bar as this is the pressure required by the membrane separator in the WGSR. The temperature of the gas produced is 1350 ◦ C. In the scientific literature it was not possible to find data for a coal gasifier with exactly the same operating parameters. Data for coal gasifier with the same technology and different operating parameters were used to test the model used for a simulation of the coal gasifier. Such a model is based on the chemical equilibrium theory. To apply such a theory it can be used the equilibrium constant method or the minimisation of the Gibbs free energy. By applying the first method all the independent stoichiometric equation of formation of the syngas species must be recognised and an equilibrium constant obtained for each one. The Gibbs free energy is a thermodynamic function that can be interpreted as a driving force of a reaction, function of the number of moles of each species: it has a null value at equilibrium. When considering two or more reactions the null value cannot be reached but there is equilibrium when a minimum is reached.
Table 1 Main characteristics of coal considered Coal Ultimate analysis (Wt%) C H N Cl S O Moisture Ash HHV (MJ/kg)
Illinois #6
63.75 4.50 1.25 0.29 2.51 6.88 11.12 9.70 27.13
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Fig. 1. Schematic of the integrated gasifier hydrogen production (IGHP).
Table 3 Main characteristics of IGHP
Table 2 Common assumptions for IGHP and CPH systems Operating temperature of the gasifier (◦ C) Operating pressure of the gasifier (bar) Inlet pressure of the ASU (bar) Outlet pressure of the ASU (bar) Operating temperature of the WGSR (◦ C) Inlet pressure of the WGSR (bar) Outlet pressure of the WGSR (bar) Pump isoentropic efficiency Turbine isoentropic efficiency Compressor isoentropic efficiency
1350 16 19 6 322 16 1 0.75 0.85 0.85
The gas produced contains different kinds of substances: • fuels (e.g. hydrogen); • diluents (e.g. the residual nitrogen fed to the gasifier together with oxygen); • noxious compounds (e.g. hydrogen sulphide). The required degree of conversion of the fuels other than hydrogen present in the flue gas is determined on the endused envisaged for the latter: as an example the use of the
Oxidant (O2 95%) fed to the gasifier (kg/kgCOAL ) Steam fed to the gasifier (kg/kgCOAL ) Steam temperature (◦ C) H2 (from WGSR) (kg/kgCOAL ) Tailgas (at the final stage) (kg/kgCOAL ) H2 content in the Tailgas (at the final stage) (% vol)
0.83 0.50 550 0.127 0.41 53.8
produced hydrogen in a PEMFC system requires the complete conversion of carbon monoxide, which is noxious, and the minimisation of methane, which acts as a diluent. The output of an entrained bed coal gasifier contains typically methane in very low concentration and an amount of carbon monoxide comparable with the one of hydrogen. The process chosen to purify hydrogen is based on a reactor called WGSR. It is capable to accomplish simultaneously the water-gas shift reaction and the separation of hydrogen from all the other substances by using a membrane. Not all the hydrogen can be recovered, but its purity is very high (99.9999%). Operating conditions assumed are T = 322 ◦ C, pin = 16 bar, pout = 1 bar [3]. At NETL a project is devel-
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oped for a technology that is able to produce pure H2 from coal gasification using a membrane system at 600 ◦ C [4]. The residual hydrogen remains in the tail gas stream without significant pressure drop. The other main components are carbon dioxide (51.6%) and water (27.5%), which could be separated, nitrogen (4.8%), carbon monoxide (1.2%) and hydrogen sulphide (0.8%). At this lower level of temperature the sulphur separation can be carried out by means of a selective catalytic oxidation of hydrogen sulphide (SCOHS) [5]. Such a process allows also direct sulphur production and was developed especially for the coal gasification plants. Selective oxidation of hydrogen sulphide to sulphur is obtained by the combustion with air or oxygen (2–4 times the stoichiometric factor). The system is made up by two reactors: in the first one the hydrogen sulphide is removed from the gas stream with a zinc oxide sorbent while in the second one the sorbent is regenerated with air or oxygen, when it reaches 50% of its weight in sulphur. Recent studies show that below 200 ◦ C it is possible to obtain a sulphur removal as high as 99.76%. Pressures in the range 11–33 bar have been reported [6]. The purified gas contains also a small amount of oxygen which is eliminated with a combustion. At this step the composition of the tail gas is: H2 = 12.6%, CO2 = 52.2%, H2 O = 29.6%, N2 = 5.1%, CO = 0.5%. The final stages of purification for the tail gas are the separation of water and carbon dioxide and the liquefaction of carbon dioxide (T = 298 K, p = 15 MPa). A sequestration of 90% of the carbon dioxide was assumed with a power consumption of 500 kJ/kgCO2 . The final composition of the tail gas is therefore: H2 = 53.8%, CO2 = 22.4%, N2 = 21.8%, CO = 2.0%.
The presence of a unique condenser corresponds to the ideal layout; however, the thermal integration of CPH1 is not optimal: there are some heat losses located along the purification line before the condenser. Therefore a better integration could arise from expanding in a low pressure turbine a fraction of the steam flow larger than the one fed to the gasifier. This has been analysed with CPH2 (Fig. 3). The schematic is the same as the previous one, but after the last reheating the steam flow rate is divided in two streams and one of them is expanded in a third turbine (LP) down to 0.05 bar. The main characteristics of CPH2 are shown in Table 5. If the steam fed to the gasifier were a high temperature steam, a lower amount of oxygen should be required to burn the coal and maintain the gasifier at the operating temperature. In such a case a higher amount of hydrogen could be obtained. Such a solution could be carried out by burning some of the hydrogen with stoichiometric oxygen into a direct steam generator. By compressing the reactants, for example up to 64 bar, it is possible to mix the steam from HP with a proper amount of steam directly generated in such a way that the steam at the inlet of IP be at 1500 ◦ C. In this way the steam after the expansion still has a high temperature and can be fed directly to the gasifier. It is also possible to use the tail gas rather than the pure hydrogen. It is not exactly the same because of the presence of carbon monoxide, carbon dioxide and nitrogen, but the resulting gas has to be fed to the gasifier and not to work in a steam power plant under atmospheric pressure. Therefore there are no problems with uncondensable gases. The resulting layout is shown in Fig. 4 (CPH3), while the main characteristics are reported in Table 6.
3. The CPH layouts examined The concept of CPH is based on the consideration that fuel processors require steam while most of the power plants discharge it. Therefore it is possible to use the steam discharged from a power plant into a fuel processor. The basic requirement is that the discharge pressure be adequate. There are no requirements concerning the purity of the steam. A high temperature is favourable, but not necessary. A CPH should allow the maximum efficiency when the thermal integration of the two systems is such that all the steam is condensed only at the end of the gas cleaning process. This means that all the steam flowing into the power plant has to be fed to the coal gasifier. Fig. 2 shows a first layout (CPH1). In this case the water required by the coal gasifier is pumped up to 256 bar, evaporated, heated up to 550 ◦ C (HEX1) and expanded down to 64 bar in a first turbine (HP). The steam exiting from HP is then reheated up to 550 ◦ C (HEX2) and expanded in a second turbine (IP) down to the pressure of the gasifier, 16 bar, to which is fed after a final reheating up to 550 ◦ C (HEX3). Some characteristics of CPH1 are summarised in Table 4.
4. Discussion of the results The different approaches to the thermal integration of the steam power plant and the coal gasifier are responsible for the differences in the temperature of the streams. However, the working temperature of WGSR and SCOHS were assumed to be the same in all cases. The stream fed to the SCOHS was cooled without recovering the heat because of its low temperature. The stream fed to the WGSR was cooled by adding water in order to obtain a higher conversion of carbon monoxide. Due to this choice the water globally fed to the gasifier and to the WGSR was not always the same. Therefore the production of hydrogen in the reference case and in the three cases analysed is not exactly the same. Also the composition of the tail gas is different. As already stated, in the third CPH layout the higher production of pure hydrogen is also due to the lower flow of oxygen required by the gasifier. The goal of this work was to verify if there is an advantage in combining the production of hydrogen and the production
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Fig. 2. Schematic of a combined power and hydrogen system (CPH1).
of electric energy. Therefore the most significant result is the energy output from the systems. In Table 7 such output is reported in terms of chemical energy and electrical energy both expressed in kJ/kgCOAL . The chemical energy is divided in two terms: the first one is related to the stream of pure hydrogen and the other one is related to the stream of tail gas. The electrical energy is the net energy after having subtracted the consumption for compressors, pumps and carbon dioxide sequestration. The first consideration is that the power production is very low, 0.8–2.8% of total energy output. This is connected with the rather small amount of steam required by the gasifier. It is evident that, in order to have a significant production of power, a significant amount of the hydrogen yield has to be used. This involves the production of a large amount of steam, much higher than the one required by the gasifier,
Table 4 Main characteristics of CPH1 Oxidant (95% O2 ) fed to the gasifier (kg/kgCOAL ) Steam fed to the gasifier (kg/kgCOAL ) HP pressure /HP temperature (bar/◦ C) IP pressure /IP temperature (bar/◦ C) H2 (from WGSR) (kg/kgCOAL ) Tailgas (at the final stage) (kg/kgCOAL ) H2 content in the Tailgas (at the final stage) (% vol)
0.87 0.80 256/550 64/550 0.127 0.43 50.5
with a negative effect on the system efficiency due to the increased flow of steam to be condensed. A way to reduce the steam flow rate to be produced for power generation is to increase its mean temperature. At this concern the third CPH layout (CPH3) could be more interesting. The expansion could be realised in more than
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Fig. 3. Schematic of a combined power and hydrogen system (CPH2). Table 5 Main characteristics of CPH2 Oxidant (95% O2 ) fed to the gasifier (kg/kgCOAL ) Steam fed to the gasifier (kg/kgCOAL ) HP pressure /HP temperature (bar/◦ C) IP pressure /IP temperature (bar/◦ C) Steam flow rate at LP Turbine (kg/kgCOAL ) LP pressure/LP temperature (bar/◦ C) Condenser pressure (bar) H2 (from WGSR) (kg/kgCOAL ) Tailgas (at the final stage) (kg/kgCOAL ) H2 content in the Tailgas (at the final stage) (% vol)
Table 6 Main characteristics of CPH3 0.87 0.80 256/550 64/550 0.2 16/550 0.05 0.127 0.42 50.1
one turbine with multiple reheatings at 1500 ◦ C. The new steam cooling system used for the series H gas turbine could be applied to this high temperature steam turbines. When considering the efficiency of power production the results are very interesting. It is obvious that the efficiency of a counterpressure power plant is not very high. But in
Oxidant fed to the gasifier (kg/kgCOAL ) Impure steam fed to the gasifier (kg/kgCOAL ) Impure steam temperature at the gasifier (◦ C) Steam flow rate to HP turbine (kg/kgCOAL ) HP pressure /HP temperature (bar/◦ C) IP pressure /IP temperature (bar/◦ C) Steam flow rate to LP turbine (kg/kgCOAL ) LP pressure/LP temperature (bar/◦ C) Condenser pressure (bar) CC pressure (bar) H2 (from WGSR) (kg/kgCOAL ) H2 content in the Tailgas (at the final stage) (% vol) Tailgas (at the final stage) (kg/kgCOAL ) Tailgas at CC (kg/kgCOAL )
0.78 0.87 1180 0.85 256/550 64/1500 0.35 16/550 0.05 16 0.129 43.7 0.6 0.25
this case the steam is supplied to another user and therefore it has a positive effect. From Table 7 we can see that CPH
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Fig. 4. Schematic of a combined power and hydrogen system (CPH3). Table 7 Energy output (kJ/kgCOAL ) IGHP
CPH1
CPH2
CPH3
Chemical (pure H2 ) Chemical (tail gas) Electrical
18,209 3752 −224
17,714 3665 165
17,894 3486 446
18,350 2608 599
Total
21,737
21,544
21,826
21,557
layouts generate more electrical energy and less chemical energy than the reference system. In order to estimate the benefit of every layouts we could define an apparent effi-
Fig. 5. Energy balance of IGHP and CPH systems.
ciency which takes into account these differences with respect to the reference system. Referring to Figs. 5 and 6, we could consider the power generation in the CPH be due to a powerplant which uses part of the hydrogen produced. This powerplant has been
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Fig. 6. CPH energy balance as combined IGHP and apparent powerplant.
Table 8 Apparent efficiency
Electrical energy produced Chemical energy apparently consumed Apparent efficiency
CPH1
CPH2
CPH3
389 582
670 581
823 1003
0.668
1.153
0.821
of a IGCC and therefore it is expected that also increasing the power production of a CPH, the global efficiency be higher than the one obtainable with separated production. As previously stated, the analysis pointed out does not deal with the plant optimisation. We simply designed some layouts and analysed their performance in order to understand if the concept is interesting and how to address future studies. The next step will be a comparative analysis of plant layouts based on different cleaning processes, in particular low pressure and low temperature ones. It could allow to reduce the pressure of the gasifier and to avoid to fraction the thermal integration of the systems. And, of course, it will be necessary to repower the section of the plant devoted to power production, analysing at the same time the effect of the power over the global efficiency of the system in comparison with the two separate plants.
References called apparent powerplant (APP) and its efficiency has been called apparent efficiency. This apparent efficiency is therefore defined as the more electrical energy generated divided by the less chemical energy produced, which is a chemical energy apparently consumed: App =
Eel,CPH − Eel,IGHP Eel = . Ech Ech,IGHP − Ech,CPH
(1)
Table 8 shows the results. Particularly surprising is the apparent efficiency of CPH2. As already stated it is not a real efficiency. It simply means that combining the two plants it is possible to reduce the global energy losses with respect to the IGHP system. If we multiply the apparent efficiency by the conversion efficiency from coal to hydrogen, that is 0.81, we obtain a conversion factor from coal to power, apparently passing through hydrogen production. Such a conversion factor ranges from 0.54 to 0.93. As a reference case for power production from coal we could consider an Integrated Gasifier Combined Cycle (IGCC) which efficiency is about 0.45. In all cases the conversion factor is higher than the efficiency
[1] Spazzafumo G. Cogeneration of power and hydrogen with integrated fuel processor counterpressure steam cycles. Int J Hydrogen Energy 2004;29(11):1147–50. [2] Process Engineering Division. Destec gasifier base cases PEDIGCC-98-003. September 1998. Latest Revision June 2000. NETL. [3] Criscuoli A, Basile A, Drioli E, Loiacono O. An economic feasibility study for water gas shift membrane reactor. J Membrane Sci 2001;181:21–7. [4] Killmeyer R, Ciocco M, Morreale B, Enick R, Bustamante F. Water gas shift membrane reactor studies. NETL, Office of Science and Technology Fuel and Process Chemistry Division (from www.eere.energy.gov/hydrogenandfuelcells/hydrogen). [5] Selective Catalytic Oxidation of Hydrogen Sulfide for Simultaneous Coal Gas Desulfurization and Direct Sulfur Production (SCOHS) System Analysis. February 2002. Prepared for NETL. Task. No 50802. [6] Siriwardane RV, Cicero D. Durable zinc oxide containing regenerable desulfurization sorbents for both low and high temperature applications. US Department of Energy, Federal Energy Technology Center Morgantown. Prepared for presentation at AIChE Spring National Meeting, March 5–9 2000, High Temperature Gas Cleaning (Gases and Particles) Session TF004 (from www.osti.gov/bridge).
CPH systems for cogeneration of power and hydrogen from coal S.P. Cicconardi, A. Perna, G. Spazzafumo∗ , F. Tunzio Industrial Engineering Department, University of Cassino, Via G. di Biasio 43-03043 Cassino (FR), Italy Available online 8 September 2005
Abstract Three layouts with an integrated coal gasifier hydrogen production and a small powerplant section have been modelled using a computer code (ASPEN PLUSTM ). The integration allows to eliminate or to reduce the losses at the condenser of the powerplant: the steam is reheated and fed to the gasifier. The resulting counterpressure operation of the powerplant is justified like in a co-generator of heat and power (CHP). In this case we have a co-generation of power and hydrogen (CPH). Therefore the efficiency of the power plant is not high, but it shows an “apparent” efficiency very high. Even if the concept has been demonstrated, further work is required because power generation is very small with respect to the hydrogen production. 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Cogeneration; Hydrogen; Power; Coal
1. Introduction Within the next 20 years peaks in production of oil and natural gas are expected to be reached. After that time their cost will increase continuously. Renewable energy sources will not be yet able to cover the total energy demand in all countries: some countries will replace oil and natural gas with nuclear energy, some others with coal. Therefore it is important to develop coal technologies which will be clean and efficient. Coal is the fossil fuel with the highest content of carbon and therefore the conversion efficiency is important also to reduce the carbon dioxide emissions, waiting for effective sequestration systems. Electric energy and hydrogen will be the energy carriers of the future. And hydrogen will become fundamental for transport applications. Technologies to produce both these carriers from coal are already well known. The integration of clean coal technologies and coal to hydrogen production holds the promise of an improvement of the global
∗ Corresponding author. Tel.: +39 0585 51917; fax: +39 0775 881021. E-mail address: [email protected] (G. Spazzafumo).
conversion efficiency and a decrease of greenhouse gas emissions for those countries in which the exploitable renewable energy sources flows are largely offset by their exploitable coal resource endowment or where coal already plays a substantial role in the electricity generation mix. This idea arises from the observation that a steam power plant has the main energy loss at the condenser. At the same time a coal gasifier requires heat and steam. And it needs a condenser too, to dry the gas produced: an excess of steam is used to increase the chemical conversion to hydrogen. A general analysis of this concept was recently carried out [1], but the variety of layouts and components makes impossible to evaluate the real advantages. In order to know the improvement of efficiency and the reduction of carbon dioxide emissions, it is necessary to identify a reference system and to modify it following the concept introduced. In this paper a first attempt to translate this concept into a system layout has been made. A system based on a coal gasifier and devoted to the production of hydrogen has been chosen as a reference system. This first work on the subject is only a preliminary evaluation of the cogeneration of power and hydrogen (CPH) concept in the case of coal. No
0360-3199/$30.00 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.07.004
694
S.P. Cicconardi et al. / International Journal of Hydrogen Energy 31 (2006) 693 – 700
Nomenclature APP apparent powerplant ASU air separation unit CC combustion chamber Ech chemical energy Eel electrical energy HEX heat exchanger HHV high heating value HP high pressure IGCC integrated gasifier combined cycle IGHP integrated gasifier hydrogen production investigations were carried out concerning the system optimisation. Some components of the systems examined have been taken from scientific literature, while other components and the whole plants have been simulated using ASPEN PLUSTM .
2. The reference layout The production of hydrogen from coal requires: • • • • • •
the the the the the the
production of steam; separation of oxygen from air; coal gasification; cleaning of the gas from impurity; conversion of carbon monoxide; separation of carbon dioxide and water.
Each of the above listed process steps can be performed via different technology options therefore yielding to several plant layouts. To narrow the focus of the analysis the gas clean-up process is first selected, given its critical role in determining the suitability of the produced hydrogen to various different energy end-use services and technologies. Three layouts are here considered with the complex of operations and components designed around a membrane-type reactor. The ultimate analysis of coal considered and its high heating value are reported in Table 1, and a simplified schematic of such a plant is shown in Fig. 1. Coal, water and air are the inputs to the system. The air goes into the air separation unit (ASU) where the oxygen is separated and compressed. Then it is fed to the coal gasifier. The nitrogen is released to the atmosphere after having been heated and expanded to recover energy. The oxygen is produced with a purity of 95% [2]. The water is divided in two streams: to the heat recovery steam generator (HEX1) and to the water gas shift reactor (WGSR). The first is used to produce steam to be fed to the coal gasifier, while the latter is used to convert most of the residual CO.
IP intermediate pressure LP low pressure p pressure SCOHS selective catalytic oxidation of hydrogen sulphide SEP separation of water and sequestration of carbon dioxide T temperature WGSR water gas shift reactor App apparent efficiency Table 2 shows common assumptions for the integrated gasifier hydrogen production (IGHP) and the different CPH layouts proposed later. Table 3 shows the main characteristics of IGHP system. The coal gasifier has been supposed to be an entrained bed type. The operating pressure is 16 bar as this is the pressure required by the membrane separator in the WGSR. The temperature of the gas produced is 1350 ◦ C. In the scientific literature it was not possible to find data for a coal gasifier with exactly the same operating parameters. Data for coal gasifier with the same technology and different operating parameters were used to test the model used for a simulation of the coal gasifier. Such a model is based on the chemical equilibrium theory. To apply such a theory it can be used the equilibrium constant method or the minimisation of the Gibbs free energy. By applying the first method all the independent stoichiometric equation of formation of the syngas species must be recognised and an equilibrium constant obtained for each one. The Gibbs free energy is a thermodynamic function that can be interpreted as a driving force of a reaction, function of the number of moles of each species: it has a null value at equilibrium. When considering two or more reactions the null value cannot be reached but there is equilibrium when a minimum is reached.
Table 1 Main characteristics of coal considered Coal Ultimate analysis (Wt%) C H N Cl S O Moisture Ash HHV (MJ/kg)
Illinois #6
63.75 4.50 1.25 0.29 2.51 6.88 11.12 9.70 27.13
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695
Fig. 1. Schematic of the integrated gasifier hydrogen production (IGHP).
Table 3 Main characteristics of IGHP
Table 2 Common assumptions for IGHP and CPH systems Operating temperature of the gasifier (◦ C) Operating pressure of the gasifier (bar) Inlet pressure of the ASU (bar) Outlet pressure of the ASU (bar) Operating temperature of the WGSR (◦ C) Inlet pressure of the WGSR (bar) Outlet pressure of the WGSR (bar) Pump isoentropic efficiency Turbine isoentropic efficiency Compressor isoentropic efficiency
1350 16 19 6 322 16 1 0.75 0.85 0.85
The gas produced contains different kinds of substances: • fuels (e.g. hydrogen); • diluents (e.g. the residual nitrogen fed to the gasifier together with oxygen); • noxious compounds (e.g. hydrogen sulphide). The required degree of conversion of the fuels other than hydrogen present in the flue gas is determined on the endused envisaged for the latter: as an example the use of the
Oxidant (O2 95%) fed to the gasifier (kg/kgCOAL ) Steam fed to the gasifier (kg/kgCOAL ) Steam temperature (◦ C) H2 (from WGSR) (kg/kgCOAL ) Tailgas (at the final stage) (kg/kgCOAL ) H2 content in the Tailgas (at the final stage) (% vol)
0.83 0.50 550 0.127 0.41 53.8
produced hydrogen in a PEMFC system requires the complete conversion of carbon monoxide, which is noxious, and the minimisation of methane, which acts as a diluent. The output of an entrained bed coal gasifier contains typically methane in very low concentration and an amount of carbon monoxide comparable with the one of hydrogen. The process chosen to purify hydrogen is based on a reactor called WGSR. It is capable to accomplish simultaneously the water-gas shift reaction and the separation of hydrogen from all the other substances by using a membrane. Not all the hydrogen can be recovered, but its purity is very high (99.9999%). Operating conditions assumed are T = 322 ◦ C, pin = 16 bar, pout = 1 bar [3]. At NETL a project is devel-
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oped for a technology that is able to produce pure H2 from coal gasification using a membrane system at 600 ◦ C [4]. The residual hydrogen remains in the tail gas stream without significant pressure drop. The other main components are carbon dioxide (51.6%) and water (27.5%), which could be separated, nitrogen (4.8%), carbon monoxide (1.2%) and hydrogen sulphide (0.8%). At this lower level of temperature the sulphur separation can be carried out by means of a selective catalytic oxidation of hydrogen sulphide (SCOHS) [5]. Such a process allows also direct sulphur production and was developed especially for the coal gasification plants. Selective oxidation of hydrogen sulphide to sulphur is obtained by the combustion with air or oxygen (2–4 times the stoichiometric factor). The system is made up by two reactors: in the first one the hydrogen sulphide is removed from the gas stream with a zinc oxide sorbent while in the second one the sorbent is regenerated with air or oxygen, when it reaches 50% of its weight in sulphur. Recent studies show that below 200 ◦ C it is possible to obtain a sulphur removal as high as 99.76%. Pressures in the range 11–33 bar have been reported [6]. The purified gas contains also a small amount of oxygen which is eliminated with a combustion. At this step the composition of the tail gas is: H2 = 12.6%, CO2 = 52.2%, H2 O = 29.6%, N2 = 5.1%, CO = 0.5%. The final stages of purification for the tail gas are the separation of water and carbon dioxide and the liquefaction of carbon dioxide (T = 298 K, p = 15 MPa). A sequestration of 90% of the carbon dioxide was assumed with a power consumption of 500 kJ/kgCO2 . The final composition of the tail gas is therefore: H2 = 53.8%, CO2 = 22.4%, N2 = 21.8%, CO = 2.0%.
The presence of a unique condenser corresponds to the ideal layout; however, the thermal integration of CPH1 is not optimal: there are some heat losses located along the purification line before the condenser. Therefore a better integration could arise from expanding in a low pressure turbine a fraction of the steam flow larger than the one fed to the gasifier. This has been analysed with CPH2 (Fig. 3). The schematic is the same as the previous one, but after the last reheating the steam flow rate is divided in two streams and one of them is expanded in a third turbine (LP) down to 0.05 bar. The main characteristics of CPH2 are shown in Table 5. If the steam fed to the gasifier were a high temperature steam, a lower amount of oxygen should be required to burn the coal and maintain the gasifier at the operating temperature. In such a case a higher amount of hydrogen could be obtained. Such a solution could be carried out by burning some of the hydrogen with stoichiometric oxygen into a direct steam generator. By compressing the reactants, for example up to 64 bar, it is possible to mix the steam from HP with a proper amount of steam directly generated in such a way that the steam at the inlet of IP be at 1500 ◦ C. In this way the steam after the expansion still has a high temperature and can be fed directly to the gasifier. It is also possible to use the tail gas rather than the pure hydrogen. It is not exactly the same because of the presence of carbon monoxide, carbon dioxide and nitrogen, but the resulting gas has to be fed to the gasifier and not to work in a steam power plant under atmospheric pressure. Therefore there are no problems with uncondensable gases. The resulting layout is shown in Fig. 4 (CPH3), while the main characteristics are reported in Table 6.
3. The CPH layouts examined The concept of CPH is based on the consideration that fuel processors require steam while most of the power plants discharge it. Therefore it is possible to use the steam discharged from a power plant into a fuel processor. The basic requirement is that the discharge pressure be adequate. There are no requirements concerning the purity of the steam. A high temperature is favourable, but not necessary. A CPH should allow the maximum efficiency when the thermal integration of the two systems is such that all the steam is condensed only at the end of the gas cleaning process. This means that all the steam flowing into the power plant has to be fed to the coal gasifier. Fig. 2 shows a first layout (CPH1). In this case the water required by the coal gasifier is pumped up to 256 bar, evaporated, heated up to 550 ◦ C (HEX1) and expanded down to 64 bar in a first turbine (HP). The steam exiting from HP is then reheated up to 550 ◦ C (HEX2) and expanded in a second turbine (IP) down to the pressure of the gasifier, 16 bar, to which is fed after a final reheating up to 550 ◦ C (HEX3). Some characteristics of CPH1 are summarised in Table 4.
4. Discussion of the results The different approaches to the thermal integration of the steam power plant and the coal gasifier are responsible for the differences in the temperature of the streams. However, the working temperature of WGSR and SCOHS were assumed to be the same in all cases. The stream fed to the SCOHS was cooled without recovering the heat because of its low temperature. The stream fed to the WGSR was cooled by adding water in order to obtain a higher conversion of carbon monoxide. Due to this choice the water globally fed to the gasifier and to the WGSR was not always the same. Therefore the production of hydrogen in the reference case and in the three cases analysed is not exactly the same. Also the composition of the tail gas is different. As already stated, in the third CPH layout the higher production of pure hydrogen is also due to the lower flow of oxygen required by the gasifier. The goal of this work was to verify if there is an advantage in combining the production of hydrogen and the production
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Fig. 2. Schematic of a combined power and hydrogen system (CPH1).
of electric energy. Therefore the most significant result is the energy output from the systems. In Table 7 such output is reported in terms of chemical energy and electrical energy both expressed in kJ/kgCOAL . The chemical energy is divided in two terms: the first one is related to the stream of pure hydrogen and the other one is related to the stream of tail gas. The electrical energy is the net energy after having subtracted the consumption for compressors, pumps and carbon dioxide sequestration. The first consideration is that the power production is very low, 0.8–2.8% of total energy output. This is connected with the rather small amount of steam required by the gasifier. It is evident that, in order to have a significant production of power, a significant amount of the hydrogen yield has to be used. This involves the production of a large amount of steam, much higher than the one required by the gasifier,
Table 4 Main characteristics of CPH1 Oxidant (95% O2 ) fed to the gasifier (kg/kgCOAL ) Steam fed to the gasifier (kg/kgCOAL ) HP pressure /HP temperature (bar/◦ C) IP pressure /IP temperature (bar/◦ C) H2 (from WGSR) (kg/kgCOAL ) Tailgas (at the final stage) (kg/kgCOAL ) H2 content in the Tailgas (at the final stage) (% vol)
0.87 0.80 256/550 64/550 0.127 0.43 50.5
with a negative effect on the system efficiency due to the increased flow of steam to be condensed. A way to reduce the steam flow rate to be produced for power generation is to increase its mean temperature. At this concern the third CPH layout (CPH3) could be more interesting. The expansion could be realised in more than
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Fig. 3. Schematic of a combined power and hydrogen system (CPH2). Table 5 Main characteristics of CPH2 Oxidant (95% O2 ) fed to the gasifier (kg/kgCOAL ) Steam fed to the gasifier (kg/kgCOAL ) HP pressure /HP temperature (bar/◦ C) IP pressure /IP temperature (bar/◦ C) Steam flow rate at LP Turbine (kg/kgCOAL ) LP pressure/LP temperature (bar/◦ C) Condenser pressure (bar) H2 (from WGSR) (kg/kgCOAL ) Tailgas (at the final stage) (kg/kgCOAL ) H2 content in the Tailgas (at the final stage) (% vol)
Table 6 Main characteristics of CPH3 0.87 0.80 256/550 64/550 0.2 16/550 0.05 0.127 0.42 50.1
one turbine with multiple reheatings at 1500 ◦ C. The new steam cooling system used for the series H gas turbine could be applied to this high temperature steam turbines. When considering the efficiency of power production the results are very interesting. It is obvious that the efficiency of a counterpressure power plant is not very high. But in
Oxidant fed to the gasifier (kg/kgCOAL ) Impure steam fed to the gasifier (kg/kgCOAL ) Impure steam temperature at the gasifier (◦ C) Steam flow rate to HP turbine (kg/kgCOAL ) HP pressure /HP temperature (bar/◦ C) IP pressure /IP temperature (bar/◦ C) Steam flow rate to LP turbine (kg/kgCOAL ) LP pressure/LP temperature (bar/◦ C) Condenser pressure (bar) CC pressure (bar) H2 (from WGSR) (kg/kgCOAL ) H2 content in the Tailgas (at the final stage) (% vol) Tailgas (at the final stage) (kg/kgCOAL ) Tailgas at CC (kg/kgCOAL )
0.78 0.87 1180 0.85 256/550 64/1500 0.35 16/550 0.05 16 0.129 43.7 0.6 0.25
this case the steam is supplied to another user and therefore it has a positive effect. From Table 7 we can see that CPH
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Fig. 4. Schematic of a combined power and hydrogen system (CPH3). Table 7 Energy output (kJ/kgCOAL ) IGHP
CPH1
CPH2
CPH3
Chemical (pure H2 ) Chemical (tail gas) Electrical
18,209 3752 −224
17,714 3665 165
17,894 3486 446
18,350 2608 599
Total
21,737
21,544
21,826
21,557
layouts generate more electrical energy and less chemical energy than the reference system. In order to estimate the benefit of every layouts we could define an apparent effi-
Fig. 5. Energy balance of IGHP and CPH systems.
ciency which takes into account these differences with respect to the reference system. Referring to Figs. 5 and 6, we could consider the power generation in the CPH be due to a powerplant which uses part of the hydrogen produced. This powerplant has been
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Fig. 6. CPH energy balance as combined IGHP and apparent powerplant.
Table 8 Apparent efficiency
Electrical energy produced Chemical energy apparently consumed Apparent efficiency
CPH1
CPH2
CPH3
389 582
670 581
823 1003
0.668
1.153
0.821
of a IGCC and therefore it is expected that also increasing the power production of a CPH, the global efficiency be higher than the one obtainable with separated production. As previously stated, the analysis pointed out does not deal with the plant optimisation. We simply designed some layouts and analysed their performance in order to understand if the concept is interesting and how to address future studies. The next step will be a comparative analysis of plant layouts based on different cleaning processes, in particular low pressure and low temperature ones. It could allow to reduce the pressure of the gasifier and to avoid to fraction the thermal integration of the systems. And, of course, it will be necessary to repower the section of the plant devoted to power production, analysing at the same time the effect of the power over the global efficiency of the system in comparison with the two separate plants.
References called apparent powerplant (APP) and its efficiency has been called apparent efficiency. This apparent efficiency is therefore defined as the more electrical energy generated divided by the less chemical energy produced, which is a chemical energy apparently consumed: App =
Eel,CPH − Eel,IGHP Eel = . Ech Ech,IGHP − Ech,CPH
(1)
Table 8 shows the results. Particularly surprising is the apparent efficiency of CPH2. As already stated it is not a real efficiency. It simply means that combining the two plants it is possible to reduce the global energy losses with respect to the IGHP system. If we multiply the apparent efficiency by the conversion efficiency from coal to hydrogen, that is 0.81, we obtain a conversion factor from coal to power, apparently passing through hydrogen production. Such a conversion factor ranges from 0.54 to 0.93. As a reference case for power production from coal we could consider an Integrated Gasifier Combined Cycle (IGCC) which efficiency is about 0.45. In all cases the conversion factor is higher than the efficiency
[1] Spazzafumo G. Cogeneration of power and hydrogen with integrated fuel processor counterpressure steam cycles. Int J Hydrogen Energy 2004;29(11):1147–50. [2] Process Engineering Division. Destec gasifier base cases PEDIGCC-98-003. September 1998. Latest Revision June 2000. NETL. [3] Criscuoli A, Basile A, Drioli E, Loiacono O. An economic feasibility study for water gas shift membrane reactor. J Membrane Sci 2001;181:21–7. [4] Killmeyer R, Ciocco M, Morreale B, Enick R, Bustamante F. Water gas shift membrane reactor studies. NETL, Office of Science and Technology Fuel and Process Chemistry Division (from www.eere.energy.gov/hydrogenandfuelcells/hydrogen). [5] Selective Catalytic Oxidation of Hydrogen Sulfide for Simultaneous Coal Gas Desulfurization and Direct Sulfur Production (SCOHS) System Analysis. February 2002. Prepared for NETL. Task. No 50802. [6] Siriwardane RV, Cicero D. Durable zinc oxide containing regenerable desulfurization sorbents for both low and high temperature applications. US Department of Energy, Federal Energy Technology Center Morgantown. Prepared for presentation at AIChE Spring National Meeting, March 5–9 2000, High Temperature Gas Cleaning (Gases and Particles) Session TF004 (from www.osti.gov/bridge).