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Combined power and hydrogen production from coal. Part A—Analysis of IGHP plants
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Combined power and hydrogen production from coal. Part A—Analysis of IGHP plants Alessandra Perna Department of Industrial Engineering, University of Cassino, Via G. di Biasio, 43, Cassino, Italy
art i cle info
ab st rac t
Article history:
Recent studies have demonstrated the possibility of an efficient and clean employment of
Received 10 December 2007
fossil fuels by using a new concept of cogeneration of power and hydrogen. This concept is
Received in revised form
based on the considerations that steam power plants discharge heat by condensing steam,
31 March 2008
while coal gasification plants require steam to produce hydrogen: an integration of these
Accepted 2 April 2008
systems results in a very high conversion efficiency. This two-part paper analyses these plants, the so-called combined power and hydrogen
Available online 27 May 2008 Keywords: Hydrogen Coal gasification Combined power and hydrogen production CO2 capture
(CPH) systems, and compares their performances with a plant for only hydrogen production, the integrated gasifier for hydrogen production (IGHP) plant, by means of new parameters, the marginal efficiency and the apparent efficiency. The use of these parameters requires the definition of a IGHP reference plant, which must be designed to maximize the hydrogen production with minimum power consumption. Thus, Part A investigates the IGHP plants to define the reference plant, Part B analyses and evaluates different plant configurations of CPH systems by comparison with the IGHP reference plant. In order to identify the thermodynamically favourable operating conditions for the IGHP plant (maximum hydrogen production with minimum power consumption), a sensitivity analysis has been conducted at a gasification temperature of 1350 1C, varying the gasification pressure from 1 to 80 bar, the steam to carbon ratio at the gasifier, S/C, from 0.1 to 0.9, and the steam to carbon ratio at the water–gas shift reactors, SSHIFT/C, from 0.6 to 1.2. Results show that the hydrogen production depends significantly on the steam to carbon ratio at both the gasifier and the shift reactors, while the influence of gasification pressure is negligible even though it is a very important factor to minimize the power consumption (or to generate electrical power). Better performances are achieved for a gasification pressure of 70 bar, a S/C ratio of 0.2, and a SSHIFT/C ratio equal to 1.2. The hydrogen production efficiency, referred to the HHV, is 72%. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
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. In a long-term scenario renewable energy sources will become competitive but they should not be adequate to meet the energy needs in the midterm, especially for those countries with a high
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E-mail address: [email protected] 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.04.005
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Nomenclature hydrogen production efficiency (H2 HHV power/ coal HHV power) electrical efficiency (net electric power/coal HHV Zel power) thermal power available from the IGHP plant, MW QAV W net electrical power of IGHP plant without CO2 capture, MW air compression power, MW WAIR Wch, CPH chemical power in (coal) or out (H2) referred to CPH system Wch, IGHP chemical power in (coal) or out (H2) referred to IGHP plant net electric power of CPH system WCPH net electric power of IGHP plant WIGHP oxygen compression power, MW W O2 WPUMP water pumping power, MW
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WSYN WTN2
syngas expansion power, MW nitrogen expansion power, MW
ZH2
population density. Therefore, it is necessary to find an energy source less expensive than oil and natural gas. It will be used especially to produce hydrogen to replace gasoline for transportation and natural gas for heating. A chance could be to use coal as it will last for some centuries and its distribution on the Earth is not so concentrated as that of oil and natural gas. Therefore, its cost will remain more stable and competitive for a long time. However, it is important to develop coal technologies which are clean and efficient [1–9]. Coal is the fossil fuel with the highest content of carbon, and therefore, the energy conversion (e.g. the conversion of the chemical power of the fuel into electrical power) efficiency is also important to reduce the carbon dioxide emissions, waiting for effective sequestration systems. Electric energy and hydrogen will be the energy carriers of the future: hydrogen will become fundamental for transport applications. Technologies to produce both these carriers from coal are already well known [10]. Their integration holds the promise of an improvement of the global conversion efficiency and a decrease in greenhouse gas emissions for those countries in which the exploitable renewable energy source flows are largely offset by their exploitable coal resource endowment or where coal already plays a substantial role in the electricity generation mix. The co-production of power and hydrogen from coal has been recently investigated in several technical papers [11–15]. In particular, a new concept of cogeneration of power and hydrogen has been developed in [14]. This concept is based on the considerations that the steam power plants discharge heat by condensing steam, while coal gasification plants require steam to convert coal into hydrogen: an integration of these systems leads to a very high energy conversion efficiency. The resulting system is a novel plant, the so-called combined power and hydrogen (CPH) system, that is designed to co-generate electrical power already within the gasification island [14,15]. To estimate the performance of these novel systems new parameters have been introduced [14], the marginal efficiency and the apparent efficiency.
Acronyms AGR ASU CPG CPH GTCC HEX HHV IGCC IGHP PSA SAT WGHT WGLT
acid gas removal air separation unit purge gas compressor combined power and hydrogen gas turbine combined cycle heat exchanger high heating value integrated gasifier combined cycle integrated gasifier for hydrogen production pressure swing adsorption saturator high temperature water–gas shift reactor low temperature water–gas shift reactor
The marginal efficiency is defined as the ratio of the more electrical power generated to the more chemical power consumed with respect to a reference plant, that is, the integrated gasifier for hydrogen production (IGHP) plant. Since it estimates the efficiency with which the additional coal chemical power is converted into electrical power, it is a useful parameter to compare the CPH systems with an integrated gasifier combined cycle (IGCC). Thus, assuming the hydrogen produced in the IGHP plant as the reference value, the marginal efficiency is calculated as Zmarg ¼
DW a WCPH WIGHP ¼ DWch;coal ða Wch;CPH Wch;IGHP Þcoal
(1)
where the coefficient a is the ratio of the hydrogen produced by the IGHP reference plant to that coming out from the CPH system. Keeping constant the coal chemical power entering both the IGHP and CPH systems, the apparent efficiency is defined as the ratio of the more electrical power generated to the less hydrogen chemical power produced, that is the chemical power apparently consumed, with respect to the IGHP reference plant. The apparent efficiency is calculated as Zapp ¼
DW WCPH WIGHP ¼ DWch;H2 ðWch;IGHP Wch;CPH ÞH2
(2)
The use of these parameters, useful to evaluate the energy suitability of the CPH systems, requires the definition of a IGHP reference plant. The IGHP plant chosen as reference plant must be designed to maximize the hydrogen production with the minimum power consumption. In this two-paper work, Part A investigates the IGHP systems to define the reference plant, Part B analyses and evaluates different plant configurations of CPH systems by the comparison with the IGHP reference plant. In order to identify the thermodynamically favourable operating conditions for the IGHP plant, a sensitivity analysis has been carried out at a gasification temperature of 1350 1C, varying the gasification pressure, the steam to carbon ratio at
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the gasifier, S/C, and the steam to carbon ratio at the water–gas shift reactors, SSHIFT/C.
2.
Acid– gas removal unit (AGR) and carbon dioxide capture unit
The IGHP plant
The layout of the IGHP reference plant for both atmospheric (IGHP,A) and pressurized (IGHP,P) configurations, is depicted in Fig. 1. The main components are:
Air separation unit (ASU): It is a stand-alone unit operating
at 19 bar and generating pure 95% oxygen at a pressure of 6.25 bar. Fuel processor reactor (GASIFIER): The coal gasifier has been supposed to be an entrained bed type. The temperature of the syngas produced is assumed constant at 1350 1C. The gasifier is fed by Illinois #6 coal and superheated steam at 650 1C. The ultimate analysis and heating values of the coal employed are reported in Table 1. Water– gas shift reactors (WGHT and WGLT): In order to obtain the maximum hydrogen production, the water–gas shift reaction (WGSR), CO+H2O2H2+CO2, is carried out in two reactors placed in series (high temperature water–gas shift reactor, WGHT, and low temperature water–gas shift reactor, WGLT). They work at temperatures of about 400 and 200 1C, respectively. The catalysts employed to improve the reaction rate are sulphur tolerant catalysts, thus the sulphur removal unit can be put downstream the shifters [13].
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(CO2 capture): These units are based on chemical absorption and physical absorption for atmospheric and pressurized configuration, respectively. The AGR removes 99.9% of the H2S, which is converted to elemental sulphur via Claus and SCOT plants. The efficiency of the CO2 capture unit is assumed equal to 95%. Pressure swing adsorption unit (PSA): The PSA unit produces hydrogen with 99.999% of purity, operates at about 20 bar and reaches H2 separation efficiencies in the range 85–90%. The PSA purge gas can be used to superheat steam or can be combusted for power generation in a gas turbine combined cycle.
Table 1 – Coal properties Illinois #6 coal
Ultimate analysis (wt%)
C H N Cl S O Moisture Ash
63.75 4.50 1.25 0.29 2.51 6.88 11.12 9.7
HHV (MJ/kg) LHV (MJ/kg)
27.13 25.88
SYNGAS
H2 PURGE GAS PSA
WATER
HEX2 N2
TN2 PUMP CSYN HEX3
HEX1
ASU CAIR
STEAM
CO2 CO2 CAPTURE
N2
SYNGAS
WGHT
O2
Sulfur
COAL SYNGAS
AIR
COX
AGR
H2S
Claus/SCOT Plants
GASIFIER SLAG
Components surrounded with dashed boxes and streams drawn by dashed lines are present only in IGHP,P plants. CSYN is not present in IGHP,P plants.
WGLT
Fig. 1 – The IGHP plant layout.
SYNGAS
TSYN
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When the IGHP,A is considered, the syngas exiting the low temperature water–gas shift reactor, is cooled at the operating temperature of the AGR unit, which is included between 35 and 50 1C. In addition, the compression of the free CO2 hydrogen stream (compressor CSYN) is needed to achieve the operating conditions of the PSA unit. In the case of IGHP,P, an oxygen compressor (COX) is placed after the ASU and a syngas expander (TSYN) replaces the heat exchanger downstream of the low temperature water–gas shift reactor. The thermal recovery of the syngas coming out from the gasifier allows to generate the superheated steam for the gasification reactions (HEX1 and HEX3) and to heat the nitrogen flow rate from the ASU unit (HEX2) before its expansion in the TN2 turbine. The water entering the system is pumped to the gasification pressure and split in two fluxes fed to the gasifier and the shifters. The compressors and turbines politropic efficiencies have been assumed equal to 0.88 and 0.9, respectively, while the isoentropic efficiency of the pump is 0.75. The minimum DT for heat transfer in exchangers HEX1, HEX2, and HEX3 has been assumed equal to 10 1C. Concerning the modelling assumptions, the chemical reactors, gasifier and shifters, are simulated considering the chemical equilibrium which is solved by a non-stoichiometric formulation. In this approach the equilibrium composition is found by the direct minimization of the Gibbs free energy for a given set of species without any specification of the possible reactions which might take place in the system. The equilibrium compositions have been calculated for a given operating condition, and in order to evaluate the overall efficiency, the material and energy balances are solved for each configuration. The sensitivity analysis of the IGHP systems has been performed by using the thermochemical code AspenPlusTM.
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the sensitivity analysis has been carried out varying the following parameters:
Gasification pressure, pGASIFIER, in the range 1–80 bar. Steam to carbon ratio, S/C, at the gasifier, defined as the
ratio of the mass flow rate of the feeding coal to the mass flow rate of the steam fed to the gasifier, in the range 0.1–0.9. Steam to carbon ratio, SSHIFT/C, at the water–gas shift reactors, defined as the ratio between the mass flow rate of the feeding coal and the mass flow rate of the water fed to the shifters, in the range 0.6–1.2; it is assumed that the mass fraction sent to the WGHT is 0.67.
3.1. Influence of gasification pressure, pGASIFIER, and steam to carbon ratio, S/C Figs. 2a and b show the effects of gasification pressure and steam to carbon ratio on the equilibrium composition, on wet and dry basis, respectively, of the syngas leaving the gasifier, mainly formed by H2, CO, and CO2. The gasification pressure does not impact significantly the equilibrium composition: by changing the pressure all curves are almost overlapped. As the S/C ratio increases, the molar fractions of H2 and CO2 increase while the CO content diminishes (Fig. 2b). However, the hydrogen concentration rises with a decreasing slope with the S/C ratio, because a higher amount of heat, involving a greater extent of the exothermic reactions, is needed to sustain the gasification process. Moreover, high steam to carbon ratios imply an increase in the air flow rates entering the ASU, so that the compression power required by the air and oxygen compressors and the power generated by the nitrogen expansion increase too (Fig. 3).
3.2. Influence of steam to carbon ratio at the shifters SSHIFT/C
3.
Sensitivity analysis of IGHP plant
In order to identify the thermodynamically favourable operating conditions for the maximum hydrogen production,
The SSHIFT/C parameter has a very important effect on the hydrogen production. Fig. 4 shows the hydrogen flow rate in the syngas (the stream exiting the WGLT reactor) as a function
0.7
0.7 1
0.5 0.4
H2
CO
0.3 0.2 0.1 0
H2O
CO2
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 S/C
1
0.6 Molar Fraction
Molar Fraction
0.6
80
0.5 0.4
80
CO H2
0.3 0.2
CO2
0.1 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 S/C
Fig. 2 – Molar fractions of syngas components from the gasifier (for pGASIFIER equal to 1 and 80 bar). (a) Wet basis; (b) dry basis.
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1.25 1
1
80 WN2
0.75
Power (MW)
0.5 0.25 0 WO2
-0.25 -0.5 -0.75
WAIR
-1 -1.25 0.1
0.2
0.3
0.4
0.5 S/C
0.6
0.7
0.8
0.9
Fig. 3 – Power consumptions and power generated from the nitrogen expansion (for pGASIFIER equal to 1 and 80 bar).
0.165
conversion of carbon monoxide to carbon dioxide, is an exothermic reaction and, therefore, it is favoured at lower temperatures. In addition, an increase in the reactants concentration (CO and H2O) moves the chemical equilibrium to the products (CO2 and H2). This means that, in order to obtain the maximum hydrogen production, it is necessary to reduce the S/C ratio at the gasifier and to increase the SSHIFT/C ratio to the shifters. Furthermore, with respect to the gasification pressure, the SSHIFT/C ratio does not present a significant influence on the hydrogen production, as it is previously found. Table 2 reports the molar composition of the syngas exiting the WGLT reactor calculated in the conditions of the maximum hydrogen production. The hydrogen is maximum for a S/C ratio of 0.2 and a SSHIFT/ C ratio equal to 1.2.
3.3. The power balance of the IGHP plants without the cleanup system The power balance, before the syngas cleanup section, for different plant operating parameters is summarized in Table 3. The gasification pressures considered correspond to the atmospheric (IGHP,A), medium pressure (IGHP,30), and high pressure (IGHP,70). From these results some considerations can be made:
1.2 0.16 Hydrogen flow rate (kg/kg coal)
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1 0.155 0.15 0.8 0.145
At atmospheric pressure the power consumption of the
SSHIFT/C
0.6 0.14
0.135 0.13
1
80
0.125 0.1
0.2
0.3
0.4
0.5 S/C
0.6
0.7
0.8
0.9
Fig. 4 – Hydrogen flow rate vs. S/C ratios for different SSHIFT/C ratio (for pGASIFIER equal to 1 and 80 bar).
of S/C, for given SSHIFT/C values, varying the gasification pressure. It is worth noting that:
water feeding pump can be neglected, because only the pressure losses has to be taken into account. By increasing the S/C ratio, the power required for the oxygen feeding and the power obtained by the nitrogen expansion increase. In the pressurized configurations, the power resulting by the syngas expansion in the TSYN turbine contributes to increase the net electrical power generated (W). In the atmospheric configuration, the thermal power available (QAV) from the syngas cooling in the heat exchanger between the shifters and that before the AGR unit, decreases with increasing SSHIFT/C ratio. In the
Table 2 – Characteristics of the syngas from WGLT at different SSHIFT/C ratios in the conditions of maximum hydrogen production
The hydrogen production presents an increasing–decreasing trend as the S/C ratio increases.
The hydrogen production grows with the increasing SSHIFT/ C ratios.
By increasing the SSHIFT/C ratio, the maximum hydrogen
production is achieved for ever decreasing S/C ratios; as a consequence, an optimal value of the S/C ratio exists for each SSHIFT/C. For S/C ratios greater then 0.8, the value of SSHIFT/C becomes irrelevant and all curves are practically coincident.
This behaviour can be explained by analysing the WGSR that occurs in both the gasifier and the shifters. The WGSR, which determines the amount of hydrogen produced and the
SSHIFT/C S/C
0.6 0.7
0.8 0.6
1 0.4
1.2 0.2
Molar composition (%) 49.95 H2 33.28 CO2 CO 0.89 H2O 14.34 0.50 H2S 0.00 CH4 1.00 N2+Ar
48.92 32.44 0.55 16.61 0.49 0.00 1.00
49.58 32.39 0.61 15.96 0.49 0.00 1.00
50.15 32.28 0.72 15.40 0.49 0.01 1.00
Hydrogen flow rate H2 (kg/kgcoal)
0.159
0.161
0.162
0.156
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Table 3 – Power balance of the IGHP plants without the cleanup system IGHP,A SSHIFT/C S/C Gasification pressure Coal input (HHV) WAIR WO2 WN2 WPUMP 103 WSYN W QAV H2 (HHV) ZH2 (HHV)
bar MW MW MW MW MW MW MW MW MW %
0.6 0.7
0.8 0.6
27.13 1.07 – 0.93 0 0 0.14 2.33 20.63 76.0
27.13 1.05 – 0.91 0 0 0.14 2.12 21.72 80.1
IGHP,30
1 0.4
1.2 0.2
0.6 0.7
0.8 0.6
27.13 1.03 – 0.89 0 0 0.14 1.81 22.36 82.4
27.13 1 – 0.87 0 0 0.13 1.39 23.02 84.9
27.13 1.07 0.12 0.92 5.1 0.89 0.62 0.95 22.25 82.0
27.13 1.05 0.11 0.91 5.5 0.90 0.65 0.67 22.57 83.2
1
IGHP,70
1 0.4
1.2 0.2
0.6 0.7
0.8 0.6
27.13 1.02 0.11 0.89 5.5 0.90 0.66 0.29 22.86 84.3
27.13 1 0.11 0.86 5.5 0.91 0.65 0 23.04 84.9
27.13 1.07 0.19 0.93 12.2 1.15 0.81 1.08 22.47 82.8
27.13 1.05 0.18 0.91 13.1 1.17 0.84 0.81 22.65 83.5
30
1 0.4
1.2 0.2
27.13 1.02 0.18 0.89 13.1 1.18 0.85 0.42 22.97 84.7
27.13 0.99 0.17 0.86 13.1 1.18 0.87 0 23.06 85.0
70
PURGE GAS POWER H2 CPG
PSA
GTCC
SAT
CO2 CAPTURE
CO2 emitted
CO2 captured
steam for H2 Sstripping
SYNGAS from WGLT TSYN AGR
H2S
Claus/SCOT PLANT
Sulphur
Fig. 5 – Schematic of the cleanup system and the power island.
pressurized configurations, it is only due to the syngas cooling between the shifters and it grows by increasing the gasification pressure because of the decrease in evaporation latent heat of water sent to the WGHT; moreover, it diminishes with the increasing SSHIFT/C ratio. Unlike the IGHP,A plant, in which electrical power is required, the pressurized configurations produce a net power that increases with the gasification pressure. The IGHP,70 is the better configuration both for the hydrogen production and power consumption since net electrical power is generated.
4. Energy balance of the IGHP reference plant with the syngas cleanup system The energy balance summarized in Table 3 is referred to the IGHP plant without the cleanup system. In this section the
effect of the cleanup system on the IGHP reference plant is evaluated. The physical absorption in dimethyl ether of polyethylene glycol (Selexol) is used to remove H2S and CO2 from the syngas in the IGHP,70 plant which is, as it results from the sensitivity analysis, the IGHP reference plant. In order to remove H2S and CO2 separately from the hydrogen product stream, a double-stage Selexol unit [2,13], operating at 30 bar, was selected. This process removes H2S from the cooled syngas and then removes CO2 from the desulphurized syngas. The heat required for H2S stripping is assumed to be 6 MJ of 6 bar steam per kg of sulphur stripped [13]. The carbon dioxide streams are released at different pressure levels in four flash drums and delivered for final compression to 150 bar. The free CO2 hydrogen stream, coming out from the CO2 capture unit, is sent to the PSA unit in which the hydrogen is separated with an efficiency of 85%. Finally, the purge gas from the PSA unit, which consists mainly of H2 (HHV ¼ 14.1 MJ/kg), after its compression and
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Table 4 – Performance of the IGHP reference plant with the cleanup system and the GTCC IGHP,70 Coal input (HHV)
MW
1810
SELEXOL/CO2 compressor WIGHP,70
MW MW
68.0 7.7
Gas Turbine Combined Cycle (GTCC) Purge gas (HHV) Purge gas compressor (CPG) Gas turbine Steam turbinea
MW MW MW MW
261 12.0 79.8 37.8
IGHP,70-GTCC H2 (HHV) Net power output Cogeneration ratiob ZH2 (HHV) Zel (HHV) CO2 Captured CO2 emissions CO2 emissions
MW MW % %
1303 45.3 3.5 72.0
% % g/kWh kg/GJHHV
2.5 95 61.3 6.9
a The power generated from the steam power section has been calculated taking into account the heat needed for H2S stripping, which is obtained by bleeding steam from steam turbine. b The cogeneration ratio is defined as net power output/H2 HHV power.
saturation in CPG and SAT, is used as fuel in a triple-pressure, reheat combined cycle (GTCC), based on a commercial gas turbine. The system described is depicted in Fig. 5. The energy balance of the whole system is reported in Table 4. The coal feed flow rate has been calculated according to the purge gas flow rate required by the gas turbine (the air flow rate is 188 kg/s and the turbine inlet temperature is 1289 1C) and the specific CO2 emissions (g/kWh) related to the power generation have been estimated assuming that the free CO2 hydrogen stream is directly burned in a gas turbine combined cycle with the same efficiency of that fed by the purge gas.
5.
Conclusions
The aim of this Part A was to define the IGHP reference plant needed to analyse and compare novel systems, the CPH systems, investigated in Part B. The IGHP reference plant has been designed to optimize the hydrogen yield with the minimum power consumption. In order to identify the thermodynamically favourable operating conditions for the maximum hydrogen production, a sensitivity analysis has been carried out, varying the gasification pressure, the S/C ratio, and the SSHIFT/C ratio. The sensitivity analysis points out that the hydrogen production depends on both S/C and SSHIFT/C ratios which have to be chosen together: in order to maximize the hydrogen production, it is necessary to reduce the S/C ratio at the gasifier and to increase the SSHIFT/C ratio to the shifters. In particular, the maximum hydrogen flow rate is achieved for a steam to carbon ratio at the gasifier of 0.2 and a steam to carbon ratio at the shifters of 1.2 (Fig. 4). The power balance of the IGHP plants, reported in Table 3, presents that, in comparison with the IGHP,A, the IGHP,P plants produce the same hydrogen flow rate, but a net electrical power is also generated. This net electrical power
increases with the gasification pressure, so that the IGHP,70 is chosen as the IGHP reference plant. Finally, in order to evaluate the effect of CO2 capture on the plant performance, the IGHP,70 has been analysed, considering the cleanup system (AGR, CO2 capture unit, and PSA) and using the purge gas from the PSA unit as fuel in a conventional gas turbine combined cycle (the GTCC efficiency is 50%). The electrical power supplied by the gas turbine combined cycle covers the power penalty due to the cleanup system. The hydrogen production efficiency, referred to the HHV, is 72%. R E F E R E N C E S
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Combined power and hydrogen production from coal. Part A—Analysis of IGHP plants Alessandra Perna Department of Industrial Engineering, University of Cassino, Via G. di Biasio, 43, Cassino, Italy
art i cle info
ab st rac t
Article history:
Recent studies have demonstrated the possibility of an efficient and clean employment of
Received 10 December 2007
fossil fuels by using a new concept of cogeneration of power and hydrogen. This concept is
Received in revised form
based on the considerations that steam power plants discharge heat by condensing steam,
31 March 2008
while coal gasification plants require steam to produce hydrogen: an integration of these
Accepted 2 April 2008
systems results in a very high conversion efficiency. This two-part paper analyses these plants, the so-called combined power and hydrogen
Available online 27 May 2008 Keywords: Hydrogen Coal gasification Combined power and hydrogen production CO2 capture
(CPH) systems, and compares their performances with a plant for only hydrogen production, the integrated gasifier for hydrogen production (IGHP) plant, by means of new parameters, the marginal efficiency and the apparent efficiency. The use of these parameters requires the definition of a IGHP reference plant, which must be designed to maximize the hydrogen production with minimum power consumption. Thus, Part A investigates the IGHP plants to define the reference plant, Part B analyses and evaluates different plant configurations of CPH systems by comparison with the IGHP reference plant. In order to identify the thermodynamically favourable operating conditions for the IGHP plant (maximum hydrogen production with minimum power consumption), a sensitivity analysis has been conducted at a gasification temperature of 1350 1C, varying the gasification pressure from 1 to 80 bar, the steam to carbon ratio at the gasifier, S/C, from 0.1 to 0.9, and the steam to carbon ratio at the water–gas shift reactors, SSHIFT/C, from 0.6 to 1.2. Results show that the hydrogen production depends significantly on the steam to carbon ratio at both the gasifier and the shift reactors, while the influence of gasification pressure is negligible even though it is a very important factor to minimize the power consumption (or to generate electrical power). Better performances are achieved for a gasification pressure of 70 bar, a S/C ratio of 0.2, and a SSHIFT/C ratio equal to 1.2. The hydrogen production efficiency, referred to the HHV, is 72%. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
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. In a long-term scenario renewable energy sources will become competitive but they should not be adequate to meet the energy needs in the midterm, especially for those countries with a high
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E-mail address: [email protected] 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.04.005
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Nomenclature hydrogen production efficiency (H2 HHV power/ coal HHV power) electrical efficiency (net electric power/coal HHV Zel power) thermal power available from the IGHP plant, MW QAV W net electrical power of IGHP plant without CO2 capture, MW air compression power, MW WAIR Wch, CPH chemical power in (coal) or out (H2) referred to CPH system Wch, IGHP chemical power in (coal) or out (H2) referred to IGHP plant net electric power of CPH system WCPH net electric power of IGHP plant WIGHP oxygen compression power, MW W O2 WPUMP water pumping power, MW
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WSYN WTN2
syngas expansion power, MW nitrogen expansion power, MW
ZH2
population density. Therefore, it is necessary to find an energy source less expensive than oil and natural gas. It will be used especially to produce hydrogen to replace gasoline for transportation and natural gas for heating. A chance could be to use coal as it will last for some centuries and its distribution on the Earth is not so concentrated as that of oil and natural gas. Therefore, its cost will remain more stable and competitive for a long time. However, it is important to develop coal technologies which are clean and efficient [1–9]. Coal is the fossil fuel with the highest content of carbon, and therefore, the energy conversion (e.g. the conversion of the chemical power of the fuel into electrical power) efficiency is also important to reduce the carbon dioxide emissions, waiting for effective sequestration systems. Electric energy and hydrogen will be the energy carriers of the future: hydrogen will become fundamental for transport applications. Technologies to produce both these carriers from coal are already well known [10]. Their integration holds the promise of an improvement of the global conversion efficiency and a decrease in greenhouse gas emissions for those countries in which the exploitable renewable energy source flows are largely offset by their exploitable coal resource endowment or where coal already plays a substantial role in the electricity generation mix. The co-production of power and hydrogen from coal has been recently investigated in several technical papers [11–15]. In particular, a new concept of cogeneration of power and hydrogen has been developed in [14]. This concept is based on the considerations that the steam power plants discharge heat by condensing steam, while coal gasification plants require steam to convert coal into hydrogen: an integration of these systems leads to a very high energy conversion efficiency. The resulting system is a novel plant, the so-called combined power and hydrogen (CPH) system, that is designed to co-generate electrical power already within the gasification island [14,15]. To estimate the performance of these novel systems new parameters have been introduced [14], the marginal efficiency and the apparent efficiency.
Acronyms AGR ASU CPG CPH GTCC HEX HHV IGCC IGHP PSA SAT WGHT WGLT
acid gas removal air separation unit purge gas compressor combined power and hydrogen gas turbine combined cycle heat exchanger high heating value integrated gasifier combined cycle integrated gasifier for hydrogen production pressure swing adsorption saturator high temperature water–gas shift reactor low temperature water–gas shift reactor
The marginal efficiency is defined as the ratio of the more electrical power generated to the more chemical power consumed with respect to a reference plant, that is, the integrated gasifier for hydrogen production (IGHP) plant. Since it estimates the efficiency with which the additional coal chemical power is converted into electrical power, it is a useful parameter to compare the CPH systems with an integrated gasifier combined cycle (IGCC). Thus, assuming the hydrogen produced in the IGHP plant as the reference value, the marginal efficiency is calculated as Zmarg ¼
DW a WCPH WIGHP ¼ DWch;coal ða Wch;CPH Wch;IGHP Þcoal
(1)
where the coefficient a is the ratio of the hydrogen produced by the IGHP reference plant to that coming out from the CPH system. Keeping constant the coal chemical power entering both the IGHP and CPH systems, the apparent efficiency is defined as the ratio of the more electrical power generated to the less hydrogen chemical power produced, that is the chemical power apparently consumed, with respect to the IGHP reference plant. The apparent efficiency is calculated as Zapp ¼
DW WCPH WIGHP ¼ DWch;H2 ðWch;IGHP Wch;CPH ÞH2
(2)
The use of these parameters, useful to evaluate the energy suitability of the CPH systems, requires the definition of a IGHP reference plant. The IGHP plant chosen as reference plant must be designed to maximize the hydrogen production with the minimum power consumption. In this two-paper work, Part A investigates the IGHP systems to define the reference plant, Part B analyses and evaluates different plant configurations of CPH systems by the comparison with the IGHP reference plant. In order to identify the thermodynamically favourable operating conditions for the IGHP plant, a sensitivity analysis has been carried out at a gasification temperature of 1350 1C, varying the gasification pressure, the steam to carbon ratio at
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the gasifier, S/C, and the steam to carbon ratio at the water–gas shift reactors, SSHIFT/C.
2.
Acid– gas removal unit (AGR) and carbon dioxide capture unit
The IGHP plant
The layout of the IGHP reference plant for both atmospheric (IGHP,A) and pressurized (IGHP,P) configurations, is depicted in Fig. 1. The main components are:
Air separation unit (ASU): It is a stand-alone unit operating
at 19 bar and generating pure 95% oxygen at a pressure of 6.25 bar. Fuel processor reactor (GASIFIER): The coal gasifier has been supposed to be an entrained bed type. The temperature of the syngas produced is assumed constant at 1350 1C. The gasifier is fed by Illinois #6 coal and superheated steam at 650 1C. The ultimate analysis and heating values of the coal employed are reported in Table 1. Water– gas shift reactors (WGHT and WGLT): In order to obtain the maximum hydrogen production, the water–gas shift reaction (WGSR), CO+H2O2H2+CO2, is carried out in two reactors placed in series (high temperature water–gas shift reactor, WGHT, and low temperature water–gas shift reactor, WGLT). They work at temperatures of about 400 and 200 1C, respectively. The catalysts employed to improve the reaction rate are sulphur tolerant catalysts, thus the sulphur removal unit can be put downstream the shifters [13].
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(CO2 capture): These units are based on chemical absorption and physical absorption for atmospheric and pressurized configuration, respectively. The AGR removes 99.9% of the H2S, which is converted to elemental sulphur via Claus and SCOT plants. The efficiency of the CO2 capture unit is assumed equal to 95%. Pressure swing adsorption unit (PSA): The PSA unit produces hydrogen with 99.999% of purity, operates at about 20 bar and reaches H2 separation efficiencies in the range 85–90%. The PSA purge gas can be used to superheat steam or can be combusted for power generation in a gas turbine combined cycle.
Table 1 – Coal properties Illinois #6 coal
Ultimate analysis (wt%)
C H N Cl S O Moisture Ash
63.75 4.50 1.25 0.29 2.51 6.88 11.12 9.7
HHV (MJ/kg) LHV (MJ/kg)
27.13 25.88
SYNGAS
H2 PURGE GAS PSA
WATER
HEX2 N2
TN2 PUMP CSYN HEX3
HEX1
ASU CAIR
STEAM
CO2 CO2 CAPTURE
N2
SYNGAS
WGHT
O2
Sulfur
COAL SYNGAS
AIR
COX
AGR
H2S
Claus/SCOT Plants
GASIFIER SLAG
Components surrounded with dashed boxes and streams drawn by dashed lines are present only in IGHP,P plants. CSYN is not present in IGHP,P plants.
WGLT
Fig. 1 – The IGHP plant layout.
SYNGAS
TSYN
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When the IGHP,A is considered, the syngas exiting the low temperature water–gas shift reactor, is cooled at the operating temperature of the AGR unit, which is included between 35 and 50 1C. In addition, the compression of the free CO2 hydrogen stream (compressor CSYN) is needed to achieve the operating conditions of the PSA unit. In the case of IGHP,P, an oxygen compressor (COX) is placed after the ASU and a syngas expander (TSYN) replaces the heat exchanger downstream of the low temperature water–gas shift reactor. The thermal recovery of the syngas coming out from the gasifier allows to generate the superheated steam for the gasification reactions (HEX1 and HEX3) and to heat the nitrogen flow rate from the ASU unit (HEX2) before its expansion in the TN2 turbine. The water entering the system is pumped to the gasification pressure and split in two fluxes fed to the gasifier and the shifters. The compressors and turbines politropic efficiencies have been assumed equal to 0.88 and 0.9, respectively, while the isoentropic efficiency of the pump is 0.75. The minimum DT for heat transfer in exchangers HEX1, HEX2, and HEX3 has been assumed equal to 10 1C. Concerning the modelling assumptions, the chemical reactors, gasifier and shifters, are simulated considering the chemical equilibrium which is solved by a non-stoichiometric formulation. In this approach the equilibrium composition is found by the direct minimization of the Gibbs free energy for a given set of species without any specification of the possible reactions which might take place in the system. The equilibrium compositions have been calculated for a given operating condition, and in order to evaluate the overall efficiency, the material and energy balances are solved for each configuration. The sensitivity analysis of the IGHP systems has been performed by using the thermochemical code AspenPlusTM.
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the sensitivity analysis has been carried out varying the following parameters:
Gasification pressure, pGASIFIER, in the range 1–80 bar. Steam to carbon ratio, S/C, at the gasifier, defined as the
ratio of the mass flow rate of the feeding coal to the mass flow rate of the steam fed to the gasifier, in the range 0.1–0.9. Steam to carbon ratio, SSHIFT/C, at the water–gas shift reactors, defined as the ratio between the mass flow rate of the feeding coal and the mass flow rate of the water fed to the shifters, in the range 0.6–1.2; it is assumed that the mass fraction sent to the WGHT is 0.67.
3.1. Influence of gasification pressure, pGASIFIER, and steam to carbon ratio, S/C Figs. 2a and b show the effects of gasification pressure and steam to carbon ratio on the equilibrium composition, on wet and dry basis, respectively, of the syngas leaving the gasifier, mainly formed by H2, CO, and CO2. The gasification pressure does not impact significantly the equilibrium composition: by changing the pressure all curves are almost overlapped. As the S/C ratio increases, the molar fractions of H2 and CO2 increase while the CO content diminishes (Fig. 2b). However, the hydrogen concentration rises with a decreasing slope with the S/C ratio, because a higher amount of heat, involving a greater extent of the exothermic reactions, is needed to sustain the gasification process. Moreover, high steam to carbon ratios imply an increase in the air flow rates entering the ASU, so that the compression power required by the air and oxygen compressors and the power generated by the nitrogen expansion increase too (Fig. 3).
3.2. Influence of steam to carbon ratio at the shifters SSHIFT/C
3.
Sensitivity analysis of IGHP plant
In order to identify the thermodynamically favourable operating conditions for the maximum hydrogen production,
The SSHIFT/C parameter has a very important effect on the hydrogen production. Fig. 4 shows the hydrogen flow rate in the syngas (the stream exiting the WGLT reactor) as a function
0.7
0.7 1
0.5 0.4
H2
CO
0.3 0.2 0.1 0
H2O
CO2
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 S/C
1
0.6 Molar Fraction
Molar Fraction
0.6
80
0.5 0.4
80
CO H2
0.3 0.2
CO2
0.1 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 S/C
Fig. 2 – Molar fractions of syngas components from the gasifier (for pGASIFIER equal to 1 and 80 bar). (a) Wet basis; (b) dry basis.
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1.25 1
1
80 WN2
0.75
Power (MW)
0.5 0.25 0 WO2
-0.25 -0.5 -0.75
WAIR
-1 -1.25 0.1
0.2
0.3
0.4
0.5 S/C
0.6
0.7
0.8
0.9
Fig. 3 – Power consumptions and power generated from the nitrogen expansion (for pGASIFIER equal to 1 and 80 bar).
0.165
conversion of carbon monoxide to carbon dioxide, is an exothermic reaction and, therefore, it is favoured at lower temperatures. In addition, an increase in the reactants concentration (CO and H2O) moves the chemical equilibrium to the products (CO2 and H2). This means that, in order to obtain the maximum hydrogen production, it is necessary to reduce the S/C ratio at the gasifier and to increase the SSHIFT/C ratio to the shifters. Furthermore, with respect to the gasification pressure, the SSHIFT/C ratio does not present a significant influence on the hydrogen production, as it is previously found. Table 2 reports the molar composition of the syngas exiting the WGLT reactor calculated in the conditions of the maximum hydrogen production. The hydrogen is maximum for a S/C ratio of 0.2 and a SSHIFT/ C ratio equal to 1.2.
3.3. The power balance of the IGHP plants without the cleanup system The power balance, before the syngas cleanup section, for different plant operating parameters is summarized in Table 3. The gasification pressures considered correspond to the atmospheric (IGHP,A), medium pressure (IGHP,30), and high pressure (IGHP,70). From these results some considerations can be made:
1.2 0.16 Hydrogen flow rate (kg/kg coal)
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1 0.155 0.15 0.8 0.145
At atmospheric pressure the power consumption of the
SSHIFT/C
0.6 0.14
0.135 0.13
1
80
0.125 0.1
0.2
0.3
0.4
0.5 S/C
0.6
0.7
0.8
0.9
Fig. 4 – Hydrogen flow rate vs. S/C ratios for different SSHIFT/C ratio (for pGASIFIER equal to 1 and 80 bar).
of S/C, for given SSHIFT/C values, varying the gasification pressure. It is worth noting that:
water feeding pump can be neglected, because only the pressure losses has to be taken into account. By increasing the S/C ratio, the power required for the oxygen feeding and the power obtained by the nitrogen expansion increase. In the pressurized configurations, the power resulting by the syngas expansion in the TSYN turbine contributes to increase the net electrical power generated (W). In the atmospheric configuration, the thermal power available (QAV) from the syngas cooling in the heat exchanger between the shifters and that before the AGR unit, decreases with increasing SSHIFT/C ratio. In the
Table 2 – Characteristics of the syngas from WGLT at different SSHIFT/C ratios in the conditions of maximum hydrogen production
The hydrogen production presents an increasing–decreasing trend as the S/C ratio increases.
The hydrogen production grows with the increasing SSHIFT/ C ratios.
By increasing the SSHIFT/C ratio, the maximum hydrogen
production is achieved for ever decreasing S/C ratios; as a consequence, an optimal value of the S/C ratio exists for each SSHIFT/C. For S/C ratios greater then 0.8, the value of SSHIFT/C becomes irrelevant and all curves are practically coincident.
This behaviour can be explained by analysing the WGSR that occurs in both the gasifier and the shifters. The WGSR, which determines the amount of hydrogen produced and the
SSHIFT/C S/C
0.6 0.7
0.8 0.6
1 0.4
1.2 0.2
Molar composition (%) 49.95 H2 33.28 CO2 CO 0.89 H2O 14.34 0.50 H2S 0.00 CH4 1.00 N2+Ar
48.92 32.44 0.55 16.61 0.49 0.00 1.00
49.58 32.39 0.61 15.96 0.49 0.00 1.00
50.15 32.28 0.72 15.40 0.49 0.01 1.00
Hydrogen flow rate H2 (kg/kgcoal)
0.159
0.161
0.162
0.156
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Table 3 – Power balance of the IGHP plants without the cleanup system IGHP,A SSHIFT/C S/C Gasification pressure Coal input (HHV) WAIR WO2 WN2 WPUMP 103 WSYN W QAV H2 (HHV) ZH2 (HHV)
bar MW MW MW MW MW MW MW MW MW %
0.6 0.7
0.8 0.6
27.13 1.07 – 0.93 0 0 0.14 2.33 20.63 76.0
27.13 1.05 – 0.91 0 0 0.14 2.12 21.72 80.1
IGHP,30
1 0.4
1.2 0.2
0.6 0.7
0.8 0.6
27.13 1.03 – 0.89 0 0 0.14 1.81 22.36 82.4
27.13 1 – 0.87 0 0 0.13 1.39 23.02 84.9
27.13 1.07 0.12 0.92 5.1 0.89 0.62 0.95 22.25 82.0
27.13 1.05 0.11 0.91 5.5 0.90 0.65 0.67 22.57 83.2
1
IGHP,70
1 0.4
1.2 0.2
0.6 0.7
0.8 0.6
27.13 1.02 0.11 0.89 5.5 0.90 0.66 0.29 22.86 84.3
27.13 1 0.11 0.86 5.5 0.91 0.65 0 23.04 84.9
27.13 1.07 0.19 0.93 12.2 1.15 0.81 1.08 22.47 82.8
27.13 1.05 0.18 0.91 13.1 1.17 0.84 0.81 22.65 83.5
30
1 0.4
1.2 0.2
27.13 1.02 0.18 0.89 13.1 1.18 0.85 0.42 22.97 84.7
27.13 0.99 0.17 0.86 13.1 1.18 0.87 0 23.06 85.0
70
PURGE GAS POWER H2 CPG
PSA
GTCC
SAT
CO2 CAPTURE
CO2 emitted
CO2 captured
steam for H2 Sstripping
SYNGAS from WGLT TSYN AGR
H2S
Claus/SCOT PLANT
Sulphur
Fig. 5 – Schematic of the cleanup system and the power island.
pressurized configurations, it is only due to the syngas cooling between the shifters and it grows by increasing the gasification pressure because of the decrease in evaporation latent heat of water sent to the WGHT; moreover, it diminishes with the increasing SSHIFT/C ratio. Unlike the IGHP,A plant, in which electrical power is required, the pressurized configurations produce a net power that increases with the gasification pressure. The IGHP,70 is the better configuration both for the hydrogen production and power consumption since net electrical power is generated.
4. Energy balance of the IGHP reference plant with the syngas cleanup system The energy balance summarized in Table 3 is referred to the IGHP plant without the cleanup system. In this section the
effect of the cleanup system on the IGHP reference plant is evaluated. The physical absorption in dimethyl ether of polyethylene glycol (Selexol) is used to remove H2S and CO2 from the syngas in the IGHP,70 plant which is, as it results from the sensitivity analysis, the IGHP reference plant. In order to remove H2S and CO2 separately from the hydrogen product stream, a double-stage Selexol unit [2,13], operating at 30 bar, was selected. This process removes H2S from the cooled syngas and then removes CO2 from the desulphurized syngas. The heat required for H2S stripping is assumed to be 6 MJ of 6 bar steam per kg of sulphur stripped [13]. The carbon dioxide streams are released at different pressure levels in four flash drums and delivered for final compression to 150 bar. The free CO2 hydrogen stream, coming out from the CO2 capture unit, is sent to the PSA unit in which the hydrogen is separated with an efficiency of 85%. Finally, the purge gas from the PSA unit, which consists mainly of H2 (HHV ¼ 14.1 MJ/kg), after its compression and
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Table 4 – Performance of the IGHP reference plant with the cleanup system and the GTCC IGHP,70 Coal input (HHV)
MW
1810
SELEXOL/CO2 compressor WIGHP,70
MW MW
68.0 7.7
Gas Turbine Combined Cycle (GTCC) Purge gas (HHV) Purge gas compressor (CPG) Gas turbine Steam turbinea
MW MW MW MW
261 12.0 79.8 37.8
IGHP,70-GTCC H2 (HHV) Net power output Cogeneration ratiob ZH2 (HHV) Zel (HHV) CO2 Captured CO2 emissions CO2 emissions
MW MW % %
1303 45.3 3.5 72.0
% % g/kWh kg/GJHHV
2.5 95 61.3 6.9
a The power generated from the steam power section has been calculated taking into account the heat needed for H2S stripping, which is obtained by bleeding steam from steam turbine. b The cogeneration ratio is defined as net power output/H2 HHV power.
saturation in CPG and SAT, is used as fuel in a triple-pressure, reheat combined cycle (GTCC), based on a commercial gas turbine. The system described is depicted in Fig. 5. The energy balance of the whole system is reported in Table 4. The coal feed flow rate has been calculated according to the purge gas flow rate required by the gas turbine (the air flow rate is 188 kg/s and the turbine inlet temperature is 1289 1C) and the specific CO2 emissions (g/kWh) related to the power generation have been estimated assuming that the free CO2 hydrogen stream is directly burned in a gas turbine combined cycle with the same efficiency of that fed by the purge gas.
5.
Conclusions
The aim of this Part A was to define the IGHP reference plant needed to analyse and compare novel systems, the CPH systems, investigated in Part B. The IGHP reference plant has been designed to optimize the hydrogen yield with the minimum power consumption. In order to identify the thermodynamically favourable operating conditions for the maximum hydrogen production, a sensitivity analysis has been carried out, varying the gasification pressure, the S/C ratio, and the SSHIFT/C ratio. The sensitivity analysis points out that the hydrogen production depends on both S/C and SSHIFT/C ratios which have to be chosen together: in order to maximize the hydrogen production, it is necessary to reduce the S/C ratio at the gasifier and to increase the SSHIFT/C ratio to the shifters. In particular, the maximum hydrogen flow rate is achieved for a steam to carbon ratio at the gasifier of 0.2 and a steam to carbon ratio at the shifters of 1.2 (Fig. 4). The power balance of the IGHP plants, reported in Table 3, presents that, in comparison with the IGHP,A, the IGHP,P plants produce the same hydrogen flow rate, but a net electrical power is also generated. This net electrical power
increases with the gasification pressure, so that the IGHP,70 is chosen as the IGHP reference plant. Finally, in order to evaluate the effect of CO2 capture on the plant performance, the IGHP,70 has been analysed, considering the cleanup system (AGR, CO2 capture unit, and PSA) and using the purge gas from the PSA unit as fuel in a conventional gas turbine combined cycle (the GTCC efficiency is 50%). The electrical power supplied by the gas turbine combined cycle covers the power penalty due to the cleanup system. The hydrogen production efficiency, referred to the HHV, is 72%. R E F E R E N C E S
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