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Performance evaluation of chemically recuperated gas turbine (CRGT) power plants fuelled by di-methyl-ether (DME)
Energy 31 (2006) 1446–1458 www.elsevier.com/locate/energy
Performance evaluation of chemically recuperated gas turbine (CRGT) power plants fuelled by di-methyl-ether (DME) Daniele Cocco*, Vittorio Tola, Giorgio Cau Department of Mechanical Engineering, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy
Abstract This paper reports a performance analysis on CRGT power plants fuelled by DME, which is a potentially attractive fuel for gas turbines. The study also includes a performance comparison of simple cycle gas turbines fuelled by natural gas, DME and methanol. The study shows that, owing to the exhaust heat recovery carried out through DME pre-heating and vaporization before combustion, the efficiency of the DME fuelled turbine improves by about 1 point, without any significant change in power output. Thermochemical recuperation in DME fuelled turbines allows to achieve a significant performance improvement. For the highest water/DME molar ratio allowed by the minimum temperature difference here assumed, the power output of the CRGT plant is 44% higher than that of the reference plant, with a corresponding 54% efficiency (versus 41% of the reference unit) and a 8% decrease of the specific CO2 emissions. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction As known, the most promising option for the substitution of conventional fossil fuels appears to be the wide use of hydrogen. Hydrogen is a very attractive energy carrier, as it can be fed to high efficiency energy systems, such as fuel cells and advanced hydrogen combustion turbines [1,2]. However, hydrogen must be produced from water electrolysis through electrical energy generated by renewable sources or nuclear power plants; moreover, some problems are still involved in its long distance transport and long term storage. Owing to these features, many R&D programs therefore focuses on alternative energy carriers, like methanol, dimethylether (DME), ethanol and others. These fuels could compete with hydrogen, especially in the mid term perspective and for distributed power generation, where the hydrogen supply network could prove uneconomical [3]. * Corresponding author. Tel.: C39 070 6755720; fax: C39 070 6755717. E-mail address: [email protected] (D. Cocco). 0360-5442/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.05.015
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Nomenclature n Water/DME molar ratio p Pressure RDME DME conversion RN,DME DMECmethanol conversion T Temperature, turbine DT Temperature difference Acronyms CFC chlorine-fluor-carbons CRGT chemically recuperated gas turbine DME di-methyl-ether LHV lower heating value LNG liquefied natural gas LPG liquefied petroleum gases Moreover, most of these alternative fuels can be produced from a wide range of primary fuels (natural gas, coal, biomass, etc.), they can be easily transported (they are liquid at ambient conditions) and fed to many energy conversion systems with high efficiency and low pollutant emissions (fuel cells, gas turbines, internal combustion engines, etc.). In particular, recent studies show the growing interest of DME as a new clean fuel for diesel engines, gas turbines and household uses [4–11]. DME is very similar to liquefied petroleum gas (LPG) so that it can be handled and transported through the same LPG devices. DME contains no metals, sulphur and aromatics and recent tests on DME fuelled gas turbines show performance and pollutant emissions comparable to natural gas fuelled turbines [4,5]. On the other hand, it is well known that one of the most interesting options to increase the gas turbine performance relies on exhaust heat recovery trough thermochemical recuperation (or chemically recuperated gas turbine, CRGT). Most of the CRGT solutions recently proposed resort to methane steam reforming. However, the use of methane in CRGT power plants is problematic due to its high reforming temperature (about 600–800 8C), higher than the exhaust temperature of commercial gas turbines. On the contrary, alternative fuels like methanol, ethanol and DME, can be more suitable for CRGT power plants owing to their lower reforming temperature (about 250–300 8C for methanol, 300–350 8C for DME and 400–500 8C for ethanol) [12–16]. This paper concerns a performance assessment of DME fuelled gas turbines. In particular, a comparative performance analysis on a simple cycle and a CRGT gas turbine power plant fuelled by natural gas, DME and methanol has been carried out.
2. DME production and use Owing to its chemical formula (CH3OCH3), dimethylether (DME) is the simplest ether compound. It is a non-toxic, non-corrosive, colourless and chemically stable liquid. Today, it is widely used for
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Table 1 Physical and chemical characteristics of DME and other fuels
Chemical formula Molar weight (kg/kmol) Boiling point, 1 atm (8C) Vapor pressure, 20 8C (bar) Liquid density, 20 8C (kg/m3) LHV, (MJ/kg) Ignition temperature, (8C) Cetane number Explosion limits in air, (vol%)
Methane
Propane
Butane
Methanol
DME
Diesel fuel
CH4 16.07 K161.5 243 N.A. 50.2 650 0 5–15
C3H8 44.11 K42.1 8.4 500 46.4 470 5 2.1–9.4
C4H10 58.13 K0.5 3.1 610 45.7 365 20 1.9–8.4
CH3OH 32.05 64.6 0.13 790 19.9 450 5 5.5–36
CH3OCH3 46.07 K25.1 5.1 670 28.4 235 55–60 3.4–17
N.A. N.A. 180–370 N.A. 840 42.2 250 40–55 0.6–6.5
the manufacture of aerosol propellants, agricultural chemicals and cosmetics. In fact, unlike CFC, DME is not harmful for the ozone layer because it decomposes in several hours into the troposphere [6–9]. Owing to its fuel characteristics (it contains no metals, sulphur and aromatics), DME is considered a very interesting, multi-purpose clean energy carrier for the next future. DME could substitute many conventional fuels in diesel engines, power generation plants and household uses. Table 1 shows a comparison of the main properties of DME and other conventional and alternative fuels. Many DME properties are very similar to those of propane and butane and their commercial mixture LPG (liquefied petroleum gases). In particular, the DME boiling point and vapor pressure are very close to those of LPG, so that it can be easily liquefied under pressure as well as stored and handled by means of the LPG devices. The lower heating value (LHV) of DME is lower than those of most commercial fuels (methane, propane, butane and diesel fuel), but it is higher than that of methanol, one of the other alternative energy carriers today under evaluation. DME could substitute diesel fuel in the transportation sector, allowing to reduce oil dependence. Owing to its high cetane number and low boiling point, DME use in diesel engines provide fast fuel/air mixing formation, reduced ignition delay and excellent cold start properties. Moreover, the use of DME only requires minor modification on the fuel injection system. As demonstrated by some tests, DME use in diesel engines allows to reduce SOX, NOX and particulate emissions, as well as engine noise, with equivalent thermal efficiency [6,7]. DME could also be a natural gas substitute for distributed power generation, especially for small scale power plants not reached by the natural gas supply network. Recent tests show that DME use in gas turbines allows pollutant emissions, dynamic pressures and combustor metal temperatures which are comparable to those of natural gas fuelled gas turbines [4,5]. Moreover, steam reforming of DME seems a very interesting hydrogen source to feed fuel cells for stationary and vehicle applications [10,11]. Finally, DME could substitute LPG for domestic uses, like household boilers and cooking stoves. With reference to this application, many combustion tests were carried out in Japan, a large LPG consumer, with good results [6,7]. Today, high-purity DME is produced from methanol, through a dehydration process. However, it can be directly manufactured from synthesis gas (mainly composed by CO and H2), produced by coal gasification or natural gas reforming processes. For power generation use, manufacturing of a fuel grade
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Table 2 Fuel grade DME specifications DME Methanol Water Oxygenates, high ethers, gases Metals, nitrogen and sulphur Lower heating value
88.9G0.9 wt% 7.5G1.0 wt% 3.2G0.3 wt% 0.4G0.1 wt% Nil 26.7G0.5 MJ/kg
DME instead of pure DME is a better option, due to its lower investment and energy consumption for the fuel purification section. Fuel grade DME specifications are reported in Table 2 [9]. In particular, DME appears today a valuable option to improve the economic performance of low-cost natural gas reserves. However, in the near future, DME manufactured from coal synthesis gas could allow to reduce the dependence on oil and natural gas, as well as to utilize the worldwide coal reserves. Furthermore, coproduction of methanol and DME could allow to improve the economic performance of the plant [8,9]. In the last few years many projects on DME production and commercialization have been announced with reference to countries without a natural gas supply network (Japan, Korea, Taiwan, India, etc.), where DME supply appears cheaper than LNG or LPG. Ocean transport of DME can be carried out through conventional LPG tankers. Like LPG, liquid DME can be stored under pressure (5–6 bar) at atmospheric temperature, or as refrigerated liquid (K15 to K25 8C) at atmospheric pressure [4,5,8,9].
3. DME fuelled gas turbines Performance of gas turbine power plants fuelled by DME has been analysed in this paper with reference to a GE LM6000 aeroderivative gas turbine. The LM6000 unit is rated 43.2 MW at 41.5% efficiency, burning natural gas at ISO conditions. Firstly, gas turbine performance has been assessed by simply replacing natural gas with pure or fuel grade DME. For comparison purposes, the performance of the same gas turbine fuelled by methanol has been also reported. The performance analysis has been carried out through the AspenPlusw simulation software, version 11.1. Table 3 reports the main assumptions adopted for studying the different plant configurations. Table 3 Main assumptions adopted in the analysis Air mass flow at the compressor inlet Compression ratio Compressor isentropic efficiency Turbine isentropic efficiency Inlet pressure drop (% of pIN) Combustion pressure drop (% of pIN) Exhaust pressure drop (% of pIN) Combustion efficiency Mech. losses and aux. consumption
127 kg/s 29.6 86% 88% 1% 3% 1% 99.5% 4%
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Table 4 Main performance of the LM6000 gas turbine fuelled by natural gas, DME and methanol
Fuel conditions at battery limits Fuel LHV, (MJ/kg) Fuel injection temperature (8C) Power output (MW) Net efficiency (%) Fuel mass flow (kg/s) Flue gas mass flow (kg/s) Turbine exit temperature (8C) Flue gas temperature (8C) CO2 specific emission (g/kWh)
Natural gas
Pure DME
Fuel grade DME
Methanol
15 8C/1atm 48.0 15 42.0 40.3 2.17 129.2 452.4 452.4 491.3
15 8C/6 bar 28.4 250 15 42.6 42.8 41.2 40.1 3.65 3.76 130.2 130.3 453.7 454.3 433.9 454.3 588.6 604.7
15 8C/6 bar 26.8 250 15 42.9 43.1 41.6 40.2 3.86 4.01 130.4 130.6 454.1 454.8 430.0 454.8 585.3 605.7
15 8C/1atm 19.9 250 15 44.7 45.4 43.9 40.9 5.11 5.56 131.7 132.1 456.0 457.7 404.0 457.7 566.6 608.2
For the analysis, all fuels are assumed available at 15 8C, so that DME and methanol are both liquid. Pure and fuel grade DME (here assumed composed by 89 wt% DME, 8 wt% methanol and 3 wt% water) are assumed available at 6 bar, methanol at atmospheric pressure. A pump increases the pressure up to the value required by the fuel injection system. Moreover, in order to take advantage of the Dry-Low NOX (DLN) combustion systems used by most advanced gas turbines, as well as to increase thermal efficiency, the liquid DME and methanol are vaporized and heated before the combustion chamber through a flue gas waste heat recovery. Natural gas preheating has not been here considered mainly due to its lower mass flow and specific heat. Table 4 summarizes the performance of the LM6000 gas turbine fuelled by natural gas, DME and methanol. For the latter two fuels, the performance has been evaluated with and without considering the fuel preheating and vaporization before combustion. Owing to the computation of both inlet and exhaust pressure drop, as well as the auxiliaries energy consumption, the performance of the natural gas fuelled turbine slightly differs from those referred to ISO conditions. Under the assumptions adopted here, replacing natural gas with DME results in a slight increase of both power output and thermal efficiency. This is mainly due to the exhaust heat recovery carried out by DME pre-heating and vaporization. In particular, the slight increase of the power output is essentially due to the higher flue gas mass flow produced by the lower LHV of DME compared to natural gas. With respect to natural gas, DME use results in a minor change (about 2 8C) of the turbine exit temperature, and the presence of the exhaust heat recovery results in a more significant decrease of the flue gas temperature (20–25 8C). As shown in Table 4, only negligible differences are involved in the use of pure or fuel grade DME. However, it is interesting to observe that, the methanol contained in the fuel grade DME allows to slightly improve the exhaust heat recovery and therefore the plant performance. In fact, methanol vaporization and preheating requires more heat with respect to DME. Moreover, the lower LHV of methanol determines the increase of the flue gas mass flow and to the power output. This is confirmed by the performance of the methanol fuelled gas turbine, which are better than those of both natural gas and DME fuelled turbines. Owing to the higher carbon content, the CO2 specific emissions of methanol and DME fuelled gas turbines are 15–25% higher than that of the reference turbine. Finally, the performance assessment carried out assuming pure and fuel grade DME both available at K25 8C and atmospheric pressure does not show any significant change in gas turbine performance with respect to those reported in Table 4.
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4. DME reforming thermodynamics Performance and CO2 emissions of gas turbines can be improved if the exhaust waste heat is recovered through the endothermic steam reforming process of the primary fuel. Most of the CRGT power plants recently proposed resort to methane (natural gas) steam reforming, owing to its wide availability. However, methane use in CRGT power plants is problematic due to its high reforming temperature (about 600–800 8C), which is higher than the flue gas temperature of current heavy-duty (500–600 8C) and aeroderivative (400–500 8C) turbines. Methanol, DME and ethanol are more suitable fuels for CRGT plants owing to their lower reforming temperature [14–16]. Up to now, the DME reforming process has gained less attention than, for example, methane or methanol steam reforming. Only in the last few years some studies have been devoted to the assessment of the hydrogen production for fuel cells by means of DME steam reforming [10,11]. The overall DME steam reforming process can be described by means of three independent chemical reactions: CH3 OCH3 C H2 O5 2CH3 OH
(1)
CH3 OH5 CO C 2H2
(2)
CO C H2 O5 CO2 C H2
(3)
The first reaction corresponds to the endothermic DME hydration process, which produces methanol, whereas the second and the third reactions give the endothermic methanol steam reforming process. As can be seen from reactions (1)–(3), the complete conversion of DME and methanol requires at least 3 mol of water for each mole of DME. On the overall, as shown by Table 5 the complete steam reforming of DME is more endothermic than methanol steam reforming. In particular, on molar basis, the heat required by the DME reforming is about 2.5 times higher than that required by methanol (about 1.71 times on mass basis). Moreover, the heat of reaction for methanol and DME is about 9.4 and 7.7% of the respective LHV. Obviously, the exhaust heat recovered by the fuel reforming process also depends on the final temperature of the reformed fuel, as well as on its overall mass flow. The overall DME steam reforming process depends on several operating parameters, such as temperature and pressure, water/DME molar ratio n, catalyst activity, as well as the reformer configuration and its geometric arrangement. Experimental studies carried out on DME steam reforming have demonstrated that, using catalysts based on a mixture of HPA/Al2O3 and Cu/SiO2, DME conversion is almost complete at about 290 8C and the reaction products are essentially composed by H2, CO and CO2, Table 5 Energy characteristics of the DME and methanol steam reforming processes
Chemical formula Molar weight (kg/kmol) LHV (MJ/kg) Heat of reaction (MJ/kmol) Heat of reaction (MJ/kg) Heat of reaction/LHV (%)
Methanol
DME
CH3OH 32.05 19.9 49.2 1.54 7.71
CH3OCH3 46.07 28.4 121.4 2.64 9.38
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having DME and methanol concentrations below detection limits. Moreover, the study showed that calculated equilibrium concentrations of both DME and methanol were similar to the experimental values, whereas calculated concentrations of H2 and CO2 were slightly lower, and those of CO and H2O were higher than the respective experimental values. On the overall, the DME steam reforming reactions are practically governed by equilibrium if the temperature is higher than about 300 8C [11]. As in methanol steam reforming, equilibrium in DME steam reforming is favoured by low pressure, high temperature and high water/DME molar ratios. Moreover, water/DME molar ratios higher than about 2.5–3.0 are required in order to avoid carbon (graphite) formation [10]. On the basis of the above considerations, the influence of temperature T, pressure p and water/DME molar ratio n on the reforming process of pure DME has been analysed, under the assumption of thermodynamic equilibrium and with reference to reactions (1)–(3). The temperature T and the molar ratio n have been varied in the range 250–500 8C and 1–8, respectively. Moreover, in order to fulfil the fuel pressure requirements of both heavy-duty and aeroderivative gas turbines, two pressure values, 20 and 40 bar, have been considered. Fig. 1 gives the DME conversion ratio, RDME, versus the reforming temperature T, for different n and p values. DME conversion increases very fast with temperature, being almost complete for a reforming temperature higher than about 450–500 8C, for any value of n and p. For a given temperature, DME conversion is favoured by high values of n and low pressures. For TZ250 8C and pZ20 bar, DME conversion is about 40% for nZ1, 90% for nZ3 and close to 100% for nZ5. For the same temperature, the increase of pressure from 20 to 40 bar results in a lower DME conversion, about 33% for nZ1, 76% for nZ3 and 96% for nZ5. On the overall, for the pressure range here considered, DME conversion is almost complete for reforming temperatures higher than about 350 8C, so long as n is higher than about 3 (such n values are also required in order to avoid carbon formation). 1.0 0.9
DME conversion, RDME
0.8
n=1 n=2 n=3 n=4 n=5 n=8
0.7 0.6 0.5 0.4
p=20 bar p=40 bar
0.3 0.2 250
300
350
400
450
500
Reforming temperature (°C)
Fig. 1. DME conversion versus temperature, pressure and water/DME molar ratio.
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1.0
DME net conversion, RN,DME
0.9 0.8 n=1 n=2 n=3 n=4 n=5 n=8
0.7 0.6 0.5 0.4 p=2 0 bar p=4 0 bar
0.3 0.2 250
300
350
400
450
500
Reforming temperature (°C)
Fig. 2. Net DME conversion versus temperature, pressure and water/DME molar ratio.
Due to reaction (1), some of the converted DME can be found in the reformed gas as methanol, which hides the corresponding heat of reaction. For this reason, Fig. 2 reports the net DME conversion, RN,DME, which takes into account both the DME and the methanol contained in the reformed gas. Comparison of Figs. 1 and 2 shows that RN,DME is lower than RDME especially for high pressures and low water/DME molar ratios. However, as RDME, also RN,DME is near 100% for reforming temperatures higher than about 350 8C, so long as n is higher than about 5. Fig. 3 gives the molar composition of the gaseous mixture produced by the reforming process of 1 kmol of DME at 350 8C, versus the water/DME molar ratio n, for the two pressures here considered. Since RN,DME increases with n, both DME and methanol are practically absent for n ratios higher than about 3. A further increase of n enhances CO conversion through reaction (3), with a consequent increase of both CO2 and H2 content of the reformed gas. For n greater than 4–5, the reformed fuel gas is composed essentially of H2, H2O and CO2. Further increasing of n results in a greater dilution of the reformed gas with steam. As mentioned, Figs. 1–3 refer to the steam reforming process of pure DME. However, the same study carried out with fuel grade DME has shown only slight differences in the reformed fuel gas composition and DME conversion.
5. DME fuelled CRGT power plants Fig. 4 shows the simplified scheme of a CRGT power plant, where inside the reforming section, the flue gas cooling provides the heat required to preheat, evaporate and reform the water-DME mixture. The reforming section is similar to a heat recovery steam generator, where the steam superheater
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D. Cocco et al. / Energy 31 (2006) 1446–1458 6 20 bar Reformed gas composition (kmol/kmolDME)
5
40 bar H2
4
H2O CO 2 CO
3
MeOH DME
2
1
0 1
2
3
4
5
6
7
8
Water/DME molar ratio, n
Fig. 3. Reformed gas composition versus pressure and water/DME molar ratio.
is replaced by the catalytic reforming section of the fuel gas produced by the previous preheating and evaporation sections of the water–DME mixture. Owing to the low density and the low LHV of the reformed fuel, the fuel feeding system requires a proper design, as in IGCC gas turbines, which are fed by a coal synthesis gas. The CRGT power plant here analysed is based on the same LM6000 gas turbine previously considered. The performance assessment has been carried out with reference to a fuel grade DME and to different water/DME molar ratios. A reforming pressure of 40 bar has been here assumed on accounting Water + DME
P
RF
Reformed fuel Air CC
C
T
Fig. 4. Simplified scheme of the CRGT plant.
Flue gases
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1455 56
52 50 Reforming temp. 350 °C 300 ˚C
Power, (MW)
65
48
Efficiency (%)
54
46
60 55 50 45 1
2
3
4
5
6
Water / DME molar ratio, n
Fig. 5. Power and efficiency of the CRGT power plant versus water/methanol molar ratio.
for the compression ratio (close to 30) and the reformer and combustor pressure drops (2 and 3% of the inlet pressure, respectively). Moreover, two reforming temperatures have been considered for the analysis: 300 and 350 8C. Fig. 5 shows the efficiency and the power output of the CRGT plant in function of the water/ DME molar ratio and for the two different reforming temperatures. The exhaust heat recovery carried out through the DME reforming process allows to obtain a significant increase in both efficiency and power output of the CRGT plant compared to the simple cycle one (Table 4). The performance improvement is remarkable especially for high n values, so that for nZ6 the CRGT plant yields a power output of more than 60 MW (44% higher than the simple cycle plant) at about 54% efficiency (32% higher than the reference gas turbine). In fact, the presence of water ensures a more effective waste heat recovery, owing to the higher DME conversion and, as in steam injected gas turbines, the heat required by the water evaporation and superheating processes. For a given n ratio, the increase of the reforming temperature results in a better plant efficiency (1–2 efficiency points), with only a minor influence on the power output. Moreover, the increase of the thermal efficiency determines a corresponding decrease of the CO2 specific emissions of the CRGT plant. For example, for nZ6, the CO2 specific emissions are about 450 g/kWh, that is 8% lower than the reference plant. The maximum water/DME molar ratio here considered was evaluated by specifying the minimum temperature difference between the flue gases and the water–DME mixture inside the reformer. As shown by Fig. 6, the minimum temperature differences are located, respectively, between the preheating and evaporation sections (DT1) and between the superheating and reforming sections (DT2). Fig. 7 shows that both DT1 and DT2 decrease with increasing values of the reforming temperature, due to the higher exhaust heat recovery. With the lowest n values the minimum temperature difference is DT2, whereas DT1 is the minimum one with reference to the highest n values. In particular, DT1 attains its minimum value here assumed (10 8C) for n equal to about 6 (TZ350 8C) and 6.5 (TZ300 8C), which are
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D. Cocco et al. / Energy 31 (2006) 1446–1458 500 DT 2 Temperature (°C)
400 DT1 300 200 Superheating 100 Reforming
Preheating
Vaporization
0 0
10
20
30
40
50
Heat recovered (MW)
Fig. 6. Pinch-point temperature differences inside the waste hear recovery section.
therefore the maximum allowable n values for the CRGT power plant considered here. As can be concluded by Fig. 6, a lower value of DT1 or DT2 can allow to decrease the outlet flue gas temperature and then to improve the waste heat recovery and the plant efficiency. Obviously, the maximum value of n also depends on other boundary conditions, such as the maximum permissible mass flow of the turbine and the maximum power allowed by the main plant components (shaft, gear, electrical generator, etc.). Fig. 8 shows the increase in turbine mass flow of the CRGT plant with respect to the corresponding mass flow of the reference plant, again as a function of n. Fig. 8 shows that for nZ6 the turbine mass flow is 10% higher than that of the reference plant. It can be pointed out 180 DT1
160
Temperature difference, (°C)
DT2 140 300 °C 120
350 °C
100 80 60 40 20 0 1
2
3 4 5 Water / DME molar ratio, n
6
7
Fig. 7. Pinch-point temperature difference as a function of water/DME molar ratio.
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12 300 °C
Turbine flow increase, (%)
10
350 °C
8
6
4
2
0 1
2
3
4
5
6
7
Water / DME molar ratio, n
Fig. 8. Turbine flow increase as a function of the water/DME molar ratio.
that aeroderivative gas turbines allow to operate with 10–12% turbine mass flow increase, as demonstrated by the wide use of steam injected gas turbines in commercial application. Finally, it is noteworthy to observe that the CRGT power plant here analysed allows to achieve a maximum efficiency (54%) about equal to that of a combined cycle power plant fuelled by DME [4]. However, the CRGT plant is a less complex solution, and it can be the better option especially for midsize power generation plants.
6. Conclusions DME is a potentially attractive fuel for gas turbines since, as demonstrated by recent tests, DME fuelled gas turbines show performance and pollutant emissions comparable to, and even better than, natural gas fuelled turbines. The study presented in this paper has shown that, owing to the exhaust heat recovery carried out through DME pre-heating and vaporization, thermal efficiency improves of about 1 point, while the power output is almost unchanged. Owing to the higher carbon content of the DME, with respect to natural gas, only CO2 emissions increase. Performance and CO2 emissions are improved by using the DME in CRGT power plants. The performance improvement is remarkable especially for the highest water/DME molar ratio allowed by the minimum temperature difference into the reformer. In this case, the power output of the CRGT plant is 44% higher than that of the reference plant, with a corresponding 54% thermal efficiency (32% higher than the reference plant).
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Performance evaluation of chemically recuperated gas turbine (CRGT) power plants fuelled by di-methyl-ether (DME) Daniele Cocco*, Vittorio Tola, Giorgio Cau Department of Mechanical Engineering, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy
Abstract This paper reports a performance analysis on CRGT power plants fuelled by DME, which is a potentially attractive fuel for gas turbines. The study also includes a performance comparison of simple cycle gas turbines fuelled by natural gas, DME and methanol. The study shows that, owing to the exhaust heat recovery carried out through DME pre-heating and vaporization before combustion, the efficiency of the DME fuelled turbine improves by about 1 point, without any significant change in power output. Thermochemical recuperation in DME fuelled turbines allows to achieve a significant performance improvement. For the highest water/DME molar ratio allowed by the minimum temperature difference here assumed, the power output of the CRGT plant is 44% higher than that of the reference plant, with a corresponding 54% efficiency (versus 41% of the reference unit) and a 8% decrease of the specific CO2 emissions. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction As known, the most promising option for the substitution of conventional fossil fuels appears to be the wide use of hydrogen. Hydrogen is a very attractive energy carrier, as it can be fed to high efficiency energy systems, such as fuel cells and advanced hydrogen combustion turbines [1,2]. However, hydrogen must be produced from water electrolysis through electrical energy generated by renewable sources or nuclear power plants; moreover, some problems are still involved in its long distance transport and long term storage. Owing to these features, many R&D programs therefore focuses on alternative energy carriers, like methanol, dimethylether (DME), ethanol and others. These fuels could compete with hydrogen, especially in the mid term perspective and for distributed power generation, where the hydrogen supply network could prove uneconomical [3]. * Corresponding author. Tel.: C39 070 6755720; fax: C39 070 6755717. E-mail address: [email protected] (D. Cocco). 0360-5442/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.05.015
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Nomenclature n Water/DME molar ratio p Pressure RDME DME conversion RN,DME DMECmethanol conversion T Temperature, turbine DT Temperature difference Acronyms CFC chlorine-fluor-carbons CRGT chemically recuperated gas turbine DME di-methyl-ether LHV lower heating value LNG liquefied natural gas LPG liquefied petroleum gases Moreover, most of these alternative fuels can be produced from a wide range of primary fuels (natural gas, coal, biomass, etc.), they can be easily transported (they are liquid at ambient conditions) and fed to many energy conversion systems with high efficiency and low pollutant emissions (fuel cells, gas turbines, internal combustion engines, etc.). In particular, recent studies show the growing interest of DME as a new clean fuel for diesel engines, gas turbines and household uses [4–11]. DME is very similar to liquefied petroleum gas (LPG) so that it can be handled and transported through the same LPG devices. DME contains no metals, sulphur and aromatics and recent tests on DME fuelled gas turbines show performance and pollutant emissions comparable to natural gas fuelled turbines [4,5]. On the other hand, it is well known that one of the most interesting options to increase the gas turbine performance relies on exhaust heat recovery trough thermochemical recuperation (or chemically recuperated gas turbine, CRGT). Most of the CRGT solutions recently proposed resort to methane steam reforming. However, the use of methane in CRGT power plants is problematic due to its high reforming temperature (about 600–800 8C), higher than the exhaust temperature of commercial gas turbines. On the contrary, alternative fuels like methanol, ethanol and DME, can be more suitable for CRGT power plants owing to their lower reforming temperature (about 250–300 8C for methanol, 300–350 8C for DME and 400–500 8C for ethanol) [12–16]. This paper concerns a performance assessment of DME fuelled gas turbines. In particular, a comparative performance analysis on a simple cycle and a CRGT gas turbine power plant fuelled by natural gas, DME and methanol has been carried out.
2. DME production and use Owing to its chemical formula (CH3OCH3), dimethylether (DME) is the simplest ether compound. It is a non-toxic, non-corrosive, colourless and chemically stable liquid. Today, it is widely used for
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Table 1 Physical and chemical characteristics of DME and other fuels
Chemical formula Molar weight (kg/kmol) Boiling point, 1 atm (8C) Vapor pressure, 20 8C (bar) Liquid density, 20 8C (kg/m3) LHV, (MJ/kg) Ignition temperature, (8C) Cetane number Explosion limits in air, (vol%)
Methane
Propane
Butane
Methanol
DME
Diesel fuel
CH4 16.07 K161.5 243 N.A. 50.2 650 0 5–15
C3H8 44.11 K42.1 8.4 500 46.4 470 5 2.1–9.4
C4H10 58.13 K0.5 3.1 610 45.7 365 20 1.9–8.4
CH3OH 32.05 64.6 0.13 790 19.9 450 5 5.5–36
CH3OCH3 46.07 K25.1 5.1 670 28.4 235 55–60 3.4–17
N.A. N.A. 180–370 N.A. 840 42.2 250 40–55 0.6–6.5
the manufacture of aerosol propellants, agricultural chemicals and cosmetics. In fact, unlike CFC, DME is not harmful for the ozone layer because it decomposes in several hours into the troposphere [6–9]. Owing to its fuel characteristics (it contains no metals, sulphur and aromatics), DME is considered a very interesting, multi-purpose clean energy carrier for the next future. DME could substitute many conventional fuels in diesel engines, power generation plants and household uses. Table 1 shows a comparison of the main properties of DME and other conventional and alternative fuels. Many DME properties are very similar to those of propane and butane and their commercial mixture LPG (liquefied petroleum gases). In particular, the DME boiling point and vapor pressure are very close to those of LPG, so that it can be easily liquefied under pressure as well as stored and handled by means of the LPG devices. The lower heating value (LHV) of DME is lower than those of most commercial fuels (methane, propane, butane and diesel fuel), but it is higher than that of methanol, one of the other alternative energy carriers today under evaluation. DME could substitute diesel fuel in the transportation sector, allowing to reduce oil dependence. Owing to its high cetane number and low boiling point, DME use in diesel engines provide fast fuel/air mixing formation, reduced ignition delay and excellent cold start properties. Moreover, the use of DME only requires minor modification on the fuel injection system. As demonstrated by some tests, DME use in diesel engines allows to reduce SOX, NOX and particulate emissions, as well as engine noise, with equivalent thermal efficiency [6,7]. DME could also be a natural gas substitute for distributed power generation, especially for small scale power plants not reached by the natural gas supply network. Recent tests show that DME use in gas turbines allows pollutant emissions, dynamic pressures and combustor metal temperatures which are comparable to those of natural gas fuelled gas turbines [4,5]. Moreover, steam reforming of DME seems a very interesting hydrogen source to feed fuel cells for stationary and vehicle applications [10,11]. Finally, DME could substitute LPG for domestic uses, like household boilers and cooking stoves. With reference to this application, many combustion tests were carried out in Japan, a large LPG consumer, with good results [6,7]. Today, high-purity DME is produced from methanol, through a dehydration process. However, it can be directly manufactured from synthesis gas (mainly composed by CO and H2), produced by coal gasification or natural gas reforming processes. For power generation use, manufacturing of a fuel grade
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Table 2 Fuel grade DME specifications DME Methanol Water Oxygenates, high ethers, gases Metals, nitrogen and sulphur Lower heating value
88.9G0.9 wt% 7.5G1.0 wt% 3.2G0.3 wt% 0.4G0.1 wt% Nil 26.7G0.5 MJ/kg
DME instead of pure DME is a better option, due to its lower investment and energy consumption for the fuel purification section. Fuel grade DME specifications are reported in Table 2 [9]. In particular, DME appears today a valuable option to improve the economic performance of low-cost natural gas reserves. However, in the near future, DME manufactured from coal synthesis gas could allow to reduce the dependence on oil and natural gas, as well as to utilize the worldwide coal reserves. Furthermore, coproduction of methanol and DME could allow to improve the economic performance of the plant [8,9]. In the last few years many projects on DME production and commercialization have been announced with reference to countries without a natural gas supply network (Japan, Korea, Taiwan, India, etc.), where DME supply appears cheaper than LNG or LPG. Ocean transport of DME can be carried out through conventional LPG tankers. Like LPG, liquid DME can be stored under pressure (5–6 bar) at atmospheric temperature, or as refrigerated liquid (K15 to K25 8C) at atmospheric pressure [4,5,8,9].
3. DME fuelled gas turbines Performance of gas turbine power plants fuelled by DME has been analysed in this paper with reference to a GE LM6000 aeroderivative gas turbine. The LM6000 unit is rated 43.2 MW at 41.5% efficiency, burning natural gas at ISO conditions. Firstly, gas turbine performance has been assessed by simply replacing natural gas with pure or fuel grade DME. For comparison purposes, the performance of the same gas turbine fuelled by methanol has been also reported. The performance analysis has been carried out through the AspenPlusw simulation software, version 11.1. Table 3 reports the main assumptions adopted for studying the different plant configurations. Table 3 Main assumptions adopted in the analysis Air mass flow at the compressor inlet Compression ratio Compressor isentropic efficiency Turbine isentropic efficiency Inlet pressure drop (% of pIN) Combustion pressure drop (% of pIN) Exhaust pressure drop (% of pIN) Combustion efficiency Mech. losses and aux. consumption
127 kg/s 29.6 86% 88% 1% 3% 1% 99.5% 4%
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Table 4 Main performance of the LM6000 gas turbine fuelled by natural gas, DME and methanol
Fuel conditions at battery limits Fuel LHV, (MJ/kg) Fuel injection temperature (8C) Power output (MW) Net efficiency (%) Fuel mass flow (kg/s) Flue gas mass flow (kg/s) Turbine exit temperature (8C) Flue gas temperature (8C) CO2 specific emission (g/kWh)
Natural gas
Pure DME
Fuel grade DME
Methanol
15 8C/1atm 48.0 15 42.0 40.3 2.17 129.2 452.4 452.4 491.3
15 8C/6 bar 28.4 250 15 42.6 42.8 41.2 40.1 3.65 3.76 130.2 130.3 453.7 454.3 433.9 454.3 588.6 604.7
15 8C/6 bar 26.8 250 15 42.9 43.1 41.6 40.2 3.86 4.01 130.4 130.6 454.1 454.8 430.0 454.8 585.3 605.7
15 8C/1atm 19.9 250 15 44.7 45.4 43.9 40.9 5.11 5.56 131.7 132.1 456.0 457.7 404.0 457.7 566.6 608.2
For the analysis, all fuels are assumed available at 15 8C, so that DME and methanol are both liquid. Pure and fuel grade DME (here assumed composed by 89 wt% DME, 8 wt% methanol and 3 wt% water) are assumed available at 6 bar, methanol at atmospheric pressure. A pump increases the pressure up to the value required by the fuel injection system. Moreover, in order to take advantage of the Dry-Low NOX (DLN) combustion systems used by most advanced gas turbines, as well as to increase thermal efficiency, the liquid DME and methanol are vaporized and heated before the combustion chamber through a flue gas waste heat recovery. Natural gas preheating has not been here considered mainly due to its lower mass flow and specific heat. Table 4 summarizes the performance of the LM6000 gas turbine fuelled by natural gas, DME and methanol. For the latter two fuels, the performance has been evaluated with and without considering the fuel preheating and vaporization before combustion. Owing to the computation of both inlet and exhaust pressure drop, as well as the auxiliaries energy consumption, the performance of the natural gas fuelled turbine slightly differs from those referred to ISO conditions. Under the assumptions adopted here, replacing natural gas with DME results in a slight increase of both power output and thermal efficiency. This is mainly due to the exhaust heat recovery carried out by DME pre-heating and vaporization. In particular, the slight increase of the power output is essentially due to the higher flue gas mass flow produced by the lower LHV of DME compared to natural gas. With respect to natural gas, DME use results in a minor change (about 2 8C) of the turbine exit temperature, and the presence of the exhaust heat recovery results in a more significant decrease of the flue gas temperature (20–25 8C). As shown in Table 4, only negligible differences are involved in the use of pure or fuel grade DME. However, it is interesting to observe that, the methanol contained in the fuel grade DME allows to slightly improve the exhaust heat recovery and therefore the plant performance. In fact, methanol vaporization and preheating requires more heat with respect to DME. Moreover, the lower LHV of methanol determines the increase of the flue gas mass flow and to the power output. This is confirmed by the performance of the methanol fuelled gas turbine, which are better than those of both natural gas and DME fuelled turbines. Owing to the higher carbon content, the CO2 specific emissions of methanol and DME fuelled gas turbines are 15–25% higher than that of the reference turbine. Finally, the performance assessment carried out assuming pure and fuel grade DME both available at K25 8C and atmospheric pressure does not show any significant change in gas turbine performance with respect to those reported in Table 4.
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4. DME reforming thermodynamics Performance and CO2 emissions of gas turbines can be improved if the exhaust waste heat is recovered through the endothermic steam reforming process of the primary fuel. Most of the CRGT power plants recently proposed resort to methane (natural gas) steam reforming, owing to its wide availability. However, methane use in CRGT power plants is problematic due to its high reforming temperature (about 600–800 8C), which is higher than the flue gas temperature of current heavy-duty (500–600 8C) and aeroderivative (400–500 8C) turbines. Methanol, DME and ethanol are more suitable fuels for CRGT plants owing to their lower reforming temperature [14–16]. Up to now, the DME reforming process has gained less attention than, for example, methane or methanol steam reforming. Only in the last few years some studies have been devoted to the assessment of the hydrogen production for fuel cells by means of DME steam reforming [10,11]. The overall DME steam reforming process can be described by means of three independent chemical reactions: CH3 OCH3 C H2 O5 2CH3 OH
(1)
CH3 OH5 CO C 2H2
(2)
CO C H2 O5 CO2 C H2
(3)
The first reaction corresponds to the endothermic DME hydration process, which produces methanol, whereas the second and the third reactions give the endothermic methanol steam reforming process. As can be seen from reactions (1)–(3), the complete conversion of DME and methanol requires at least 3 mol of water for each mole of DME. On the overall, as shown by Table 5 the complete steam reforming of DME is more endothermic than methanol steam reforming. In particular, on molar basis, the heat required by the DME reforming is about 2.5 times higher than that required by methanol (about 1.71 times on mass basis). Moreover, the heat of reaction for methanol and DME is about 9.4 and 7.7% of the respective LHV. Obviously, the exhaust heat recovered by the fuel reforming process also depends on the final temperature of the reformed fuel, as well as on its overall mass flow. The overall DME steam reforming process depends on several operating parameters, such as temperature and pressure, water/DME molar ratio n, catalyst activity, as well as the reformer configuration and its geometric arrangement. Experimental studies carried out on DME steam reforming have demonstrated that, using catalysts based on a mixture of HPA/Al2O3 and Cu/SiO2, DME conversion is almost complete at about 290 8C and the reaction products are essentially composed by H2, CO and CO2, Table 5 Energy characteristics of the DME and methanol steam reforming processes
Chemical formula Molar weight (kg/kmol) LHV (MJ/kg) Heat of reaction (MJ/kmol) Heat of reaction (MJ/kg) Heat of reaction/LHV (%)
Methanol
DME
CH3OH 32.05 19.9 49.2 1.54 7.71
CH3OCH3 46.07 28.4 121.4 2.64 9.38
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having DME and methanol concentrations below detection limits. Moreover, the study showed that calculated equilibrium concentrations of both DME and methanol were similar to the experimental values, whereas calculated concentrations of H2 and CO2 were slightly lower, and those of CO and H2O were higher than the respective experimental values. On the overall, the DME steam reforming reactions are practically governed by equilibrium if the temperature is higher than about 300 8C [11]. As in methanol steam reforming, equilibrium in DME steam reforming is favoured by low pressure, high temperature and high water/DME molar ratios. Moreover, water/DME molar ratios higher than about 2.5–3.0 are required in order to avoid carbon (graphite) formation [10]. On the basis of the above considerations, the influence of temperature T, pressure p and water/DME molar ratio n on the reforming process of pure DME has been analysed, under the assumption of thermodynamic equilibrium and with reference to reactions (1)–(3). The temperature T and the molar ratio n have been varied in the range 250–500 8C and 1–8, respectively. Moreover, in order to fulfil the fuel pressure requirements of both heavy-duty and aeroderivative gas turbines, two pressure values, 20 and 40 bar, have been considered. Fig. 1 gives the DME conversion ratio, RDME, versus the reforming temperature T, for different n and p values. DME conversion increases very fast with temperature, being almost complete for a reforming temperature higher than about 450–500 8C, for any value of n and p. For a given temperature, DME conversion is favoured by high values of n and low pressures. For TZ250 8C and pZ20 bar, DME conversion is about 40% for nZ1, 90% for nZ3 and close to 100% for nZ5. For the same temperature, the increase of pressure from 20 to 40 bar results in a lower DME conversion, about 33% for nZ1, 76% for nZ3 and 96% for nZ5. On the overall, for the pressure range here considered, DME conversion is almost complete for reforming temperatures higher than about 350 8C, so long as n is higher than about 3 (such n values are also required in order to avoid carbon formation). 1.0 0.9
DME conversion, RDME
0.8
n=1 n=2 n=3 n=4 n=5 n=8
0.7 0.6 0.5 0.4
p=20 bar p=40 bar
0.3 0.2 250
300
350
400
450
500
Reforming temperature (°C)
Fig. 1. DME conversion versus temperature, pressure and water/DME molar ratio.
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1.0
DME net conversion, RN,DME
0.9 0.8 n=1 n=2 n=3 n=4 n=5 n=8
0.7 0.6 0.5 0.4 p=2 0 bar p=4 0 bar
0.3 0.2 250
300
350
400
450
500
Reforming temperature (°C)
Fig. 2. Net DME conversion versus temperature, pressure and water/DME molar ratio.
Due to reaction (1), some of the converted DME can be found in the reformed gas as methanol, which hides the corresponding heat of reaction. For this reason, Fig. 2 reports the net DME conversion, RN,DME, which takes into account both the DME and the methanol contained in the reformed gas. Comparison of Figs. 1 and 2 shows that RN,DME is lower than RDME especially for high pressures and low water/DME molar ratios. However, as RDME, also RN,DME is near 100% for reforming temperatures higher than about 350 8C, so long as n is higher than about 5. Fig. 3 gives the molar composition of the gaseous mixture produced by the reforming process of 1 kmol of DME at 350 8C, versus the water/DME molar ratio n, for the two pressures here considered. Since RN,DME increases with n, both DME and methanol are practically absent for n ratios higher than about 3. A further increase of n enhances CO conversion through reaction (3), with a consequent increase of both CO2 and H2 content of the reformed gas. For n greater than 4–5, the reformed fuel gas is composed essentially of H2, H2O and CO2. Further increasing of n results in a greater dilution of the reformed gas with steam. As mentioned, Figs. 1–3 refer to the steam reforming process of pure DME. However, the same study carried out with fuel grade DME has shown only slight differences in the reformed fuel gas composition and DME conversion.
5. DME fuelled CRGT power plants Fig. 4 shows the simplified scheme of a CRGT power plant, where inside the reforming section, the flue gas cooling provides the heat required to preheat, evaporate and reform the water-DME mixture. The reforming section is similar to a heat recovery steam generator, where the steam superheater
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D. Cocco et al. / Energy 31 (2006) 1446–1458 6 20 bar Reformed gas composition (kmol/kmolDME)
5
40 bar H2
4
H2O CO 2 CO
3
MeOH DME
2
1
0 1
2
3
4
5
6
7
8
Water/DME molar ratio, n
Fig. 3. Reformed gas composition versus pressure and water/DME molar ratio.
is replaced by the catalytic reforming section of the fuel gas produced by the previous preheating and evaporation sections of the water–DME mixture. Owing to the low density and the low LHV of the reformed fuel, the fuel feeding system requires a proper design, as in IGCC gas turbines, which are fed by a coal synthesis gas. The CRGT power plant here analysed is based on the same LM6000 gas turbine previously considered. The performance assessment has been carried out with reference to a fuel grade DME and to different water/DME molar ratios. A reforming pressure of 40 bar has been here assumed on accounting Water + DME
P
RF
Reformed fuel Air CC
C
T
Fig. 4. Simplified scheme of the CRGT plant.
Flue gases
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1455 56
52 50 Reforming temp. 350 °C 300 ˚C
Power, (MW)
65
48
Efficiency (%)
54
46
60 55 50 45 1
2
3
4
5
6
Water / DME molar ratio, n
Fig. 5. Power and efficiency of the CRGT power plant versus water/methanol molar ratio.
for the compression ratio (close to 30) and the reformer and combustor pressure drops (2 and 3% of the inlet pressure, respectively). Moreover, two reforming temperatures have been considered for the analysis: 300 and 350 8C. Fig. 5 shows the efficiency and the power output of the CRGT plant in function of the water/ DME molar ratio and for the two different reforming temperatures. The exhaust heat recovery carried out through the DME reforming process allows to obtain a significant increase in both efficiency and power output of the CRGT plant compared to the simple cycle one (Table 4). The performance improvement is remarkable especially for high n values, so that for nZ6 the CRGT plant yields a power output of more than 60 MW (44% higher than the simple cycle plant) at about 54% efficiency (32% higher than the reference gas turbine). In fact, the presence of water ensures a more effective waste heat recovery, owing to the higher DME conversion and, as in steam injected gas turbines, the heat required by the water evaporation and superheating processes. For a given n ratio, the increase of the reforming temperature results in a better plant efficiency (1–2 efficiency points), with only a minor influence on the power output. Moreover, the increase of the thermal efficiency determines a corresponding decrease of the CO2 specific emissions of the CRGT plant. For example, for nZ6, the CO2 specific emissions are about 450 g/kWh, that is 8% lower than the reference plant. The maximum water/DME molar ratio here considered was evaluated by specifying the minimum temperature difference between the flue gases and the water–DME mixture inside the reformer. As shown by Fig. 6, the minimum temperature differences are located, respectively, between the preheating and evaporation sections (DT1) and between the superheating and reforming sections (DT2). Fig. 7 shows that both DT1 and DT2 decrease with increasing values of the reforming temperature, due to the higher exhaust heat recovery. With the lowest n values the minimum temperature difference is DT2, whereas DT1 is the minimum one with reference to the highest n values. In particular, DT1 attains its minimum value here assumed (10 8C) for n equal to about 6 (TZ350 8C) and 6.5 (TZ300 8C), which are
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D. Cocco et al. / Energy 31 (2006) 1446–1458 500 DT 2 Temperature (°C)
400 DT1 300 200 Superheating 100 Reforming
Preheating
Vaporization
0 0
10
20
30
40
50
Heat recovered (MW)
Fig. 6. Pinch-point temperature differences inside the waste hear recovery section.
therefore the maximum allowable n values for the CRGT power plant considered here. As can be concluded by Fig. 6, a lower value of DT1 or DT2 can allow to decrease the outlet flue gas temperature and then to improve the waste heat recovery and the plant efficiency. Obviously, the maximum value of n also depends on other boundary conditions, such as the maximum permissible mass flow of the turbine and the maximum power allowed by the main plant components (shaft, gear, electrical generator, etc.). Fig. 8 shows the increase in turbine mass flow of the CRGT plant with respect to the corresponding mass flow of the reference plant, again as a function of n. Fig. 8 shows that for nZ6 the turbine mass flow is 10% higher than that of the reference plant. It can be pointed out 180 DT1
160
Temperature difference, (°C)
DT2 140 300 °C 120
350 °C
100 80 60 40 20 0 1
2
3 4 5 Water / DME molar ratio, n
6
7
Fig. 7. Pinch-point temperature difference as a function of water/DME molar ratio.
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12 300 °C
Turbine flow increase, (%)
10
350 °C
8
6
4
2
0 1
2
3
4
5
6
7
Water / DME molar ratio, n
Fig. 8. Turbine flow increase as a function of the water/DME molar ratio.
that aeroderivative gas turbines allow to operate with 10–12% turbine mass flow increase, as demonstrated by the wide use of steam injected gas turbines in commercial application. Finally, it is noteworthy to observe that the CRGT power plant here analysed allows to achieve a maximum efficiency (54%) about equal to that of a combined cycle power plant fuelled by DME [4]. However, the CRGT plant is a less complex solution, and it can be the better option especially for midsize power generation plants.
6. Conclusions DME is a potentially attractive fuel for gas turbines since, as demonstrated by recent tests, DME fuelled gas turbines show performance and pollutant emissions comparable to, and even better than, natural gas fuelled turbines. The study presented in this paper has shown that, owing to the exhaust heat recovery carried out through DME pre-heating and vaporization, thermal efficiency improves of about 1 point, while the power output is almost unchanged. Owing to the higher carbon content of the DME, with respect to natural gas, only CO2 emissions increase. Performance and CO2 emissions are improved by using the DME in CRGT power plants. The performance improvement is remarkable especially for the highest water/DME molar ratio allowed by the minimum temperature difference into the reformer. In this case, the power output of the CRGT plant is 44% higher than that of the reference plant, with a corresponding 54% thermal efficiency (32% higher than the reference plant).
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