Energy Fuels 2010, 24, 1529–1537 Published on Web 02/05/2010
: DOI:10.1021/ef901113b
Study of the Theoretical Potential of Thermochemical Exhaust Heat Recuperation for Internal Combustion Engines V. Kalyana Chakravarthy, C. Stuart Daw, Josh A. Pihl,* and James C. Conklin Fuels, Engines, and Emissions Research Center, Oak Ridge National Laboratory, 2360 Cherahala Boulevard, Knoxville, Tennessee 37932 Received September 30, 2009. Revised Manuscript Received December 21, 2009
A detailed thermodynamic analysis of thermochemical recuperation (TCR) applied to an idealized internal combustion engine with single-stage work extraction is presented. Results for several different fuels are included. For a stoichiometric mixture of methanol and air, TCR can increase the estimated ideal engine second law efficiency by about 3% for constant pressure reforming and over 5% for constant volume reforming. For ethanol and isooctane, the estimated second law efficiency increases for constant volume reforming are 9 and 11%, respectively. The second law efficiency improvements from TCR result primarily from the higher intrinsic exergy of the reformed fuel and pressure boost associated with the gas mole increase. Reduced combustion irreversibility may also yield benefits for future implementations of combined cycle work extraction.
because of the partial oxidation of fuel.1,4 One study reports a significant gain in first law efficiency of 13% with TCR using dimethyl ether (DME).5 This increase, however, was achieved using a very high water/DME ratio; therfore, some of the increased efficiency is likely due to thermal recuperation rather than reforming. Two other studies applied exergy analysis to demonstrate a significant benefit from TCR, but they considered complicated combined cycles that make it difficult to isolate the impact of TCR alone.7,8 The general idea of applying TCR to IC engines has also been discussed before but without second law considerations.9-12 The experimental study by Ivanic and co-workers reports a slight increase in work output with only 30% fuel reformation.9 Pratapas and co-workers present a simple engine-TCR system that, when theoretically optimized, would increase efficiency by 10% over a case with no reformation.10 This increase is surprising because it seems to result from an increase in the peak temperature of only 50 K and a 30% decrease in the peak pressure. It is also not clear what parameters were varied in the optimization process. Kweon and co-workers show a 13% increase in work output when about 77% of the natural gas fuel is reformed and used in an adiabatic engine based on a Miller cycle.11 They imply that the efficiency increase results primarily from a change in the lower heating value of the fuel mixture because of reformation. The associated analysis seems to be based on the first law of thermodynamics (losses and work add up to the lower heating value), although they include losses associated with combustion that should only be present in the context of a second law
Introduction Thermochemical recuperation (TCR) is currently receiving renewed interest as a possible means for increasing the efficiency of internal combustion (IC) engines. The basic concept involves using exhaust heat to promote on-board reforming of hydrocarbon fuels into syngas (a mixture of carbon monoxide and hydrogen). The syngas is then burned in the engine in place of some or all of the original fuel. Because the reforming reactions are endothermic, they provide a means for recycling exhaust energy in a chemical form. For one specific fuel (methanol), the original fuel can be converted directly to syngas by adding heat alone in the presence of a catalyst. For other fuels (for example, ethanol and isooctane), additional oxygen (or water) is needed for complete conversion of the fuel to syngas. A number of TCR studies have been published, with most of them focused on gas turbine applications.1-8 All of these studies highlight the increase in the lower heating value of fuel reformate compared to the original fuel. Harvey and coworkers also mention that the combustion irreversibility of reformed fuel is lower than that of the original fuel,1 but most other studies do not analyze the exergy losses in the system. Conclusions about the benefits of TCR vary widely because many different system configurations have been considered, including those with compound cycles. Quantitative analysis is missing in some,2,3 while others report only modest gains *To whom correspondence should be addressed. E-mail: pihlja@ ornl.gov. (1) Harvey, S. P.; Knoche, K. F.; Richter, H. J. J. Eng. Gas Turbines Power 1995, 117, 24–30. (2) Huber, D. J.; Banister, R. L.; Khinkis, M. J.; Rabovitser, K. U.S. Patent 5,595,059, 1997. (3) Basu, A.; Rajagopal, S. U.S. Patent 6,223,519 B1, 2001. (4) Carapellucci, R.; Milazzo, A. Energy Convers. Manage. 2005, 46, 2936–2953. (5) Cocco, D.; Tola, V.; Cau, G. Energy 2006, 31, 1446–1458. (6) Carapellucci, R. Energy Convers. Manage. 2009, 50, 1218–1226. (7) Verkhivker, G.; Kravchenko, V. Energy 2004, 29, 379–388. (8) Mohamed, H. A. Energy Fuels 2003, 17, 1492–1500. r 2010 American Chemical Society
(9) Ivanic, Z.; Ayala, F.; Goldwitz J. A.; Heywood J. B. SAE Tech. Pap. 2005-01-0253, 2005. (10) Pratapas, J. M.; Khinkis, M. J.; Kweon, C.-B.; Zabransky, R.; Kozlov, A.; Aronchik, G. Thermochemical fuel reformer development project. Electric Power Research Institute (EPRI), Palo Alto, CA, 2006; EPRI Report 1012774. (11) Kweon, C.-B.; Khinkis, M. J.; Nosach, V. G.; Zabransky, R. F. U.S. Patent 0279333 A1, 2005. (12) Posada, F.; Bedick, C.; Clark, N. N.; Kozlov, A.; Linck, M.; Boulanov, D.; Pratapas, J. SAE Tech. Pap. 2007-01-4074, 2007.
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analysis. Posada and co-workers compare lower temperature combustion enabled by TCR with regular compression ignition direct-injection (CIDI) engine operation and cite a mechanical efficiency advantage of 4.4% for the former at high loads.12 Although the basic idea behind TCR is straightforward, the actual process involves multiple competing effects and important limitations that need to be carefully considered. Our objective here is to help clarify the thermodynamic potential of TCR by conducting a methodical, straightforward accounting of key parameters for the simplest possible case involving an IC engine. As noted above there, have been many studies of TCR in complex systems, but we are not aware of any systematic studies that eliminate confounding factors, such as cycle compounding, mechanical friction, and air preheating, to focus exclusively on the efficiency benefits of TCR as it would apply to single-stage piston engines. One important factor that we include in this study is combustion irreversibility. For most fuels, the entropy generated by unconstrained combustion destroys up to a third of the original fuel exergy, making that portion of the fuel energy unavailable for generating work.9,11,13 As we explain below, carbon monoxide (CO) and hydrogen (H2) fuels have the potential to significantly reduce this exergy loss because of their unique thermodynamic properties. The extremely low flammability limit of hydrogen can also be used to extend the lean combustion limit, which can increase expansion work and reduce emissions.9,10,14 We therefore considered it important to include the expected changes in combustion exergy losses as part of our analysis. TCR is particularly relevant to alcohol-based engine fuels, because combustion exergy losses for direct combustion of alcohols are typically higher than for alkanes and alkenes.13 In addition, ethanol produced by fermentation typically contains considerable water. Much of the cost associated with removing the water during ethanol production might be avoided if hydrous ethanol could be directly used by engines.15 Onboard, precombustion reforming could potentially help make use of hydrous ethanol more practical. In the following section, we describe our approach for analyzing the fundamental thermodynamics of TCR using a conceptual model of an ideal, single-stage IC engine. Following that, we present example results for a range of fuels burned in the conceptual engine with and without TCR. We conclude with some general observations about the potential benefits and limits of TCR as an approach to achieving higher efficiency engines.
Figure 1. Schematic of the conceptual ideal piston engine used for the TCR study. The outer control volume (CV0) encloses the entire system. The upper inner control volume (CV1) contains the combustion chamber and piston. The lower inner control volume (CV2) contains the reformer and intercooler. When reforming is not used, the fuel and exhaust pass through CV2 unchanged.
state, so that the engine state repeats precisely at each point in the cycle. We also limit ourselves to work generation by a single-stage expansion of the combustion gases. We do not consider other ways to extract work from the exhaust gases, such as Rankine bottoming cycles.1-3 As noted above, additional oxygen is required to reform hydrocarbon molecules other than methanol (e.g., ethanol and isooctane). We assume in this study that water is injected with the fuel to supply the additional oxygen. In each case, the amount of water added is equal to the amount required to completely steam reform the fuel to CO and H2. As an example, eq 1 shows the steam-reforming reaction for isooctane, for which eight water molecules are required to completely reform each molecule of isooctane. C8 H18 þ 8H2 O f 8CO þ 8H2 ð1Þ Both the water and any liquid fuel are vaporized at atmospheric pressure by indirect heating with exhaust gas prior to entering the reforming step. While we use this approach to simplify our analyses, other studies have pointed out that the additional oxygen can also be supplied by mixing the fuel directly with hot exhaust.1-3 As we discuss below, the heat required for water and fuel vaporization is nearly constant for all fuels studied and amounts to less than 15% of the exhaust enthalpy. Fully understanding IC engine efficiency requires application of both the first and second laws of thermodynamics. By the first law, the amount of energy entering any control volume at steady state must equal the energy leaving. By the second law, the entropy change between the exiting and entering streams plus the entropy change of the surroundings must be zero or positive. We apply these laws to our conceptual engine for the control volumes illustrated schematically in Figure 1. The outermost control volume (CV0) forms the outer boundary of the engine system. Control volume one (CV1) contains the combustion chamber, and control volume two (CV2) contains the reformer. The fuel-water prevaporizer is located inside CV0 but outside CV1 and CV2. The only streams entering the combustion chamber are the inlet air and the reformer gas, which contains varying mixtures of the raw fuel and syngas. In the reformer, heat is indirectly extracted from the hot combustion exhaust to reform some or all of the hydrocarbons to syngas, which is then sent to the intercooler. In the intercooler, the fuel/reformate is cooled to the boiling point of the initial fuel/water mixture before it enters the combustion chamber to minimize preheating. For the case of constant pressure reformation, the cooling is performed at constant pressure and the cooled reformate and air streams are mixed at constant (ambient) pressure prior to entering the combustion chamber. For constant volume reforming, the intercooler is also
Analytical Approach To address the fundamental thermodynamics with minimal mechanical complexity, we confined our analysis to a frictionless, single-stage IC engine operating over an ideal cycle with the following features: (i) constant pressure or constant volume mixing of gaseous fuel and air in the combustion chamber, (ii) isentropic compression of the fuel and air mixture, (iii) adiabatic constant volume combustion of the mixture at the point of maximum compression, (iv) isentropic expansion of the combustion gases to atmospheric pressure, and (v) operation at steady (13) Sivadas, H. S.; Caton, J. A. Int. J. Energy Res. 2008, 32, 896–910. (14) Korobitsyn, M. A. New and advanced energy conversion technologies: Analysis of cogeneration, combined and integrated cycles. Ph. D. Thesis, University of Twente, Enschede, The Netherlands, 1998. (15) Mack, J. H.; Aceves, S. M.; Dibble, R. W. Energy 2009, 34, 782– 787.
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above is appropriate. During the times when the combustion chamber is open (when reactants are entering or when products are leaving), the pressure is Po and the two definitions of exergy are equivalent. In cases with constant volume reformation, a closed constant volume fuel packet is tracked through the vaporizer, the reformer, and the combustion chamber. Constant volume reformation would not be possible with continuous flow of fuel through the system. Given that a closed volume of fuel/ reformate is tracked throughout until it enters the combustion chamber, use of non-flow exergy is deemed appropriate in this analysis. Once the fuel packet enters the combustion chamber and mixes with the air, the fuel-air mixture is a closed system, which, as mentioned previously, is best described using non-flow exergy. When eq 2 is combined with the first law, the resulting relationship for the adiabatic combustion chamber at steady state is
operated under constant volume conditions; therefore, the fuel/ reformate exits at higher than atmospheric pressure. Mixing of the fuel/reformate with air is also conducted under constant volume; therefore, the initial pressure in the combustion chamber is higher than atmospheric pressure. Removing the preheat helps to isolate the benefits resulting from changes in fuel composition alone (chemical recovery) from purely thermal effects (thermal recovery). Energy outflows from the combustion chamber consist of the hot exhaust and piston work. There is no heat loss from the combustion chamber to the surroundings because it is adiabatic. To further simplify the analyses, we also assume the following: (i) Before crossing the outer control volume boundary, inlet air and fuel/water are both initially at ambient temperature and pressure. (ii) The fuel (with any added water) is fully vaporized in the vaporizer at atmospheric pressure before entering the reformer. (iii) Air enters the combustion chamber at ambient temperature and pressure. (iv) Exhaust gases exit the combustion chamber at ambient pressure. Application of the first law to the combustion chamber requires that the energy entering in the fuel/water and air streams must equal the energy leaving in the form of piston work and exhaust energy (including consideration of work on or by the surrounding atmosphere). For purposes of this discussion, it is useful to define all thermodynamic properties relative to a reference or “dead” state, which we specify here as the uncondensed (gaseous) products of complete combustion at ambient temperature and pressure. As is the usual convention for combustion engines, we do not consider non-chemical and non-thermal forms of energy in the inlet and exhaust streams (for example, kinetic, magnetic, and gravitational terms are neglected). We also do not consider any potential work that might be produced by condensing water from the combustion products or by exploiting the chemical gradient between the complete combustion products and the ambient atmosphere. The first law alone cannot be used to determine how the total inlet energy contained in the fuel/water and air is split among piston work and hot exhaust, because any combination of work and exhaust energy that sum to the correct total will be sufficient. Thus, application of the second law is required to uniquely determine the combustion chamber energy split. The central requirement of the second law is that the net entropy change (considering all parts of the universe involved) for any step must be greater than or equal to zero. Because we assume that the compression and expansion steps are isentropic, the only steps in the combustion chamber where entropy can increase are (1) during fuel and air mixing and (2) during combustion. One consequence of increasing entropy is that some of the original energy present becomes irretrievably lost for producing work. Processes involving net entropy increases are said to be thermodynamically irreversible. Thermodynamic exergy provides a convenient way to introduce the second law constraint in a way that explicitly connects engine work output and irreversibility. Consistent with previous literature,16,17 we define the extensive exergy (Xi) of gas stream i as Xi ¼ Ui - Uo þ Po ðVi - Vo Þ - To ðSi - So Þ
W ¼ Xa þ Xrc - Xe - ΔXirr
ð3Þ
where W is the net piston work, Xa is the exergy of the inlet air, Xrc is the exergy of the fuel/reformate entering the combustion chamber, Xe is the exergy remaining in the combustion products leaving the combustion chamber after work extraction, and ΔXirr is the loss of exergy as a result of irreversible processes in the combustion chamber. In our ideal engine, the exergy loss (ΔXirr) results from two steps: (1) mixing of the fuel and air and (2) the constant volume combustion reaction. These losses are related to the corresponding entropy changes by ΔXirr ¼ ΔXm þ ΔXc ¼ To ðΔSm þ ΔSc Þ ð4Þ where ΔXm is the mixing exergy loss, ΔXc is the combustion exergy loss, ΔSm is the mixing entropy change, and ΔSc is the combustion entropy change. We see from eqs 3 and 4 that the output piston work is directly reduced by the amount of the exergy losses from mixing and combustion. If either of these irreversibilities is reduced, work output (W) and/or exhaust exergy (Xe) will be increased. The chemical recuperation process consists of two steps occurring within the reformer control volume. In the first step, heat flows from the exhaust gas to the fuel, which is in contact with a reforming catalyst, and transforms into syngas. The extent of reforming is limited by chemical equilibrium and the final reformate temperature and pressure, which are determined as explained below. In the second step, reformate gas is subsequently cooled (with no catalyst present) by exchanging sensible heat with the environment prior to being sent to the combustion chamber. We consider two different scenarios for reformer operation: constant pressure and constant volume. In both scenarios, we specify an equilibrium reaction temperature, which in turn fixes the composition by simultaneously satisfying the ideal gas law, the atomic mass balances imposed by reaction stoichiometry, and the chemical equilibrium constraint (i.e., the reformed gas mixture is at equilibrium as it exits the reformer). For the latter two, we have aðfuelÞ þ bðH2 OÞ T cðCOÞ þ dðH2 Þ ð5aÞ
ð2Þ
ðnCO Þc ðnH2 Þd ðnfuel Þa ðnH2 O Þb
where the extensive internal energy (U), volume (V), and entropy (S) are evaluated relative to the properties of the gas at the reference dead state and Po and To are the dead state temperature and pressure. The above definition of exergy applies strictly to closed systems. For exergy analysis of flows, a more appropriate “flow exergy”, which includes effects of flow work, is used. The two definitions of exergy are equivalent when the pressure is Po. In the case of constant pressure reformation and mixing that we consider here, the pressure is different from Po only in the combustion chamber when the gas mixture is closed from surroundings and, therefore, the use of non-flow exergy as defined
¼ Keq
ð5bÞ
In the above equations, a, b, c, and d are the stoichiometric coefficients for the reforming reaction, Keq is the equilibrium constant at the specified temperature and pressure, and nCO, nH2, nfuel, and nH2O are the number of moles of each species present at equilibrium. Note that we have constrained the reaction products to produce the maximum possible amount of syngas. Less desirable byproducts, such as solid carbon, CH4, and CO2, are also possible, but we assume that our reforming catalyst has optimum selectivity for syngas for a best-case scenario. Once the equilibrium reformate composition and temperature are known, all of its thermodynamic properties are also known. The composition of the reformate remains fixed during cooling, and because the
(16) Caton, J. A. SAE Tech. Pap. 2000-01-1081, 2000. (17) Caton, J. A. Energy 2000, 25, 1097–1117.
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temperature after cooling is specified, the reformate thermodynamic properties entering the combustion chamber are known as well. For constant pressure reforming, we assume that the pressure is ambient and the first law balance for the reformer control volume at steady state becomes Hec ¼ Heh - Q þ Hv - Hr
Table 1. Summary of Thermodynamic Differences among Selected Engine Fuels Fed to an Ideal Engine in a Gaseous State
ð6Þ
where Hec is the enthalpy of the cool exhaust, Heh is the enthalpy of the hot exhaust, Q is the sensible heat lost to the surroundings, Hv is the enthalpy of fuel/water entering the reformer, and Hr is the enthalpy of the fuel/reformate entering the combustion chamber. Prior to entering the combustion chamber, air and cooled reformate are mixed at ambient pressure. For constant volume reforming, the first law balance becomes instead Hec ¼ Heh - Q þ Uv - Ur
fuel
Xa
Va
ΔXm/X
ΔXc/X
Wa
γe
H2 CO H2/CO methanol ethanol n-butanol methane propane n-heptane isooctane
1.00 1.12 1.04 1.01 0.957 0.940 0.879 0.910 0.917 0.920
1.00 1.00 1.00 0.803 0.754 0.729 0.778 0.734 0.718 0.716
0.0219 0.0198 0.0214 0.0108 0.0070 0.0042 0.0102 0.0050 0.0027 0.0024
0.1162 0.0887 0.1070 0.1990 0.2019 0.2033 0.1809 0.1943 0.2005 0.2031
1.00 1.07 1.02 0.971 0.924 0.910 0.862 0.888 0.892 0.893
1.277 1.239 1.263 1.273 1.276 1.277 1.281 1.279 1.279 1.279
a Normalized to a corresponding value for H2. For 1 mol of air and 0.42 mol of H2, X = 96.6 kJ and W = 57.4 kJ.
facilitate comparisons. In general, all of the fuels, except CO and syngas, have lower exergy than H2. CO has the highest exergy of all. The second column in Table 1 (V) compares the relative volumes of stoichiometric mixtures of each fuel with air at the same temperature and pressure. All volumes are normalized to the volume of a stoichiometric mixture of air and H2. The different volumes for each fuel reflect differences in the moles of fuel required to consume the same amount of oxygen. In practice, these volumetric differences would translate into differences in engine displacement for a fixed inlet temperature and pressure. For example, an equivalent charge of air and isooctane would only be about 72% of the volume of a H2-air charge. Alternately, charging stoichiometrically equivalent amounts of H2 and isooctane in an engine with a fixed displacement would require a boost in the H2 inlet pressure of about 40%. The remaining columns in Table 1 refer to differences among the fuels that arise specifically in the context of the ideal engine combustion that we described above. In the third and fourth columns of Table 1, we list the fraction of the original exergy in each fuel that would be destroyed by fuel-air mixing (ΔXm/X) and combustion irreversibility (ΔXc/X), respectively. We see here that the fraction of exergy destroyed by air-fuel mixing is relatively low for all fuels (typically less than 2%), with the highest losses for H2, CO, and syngas because of the larger number of fuel moles involved. However, the irreversibility losses for combustion (column four) are much larger, and H2 and CO clearly stand out from the other fuels in having the lowest losses. Relative work output from the ideal engine is listed in the fifth column (W) of Table 1 for each fuel, where we can see most directly how fuel differences translate to the potential work that can be extracted by combustion gas expansion. When these results are plotted graphically as in Figure 2, all of the hydrocarbon and alcohol fuels evaluated exhibit a nearly linear relationship between their initial exergy and the amount of work generated, implying that the work potential for these fuels is determined solely by their intensive exergy. However, H2 and CO clearly deviate from the linear trend followed by the other fuels, generating more expansion work than might be expected on the basis of their intensive exergy. One reason for the unexpected work/exergy ratio of H2 and CO is their lower combustion irreversibility. That is, less of their initial exergy is destroyed by combustion; therefore, more is left to generate work. The reduced combustion irreversibility for CO and H2 is a consequence of the fact that, for these particular fuels, the total number of gas molecules decreases with the reaction, thereby lowering the
ð7Þ
where the internal energies of the reformate entering the combustion chamber (Ur) and the fuel entering the reformer (Uv) are substituted for the enthalpies Hr and Hv, respectively. Prior to entering the combustion chamber, the cooled reformate and air are mixed at a constant volume. In all cases presented here, there is substantially more heat in the expanded combustion products than what is needed for the reformation process. Entropy is generated in the process of transferring heat from expanded combustion products to the fuel (as it is reformed). The entropy generated by heat transfer increases monotonically with the temperature difference between the two gas mixtures entering the reformer/heat exchanger. This entropy loss could be minimized to preserve the thermal exergy in the combustion products if it were to be used for additional work extraction. However, using this exergy would require an additional bottoming cycle, which, as mentioned previously, is not considered here. Therefore, we have not attempted to minimize the second law efficiency of the heat-exchange process, and it is very low in all cases considered here. Using the above relationships, we computed estimates of the ideal engine work, for example, cases with and without TCR using methanol, ethanol, and isooctane as the fuel. In all of these cases, we used realistic (i.e., temperature-dependent) thermodynamic properties for the fuel, air, and combustion products. We also assumed an ambient temperature and pressure of 300 K and 1 atm, respectively, and an initial compression ratio of 10. Because we used a cycle with full exhaust gas expansion to atmospheric pressure, the expansion ratio varies but is always greater than 10.
Results Inherent Fuel Differences. To better understand the potential impact of TCR, it is first helpful to consider the differences among fuels when they are used in the ideal engine without any reforming. In this case, we assume that all fuels enter the combustion chamber as gases; therefore, any fuels that are normally liquid under ambient conditions are vaporized. The first column of Table 1 (X) compares the relative chemical exergy of stoichiometrically equivalent amounts of several common engine fuels (in a gaseous state) at ambient conditions, including hydrocarbons, alcohols, CO, H2, and an equimolar mixture of CO and H2. By stoichiometrically equivalent, we mean the amount of fuel that would be fully consumed when burned with a fixed amount of air. This format provides us with a comparison of the inherent work potential of each fuel without regard to any particular engine concept or size (i.e., it measures an intrinsic thermodynamic property of the fuels). The exergy value listed for each fuel is also normalized to that of H2 to 1532
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Figure 2. Comparison of initial fuel exergy and work generated by the conceptual ideal piston engine (both normalized to H2) for a range of selected fuels without TCR. The trend line was fit to hydrocarbon fuels only (not H2 or CO).
entropy of the products. Combustion entropy is also generated by heat transfer between the reactant and product molecules;18 therefore, the overall entropy change for H2 and CO combustion is still positive, but the net entropy increase is smaller than it would be otherwise. For all of the other fuels, the total number of molecules increases with combustion and the final entropy is accordingly higher. However, if combustion irreversibility were the only important factor besides the initial exergy in controlling work output, one would expect both CO and H2 to be elevated above the trend line of the other fuels in Figure 2. In fact, CO has the lowest irreversibility of all, and its work output might thus be expected to lie substantially above the trendline for the other fuels; however, the actual work production for CO lies fairly close to the trendline. This apparent suppression of work output for CO can be explained by the differences in γ (ratio of constant pressure to constant volume heat capacity) for the fully expanded exhaust from each of the fuels in the last column of Table 1. Higher values of γ translate to increased expansion work. Even though γ varies with temperature, the relative variations in γe among fuels parallel the variations at intermediate states of expansion in the ideal engine. Thus, it appears that engine work output for CO combustion is suppressed from what it would be otherwise because of a lower γ. In summary, the following observations are noted regarding the potential effects of reforming on work output from the ideal engine analyzed here: (i) The CO and H2 reformates have a higher exergy than the original fuel. Lower combustion irreversibility alone does not necessarily lead to increased work output. For this reason, methane is an undesirable reformate. (ii) The CO and H2 reformates have a higher γ, resulting in increased compression work and higher precombustion temperature and pressure. This property of the reformate-air mixture has a similar effect to slightly raising the compression ratio. (iii) The moles of CO and H2 reformate are greater than the original moles of fuel. If the reforming is performed at a constant volume, this produces a pressure boost that will increase the effective
Figure 3. (a) Temperature of the reformate mixture exiting the reformer, (b) fraction of the combustion exhaust energy used for vaporization and reformation of the fuel, and (c) second law efficiency (piston work output/fuel exergy) as a function of the degree of reforming (0 = no reformation, and 1 = complete conversion to syngas) for an ideal combustion engine with constant pressure (violet - - -) and constant volume (blue ;) TCR of methanol.
compression ratio. If the reforming is performed at a constant pressure, the pressure boost effect is lost. The exergy loss because of the mixing of the larger number of gas moles is much smaller than the pressure boost effect. All of the above effects of reforming are summarized in the comparisons made in Table 1 and Figure 2. Impact of TCR for Methanol, Ethanol, and Isooctane. Figure 3 summarizes the key results of our analysis of the ideal combustion engine with TCR illustrated in Figure 1 when methanol is the fuel. The figure includes two cases: one with TCR at atmospheric pressure (- - -) and the other with constant volume TCR (;). Because the plots in Figures 4 and 5 are similar to those in Figure 3, we explain here how the values for Figure 3 were obtained in some detail.
(18) Rakopoulos, C. D.; Giakoumis, E. G. Prog. Energy Combust. Sci. 2006, 32, 2–47.
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Figure 4. Effect of the fuel/air equivalence ratio on the (a) fraction of combustion exhaust enthalpy used for vaporization and reformation of the fuel and (b) second law efficiency (piston work output/fuel exergy) for an ideal combustion engine with constant pressure (violet - - -) and constant volume (blue ;) TCR of methanol at 600 K.
Figure 3a shows the temperature of the reformate exiting the reformer at various reformer conversions (a degree of reforming of 0 means that the methanol passes through the reformer unreacted, and a degree of reforming of 1 means that all of the methanol is converted to syngas). Because we assume that the reformer operates at equilibrium, the reformate temperature and composition (described here as the degree of reforming) are directly related: setting one of these properties fixes the value of the other. We choose the degree of reformation here as the independent variable to simplify comparisons between the various fuels and operating parameters. Figure 3b illustrates the fraction of the exhaust enthalpy required for vaporizing and reforming the fuel as a function of the degree of reforming. The amount of reforming heat required is calculated from the enthalpies of vaporization of the fuel (and water in subsequent cases), the heat capacity of the gas in the reformer, and the enthalpy of the reforming reactions. Figure 3c shows the calculated second law efficiency (defined as the work output divided by the initial fuel exergy) for the entire combustion engine and TCR system, again as a function of the degree of reforming. Figure 3a reveals that, for constant pressure TCR with methanol fuel, reformation is nearly complete when the reformer exit temperature is above 500 K (which is much lower than the temperature of the exhaust coming out of the combustion chamber). Figure 3b illustrates that, in the limit of nearly complete reforming, over a third of the enthalpy in the exhaust from the combustion chamber must be used to drive the endothermic methanol reforming reaction. While this amount of heat recuperation may seem large, it could be achieved with
Figure 5. (a) Temperature of the reformate mixture exiting the reformer, (b) fraction of the combustion exhaust energy used for vaporization and reformation of the fuel, and (c) second law efficiency (piston work output/fuel exergy) as a function of the degree of reforming (0 = no reformation, and 1 = complete conversion to syngas) for an ideal combustion engine with constant volume TCR of an isooctane and water mixture (red - - -), an ethanol and water mixture (green - 3 -), and methanol (blue ;).
counterflow heat exchange. In Figure 3c, we observe that the second law efficiency (work output divided by fuel exergy) increases linearly with the degree of reformation. Net work increases by about 3% of the original exergy when the methanol is fully reformed. When we compare the small boost in work output with the amount of heat recuperated from the exhaust, it would appear that the recuperated exhaust heat is not very effectively used by the engine for this case. This latter observation implies that the addition of a separate bottoming cycle for using the exhaust heat might be beneficial. We next consider the impact of changing the methanol reformer operating conditions from constant pressure to constant volume. Here, we only consider the pressure increase associated with the increased gas moles from reforming, and we do not account for the pressure boost that occurs 1534
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: DOI:10.1021/ef901113b
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when vaporizing the liquid fuel and any associated water prior to reforming. As illustrated in Figure 3a, the constant volume reformer outlet temperature must increase because the increased pressure from molar expansion inhibits the reforming reactions (for example, by about 50 K for 90% conversion). This temperature increase tends to offset the fact that the constant volume specific heat of methanol is less than the constant pressure specific heat. Thus, the amount of exhaust energy needed to reform the fuel at constant volume (Figure 3b) is very similar to that for constant pressure. On the other hand, the higher syngas pressure also raises the initial cylinder pressure, increasing the net expansion work to more than 5% of the fuel exergy (Figure 3c). Thus, it appears that pressure boost can be an important factor in methanol TCR. Recent engine development has emphasized lean fueling, in part because of expected increases in efficiency. This lean fueling benefit is due to the fact that the effective γ increases as the fuel/air equivalence ratio decreases and the mole fractions of diatomic molecules (N2 and O2) in the exhaust increase. A higher exhaust γ tends to increase expansion work, with all else being equal. The work increase occurs even despite the fact that, as the fuel-air mixture becomes leaner, the combustion irreversibility also increases. Also, perhaps even more importantly in real world applications, lean combustion reduces the peak flame temperature, which reduces cylinder wall heat losses in real engines. In the present study, however, our ideal engine is adiabatic; therefore, heat loss is not a consideration. To evaluate potential interactions between TCR and lean fueling, we also considered constant pressure and constant volume TCR for methanol with varying ratios of inlet fuel and air. In both cases, we fixed the reformer exit temperature at 600 K. The resulting trends with a fuel/air equivalence ratio are summarized in Figure 4. The second law efficiency increases with a decreasing equivalence ratio as might be expected. The benefit associated with lean fueling appears to be more significant for constant pressure TCR than constant volume, but the maximum work output is still achieved with constant volume TCR. To assess the impact of fuel type on TCR, we next considered ethanol and isooctane fuels. Figure 5 illustrates the results for constant volume steam reforming of ethanol and isooctane, both blended with stoichiometric quantities of water (1:1 for ethanol and 8:1 for isooctane). The results for constant volume TCR of methanol are also included for the sake of comparison. Figure 5a shows that the complete reformation of hydrous ethanol requires a temperature of about 800 K and full reformation of isooctane is only approached when the reformer exit temperature exceeds 900 K. These higher operating temperatures require increasing fractions of the post-expansion exhaust enthalpy (Heh) to drive the reformer, as illustrated in Figure 5b. At full conversion of the fuel to H2 and CO, ethanol TCR requires over half of the exhaust enthalpy to drive the reformer and isooctane TCR uses roughly two-thirds of the exhaust enthalpy. These increases in exhaust energy recuperation result in more piston work. Figure 5c shows that complete reformation of ethanol increases the second law efficiency by 9% compared to the baseline case of no reformation. For isooctane, TCR boosts work output as a fraction of fuel exergy by over 11% (which corresponds to an 18% increase in work output over the baseline case with no reformation) to achieve nearly 70% overall engine second law efficiency.
Figure 6. Postcombustion (a) pressure and (b) temperature as a function of the degree of reforming (0 = no reformation, and 1 = complete conversion to syngas) for an ideal combustion engine with constant volume TCR of an isooctane and water mixture (red - - -), an ethanol and water mixture (green - 3 -), and methanol (blue ;).
Effectively, this improvement amounts to about half of the losses associated with combustion irreversibility. Even accounting for typical inefficiencies in real engines, this is substantial. To better explain the second law efficiency trends with increasing reformation and between the three fuels, we have plotted the peak combustion temperatures and pressures for constant volume TCR of methanol, ethanol, and isooctane in Figure 6. Increased fuel reforming generates higher peak combustion temperatures and pressures. These higher temperatures and pressures are due to the higher enthalpy of combustion and lower combustion irreversibility (see Table 1) of the reformate mixtures relative to the original fuel molecules. Recall that our analysis assumes an adiabatic combustion process; therefore, all of the increased exergy in the combustion products can be converted to piston work in the subsequent expansion step. In short, in the absence of heat-loss effects, TCR effectively uses waste exhaust heat to upgrade the hydrocarbon fuel to reformate, which burns with higher heat release and lower irreversibility. The source of the differences among the three fuels is less obvious but still explainable. In the absence of reforming, isooctane generates a lower peak combustion temperature and pressure, which should result in less piston work output than the other fuels. However, all three fuels result in nearly the same second law efficiency (see Figure 5c) with no reforming. Recall that second law efficiency is defined as the total piston work divided by the fuel exergy. Thus, even though the total piston work from unreformed isooctane is lower, the inherent fuel exergy is lower and the ratio of the 1535
Energy Fuels 2010, 24, 1529–1537
: DOI:10.1021/ef901113b
Chakravarthy et al.
Figure 7. Distribution of exergy destruction/conversion for an idealized combustion engine with constant volume TCR. Results are shown for methanol, ethanol, and isooctane with no reformation (0% reformed) and high (approximately 95%) conversion to syngas.
piston work to the fuel exergy turns out to be about the same as that for methanol and ethanol. As the degree of reforming is increased, the combustion temperatures and pressures increase and converge for the three fuels. This convergence (of not only peak pressures and temperatures but work output) is due to the increasing similarity in the combustor feed composition. In the limit of complete reformation, the composition of the fuel fed to the combustor from the reformer is a nearly identical mixture of H2 and CO for all three fuels (there are subtle differences in initial pressure and temperature because of the different mole changes during the reforming reactions and the varying boiling points of the three fuels and subtle differences in composition because of the slight variation in H/C ratios in the fuel molecules). Because the combustor feed composition is so similar, we would expect to see similar combustion temperatures and pressures, which can be observed in Figure 6. However, the second law efficiency diverges with increasing reformation. Once again, the explanation can be found in the different fuel exergies that are used to normalize the work output. Isooctane reformate may generate similar piston work output to methanol and ethanol reformate, but because it has a lower inherent fuel exergy, the calculated second law efficiency is much higher. To further explore the effects of TCR, we illustrate the distributions of exergy destruction and conversion within the components of the idealized TCR combustion engine in Figure 7. This figure compares the exergy distributions for all three fuels for two cases: no reforming and roughly 95% reformer conversion. The plot reveals the following impacts of TCR: (i) In addition to losses because of vaporization, mixing, and combustion irreversibility seen for the baseline cases (no reforming), exergy is also destroyed by entropy generation in the reformer and heat transfer in the intercooler for the TCR process. Interestingly, the losses in the intercooler are quite small. Even for the case of isooctane, where the reformate mixture must be cooled from 900 K, the heat released from the intercooler amounts to less than 3% of the initial fuel exergy. (ii) The entropy of mixing increases with reformate and air compared to fuel and air. (iii) Combustion irreversibility decreases as the fuel is converted to H2 and CO in the reformer. (iv) Because of the exhaust heat recuperation, less exergy is lost to the exhaust. Because we have limited ourselves to work extraction only via the piston (there is no bottoming cycle), net work output is increased. When the exergy balances in Figure 7 are compared across the three fuels, the losses associated with the reformer,
intercooler, and reformate/air mixing add up to about the same fraction of the initial fuel exergy. Also, because the composition and temperature of the reformate entering the combustion chamber are nearly identical for all three fuels, the combustion irreversibility is about the same. Thus, the only major difference between the reformed fuels is in the partitioning between the exergy lost to the exhaust and the piston work output. As the exhaust enthalpy required for fuel reforming increases from methanol to ethanol and again to isooctane, the exergy lost to the exhaust decreases and the piston work output increases. This explains the larger increases in second law efficiency observed with TCR of ethanol and isooctane. Summary and Conclusions Our initial investigation of TCR indicates that it could result in substantial boosts in second law efficiency (as measured in terms of single-stage work output from an ideal IC engine) for a range of fuels. For an ideal stoichiometric engine fueled with methanol, TCR can increase the estimated second law efficiency by about 3% for constant pressure reforming and over 5% for constant volume reforming. The improvement of constant volume reforming over constant pressure reforming results from the pressure boost caused by the molar expansion. When the engine is operated lean (e.g., at a fuel/air equivalence ratio of 0.4), the expected second law efficiency benefits for methanol could be raised an additional 2%. The estimated second law efficiency increases for constant volume TCR of ethanol and isooctane are 9 and 11%, respectively. The second law efficiency benefits from TCR in the present study are mainly due to the higher cylinder input exergy for reformate and the pressure boost in the case of constant volume reformation. We note, however, that it will be important in future studies to consider the possibility for using combined cycle work extraction. When additional work can be extracted from the exhaust, the benefits of the reduced combustion irreversibility are likely to be more evident. In the ideal engine system used here, there is significant potential exergy loss associated with the reformer, where we have made no attempt to minimize the temperature gradient or generate work from the heat transferred between the post-expansion exhaust and reformer. If the proposed engine concept is modified to include a bottoming cycle that uses this heat, one would expect considerable increases in the potential work. Still, even for the relatively simple system considered here, TCR could yield substantial efficiency gains. 1536
Energy Fuels 2010, 24, 1529–1537
: DOI:10.1021/ef901113b
Chakravarthy et al.
Keq = reforming equilibrium constant (kPa(c þ d - a - b)) n = number of gas moles P = pressure (kPa) Q = heat (J) Rg = ideal gas constant (J mol-1 K-1) S = entropy (J/K) T = absolute temperature (K) U = internal energy (J) V = volume (m3) W = piston work (J) X = exergy (J) γ = ratio of specific heats
Acknowledgment. This research was sponsored by the U.S. Department of Energy (DOE) under Contract DE-AC0500OR22725 with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC. The authors specifically thank Gurpreet Singh of the Office of Vehicle Technologies, DOE, for sponsoring this work. The authors also thank Dr. K. Dean Edwards of Oak Ridge National Laboratory for enlightening discussions and insights.
Nomenclature a, b, c, and d = stoichiometric coefficients for reforming reactions H = enthalpy (J)
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