- Email: [email protected]
Extension of the lean limit through hydrogen enrichment of a LFG-fueled spark-ignition engine and emissions reduction
international journal of hydrogen energy 35 (2010) 1412–1419
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Extension of the lean limit through hydrogen enrichment of a LFG-fueled spark-ignition engine and emissions reduction Kurt Kornbluth, Jason Greenwood, Zach McCaffrey, David Vernon, Paul Erickson* Mechanical and Aerospace Engineering Department, UC Davis, CA 95616, USA
article info
abstract
Article history:
In this experimental investigation the affect of hydrogen addition to a landfill gas-fueled
Received 6 August 2009
naturally-aspirated spark-ignition engine was explored. Hydrogen concentrations of 0%,
Accepted 7 November 2009
30%, 40%, and 50% by volume were added to simulated landfill gas (60% CH4 and 40% CO2).
Available online 22 December 2009
Efficiency, coefficient of variance of indicated mean effective pressure, and CO emissions were measured from near stoichiometric mixtures up to the lean operating limit. Engine-
Keywords:
out NOx emissions were compared to predicted future best available control technology
Hydrogen enrichment
targets for NOx emissions in landfill gas-to-energy projects. From this study, it was
Bi-fueling
determined that with 40% hydrogen by volume untreated exhaust NOx emissions can meet
Emissions
the 0.22 g/kWh NOx target while retaining 95% of baseline power and low CO emissions.
Spark ignition
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Lean operating limit Landfill gas
1.
Introduction
Landfill gas (LFG) is produced by anaerobic decomposition of biodegradable material in a landfill. LFG is typically composed of 40–60% methane by volume, with carbon dioxide and minor amounts of other compounds including nitrogen, oxygen, ammonia, sulfides, mercaptans, hydrogen, carbon monoxide, and non-methane organic compounds [1–3]. The particular ratio of gases is landfill specific and depends on the environmental conditions and waste compositions. For instance, LFG from municipal landfills can be as low as 40% CH4 with variable concentrations of H2S and other volatile compounds [1]; in landfills with substantial entrainment of air, nitrogen concentrations as high as 15% have been found [3]. Currently, almost 1% of California’s electricity is produced by combustion of LFG. However, 37% of the LFG available is flared instead of being recovered for energy production, in part due to tightening nitrous oxide (NOx) emission
regulations for landfill gas-to-energy (LFGTE) projects. LFGTE projects both dispose of a waste stream and generate energy. Emissions regulations for these projects should take into account the avoided emissions from both of these services and not simply compare the emissions of the projects to one standard or another. In this way we propose that emissions regulations for LFGTE projects be set at a level that is comparable to the addition of the avoided emissions from disposal, including the energy use in these systems, as well as the avoided emissions from energy generation. Notwithstanding, the stringent NOx emission regulations specified by California Senate Bill 1298 require that the best available control technology (BACT) for NOx in distributed generation units must be utilized by a facility at the earliest practicable date. As shown in Table 1, the California Air Resources Board (CARB) has defined the future best available control technology for central station power plants using waste gas as
* Corresponding author. Tel.: þ1 530 752 5360; fax: þ1 530 752 4158. E-mail addresses: [email protected] (K. Kornbluth), [email protected] (J. Greenwood), [email protected] (Z. McCaffrey), [email protected] (D. Vernon), [email protected] (P. Erickson). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.11.031
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international journal of hydrogen energy 35 (2010) 1412–1419
Nomenclature ATDC After top dead center BACT Best available control technology g/bhp-hr Gram per brake-horsepower hour BTDC Before top dead center CIWMB California Integrated Waste Management Board COV Coefficient of variance CNG Compressed natural gas EGR Exhaust gas recirculation EPA Environmental Protection Agency ER Equivalence ratio HCNG Hydrogen-enriched compressed natural gas HHV Higher heating value HLFG Hydrogen-enriched landfill gas IC Internal combustion IMEP Indicated mean effective pressure
0.03 g/kWh for NOx emissions [4]. This is a major barrier for new landfill gas-to-energy projects since current technologies may not be able to meet these standards economically [5]. For LFG combustion via the internal combustion engine, selective catalytic reduction (SCR) aftertreatment systems are currently used by some landfill operators. These systems are predicted to lower NOx emissions from the typical 0.22 g/ kWh engine-out emission level to 0.06 g/kWh [6]. In practice, however, these catalytic systems are not only expensive, but are easily poisoned by trace amounts of sulfur present in LFG. Hydrogen enrichment of compressed natural gas (HCNG) is a technology currently being utilized to lower engine-out NOx in IC engines and has been proposed for LFG-fueled engines [6–8]. Hydrogen has favorable combustion characteristics which can enhance flame stability when mixed with other fuels. In natural-gas fired engines, hydrogen enrichment allows the use of high charge-dilution strategies to reduce combustion temperature and dramatically reduce engine-out NOx emissions. Charge dilution can be achieved through lean operation or exhaust gas recirculation (EGR). A similar reduction in NOx emissions for LFG-fired engines could eliminate the need for aftertreatment systems. In a 1996 study, Collier conducted field tests for heavy duty HCNG engines; the average fuel savings using a high chargedilution engine was 13% [9]. The fuel savings also resulted in engine power de-rating. A similar de-rating is expected for lean operation of LFG; however, other researchers have shown this may be offset by the use of higher compression ratios. Available studies show that LFG IC engines can achieve similar performance to CNG at higher equivalence ratios due to the higher resistance of LFG to knock enabling the use of higher compression ratios [10]. Using a similar strategy as HCNG, HLFG should allow stable operation in high charge-dilution combustion regimes previously unattainable in LFG engines, potentially resulting in significant improvement of engine-out NOx emissions. With HLFG, higher dilution can be used to lower flame temperatures, resulting in lower formation of NOx
LCE LFG LFGTE LHV LNG LOL MBT O&M ppm RT SCR SMR SNCR VOC WOT
Levelized Cost of Electricity Landfill gas Landfill gas-to-energy Lower heating value Liquefied natural gas Lean operating limit Maximum Brake Torque Operations and maintenance parts per million Reference Timing Selective catalytic reduction Steam methane reforming Selective non-catalytic reduction Volatile organic compounds Wide Open Throttle
through the Zeldovich mechanism. Lean operation using excess air increases the concentration of oxygen and promotes complete combustion if a flame can propagate through the dilute mixture. LFG contains significant amounts of CO2 that also acts as a diluent similarly to use of cooled EGR. There is an upper limit to charge-dilution which a particular engine can withstand due to low reaction rates, low heat release, combustion instability, and flame extinction. The point at which combustion instabilities reach a defined threshold is referred to as the lean operating limit (LOL). With increasing charge dilution beyond this point of LOL, NOx continues to decrease but CO, HC, and unburned fuel emissions increase until flame extinction. This study investigates the potential for hydrogen enrichment to extend the LOL of a LFG-fired internal combustion engine and simultaneously reduce engine-out emissions.
2.
Experimental setup and procedure
The test engine used in this study was a 0.745 L, two-cylinder, liquid-cooled, port fuel-injected, Kawasaki FD 791 DFI engine, with LFG and hydrogen mixtures fumigated into the intake port. Table 2 gives engine specifications and Fig. 1 shows the experimental setup. Fuel flow of both the simulated LFG and hydrogen gases were regulated and metered via a calibrated Cole-Palmer differential pressure mass flow controller (MFC), model 32907-79.
Table 1 – Waste Gas Emission Standards [4] Pollutant
NOx CO VOCs
Emission Standard [g/kWh] On or after January 1, 2008
On or after January 1, 2013
0.22 2.72 0.45
0.03 0.05 0.09
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international journal of hydrogen energy 35 (2010) 1412–1419
Table 2 – Test Engine Specifications. Model
Kawasaki FD 791 DFI
Year of manufacture Construction
2007 Aluminum block and heads 0.745 L 8.5:1 78 mm 78 mm 3.25
Displacement volume Compression ration Bore Stroke l/a ratio
After a 20 min warm-up period on gasoline, the appropriate test gas mixture was connected to the MFC. The engine was re-started and the speed was gradually ramped up to 2600 rpm at which point the automatic load control was engaged and allowed to stabilize. Ignition timing was adjusted to find the maximum brake torque (MBT); the timing was then retarded by 2 from MBT timing to reduce the uncertainty in NOx measurements [11]. This retarding of spark timing resulted in less than 5% reduction in brake torque. Equivalence ratio was determined by comparing exhaust gas oxygen concentration to predicted values based on stoichiometry. The system was allowed to stabilize for 2.5 min (five times the time constant of the NOx emissions measurement system). Data were recorded in 2 min intervals at 1 Hz. Between each test gas data set, the exhaust gas analyzer was purged with nitrogen and recalibrated.
3.
Instrumentation
Spark timing was varied using an XDI II electromotive ignition system incorporating a crank encoder, magnetic sensor, and high energy ignition coil. Using this system, spark timing was adjustable in increments of 1 up to 30 either side of top dead center (TDC). Cylinder pressure data were acquired using a Kistler 6117B piezoelectric spark plug-mounted pressure sensor and Kistler 5010B charge amplifier.
To measure the engine power and torque, a Dynomite 75 kW-peak water-brake dynamometer, with an auto-load control was used. Omega K-type thermocouples were installed to measure ambient air temperature, engine coolant temperature into and out of the coolant radiator, exhaust gas temperature for both cylinders, and cylinder head temperature. The torque, engine speed, intake air temperature, exhaust temperatures, coolant temperatures, humidity, and ambient pressure were recorded. A California Analytical exhaust gas analyzer, model 300, was used to measure CO and CO2 concentrations using nondispersive infrared, and O2 concentrations using a galvanic fuel cell. A California Analytical heated chemiluminescence photodiode detector, model 400, was used to measure NO and NOx measurements.
4.
Experimental design
All tests were run at 2600 rpm at wide open throttle (WOT) with simulated landfill gas using hydrogen enrichment and equivalence ratio levels as shown in Table 3. In the present study, the LOL is defined as the minimum equivalence ratio such that all three criteria for combustion stability have been satisfied, in particular: Coefficient of variance (COV) of indicated mean effective pressure has not increased more than 20% above baseline values. CO emissions have not increased more than 10% above baseline values. Brake thermal efficiency (BTE) has not decreased more than 15% below baseline values. Baseline values refer to the measured values with no hydrogen enrichment at the most stable equivalence ratio. The COV defines the cycle-to-cycle variation in indicated work per cycle, from the compression and expansion strokes, divided by the cylinder displacement volume. The BTE is defined as the ratio of mechanical energy out (measured at the dynamometer) to the amount of chemical energy input (based on the LHV of the fuel). The values for the LOL criteria detailed for the most stable LFG operating point are summarized in Table 4. By using three separate criteria for the LOL, the onset of combustion instability was better captured than using a single metric. In this study, error was determined using the student-t distribution, as shown in Equation (1), where tv;p is the test statistic, obtained from the student-t table for a corresponding degree of freedom n and a confidence interval p. In this study, a confidence interval of 95% is used.
Table 3 – Experimental Factors and Levels for CNG. Factor Hydrogen enrichment (% H2) Equivalence ratio (F)
Fig. 1 – Experimental setup.
Levels 0, 30,40,50 8 levels between max power and FL
international journal of hydrogen energy 35 (2010) 1412–1419
Table 4 – LOL Parameters; based on F [ 0.84, 7.4 kW at 2600 rpm, and 408 BTDC spark timing. Parameter
Value
COV (%) COVLOL (%) CO (ppm) COLOL (ppm) BTE (%) BTELOL (%)
9.5 11.4 586 645 26.8 22.3
xi ¼ x tv;p sx
The corresponding value of tv;p for a large sample set is 1.960 [12]; this value is used in the error calculations shown as error bars in the figures below.
5.
Theoretical background
Hydrogen possesses properties that positively influence the laminar flame speed, flammability limits, and engine performance to make hydrogen uniquely suited as a fuel additive. The laminar flame speed is directly related to how fast a flame propagates through a combustible mixture. As a first approximation, the flame speed is proportional to the square root of the thermal diffusivity and the reaction rate [13]. As shown in Table 5, hydrogen has a much greater thermal diffusivity when compared to methane or LFG, resulting in an increase in the flame speed. The faster reaction rates of the H2–O2 mechanism also allows for reactions to occur in a shorter amount of time [14], decreasing the burn duration. When hydrogen is added to a hydrocarbon fuel, both numerical simulations and experimental results show that the hydrogen causes an increase of H, O, and OH radical mole fractions, leading to faster chain branching and decreased burn duration [15,16]. The effect of hydrogen on laminar flame speed has been well documented for methane-hydrogen-air mixtures [17–19]. For example, Ayala et al. numerically showed a large increase in the flame speed near stoichiometric mixtures with decreasing influence at leaner mixtures [17]. The flammability limits of a fuel-oxidant mixture depend on the thermal diffusivity, reaction rate, and specific heat of the reactants. As previously mentioned, both the thermal diffusivity and reaction rate are increased with the addition of hydrogen. When the flammability limit is approached, the chain branching factor, a, approaches the critical value
Table 5 – Properties of H2, Methane, and LFG. Property Thermal conductivity [m2/s] Lower heating value [MJ/Kg] Laminar flame speed [cm/s] Density [kg/m3] Equivalence ratio at Lean limit
H2
Methane
LFGa
140.8 121 210 0.09 0.14
19.1 50 40 0.72 0.46
12.9 30 20 1.2 0.6
a LFG composed of 60% CH4 and 40% CO2 by volume.
necessary for flame propagation to occur [14]. Hydrogen, which produces more free radicals, will increase a and extend the lean flammability limit for a given fuel-oxidant mixture.
6.
(1)
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Combined effects of fuel properties
These direct effects of hydrogen addition on the laminar flame speed, chemical kinetics, and flammability limits of a fueloxidant mixture, combine to affect the coefficient of variance (COV), brake thermal efficiency (BTE), and CO and NOx emissions in a SI engine. The COV is a measure of the cyclic variability of the engine derived from the cylinder pressure. Onset of combustion instabilities cause an increase in the COV with decreasing equivalence ratio and arise from the lengthening of combustion time and incomplete flame propagation through the cylinder. COV continues to increase with leaner mixtures until misfire occurs. The increased flame speed with hydrogen enrichment allows for combustion to occur more quickly at a given equivalence ratio while the increased flammability range from chemical kinetics allows flames to propagate in leaner mixtures. This is especially important in local pockets where the equivalence ratio may be below that of the rest of the mixture due to incomplete mixing [11,13]. Brake thermal efficiency (BTE) indicates how well chemical energy is converted into mechanical energy. As the equivalence ratio is decreased, the burn duration increases and work transfer from the gases to the piston becomes less efficient. This decrease in efficiency is generally gradual, but increases rapidly with decreasing equivalence ratio past the onset of incomplete combustion or misfire. As shown in previous studies [17] the addition of hydrogen to gasoline increases the BTE and allows lower equivalence ratios. There are two reasons for this trend in lean and ultra-lean operating regimes; hydrogen has a faster flame speed and a higher specific energy, enabling shorter burn duration and better work transfer to the piston. Likewise, hydrogen enrichment has been shown to increase the brake thermal efficiency for LFG combustion [7]. On a system level, however, production of hydrogen requires energy input which may not be fully offset by the increase in engine efficiency. Carbon monoxide and HC emissions from a SI engine are dependent on equivalence ratio. Running lean of stoichiometric allows for relatively low CO and HC emissions. Further leaning typically reduces HC and CO emissions until the lean operating limit (LOL) where misfire occurs [11]. The three routes of NOx production are well known as thermal NO (Zeldovich mechanism), prompt NO, and fuel bound routes. The dominate production pathway for NO in most flames is the thermal NO mechanism [11,13,14]. Thus, a lower combustion temperature will reduce engine-out NOx emissions. Hydrogen enrichment of LFG (HLFG) stabilizes combustion allowing ultra-lean operation. In turn, ultra-lean operation of HLFG is shown to significantly reduce NOx production.
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international journal of hydrogen energy 35 (2010) 1412–1419
Fig. 2 – Effect of hydrogen enrichment on LOL.
Fig. 3 – Effect of Hydrogen on COV.
7. Results-effect of HLFG on COV, CO, and BTE on LOL In this study, hydrogen addition to simulated LFG was demonstrated to allow operation at lower equivalence ratios, extending the LOL with increasing hydrogen addition. As shown in Fig. 2, up to a 40% reduction of the LOL equivalence ratio could be achieved when using 50% hydrogen enrichment. All measured values for LOL, COV, CO, and BTE are presented in Table 6. In this study the COV was never the first stability criterion to be violated; this indicates combustion stability did not decrease significantly with increased dilution compared at constant enrichment until after the LOL was reached. The effect of increasing hydrogen addition was to extend the stable lean combustion limit before COV increased rapidly as demonstrated in Fig. 3. Fig. 4 demonstrates the effect on CO emissions as equivalence ratio was reduced. As hydrogen enrichment was increased from zero to 50%, the window where CO was kept below COLOL was extended from ER ¼ 0.8 to ER ¼ 0.5. From Fig. 5, it can be seen that the addition of hydrogen not only slightly increased the BTE compared to the baseline case, but also extended the window of efficient operation from ER ¼ 0.8 to ER ¼ 0.5. In this study, peak efficiency was observed to increase as hydrogen was added for a given equivalence ratio, see Fig. 6. Brake thermal efficiencies above 28% were found for mixtures with equivalence ratios as low as 0.62 with 50% hydrogen. Even at the lean operating limit (ER ¼ 0.52) H40LFG yielded a brake thermal efficiency only slightly lower than baseline efficiency value for LFG.
8. Effect of hydrogen enrichment on NOx emissions The effect of hydrogen addition on NOx emissions can be observed in Fig. 7. When hydrogen was added at a constant equivalence ratio, the combustion temperature increased due to hydrogen’s higher specific energy density, resulting in increased NOx emissions. As hydrogen enrichment increased from zero to 50%, however, the operating window was widened allowing for leaner operation, resulting in a decrease in NOx emissions. As demonstrated in Fig. 8, in order to take advantage of higher levels of hydrogen enrichment, the mixture should be close to, but not beyond, the LOL in equivalence ratio. This occurs just before the BTE begins to sharply decrease as shown in Fig. 5. A larger amount of hydrogen added allows a higher overall efficiency to be maintained with a larger reduction of NOx emissions in the data shown.
9.
Engine power de-rating
In normal SI engine operation with LFG, peak power is produced just lean of stoichiometric conditions and decreases
Table 6 – Experimental Results. Gas LFG H30LFG H40LFG H50LFG
LOL (FL) NOx (g/kW-h) CO (ppm) BTE (%) COV (%) 0.80 0.59 0.52 0.48
0.485 0.179 0.131 0.058
598 610 643 609
24.5 25.5 26.1 26.2
10.2 12.3 12.6 10.8
Fig. 4 – Effect of Hydrogen on CO emission.
international journal of hydrogen energy 35 (2010) 1412–1419
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Fig. 5 – BTE for HLFG mixes (LOL marked only for H40LFG). Fig. 7 – NOx production and LOL for HLFG mixes (D denotes the LOL). until the onset of misfire. As shown in Fig. 9, hydrogen enrichment enables similar power to be produced at a lower equivalence ratio.
10. Performance characteristics and tradeoffs with HLFG In IC engines there is a tradeoff between NOx emissions, CO and HC emissions, and BTE. Thus to maximize the overall benefit of hydrogen enrichment, CO and HC emissions need to be minimized along with NOx while maximizing brake thermal efficiency and power. As demonstrated previously in Fig. 4, a slight rise in CO emissions occurs immediately before the LOL is reached due to onset of combustion instabilities followed by an exponential rise after the LOL. Practically, this means that 40% hydrogen enrichment will likely need to be run at an equivalence ratio of 0.55, about 4% higher than the LOL equivalence ratio, in order to yield NOx emissions that will meet the proposed 0.22 g/kWh BACT standard. At this equivalence ratio the BTE for HLFG40 was approximately the same as for LFG combustion at near stoichiometric with similar CO emissions, but also has a 15% decrease in brake power. When running at a lower equivalence ratio, the output power of the engine will be decreased if the compression ratio (or intake pressure) is not increased at the same time. As mentioned above, similar power outputs can be realized for
Fig. 6 – Comparison of peak brake thermal efficiency for various gas mixtures.
LFG compared to CNG engines if the compression ratio is increased. In this study, the compression ratio was constant; however, the engine de-rating was decreased with added hydrogen, as shown in Fig. 9. This occurred because hydrogen, with an increased lower heating value, replaced the LFG, supplying more chemical energy per mol increasing power output. The drawback of an increased compression ratio is a potential for increased NOx emissions due to a higher combustion temperature; more research is needed to determine the effects of compression ratio, supercharging, and hydrogen enrichment on emissions in ultra-lean LFG combustion. Engine-out emissions in the particular engine tested indicate that current emission standards for NOx emissions for LFGTE projects can be met with 40% hydrogen enrichment with no aftertreatment while retaining a high overall efficiency. If lower NOx emissions are required, higher levels of hydrogen enrichment may be used to further extend the LOL. For example, with the engine used in this study, H40LFG running at an equivalence ratio of 0.55 will meet the current BACT standards for NOx emissions. To meet the future BACT standards for NOx in this particular engine, H50LFG running at an equivalence ratio of 0.48 is required.
Fig. 8 – NOx emissions vs. overall efficiency; points excluded for clarity.
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international journal of hydrogen energy 35 (2010) 1412–1419
Fig. 9 – Engine de-rating of hydrogen-enriched LFG.
Hydrogen enrichment of LFG shows promise for reducing emissions in LFGTE projects. These reduced emissions may enable further utilization of LFG for energy production where it is now restricted by emissions regulations. It should be stressed that hydrogen production energy input and cost also needs to be accounted for in the overall balance. This study focused on a normally-aspirated spark-ignition test engine of relatively small capacity (0.745 L). Although the results presented here offer valuable data on the general effects of hydrogen enrichment on NOx and CO emissions, and efficiency, it is important to note that typical LFG-fired engines differ from the test engine in two important ways: 1. Most LFGTE engines are turbocharged, making up for loss of power due to dilution. 2. LFGTE engines typically have a much larger power (100– 500 kW and above) with corresponding larger displacements. In order to prove the viability of HLFG as NOx reduction strategy a demonstration system should be built and tested using a typical LFGTE setup as described above.
11.
Conclusion
The combustion characteristics of hydrogen-enriched landfill gas in a small-displacement two-cylinder spark-ignition engine have been experimentally quantified. The lean operating limit was determined for each hydrogen-landfill gas mixture using a stability criterion based on the COV of IMEP, BTE, and CO emissions. Engine power output and de-rating were also recorded. Furthermore, NOx emissions were recorded and compared to current and future BACT emission standards for waste gas combustion. Hydrogen addition to landfill gas was shown to extend the lean operating limit while retaining low CO emissions within 10% of baseline landfill gas combustion. In this study, hydrogen enrichment up to 50% by volume resulted in an extension of the LOL equivalence ratio by almost 40%, from 4 ¼ 0.8 to 4 ¼ 0.49.
Results indicate that greater amounts of hydrogen addition can maintain the COV of IMEP, CO emissions, and the BTE at lower equivalence ratios. Hydrogen enrichment also enabled similar power to be produced at a lower equivalence ratio. Hydrogen enrichment increased BTE for LFG from 26.8% at an equivalence ratio of 0.84 to a maximum of 28.9% at an equivalence ratio of 0.68 for H50LFG. Furthermore, brake thermal efficiencies above 28% could be retained for mixtures with equivalence ratios as low as 0.62 with 50% hydrogen. At the lean operating limit, the brake thermal efficiency for H40- and H50LFG were 23.5% and 22.7%, only slightly lower than the maximum efficiency measured when operating with landfill gas. Engine-out NOx emissions were shown to decrease with decreasing equivalence ratios and hydrogen extended the LOL allowing further reductions. These results in a smalldisplacement engine with simulated landfill gas indicate that current and future BACT standards for NOx emissions with can be met with forty and fifty percent hydrogen enrichment respectively.
references
[1] Walsh JL, Ross CC, Smith MS, and Harper SR. Utilization of biogas. Biomass 1989;20(3–4):277–90. [2] Walker S. Compilation of landfill waste-in-place (WIP) and landfill gas-to-energy information, Levinson H, editor; 2005: Sacramento, CA. [3] Johnstone P. Development of a Hydrogen Power Park for Eureka, CA; 2006. [4] California Air Resource Board, Ammendments to title 17, California Code of Regulations, Sections 94200 to 94214. [5] Air pollution control technology fact sheet. Washington DC: Environmental Protection Agency; 2003. EPA-452/F-03– 032. [6] TIAX. Application of hydrogen assisted lean operation of biogas-fueled reciprocating engines (Bio-HALO). Sacramento, CA: California Energy Commission; 2002. [7] Kornbluth KL. Hydrogen Enrichment of Landfill Gas for Enhanced Combustion in Internal-Combustion Reciprocating Engines PhD Dissertation, University of California, Davis; 2008. [8] Richards GA, McMillian MM. Issues for low-emission, fuelflexible power systems. Progress in Energy and Combustion Science 2001:141–69. [9] Collier K, and Hoekstra R. ‘‘Untreated exhaust emissions of a hydrogen-enriched CNG production engine conversion’’; 1996, SAE International Paper 960858. [10] Bade S, Shrestha O, Narayanan G. Landfill gas with hydrogen addition – A fuel for SI engines. Fuel 2008:3616–26. [11] Heywood JB. Internal combustion engine fundamentals. New York: McGraw-Hill; 1988. [12] Figliola Richard, Beasley Donald. Theory and design for mechanical measurements. s.l.. Wiley and Sons, Inc; 2000. 0471350834 [13] Kenneth Kuo. Principles of combustion. Hoboken, New Jersey: John Wiley & Sons, Inc.; 2005. [14] Glassman, Irvin. Combustion. San Diego: Academic Press; 1996. [15] Wang Jinhua, Huang Zuohua, Tang Chenglong, Miao Haiyan, Wang Xibin. Numerical study of the effect of hydrogen
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addition on methane–air mixtures combustion. International Journal of Hydrogen Energy 2009:1084–96. [16] Schefer RW. Hydrogen enrichment for improved lean flame stability. International Journal of Hydrogen Energy 2003;28: 1131–41. [17] Ayala FA, Gerty MD, and Heywood JB. ‘‘Effects of combustion phrasing, relative air-fuel ratio, compression ratio, and load on SI engine efficiency’’; 2006 SAE International, Paper 2006-01-0229.
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[18] Rakopoulos CD, Kyritsis DC. Hydrogen enrichment effects on the second law analysis of natural and landfill gas combustion in engine cylinders. International Journal of Hydrogen Energy 2006:1384–93. [19] Guo Hongsheng, Smallwood Gregory J, Liu Fengshan, Ju Yiguang, Gulder Omer L. The effect of hydrogen addition on flammability limit and NOx emission in ultra-lean counterflow CH4/air premixed flames. Proceedings of the Combustion Institute 2005:303–11.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Extension of the lean limit through hydrogen enrichment of a LFG-fueled spark-ignition engine and emissions reduction Kurt Kornbluth, Jason Greenwood, Zach McCaffrey, David Vernon, Paul Erickson* Mechanical and Aerospace Engineering Department, UC Davis, CA 95616, USA
article info
abstract
Article history:
In this experimental investigation the affect of hydrogen addition to a landfill gas-fueled
Received 6 August 2009
naturally-aspirated spark-ignition engine was explored. Hydrogen concentrations of 0%,
Accepted 7 November 2009
30%, 40%, and 50% by volume were added to simulated landfill gas (60% CH4 and 40% CO2).
Available online 22 December 2009
Efficiency, coefficient of variance of indicated mean effective pressure, and CO emissions were measured from near stoichiometric mixtures up to the lean operating limit. Engine-
Keywords:
out NOx emissions were compared to predicted future best available control technology
Hydrogen enrichment
targets for NOx emissions in landfill gas-to-energy projects. From this study, it was
Bi-fueling
determined that with 40% hydrogen by volume untreated exhaust NOx emissions can meet
Emissions
the 0.22 g/kWh NOx target while retaining 95% of baseline power and low CO emissions.
Spark ignition
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Lean operating limit Landfill gas
1.
Introduction
Landfill gas (LFG) is produced by anaerobic decomposition of biodegradable material in a landfill. LFG is typically composed of 40–60% methane by volume, with carbon dioxide and minor amounts of other compounds including nitrogen, oxygen, ammonia, sulfides, mercaptans, hydrogen, carbon monoxide, and non-methane organic compounds [1–3]. The particular ratio of gases is landfill specific and depends on the environmental conditions and waste compositions. For instance, LFG from municipal landfills can be as low as 40% CH4 with variable concentrations of H2S and other volatile compounds [1]; in landfills with substantial entrainment of air, nitrogen concentrations as high as 15% have been found [3]. Currently, almost 1% of California’s electricity is produced by combustion of LFG. However, 37% of the LFG available is flared instead of being recovered for energy production, in part due to tightening nitrous oxide (NOx) emission
regulations for landfill gas-to-energy (LFGTE) projects. LFGTE projects both dispose of a waste stream and generate energy. Emissions regulations for these projects should take into account the avoided emissions from both of these services and not simply compare the emissions of the projects to one standard or another. In this way we propose that emissions regulations for LFGTE projects be set at a level that is comparable to the addition of the avoided emissions from disposal, including the energy use in these systems, as well as the avoided emissions from energy generation. Notwithstanding, the stringent NOx emission regulations specified by California Senate Bill 1298 require that the best available control technology (BACT) for NOx in distributed generation units must be utilized by a facility at the earliest practicable date. As shown in Table 1, the California Air Resources Board (CARB) has defined the future best available control technology for central station power plants using waste gas as
* Corresponding author. Tel.: þ1 530 752 5360; fax: þ1 530 752 4158. E-mail addresses: [email protected] (K. Kornbluth), [email protected] (J. Greenwood), [email protected] (Z. McCaffrey), [email protected] (D. Vernon), [email protected] (P. Erickson). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.11.031
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international journal of hydrogen energy 35 (2010) 1412–1419
Nomenclature ATDC After top dead center BACT Best available control technology g/bhp-hr Gram per brake-horsepower hour BTDC Before top dead center CIWMB California Integrated Waste Management Board COV Coefficient of variance CNG Compressed natural gas EGR Exhaust gas recirculation EPA Environmental Protection Agency ER Equivalence ratio HCNG Hydrogen-enriched compressed natural gas HHV Higher heating value HLFG Hydrogen-enriched landfill gas IC Internal combustion IMEP Indicated mean effective pressure
0.03 g/kWh for NOx emissions [4]. This is a major barrier for new landfill gas-to-energy projects since current technologies may not be able to meet these standards economically [5]. For LFG combustion via the internal combustion engine, selective catalytic reduction (SCR) aftertreatment systems are currently used by some landfill operators. These systems are predicted to lower NOx emissions from the typical 0.22 g/ kWh engine-out emission level to 0.06 g/kWh [6]. In practice, however, these catalytic systems are not only expensive, but are easily poisoned by trace amounts of sulfur present in LFG. Hydrogen enrichment of compressed natural gas (HCNG) is a technology currently being utilized to lower engine-out NOx in IC engines and has been proposed for LFG-fueled engines [6–8]. Hydrogen has favorable combustion characteristics which can enhance flame stability when mixed with other fuels. In natural-gas fired engines, hydrogen enrichment allows the use of high charge-dilution strategies to reduce combustion temperature and dramatically reduce engine-out NOx emissions. Charge dilution can be achieved through lean operation or exhaust gas recirculation (EGR). A similar reduction in NOx emissions for LFG-fired engines could eliminate the need for aftertreatment systems. In a 1996 study, Collier conducted field tests for heavy duty HCNG engines; the average fuel savings using a high chargedilution engine was 13% [9]. The fuel savings also resulted in engine power de-rating. A similar de-rating is expected for lean operation of LFG; however, other researchers have shown this may be offset by the use of higher compression ratios. Available studies show that LFG IC engines can achieve similar performance to CNG at higher equivalence ratios due to the higher resistance of LFG to knock enabling the use of higher compression ratios [10]. Using a similar strategy as HCNG, HLFG should allow stable operation in high charge-dilution combustion regimes previously unattainable in LFG engines, potentially resulting in significant improvement of engine-out NOx emissions. With HLFG, higher dilution can be used to lower flame temperatures, resulting in lower formation of NOx
LCE LFG LFGTE LHV LNG LOL MBT O&M ppm RT SCR SMR SNCR VOC WOT
Levelized Cost of Electricity Landfill gas Landfill gas-to-energy Lower heating value Liquefied natural gas Lean operating limit Maximum Brake Torque Operations and maintenance parts per million Reference Timing Selective catalytic reduction Steam methane reforming Selective non-catalytic reduction Volatile organic compounds Wide Open Throttle
through the Zeldovich mechanism. Lean operation using excess air increases the concentration of oxygen and promotes complete combustion if a flame can propagate through the dilute mixture. LFG contains significant amounts of CO2 that also acts as a diluent similarly to use of cooled EGR. There is an upper limit to charge-dilution which a particular engine can withstand due to low reaction rates, low heat release, combustion instability, and flame extinction. The point at which combustion instabilities reach a defined threshold is referred to as the lean operating limit (LOL). With increasing charge dilution beyond this point of LOL, NOx continues to decrease but CO, HC, and unburned fuel emissions increase until flame extinction. This study investigates the potential for hydrogen enrichment to extend the LOL of a LFG-fired internal combustion engine and simultaneously reduce engine-out emissions.
2.
Experimental setup and procedure
The test engine used in this study was a 0.745 L, two-cylinder, liquid-cooled, port fuel-injected, Kawasaki FD 791 DFI engine, with LFG and hydrogen mixtures fumigated into the intake port. Table 2 gives engine specifications and Fig. 1 shows the experimental setup. Fuel flow of both the simulated LFG and hydrogen gases were regulated and metered via a calibrated Cole-Palmer differential pressure mass flow controller (MFC), model 32907-79.
Table 1 – Waste Gas Emission Standards [4] Pollutant
NOx CO VOCs
Emission Standard [g/kWh] On or after January 1, 2008
On or after January 1, 2013
0.22 2.72 0.45
0.03 0.05 0.09
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Table 2 – Test Engine Specifications. Model
Kawasaki FD 791 DFI
Year of manufacture Construction
2007 Aluminum block and heads 0.745 L 8.5:1 78 mm 78 mm 3.25
Displacement volume Compression ration Bore Stroke l/a ratio
After a 20 min warm-up period on gasoline, the appropriate test gas mixture was connected to the MFC. The engine was re-started and the speed was gradually ramped up to 2600 rpm at which point the automatic load control was engaged and allowed to stabilize. Ignition timing was adjusted to find the maximum brake torque (MBT); the timing was then retarded by 2 from MBT timing to reduce the uncertainty in NOx measurements [11]. This retarding of spark timing resulted in less than 5% reduction in brake torque. Equivalence ratio was determined by comparing exhaust gas oxygen concentration to predicted values based on stoichiometry. The system was allowed to stabilize for 2.5 min (five times the time constant of the NOx emissions measurement system). Data were recorded in 2 min intervals at 1 Hz. Between each test gas data set, the exhaust gas analyzer was purged with nitrogen and recalibrated.
3.
Instrumentation
Spark timing was varied using an XDI II electromotive ignition system incorporating a crank encoder, magnetic sensor, and high energy ignition coil. Using this system, spark timing was adjustable in increments of 1 up to 30 either side of top dead center (TDC). Cylinder pressure data were acquired using a Kistler 6117B piezoelectric spark plug-mounted pressure sensor and Kistler 5010B charge amplifier.
To measure the engine power and torque, a Dynomite 75 kW-peak water-brake dynamometer, with an auto-load control was used. Omega K-type thermocouples were installed to measure ambient air temperature, engine coolant temperature into and out of the coolant radiator, exhaust gas temperature for both cylinders, and cylinder head temperature. The torque, engine speed, intake air temperature, exhaust temperatures, coolant temperatures, humidity, and ambient pressure were recorded. A California Analytical exhaust gas analyzer, model 300, was used to measure CO and CO2 concentrations using nondispersive infrared, and O2 concentrations using a galvanic fuel cell. A California Analytical heated chemiluminescence photodiode detector, model 400, was used to measure NO and NOx measurements.
4.
Experimental design
All tests were run at 2600 rpm at wide open throttle (WOT) with simulated landfill gas using hydrogen enrichment and equivalence ratio levels as shown in Table 3. In the present study, the LOL is defined as the minimum equivalence ratio such that all three criteria for combustion stability have been satisfied, in particular: Coefficient of variance (COV) of indicated mean effective pressure has not increased more than 20% above baseline values. CO emissions have not increased more than 10% above baseline values. Brake thermal efficiency (BTE) has not decreased more than 15% below baseline values. Baseline values refer to the measured values with no hydrogen enrichment at the most stable equivalence ratio. The COV defines the cycle-to-cycle variation in indicated work per cycle, from the compression and expansion strokes, divided by the cylinder displacement volume. The BTE is defined as the ratio of mechanical energy out (measured at the dynamometer) to the amount of chemical energy input (based on the LHV of the fuel). The values for the LOL criteria detailed for the most stable LFG operating point are summarized in Table 4. By using three separate criteria for the LOL, the onset of combustion instability was better captured than using a single metric. In this study, error was determined using the student-t distribution, as shown in Equation (1), where tv;p is the test statistic, obtained from the student-t table for a corresponding degree of freedom n and a confidence interval p. In this study, a confidence interval of 95% is used.
Table 3 – Experimental Factors and Levels for CNG. Factor Hydrogen enrichment (% H2) Equivalence ratio (F)
Fig. 1 – Experimental setup.
Levels 0, 30,40,50 8 levels between max power and FL
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Table 4 – LOL Parameters; based on F [ 0.84, 7.4 kW at 2600 rpm, and 408 BTDC spark timing. Parameter
Value
COV (%) COVLOL (%) CO (ppm) COLOL (ppm) BTE (%) BTELOL (%)
9.5 11.4 586 645 26.8 22.3
xi ¼ x tv;p sx
The corresponding value of tv;p for a large sample set is 1.960 [12]; this value is used in the error calculations shown as error bars in the figures below.
5.
Theoretical background
Hydrogen possesses properties that positively influence the laminar flame speed, flammability limits, and engine performance to make hydrogen uniquely suited as a fuel additive. The laminar flame speed is directly related to how fast a flame propagates through a combustible mixture. As a first approximation, the flame speed is proportional to the square root of the thermal diffusivity and the reaction rate [13]. As shown in Table 5, hydrogen has a much greater thermal diffusivity when compared to methane or LFG, resulting in an increase in the flame speed. The faster reaction rates of the H2–O2 mechanism also allows for reactions to occur in a shorter amount of time [14], decreasing the burn duration. When hydrogen is added to a hydrocarbon fuel, both numerical simulations and experimental results show that the hydrogen causes an increase of H, O, and OH radical mole fractions, leading to faster chain branching and decreased burn duration [15,16]. The effect of hydrogen on laminar flame speed has been well documented for methane-hydrogen-air mixtures [17–19]. For example, Ayala et al. numerically showed a large increase in the flame speed near stoichiometric mixtures with decreasing influence at leaner mixtures [17]. The flammability limits of a fuel-oxidant mixture depend on the thermal diffusivity, reaction rate, and specific heat of the reactants. As previously mentioned, both the thermal diffusivity and reaction rate are increased with the addition of hydrogen. When the flammability limit is approached, the chain branching factor, a, approaches the critical value
Table 5 – Properties of H2, Methane, and LFG. Property Thermal conductivity [m2/s] Lower heating value [MJ/Kg] Laminar flame speed [cm/s] Density [kg/m3] Equivalence ratio at Lean limit
H2
Methane
LFGa
140.8 121 210 0.09 0.14
19.1 50 40 0.72 0.46
12.9 30 20 1.2 0.6
a LFG composed of 60% CH4 and 40% CO2 by volume.
necessary for flame propagation to occur [14]. Hydrogen, which produces more free radicals, will increase a and extend the lean flammability limit for a given fuel-oxidant mixture.
6.
(1)
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Combined effects of fuel properties
These direct effects of hydrogen addition on the laminar flame speed, chemical kinetics, and flammability limits of a fueloxidant mixture, combine to affect the coefficient of variance (COV), brake thermal efficiency (BTE), and CO and NOx emissions in a SI engine. The COV is a measure of the cyclic variability of the engine derived from the cylinder pressure. Onset of combustion instabilities cause an increase in the COV with decreasing equivalence ratio and arise from the lengthening of combustion time and incomplete flame propagation through the cylinder. COV continues to increase with leaner mixtures until misfire occurs. The increased flame speed with hydrogen enrichment allows for combustion to occur more quickly at a given equivalence ratio while the increased flammability range from chemical kinetics allows flames to propagate in leaner mixtures. This is especially important in local pockets where the equivalence ratio may be below that of the rest of the mixture due to incomplete mixing [11,13]. Brake thermal efficiency (BTE) indicates how well chemical energy is converted into mechanical energy. As the equivalence ratio is decreased, the burn duration increases and work transfer from the gases to the piston becomes less efficient. This decrease in efficiency is generally gradual, but increases rapidly with decreasing equivalence ratio past the onset of incomplete combustion or misfire. As shown in previous studies [17] the addition of hydrogen to gasoline increases the BTE and allows lower equivalence ratios. There are two reasons for this trend in lean and ultra-lean operating regimes; hydrogen has a faster flame speed and a higher specific energy, enabling shorter burn duration and better work transfer to the piston. Likewise, hydrogen enrichment has been shown to increase the brake thermal efficiency for LFG combustion [7]. On a system level, however, production of hydrogen requires energy input which may not be fully offset by the increase in engine efficiency. Carbon monoxide and HC emissions from a SI engine are dependent on equivalence ratio. Running lean of stoichiometric allows for relatively low CO and HC emissions. Further leaning typically reduces HC and CO emissions until the lean operating limit (LOL) where misfire occurs [11]. The three routes of NOx production are well known as thermal NO (Zeldovich mechanism), prompt NO, and fuel bound routes. The dominate production pathway for NO in most flames is the thermal NO mechanism [11,13,14]. Thus, a lower combustion temperature will reduce engine-out NOx emissions. Hydrogen enrichment of LFG (HLFG) stabilizes combustion allowing ultra-lean operation. In turn, ultra-lean operation of HLFG is shown to significantly reduce NOx production.
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Fig. 2 – Effect of hydrogen enrichment on LOL.
Fig. 3 – Effect of Hydrogen on COV.
7. Results-effect of HLFG on COV, CO, and BTE on LOL In this study, hydrogen addition to simulated LFG was demonstrated to allow operation at lower equivalence ratios, extending the LOL with increasing hydrogen addition. As shown in Fig. 2, up to a 40% reduction of the LOL equivalence ratio could be achieved when using 50% hydrogen enrichment. All measured values for LOL, COV, CO, and BTE are presented in Table 6. In this study the COV was never the first stability criterion to be violated; this indicates combustion stability did not decrease significantly with increased dilution compared at constant enrichment until after the LOL was reached. The effect of increasing hydrogen addition was to extend the stable lean combustion limit before COV increased rapidly as demonstrated in Fig. 3. Fig. 4 demonstrates the effect on CO emissions as equivalence ratio was reduced. As hydrogen enrichment was increased from zero to 50%, the window where CO was kept below COLOL was extended from ER ¼ 0.8 to ER ¼ 0.5. From Fig. 5, it can be seen that the addition of hydrogen not only slightly increased the BTE compared to the baseline case, but also extended the window of efficient operation from ER ¼ 0.8 to ER ¼ 0.5. In this study, peak efficiency was observed to increase as hydrogen was added for a given equivalence ratio, see Fig. 6. Brake thermal efficiencies above 28% were found for mixtures with equivalence ratios as low as 0.62 with 50% hydrogen. Even at the lean operating limit (ER ¼ 0.52) H40LFG yielded a brake thermal efficiency only slightly lower than baseline efficiency value for LFG.
8. Effect of hydrogen enrichment on NOx emissions The effect of hydrogen addition on NOx emissions can be observed in Fig. 7. When hydrogen was added at a constant equivalence ratio, the combustion temperature increased due to hydrogen’s higher specific energy density, resulting in increased NOx emissions. As hydrogen enrichment increased from zero to 50%, however, the operating window was widened allowing for leaner operation, resulting in a decrease in NOx emissions. As demonstrated in Fig. 8, in order to take advantage of higher levels of hydrogen enrichment, the mixture should be close to, but not beyond, the LOL in equivalence ratio. This occurs just before the BTE begins to sharply decrease as shown in Fig. 5. A larger amount of hydrogen added allows a higher overall efficiency to be maintained with a larger reduction of NOx emissions in the data shown.
9.
Engine power de-rating
In normal SI engine operation with LFG, peak power is produced just lean of stoichiometric conditions and decreases
Table 6 – Experimental Results. Gas LFG H30LFG H40LFG H50LFG
LOL (FL) NOx (g/kW-h) CO (ppm) BTE (%) COV (%) 0.80 0.59 0.52 0.48
0.485 0.179 0.131 0.058
598 610 643 609
24.5 25.5 26.1 26.2
10.2 12.3 12.6 10.8
Fig. 4 – Effect of Hydrogen on CO emission.
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Fig. 5 – BTE for HLFG mixes (LOL marked only for H40LFG). Fig. 7 – NOx production and LOL for HLFG mixes (D denotes the LOL). until the onset of misfire. As shown in Fig. 9, hydrogen enrichment enables similar power to be produced at a lower equivalence ratio.
10. Performance characteristics and tradeoffs with HLFG In IC engines there is a tradeoff between NOx emissions, CO and HC emissions, and BTE. Thus to maximize the overall benefit of hydrogen enrichment, CO and HC emissions need to be minimized along with NOx while maximizing brake thermal efficiency and power. As demonstrated previously in Fig. 4, a slight rise in CO emissions occurs immediately before the LOL is reached due to onset of combustion instabilities followed by an exponential rise after the LOL. Practically, this means that 40% hydrogen enrichment will likely need to be run at an equivalence ratio of 0.55, about 4% higher than the LOL equivalence ratio, in order to yield NOx emissions that will meet the proposed 0.22 g/kWh BACT standard. At this equivalence ratio the BTE for HLFG40 was approximately the same as for LFG combustion at near stoichiometric with similar CO emissions, but also has a 15% decrease in brake power. When running at a lower equivalence ratio, the output power of the engine will be decreased if the compression ratio (or intake pressure) is not increased at the same time. As mentioned above, similar power outputs can be realized for
Fig. 6 – Comparison of peak brake thermal efficiency for various gas mixtures.
LFG compared to CNG engines if the compression ratio is increased. In this study, the compression ratio was constant; however, the engine de-rating was decreased with added hydrogen, as shown in Fig. 9. This occurred because hydrogen, with an increased lower heating value, replaced the LFG, supplying more chemical energy per mol increasing power output. The drawback of an increased compression ratio is a potential for increased NOx emissions due to a higher combustion temperature; more research is needed to determine the effects of compression ratio, supercharging, and hydrogen enrichment on emissions in ultra-lean LFG combustion. Engine-out emissions in the particular engine tested indicate that current emission standards for NOx emissions for LFGTE projects can be met with 40% hydrogen enrichment with no aftertreatment while retaining a high overall efficiency. If lower NOx emissions are required, higher levels of hydrogen enrichment may be used to further extend the LOL. For example, with the engine used in this study, H40LFG running at an equivalence ratio of 0.55 will meet the current BACT standards for NOx emissions. To meet the future BACT standards for NOx in this particular engine, H50LFG running at an equivalence ratio of 0.48 is required.
Fig. 8 – NOx emissions vs. overall efficiency; points excluded for clarity.
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Fig. 9 – Engine de-rating of hydrogen-enriched LFG.
Hydrogen enrichment of LFG shows promise for reducing emissions in LFGTE projects. These reduced emissions may enable further utilization of LFG for energy production where it is now restricted by emissions regulations. It should be stressed that hydrogen production energy input and cost also needs to be accounted for in the overall balance. This study focused on a normally-aspirated spark-ignition test engine of relatively small capacity (0.745 L). Although the results presented here offer valuable data on the general effects of hydrogen enrichment on NOx and CO emissions, and efficiency, it is important to note that typical LFG-fired engines differ from the test engine in two important ways: 1. Most LFGTE engines are turbocharged, making up for loss of power due to dilution. 2. LFGTE engines typically have a much larger power (100– 500 kW and above) with corresponding larger displacements. In order to prove the viability of HLFG as NOx reduction strategy a demonstration system should be built and tested using a typical LFGTE setup as described above.
11.
Conclusion
The combustion characteristics of hydrogen-enriched landfill gas in a small-displacement two-cylinder spark-ignition engine have been experimentally quantified. The lean operating limit was determined for each hydrogen-landfill gas mixture using a stability criterion based on the COV of IMEP, BTE, and CO emissions. Engine power output and de-rating were also recorded. Furthermore, NOx emissions were recorded and compared to current and future BACT emission standards for waste gas combustion. Hydrogen addition to landfill gas was shown to extend the lean operating limit while retaining low CO emissions within 10% of baseline landfill gas combustion. In this study, hydrogen enrichment up to 50% by volume resulted in an extension of the LOL equivalence ratio by almost 40%, from 4 ¼ 0.8 to 4 ¼ 0.49.
Results indicate that greater amounts of hydrogen addition can maintain the COV of IMEP, CO emissions, and the BTE at lower equivalence ratios. Hydrogen enrichment also enabled similar power to be produced at a lower equivalence ratio. Hydrogen enrichment increased BTE for LFG from 26.8% at an equivalence ratio of 0.84 to a maximum of 28.9% at an equivalence ratio of 0.68 for H50LFG. Furthermore, brake thermal efficiencies above 28% could be retained for mixtures with equivalence ratios as low as 0.62 with 50% hydrogen. At the lean operating limit, the brake thermal efficiency for H40- and H50LFG were 23.5% and 22.7%, only slightly lower than the maximum efficiency measured when operating with landfill gas. Engine-out NOx emissions were shown to decrease with decreasing equivalence ratios and hydrogen extended the LOL allowing further reductions. These results in a smalldisplacement engine with simulated landfill gas indicate that current and future BACT standards for NOx emissions with can be met with forty and fifty percent hydrogen enrichment respectively.
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