(343i) Gaseous Fuels Derived from Landfill-bound Waste- MSW and Plastics- for a Stationary Power Generation Engine: A Performance and Emissions Analysis

In the United States, the predominate end-of-life fate of municipal solid waste (MSW) is landfill with 50% being landfilled, and only 23% being recycled in 2018 (1). Landfilling such volume of waste poses several issues. First, burying the waste precludes its further use as a valuable feedstock for energy production (2, 3, 4), or for valuable extractable materials (5, 6). Second, anerobic decomposition of the buried waste in MSW landfills produces landfill gas (LFG), which, if permitted release to the atmosphere will contribute to the greenhouse gas effect due to the presence of CH4 and CO2.

Municipal solid waste and landfill gas:

Landfill gas is generated in U.S landfills at a rate of 24.2 to 59.1 giga-Nm3 per year (7). LFG uniquely presents both a problem and solution with respect to global warming. Standard practice at landfill facilities is to flare the gas, the products of such combustion having a lower global warming potential than the original gas (8). Without the utilization of the energy released during flaring, there presents a missed opportunity to use the LFG as a working fuel. Direct use of LFG in internal combustion (IC) engines is hindered by low and varying energy density of the fuel. Thus, additions of higher energy density fuels, such as syngas, composed of H2 and CO, improve the flame stability and decrease emissions such as NOX, CO, and unburned hydrocarbons (UHCs) (7).

Plastic waste and pyrolysis gas:

Present in MSW streams are discarded plastics. An estimated 9% of plastics produced are recycled (1). Double digit national recycling rate of plastics has not been achieved as of 2018, leaving the majority of plastic unrecycled as waste. The conversion of end-of-life plastic into liquid fuels via pyrolysis thermal treatment has been a topic of great interest in recent years (9, 10, 11). However, there has been a relative gap in studies on the use of the gaseous products of pyrolysis for use in IC engines for power generation. Co-location of power-generating engines at pyrolysis facilities could aid to offset process energy requirements, thus using waste instead of traditional non-renewable sources for energy.

OBJECTIVES

A variety of gaseous fuel blends having a base of simulated LFG or simulated plastic pyrolysis gas were used to operate a four-stroke spark ignition engine.

With a robust data matrix covering a wide band of realistic operating conditions, including engine load, three objectives were achieved.

1 - Characterization of the fuel blends to quantify emissions.

2 - Identification of lower limit for engine operation (stall condition) for each fuel blend.

3 - Creation of a robust dataset whereby given the system inputs (LFG or pyrolysis gas composition, maximum expected load), a recommended fuel blend can be referenced for suitable engine operation and desired emission targets.

METHODS

A Honda GC160E 1.6 liter displacement IC engine was retrofitted, with the removal of the carburetor intended for liquid fuel and air mixing, and was operated on a gas either representative of a LFG or a plastic pyrolysis gas.

LFG blends at a range of CO2/CH4 ratios representative of field values, which vary naturally due to temperature and waste composition fluctuations (12), were evaluated. Some LFG blends were enhanced with syngas to evaluate the impact on emissions and performance of upgrading the fuel.

Flow rates of the constituent gases for the blends were set using rotameters calibrated with mass flow controllers. Emissions were collected and analyzed with the use of the Enerac 700 emissions analyzer which measures, via a probe which pumps effluent gasses, stack temperature, stack draft pressure, and concentration of O2, UHCs, CO2, CO, NOX, NO, NO2, and SO2. A Pramac EG2800 electric generator was coupled directly to the engine’s power take-off shaft. An electrical circuit comprising of parallel wired light sockets was outfitted with 100W lightbulbs and powered from the generator to apply loading to the engine in increments of 0.2 kW. Engine operation test conditions range from idle (no load) to 2.0kW. A WattsUp Pro99333 power meter was used in-line to verify the power demand.

RESULTS

The discussed experimental setup has been implemented for the LFG-based fuels. Similar experiments on pyrolysis gas fuels are forthcoming.

NOX concentration of the engine exhaust increased with increasing adiabatic flame temperature from 2105K to 2210K. CO and UHC concentrations were found to be coupled. CO and UHCs result from incomplete combustion, which can occur from many causes from poorly mixed air and fuel to lower combustion temperature (7). CO and UHCs increased at engine operation conditions that exhibit poor combustion, such as fuel-starved condition just prior to stalling. The fuel-starved condition resulted in an 878% increase in UHCs compared to the idle condition and was accompanied by degradation in engine operability.

A reduction of the CH4/CO2 ratio by reducing CH4 volume from 84% to 51% greatly increased emissions, with the exception of NOx. This result is supported by a thermal impact on NOx, ignition delay impact on UHC and CO, and dilution effect impact on UHC and CO. NOx emissions decreased by 48% due to a reduced combustion temperature resulting from a large volume of dilutant (49% CO2). CO emissions increased by 40% when CO2 composition increased from 16% to 49% and, more significantly, the UHC emissions increased by 2643%.

Tradeoffs between power and emissions were apparent when comparing fuels with an H2:CO ratio of 0.9:1 and 1:0. Entire CO fraction replacement with H2 did not appreciably change the power generated. This is due to the similar volumetric energy density between H2, 12.76 kJ/L, and CO 12.63 kJ/L. However, that replacement resulted in much higher brake specific (BS) total UHCs. The other measured emissions, BSNOx and BSCO, and the BS fuel consumption were not greatly impacted by replacing the CO fraction with H2.

Iterations of engine testing were completed with simulated LFG of varying (0-55%) CO2 fraction, with varying total syngas addition, and with varying H2 fraction in the syngas. Data was compared across test cases to identify engine operational limits and regions of performance interest.

IMPLICATIONS

When engaging the waste management hierarchy (1), a move from the least preferred method of treatment and disposal (landfill) to a more preferred method of energy recovery, can be possible with direct utilization of either LFG or plastic pyrolysis gas. Compared to status-quo alternatives of flaring LFG (which captures no benefit from its inherent energy content) and of landfilling hard-to-recycle plastics, energy generation offers a more intentional resource use pathway.

Utilization of LFG-based fuels could be of interest especially in countries with high landfilling rates, including the United States. Utilization of plastic pyrolysis gas-based fuels should be investigated as the number of plastic pyrolysis facilities continues to grow (13) to more fully understand the environmental implications. Should such fuels be determined viable for direct use as drop-in power sources, costly changes to existing equipment and infrastructure could be largely avoided.

More widely, the methods outlined could be extended to upgrade other fuels beyond those discussed here.

REFERENCES

1. Olem, National Overview: Facts and Figures on Materials, Wastes and Recycling. US EPA, (2022).

2. About Partners of the Landfill Methane Outreach Program. US EPA, (2022).

3. H. R. Amini, D. R. Reinhart, Regional prediction of long-term landfill gas to energy potential. Waste Management 31, 2020-2026 (2011).

4. L. Levaggi, R. Levaggi, C. Marchiori, C. Trecroci, Waste-to-Energy in the EU: The Effects of Plant Ownership, Waste Mobility, and Decentralization on Environmental Outcomes and Welfare. Sustainability 12, 5743 (2020).

5. M. J. Castaldi, J. LeBlanc, A. Licata, The Case for Waste to Energy. Mechanical Engineering 144, 34-39 (2022).

6. S. C. Gutiérrez-Gutiérrez, F. Coulon, Y. Jiang, S. Wagland, Rare earth elements and critical metal content of extracted landfilled material and potential recovery opportunities. Waste Management 42, 128-136 (2015).

7. M. P. Kohn, M. J. Castaldi, R. J. Farrauto, Auto-thermal and dry reforming of landfill gas over a Rh/γAl2O3 monolith catalyst. Applied Catalysis B: Environmental 94, 125-133 (2010).

8. O. Ayalon, Y. Avnimelech, M. Shechter, Solid Waste Treatment as a High-Priority and Low- Cost Alternative for Greenhouse Gas Mitigation. Environmental Management 27, 697-704 (2001).

9. I. Kalargaris, G. Tian, S. Gu, The utilisation of oils produced from plastic waste at different pyrolysis temperatures in a DI diesel engine. Energy 131, 179-185 (2017).

10. K. Kumar, R. Puli, Effects of Waste Plastic Oil Blends on a Multi Cylinder Spark Ignition Engine. MATEC Web of Conferences 108, 08005 (2017).

11. Khairil et al., Experimental Study on the Performance of an SI Engine Fueled by Waste Plastic Pyrolysis Oil–Gasoline Blends. Energies 13, 4196 (2020).

12. L. Hailing et al., Seasonal CH4 and CO2 effluxes in a final covered landfill site in Beijing, China. Science of The Total Environment 725, 138355 (2020).

13. S. Hann, T. Connock, "Chemical Recycling: State of Play," (Eunomia, 2020).