CH₄ and CO₂ Reductions from Methanol Production Using Municipal Solid Waste Gasification with Hydrogen Enhancement

Abstract

This study evaluates the greenhouse gas (GHG) impacts of converting municipal solid waste (MSW) into methanol, focusing on both landfill methane (CH₄) emission avoidance and the provision of cleaner liquid fuels with lower carbon intensity. We conduct a life cycle assessment (LCA) to assess potential GHG reductions from MSW gasification to methanol, enhanced with hydrogen produced via natural gas pyrolysis or water electrolysis. Hydrogen enhancement effectively doubles the methanol yield from a given amount of MSW. Special attention is given to hydrogen production through natural gas pyrolysis due to its potential for lower-cost hydrogen and reduced reliance on renewable electricity compared to electrolytic hydrogen. Our analysis uses a case study of methanol production from an oxygen-fired entrained flow gasifier fed with refuse-derived fuel (RDF) simulated in Aspen HYSYS. The LCA incorporates the significant impact of landfill methane avoidance, particularly when considering the 20-year global warming potential (GWP). Based on the LCA, the process has illustrative net GHG emissions of 183 and 709 kgCO_2e/t MeOH using renewable electricity for electrolytic hydrogen and pyrolytic hydrogen, respectively, for the 100-year GWP. The net GHG emissions using 20-year GWP are −1222 and −434 kgCO_2e/t MeOH, respectively. Additionally, we analyze the sensitivity of net GHG emissions to varying levels of fugitive methane emissions.

1. Introduction

Avoiding methane emissions from landfilling of municipal waste and the production of low-carbon liquid fuels and chemicals are two important needs for decarbonization of methane-emitting practices and the transportation and chemical sectors. This study examines the hydrogen-enhanced conversion of municipal solid waste (MSW) into methanol as a means for meeting these goals since it roughly doubles the amount of methanol that can be produced from a given amount of MSW. We analyze the use of hydrogen enhancement with emphasis on the use of hydrogen from natural gas pyrolysis.

Converting MSW to renewable fuel through hydrogen enhancement of gasification syngas not only provides a lower-carbon fuel source but also mitigates greenhouse gas (GHG) emissions by avoiding methane generation from landfills. Given that methane is a potent GHG, with significant short-term global warming potential (GWP), the reduction in methane emissions from landfilling has a large near-term impact on GHG reductions.

In 2018, the United States generated 292 million tons of MSW (short tons, 265 million tonnes), with around 50% landfilled across 1278 sites [1]. Of the generated MSW in 2018, approximately 69 million tons (short tons, 63 million tonnes) were recycled (i.e., 23.6%), and more than 34 million tons (short tons, 31 million tonnes) (i.e., 11.8%) were incinerated with energy recovery. Globally, approximately two billion tonnes of MSW are produced each year, with 37% landfilled, 33% openly dumped, and only 19% recycled or composted [2]. Additionally, 11 percent is subject to incineration as the final disposal method [2].

In 2021, landfills were the third-largest contributor to U.S. anthropogenic CH₄ emissions, producing 122.6 million metric tonnes of CO₂-equivalent emissions per year based on the 100-year Global Warming Potential (GWP) for methane and making up 15.5% of total CH₄ emissions [3]. When considering the 20-year GWP, the potential impact from the avoidance of methane emissions from all US landfilling rises to an illustrative equivalent of 315 million metric tonnes of CO₂-equivalent emissions per year. Greater attention is being given to the use of the 20-year GWP because of increasing concerns about near term impacts of GHG emissions. For example, in a 2021 public letter to the Special US Presidential Envoy for Climate and National Climate Advisor, a group of scientists pointed out that the actual number of GHG produced in landfill per year could be more than a factor of two greater than the reported number [4]. Methane in the landfill is generated with a time delay from the time of landfilling of waste.

Following separation of the organic fraction from inorganic fraction of MSW and then separating recyclable and non-recyclable fractions, the remaining fraction of MSW can be utilized to generate heat and electricity by incineration or to produce fuels/chemicals.

Incineration of the non-recyclable fraction of MSW is presently employed to generate heat that is used for electricity generation. The incineration of MSW comes with concerns about air pollution, as it releases pollutants that include dioxins, NO_x, particulates, etc. [5]. In addition, because incinerator ash is traditionally disposed of in landfills, there are concerns about potential leaching of contaminants from the ash into the surrounding soil and water [6]. Although there has been significant progress in reducing air pollution from large incinerators, gasification by its nature produces less air pollution and can reduce the issue of hazardous ash (which can be turned into stable non-leachable glass-like material and melted metals with some gasification technologies). In addition, although there are a number of avenues for producing low-carbon electricity (e.g., wind, solar, nuclear power) as alternatives to electricity from incineration, there is substantially limited resource base for producing low-carbon liquid fuels and chemicals. Moreover, the use of hydrogen-enhanced MSW gasification discussed here can provide twice as much liquid fuel from a given amount of renewable carbon, which is in limited supply for making low-carbon fuels and chemicals. These issues and the need for low-carbon liquid fuels and chemicals have motivated consideration of converting MSW into low-carbon fuels and chemicals by gasification.

In MSW gasification processes, feedstock is partially oxidized at high temperatures in the presence of a controlled amount of oxygen. The syngas (a mixture of H₂ and CO) is then converted to fuel or chemical by catalytic reactions. There are a number of reviews in the literature regarding gasification of MSW, for example [7,8,9].

Adding low-carbon H₂ to the syngas from MSW gasification provides an attractive pathway for producing liquid fuel from MSW. Liquid fuel using externally provided H₂ can be thought of as a way of storing H₂ or electricity. Since the MSW gasification syngas is H₂-poor, additional H₂ is added to syngas to adjust it for synthesis of methanol or another fuel, known as H₂ enhancement. This approach doubles the carbon efficiency of the process relative to the conventional approach of utilizing a Water Gas Shift (WGS) reactor [10,11]. H₂ enhancement renders the use of WGS unnecessary, and CO₂ is not released in the process. In recent years, there has been a growing interest in enhancing MSW gasification to fuel using hydrogen.

Table 1. Recent studies of MSW gasification with H₂ enhancement to fuel (non-exhaustive).

Study	Gasification Method	H2 Enhancement Method *	Product	Year
Iaquaniello et al. [12]	High-temperature gasifier	Electrolysis	Methanol	2017
Materazzi & Holt [13]	Fluidized bed gasifier (FBG)	Electrolysis (PEM)	C1–C4 hydrocarbons	2019
Pan et al. [14]	Downdraft gasifier	Electrolysis (SOEC)	Synthetic natural gas (SNG)	2019
Salladini et al. [15]	–	Electrolysis	Methanol	2020
Ostadi et al. [10]	Entrained flow gasifier	Electrolysis (SOEC)/natural gas pyrolysis	Methanol	2022
Ku et al. [16]	Plasma gasifier	Electrolysis (SOEC)	Synthetic natural gas (SNG)	2023
Sun & Tang [17]	–	Electrolysis (SOEC)	Methane/methanol/dimethyl ether (DME)	2023
Aldamigh et al. [18]	–	Electrolysis	Methanol	2023
Wang et al. [19]	Plasma gasifier	Electrolysis (AEL)	Methanol/electricity	2023

* PEM: proton exchange membrane electrolysis; SOEC: solid oxide electrolysis cell; and AEL: alkaline electrolysis.

presents some of the studies reflecting this trend. All these studies consider H₂ enhancement through electrolysis, except our previous study [10] where we investigated flexible use of both electrolytic and pyrolytic H₂ enhancement.

Several studies have been conducted on the life cycle GHG emissions from converting MSW into liquid fuels (such as methanol, Fischer–Tropsch fuel, and aviation fuel) using gasification without hydrogen enhancement [20,21,22,23]. These studies include a range of different parameters and assumptions about gasification processes and the methane released from landfilling. However, none of these references have performed a Life Cycle Assessment (LCA) of the integration of natural gas pyrolytic hydrogen with MSW gasification for fuel production.

Methanol is one of the easiest and most efficiently synthesized liquid fuels that can be produced from synthesis gas. It is both an important chemical commodity and a fuel that can be efficiently used in internal combustion engines with high efficiency and lower air pollution. Methanol can be used as a high-octane fuel in spark ignition engines or as a fuel in compression-ignition engines that also use a small amount of diesel fuel to provide ignition [24]. It can also be converted into other fuels/chemicals including dimethyl ether (DME), which can be used as substitute for propane. In addition, methanol can be converted to gasoline or sustainable aviation fuel (SAF) via processes such as methanol to gasoline (MTG) [25], methanol to olefins (MTO) [26], or methanol to sustainable aviation fuel (MTSAF) [27].

An additional factor when assessing the potential impact of hydrogen-enhanced gasification of MSW to fuel is that one of the outcomes of the COP28 was the pledge by many countries and companies to reduce oil and gas production fugitive methane emissions to 0.2% [28] by 2030. Some companies (e.g., Equinor) have already achieved low fugitive emissions as low as 0.02% in upstream and midstream assets prior to introducing gas to long-distance pipeline [29,30]. Lower fugitive emissions from natural gas systems could substantially increase the GHG reduction benefit of using hydrogen from natural pyrolysis for enhancement of fuel produced from biomass gasification [31].

In this paper, we conduct a life cycle assessment (LCA) of GHG emissions from MSW conversion to methanol via gasification with hydrogen enhancement of syngas. We analyze both electrolytic and pyrolytic hydrogen enhancement. Production of hydrogen from natural gas pyrolysis can provide advantages of lower cost and reduced renewable electricity requirement. Process simulations are performed in Aspen HYSYS for both processes: MSW gasification with Electrolytic hydrogen enhancement (i.e., ME process), and MSW gasification with Pyrolytic hydrogen enhancement (i.e., MP process). The paper gives particular attention to the impacts of the level of fugitive natural gas emissions on life cycle analysis. This study provides valuable insights into the potential benefits of integrating natural gas pyrolysis or water electrolysis with MSW gasification for sustainable fuel production.

Section 2 of the paper discusses process concepts, outlining the integration of hydrogen from pyrolysis and electrolysis to MSW-to-methanol process. Section 3 discusses the life cycle assessment that evaluates net GHG impacts. Section 4 presents the conclusions and summarizes the main findings of the paper.

2. MSW-to-Methanol Processes

Figure 1 shows the typical fate of MSW in common landfilling practice. After separation of non-recyclables and diversion for incineration, the remaining MSW is sent to landfill. The fates of the landfilled carbon are landfill-sequestered carbon (solid carbon that does not degrade into a gas), landfill gas (which is captured and either flared or converted into electricity and heat) or emitted mainly as CH₄ or CO₂ into the atmosphere.

Thermochemical gasification conversion of MSW to methanol with increased methanol production using hydrogen enhancement is considered here as a potential path for valorization of MSW. Relative to methanol produced from natural gas (or coal), this process reduces GHGs by using the biomass-based carbon and hydrogen in MSW and by adding low-carbon hydrogen. Relative to the landfilling of MSW, its conversion to methanol also reduces greenhouse gas by avoidance of methane that could otherwise be emitted from landfilling.

Figure 2 shows an overview of both proposed processes, which are MSW gasification with Electrolytic hydrogen enhancement (i.e., ME process) and MSW gasification with Pyrolytic hydrogen enhancement (i.e., MP process). In the proposed processes, the non-recyclable MSW is pretreated and then converted into Refused Derived Fuel (RDF). The production efficiency of RDF from MSW varies depending on the composition of the MSW and the conversion process [32]. Pretreatment steps can include drying, size reduction, screening, sorting, and, in some cases, palletization [32]. The elemental composition of RDF that is used [33] is shown in

Table 2. Elemental composition of RDF (wt.%).

Carbon	52.11
Hydrogen	7.40
Nitrogen	0.85
Sulfur	0.46
Chlorine	0.07
Oxygen	39.11
LHV (MJ/kg) (5% wet)	19.6

.

In our illustrative study, RDF, or a possibly more refined version of RDF, is fed to an oxygen-fired entrained flow gasifier (EFG). The suitability of use of RDF as feed to an entrained flow gasifier or other gasifiers needs to be tested and proved. This may require further pretreatment to make RDF suitable to be used as an EFG feed. An EFG gasifier, which is fed with 38.7 t RDF/h (211 MW_th LHV), is used as an illustrative case. The gasifier produces synthesis gas (a mixture of mainly H₂ and CO), which is subsequently converted to methanol.

The syngas from the EFG is poor in hydrogen, and therefore, externally produced H₂ is added. Two processes are investigated: natural gas pyrolysis and steam electrolysis. For the case of electrolysis, solid oxide electrolysis cell (SOEC) was chosen due to favorable power consumption and the availability of excess heat from the gasification process [11].

The SOEC simulation is based on adiabatic steam splitting, with 80% of the steam converted into hydrogen and oxygen. The SOEC is modeled using a conversion reactor along with a component splitter. In our study, the natural gas pyrolysis reactor is modeled using a conversion reactor that transforms methane into hydrogen and carbon black. The technical details of the process modeling of the alternative hydrogen production methods and simulation are provided in our previous publication [10], and therefore, the technical details are not repeated here.

This study evaluates one illustrative implementation of the concept, being aware that there are different sizes of conversion plants, different gasification technologies, and other means of providing low-carbon hydrogen such as natural gas reforming with carbon capture and storage (CCS). We used an illustrative gasifier type and size to show effectiveness of this approach.

Based on the economic estimation performed in our previous works [10,31], an illustrative levelized cost of methanol from MSW could be around 400–500 USD/t MeOH. From an economic aspect, relative to other biomass wastes, processing MSW has the advantage of avoiding a waste disposal fee (a “tipping fee”), which adds to the competitiveness of the produced fuel. Tipping fees in 2021 in the US were mainly in the range of 45 to 80 USD/t (and over 100 USD/t in some locations). The average tipping fee in the US was 60 USD/t [34].

Process simulations in Aspen HYSYS have been performed for both processes: MSW gasification with Electrolytic hydrogen enhancement (i.e., ME process), and MSW gasification with Pyrolytic hydrogen enhancement (i.e., MP process). Simulation results from our previous publication [10] that are relevant for the LCA are reproduced in

Table 3. Feedstock requirement in both pathways.

	MSW Gasification with Electrolytic H2 Enhancement (ME)	MSW Gasification with Pyrolytic H2 Enhancement (MP)
RDF (5% wet) (t RDF/t MeOH)	0.8	0.8
Electrical power needed (MWh/t MeOH)	3.34	~0 *
Steam needed (MMBtu/t MeOH)	0.25	0.00
Natural gas (t/t MeOH)	0.0	0.5
Carbon byproduct (t/t MeOH)	0.0	0.3
Methanol production (t MeOH/h)	48.4	48.4

* Because of power produced in steam turbine.

. These are important inputs to the LCA, which is described in Section 3. Table 3 shows the feedstocks required for the production of 1 t of methanol from both ME and MP processes. It is assumed that the MSW-derived RDF has a moisture content of 5 wt.%. The steam is needed for the SOEC (solid oxide electrolysis cell that produces hydrogen and oxygen). The electrical power needed includes the SOEC, compressors, and the air separation unit (ASU). The process also produces limited power by running a steam turbine with steam generated by the waste heat. Note that much more power is needed in the electrolytic enhancement case than pyrolytic enhancement case, reflecting the different power intensities of the two processes. Conversely, the pyrolysis process requires natural gas.

3. Life Cycle Assessment

3.1. Life Cycle Assessment Methodology

We conducted a life cycle assessment (LCA) to assess GHG emissions of methanol production with hydrogen enhancement through electrolytic and pyrolytic hydrogen production pathways on a common basis. GREET 2021 model, developed by Argonne National Laboratory [35], was used for conducting the LCA. The cradle-to-grave (CTG) life cycle scope covers four stages, as shown in Figure 3: stage 1—emissions originating from the upstream supply chains of electricity, natural gas, and MSW; stage 2—onsite GHG emissions from methanol production, including emissions related to the production process; stage 3—byproduct carbon displacement and avoided methane emissions from landfilling avoidance; and stage 4—methanol transportation, distribution, and utilization of methanol as fuel for internal combustion engine vehicles. The comprehensive data utilized in this study are presented in

Table 4. Life cycle assessment data and references.

	Electricity supply chain emissions (g/MWh)	NG supply chain emissions (g/t NG)
	Stationary used: US Mix	NG as stationary fuels
CH4	900	9700
N2O	8	63
CO2	410,000	3,000,000
	Vehicle operation emissions (g/t MeOH)	Byproduct carbon displacement (g/t carbon)
	MeOH
CH4	75	CO2	3,666,667
N2O	20
CO2	1,400,000
Global warming potentials of greenhouse gases: benchmarking against CO2
	100 Years	20 Years
CH4	30	85
N2O	265	264
CO2	1	1

from the GREET 2021 model. The greenhouse gas emissions considered in this analysis include CO₂, CH₄, and N₂O, using their 20-year and 100-year GWP, as outlined in Table 4. Figure 3 shows the cradle-to-grave boundary of the LCA study, and the main components and feedstocks.

The cradle-to-grave greenhouse gas (GHG) emissions are expressed per 1 t of methanol produced. To calculate the emissions at each stage, the key mass and energy flows from the process simulation model (e.g., Aspen HYSYS) are multiplied by the respective emission factors listed in Table 4. The upstream emissions from MSW include all activities related to transportation, preprocessing, and handling. For supplying electricity, two cases are considered: the present US grid electricity mix and 100% renewable electricity. GREET’s numbers are used for the US electricity mix, as shown in Table 4.

In our LCA accounting, biogenic CO₂ refers to carbon dioxide emissions that would have been released from combustion of biogenic sources, as opposed to fossil sources. In the case of converting MSW to liquids, biogenic CO₂ primarily comes from the decomposition or processing of biogenic materials (such as biogenic components in food waste and paper waste) that are part of the municipal solid waste stream. In the figures presented in the LCA results section (Section 3.2), the solid green bar indicates the biogenic fraction per t of methanol produced. The ratio of the biogenic CO₂ to the CO₂ generated from methanol combustion is around 28%. This low ratio is due to the assumed pretreatment and the initial composition of MSW. The illustrative RDF that is considered has a substantial fraction of plastics and a relatively small amount of biogenic waste. “MeOH combustion” refers to emissions produced by a vehicle running on methanol, based on a modification of gasoline vehicle operations, assuming 85% of the volatile organic compounds (VOCs), 115% of the miles per gallon gasoline equivalent (MPGGE), and 100% of other emissions [35]. “Onsite emissions”, including process emissions, are assessed using simulations conducted in Aspen HYSYS. “Byproduct carbon” refers to the solid carbon generated in natural gas pyrolysis process, which can be safely stored. It is assumed that the avoidance of methane emissions (due to “landfilling displacement”) in the case of MSW is 32.4 kg of methane per t of landfilled MSW [36]. This representative number for methane emissions would vary depending on the efficiency of the methane capture from landfills or open dumps. “Landfill displacement” refers to the reduction in CO₂ emissions due to diversion of waste materials from landfills to more sustainable disposal methods or to their use as feedstocks for producing energy or materials. When organic waste decomposes anaerobically (without oxygen) in landfills, it generates methane (CH₄), a potent greenhouse gas. By diverting waste from landfills and converting it into liquid fuels, these methane emissions can be significantly reduced or eliminated. The term “displacement” is used because the process essentially avoids the generation of methane in landfills. Additionally, the process displaces fossil fuels by use of a renewable energy source (the renewable carbon in the biogenic part of MSW), further reducing CO₂ emissions associated with energy production.

3.2. Life Cycle Assessment Results

The results for using the current US electricity mix are presented in Figure 4, considering both 100-year and 20-year GWP. In this figure, the pathway exhibiting the lowest LCA emissions is the hydrogen enhancement derived from natural gas pyrolysis (MP). The common-sized bars with equal size in both ME and MP processes are related to upstream MSW, methanol combustion, biogenic fraction of carbon, and landfill displacement. For both 20-year GWP and 100-year GWP, the ME process using average grid mix electricity has about twice the LCA GHG emissions as the MP process using an assumption of 1% fugitive methane emissions. Using the 100-year GWP, emissions are about 1000 kgCO_2e/t MeOH higher than using the 20-year GWP for both ME and MP processes. Natural gas fugitive emissions have an important impact on the overall GHG emissions in the MP process. In the ME process, a large portion of the GHG emissions is from the electricity supply chain, given the current electricity mix in the US. The bar labeled “Potential CO₂ from inlet natural gas combustion” accounts for the CO₂ equivalent of natural gas that is used for methane pyrolysis.

For both processes, the avoided landfill GHG emissions (labeled “landfill displacement” in Figure 4) for the 20-year GWP are higher than 100-year GHG emissions due to higher GWP of CH₄ for 20 years. In case of the 20-year GWP, the CO₂ equivalent avoidance from landfill displacement has a large impact on the LCA. It is around the same amount as the CO₂ produced from production and use of natural gas-based methanol (2500 kgCO_2e/t MeOH [37]). Without the landfill effect, the LCA results will have positive numbers for the GHG emissions.

Considering the 20-year GWP, the MP process results in net negative GHG emissions. The negative LCA values indicate that through the life cycle of the fuel, GHG emissions to the atmosphere are reduced. The dominant effect for this reduction is the avoidance of methane emissions from landfilling, followed by a decreasing emission from electricity production.

Figure 5 displays the results obtained from using renewable electricity for both 100-year and 20-year GWP. For the MP process, having low natural gas fugitive emissions is important; however, for the ME process, having a renewable electricity source rather than the present grid energy mix is key to reducing the GHG emissions, as shown in Figure 5. ME has about three times lower GHG emissions than the MP process. The ME process exhibits significant potential for decarbonization with renewable electricity. Landfill displacement has a huge impact in 20-year GWP, resulting in negative GHG emissions for both processes.

For the MP process, the net GHG is primarily influenced by GHG emissions of fugitive natural gas, with an illustrative value of ~1% of the natural gas used is released upstream of the process. The impact of varying levels of fugitive methane emissions on net greenhouse gas emissions is analyzed in the Section 3.3.

3.3. Impact of NG Supply Chain Fugitive Emissions Percentage

The natural gas supply chain is assumed to have fugitive CH₄ emissions of 9700 g per t of natural gas, as presented in Table 4. This corresponds to about 1% of the CH₄ being released into the atmosphere.

After the COP28, countries have made a pledge to reduce fugitive emissions to 0.2% by 2030 [28]. This would have a large impact on the LCA results for the pyrolytic H₂ enhancement case (MP). The fugitive emissions percentage does not affect the ME process, as it is not a feedstock for that process. The impact of natural gas fugitive emissions on the LCA of the MP process is shown in Figure 6. It shows the GHG emission of the MP process with three different natural gas fugitive emissions for the 20-year GWP. The only item that changes in this figure is natural gas fugitive emissions, and other categories are unchanged. If the fugitive natural gas emissions from the oil and gas industry could eventually be limited to 0.2%, the net negative GHG emissions could eventually be doubled, assuming 20-year GWP.

The impact of a wider range of natural gas fugitive emissions on the GHG of MP process for both 20-year and 100-year GWP using renewable electricity is shown in Figure 7. For the fugitive emissions less than 2%, the 20-year impact is negative, meaning the produced fuel contributes to GHG emissions reduction from the atmosphere; meanwhile for fugitive emissions more than 2%, the 20-year impact becomes positive, and for percentages above 5%, the 20-year impact exceeds the 100-year impact. With 8% fugitive emissions, the GHG emissions with 20-year impact are roughly close to the LCA of methanol from conventional natural gas-based methanol (2500 kg CO_2e/t MeOH [37]). The 100-year impact is always positive, meaning the fuel adds to the inventory of GHGs in the atmosphere.

3.4. Potential Impacts

When additional hydrogen is produced through natural gas pyrolysis, Figure 5 shows that for the 20-year methane GWP, the net emissions of GHG are −434 kgCO_2e/t MeOH, assuming fugitive methane emissions of 1%. This negative net emissions value is mainly due to landfill methane avoidance along with a smaller effect from biogenic fraction in MSW. Using the conversion factor of 0.8 t RDF per t MeOH, as given in Table 3, for the 20-year methane GWP, the net GHG emissions are −542 kgCO_2e/t RDF. When the hydrogen is produced from electrolysis using renewable electricity, the net GHG emissions are −1222 kgCO_2e/t MeOH and −1528 kgCO_2e/t RDF.

A representative number for all of the MSW that is landfilled in the US per year is 133 t [1]. For an illustrative case of 50% of this MSW (66 Mt/y) being converted into methanol, there would be net emissions of −36 MtCO_2e/y with hydrogen enhancement from natural gas pyrolysis (based on Figure 5, using 20-year GWP). For hydrogen produced from electrolysis with renewable electricity, the net emissions would be −101 MtCO_2e/y. These negative numbers mean that the methanol production from MSW that is redirected from landfilling can provide a greenhouse gas reduction relative to the present landfilling process.

The amount of methanol that would be produced from 66 Mt of MSW is 27.7 billion gallons, which is equivalent to 12.6 billion gallons of diesel fuel. This is around 40% of yearly diesel fuel used for US long-haul heavy-duty trucks [38].

If the 27.7 billion gallon/y of methanol was to replace fossil-based diesel fuel, there would be a 126 MtCO_2e/y additional reduction in GHG relative to present practice of using fossil-based diesel fuel. Thus, for hydrogen produced from natural gas pyrolysis, the total greenhouse gas reduction relative to present practice would be (−126 + (−36)) = −162 MtCO_2e/y. For hydrogen produced from electrolysis using renewable electricity, the reduction relative to present practice would be −227 MtCO_2e/y.

Regarding social impacts of converting MSW to fuel instead of landfilling it, there are a number of important potential beneficial impacts. Firstly, it could reduce local air, water, and soil pollution of present landfilling sites. Secondly, it could reduce the need for new landfills. Thirdly, diversion of MSW to facilities that produce clean fuels could reduce the amount of open dumping and its adverse impacts including plastic pollution of drinking water and oceans. Additionally, the cleaner fuel that is produced could reduce air pollution, especially air pollution from heavy-duty trucks. As previously mentioned, reducing landfilling contributes to lower GHG emissions, particularly methane, thereby supporting global climate change mitigation efforts. In addition, it can also remove negative health impacts on vulnerable communities. However, in implementing this potential shift towards better technological solutions of MSW management, it is important to prevent unintended consequences such as higher levels of waste production if people believe that waste management is adequately addressed through fuel conversion technologies.

There are remaining technological and economic barriers to implementing these processes on a commercial scale. First, MSW is a heterogeneous material, and its variable composition can impact the process. Second, the use of RDF as feedstock for entrained flow gasifiers or other gasification technologies needs further testing and validation. While pilot projects exist, large-scale implementation of MSW-to-fuel technologies is relatively new and currently lacks a proven track record for widespread commercial deployment. Additionally, the technological complexity of the MSW gasification-to-fuel process could hinder its implementation especially in low-income countries.

4. Conclusions

We analyzed the potential life cycle greenhouse gas reductions in the US from conversion of MSW to methanol using gasification with hydrogen enhancement through electrolysis or natural gas pyrolysis. Hydrogen-enhanced conversion provides a factor of two increase in the amount of methanol produced from a given amount of MSW.

Our life cycle analysis (LCA) determines the GHG reduction form production of methanol from MSW when a 20-year methane global warming potential (GWP) is considered as well as the more standard 100-year GWP. It also includes an assessment of the effects of the natural gas supply chain fugitive emission ranging from 0.2% to 8%. This assessment shows the importance of limiting the fugitive methane emissions to around 1% or less.

Life cycle greenhouse gas emissions are determined for an illustrative entrained flow gasifier, which processes 38.7 t RDF/h (211 MW_th LHV) produced from MSW. Based on this analysis, for an illustrative case where 50% of the US MSW that is landfilled per year is converted into methanol, the conversion of this 66 million t/y (Mt/y) would produce 27.7 billion gallons/y of methanol (equivalent in energy to 12.6 billion gallons of diesel fuel). Relative to the present practice of fossil-based diesel fuel use and MSW landfilling, the overall net greenhouse gas emissions from producing this methanol from MSW that is redirected from landfilling are reduced by 162 MtCO_2e/y for hydrogen produced from natural gas pyrolysis (with 1% fugitive methane emissions), and they are reduced by 227 MtCO_2e/y for hydrogen produced from electrolysis using renewable electricity. This type of analysis could be expanded to other regions of the world.

Avoidance of landfilling of two billion tons of MSW per year worldwide is needed to address increasing concerns about methane emissions, particularly when the 20-year GWP is considered. It is also needed to prevent the soil, water, and air pollution that landfilling produces. Hydrogen-enhanced conversion of MSW to methanol could potentially serve as a means to address these issues and provide valuable clean fuel alternatives to diesel and other fossil fuels. Realization of full potential will involve progress in access to renewable electricity and natural gas pyrolysis with lower fugitive methane emissions.