Volume 42, Issue 4 e14002
ORIGINAL RESEARCH
Open Access

Resources for renewable natural gas: A Hawaii case study

Scott Turn

Corresponding Author

Scott Turn

Hawaii Natural Energy Institute, University of Hawaii, Honolulu, Hawaii, USA

Correspondence

Scott Turn, Hawaii Natural Energy Institute, University of Hawaii, 1680 East-West Rd., POST 109, Honolulu, HI 96822-2234, USA.

Email: [email protected]

Contribution: Conceptualization (equal), Data curation (equal), Formal analysis (equal), ​Investigation (equal), Methodology (equal), Writing - original draft (equal), Writing - review & editing (equal)

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Robert Williams

Robert Williams

Hawaii Natural Energy Institute, University of Hawaii, Honolulu, Hawaii, USA

Contribution: Conceptualization (equal), Data curation (equal), Formal analysis (equal), ​Investigation (equal), Methodology (equal), Visualization (equal), Writing - original draft (equal), Writing - review & editing (equal)

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Wai Ying Chan

Wai Ying Chan

Hawaii Natural Energy Institute, University of Hawaii, Honolulu, Hawaii, USA

Contribution: Data curation (equal), Formal analysis (equal), Methodology (equal), Visualization (equal)

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First published: 23 September 2022

Abstract

Feedstock resources for renewable natural gas (RNG) production by biological (e.g., anaerobic digestion) and thermochemical (e.g., gasification) conversion methods in Hawaii have been reviewed. Statewide estimates of RNG production potential from urban resources (wastewater, existing landfills, foodwaste, construction and demolition waste (CDW), and municipal solid waste) total 8860 TJ year−1. Honolulu has the largest resource base for these urban waste streams. Underutilized agricultural land resources in the state could support substantial RNG production from dedicated energy crops (260–520 GJ ha−1 year−1), although agronomic suitability of specific candidate energy crops would need to be evaluated and confirmed.

1 INTRODUCTION

Renewable natural gas (RNG) is composed primarily of methane derived from carbon of recent biogenic origin, unlike fossil natural gas (NG) that derives from ancient carbon commonly associated with fuels such as coal or petroleum. Either of these latter two resources can be used to produce synthetic natural gas (SNG) by thermochemical energy conversion methods. In general, RNG has lower life cycle greenhouse gas (GHG) emissions than NG. Depending on resource (feedstock) and production method, net GHG emissions for RNG can range from −480 to 66 g CO2eq MJ−1.1, 2 Fossil NG has net GHG emissions of about 70.1 g CO2eq MJ−1.1 The objective of this study is to explore production resources for RNG in Hawaii. The production of RNG makes use of biological or thermochemical conversion processes. Both are described in more detail below. Existing sources of biogenic methane in Hawaii that could be used to produce RNG are explored. Biomass resources that are used as the carbon feedstock for RNG production are also discussed and their occurrence in Hawaii reviewed.

RNG has the potential to directly displace incumbent fossil energy products (substitution) or to be part of a retrofit or new equipment package that would displace both the fossil fuel and end-use conversion technology. An example of the former is substitution of RNG for fossil gas use in process heat applications, whereas an example of the latter is a diesel engine replaced with an engine fueled by compressed RNG.

Hawaii is an island state devoid of indigenous fossil energy resources. For context, Hawaii consumption of fossil energy products with potential for displacement by RNG is approximately7 PJ year−1.3 Details are provided in Supplementary Material Section S1.

2 RENEWABLE NATURAL GAS PRODUCTION

Biological conversion processes typically occur under anaerobic conditions, where biogenic material (substrate) is consumed by a community of bacteria (anaerobes) in anoxic conditions. In the final step of the process, methane-producing (methanogenic) bacteria convert substrate to microbial biomass (i.e., via cell division) and metabolite biogas primarily composed of carbon dioxide (CO2), and methane (CH4). This conversion does not occur with 100% efficiency and some portion of the biogenic material will remain. CO2 and CH4 are gases at ambient temperatures and pressures, and the gas stream from an anaerobic process can be collected for beneficial use or disposal.

Anaerobic production of biogas occurs naturally in anoxic swampy areas and deep ocean sediments, the digestive tracts of ruminants, termites, and oceanic zooplankton,4 and a number of common waste management techniques for high moisture materials, for example, solid waste landfills and digesters designed to treat urban wastewater, livestock manure, or food wastes. Sealed landfills initially contain air, but the oxygen is quickly consumed by aerobic bacteria resulting in an anaerobic environment. Under these oxygen depleted conditions, a bacterial community dominated by anaerobes evolves and biogas production ensues. Modern landfills are designed with systems in place to extract and manage biogas with a lifetime overall recovery efficiency of about 75%.5 Digesters are designed to create and maintain anaerobic conditions for treating and stabilizing waste so that it can safely be returned to the environment or beneficially reused. Digester systems are designed to contain, collect, and manage the biogas byproduct. The potential for materials to produce biogas in a digester system is dependent on the characteristics of the solid material, among other things. Solids content is characterized as total solids (TS) and volatile solids (VS), and the latter is the component that the anaerobic digestion process partially converts to biogas. Volatile solids are determined by dry sample weight loss at 550°C in an oxidizing environment (i.e., Method 2540G in Clesceri et al).6

As noted, CO2 and CH4 are the principal components of biogas, but other compounds may be present depending on the substrate and the design and management of the landfill or digester system. Under the best conditions, CH4 can account for up to 70% of the total gas volume with CO2 as the balance. Under less favorable conditions, the biogas can contain measurable amounts of other compounds derived from the substrate, including moisture, ammonia, sulfur compounds, halogenated compounds, siloxanes, and volatile organic compounds. These compounds can delimit end-use applications, and may have negative impacts on materials, human health, and/or the environment; hence, they can be considered contaminants. Landfill gas collection and digester systems that are poorly sealed may also allow air intrusion, resulting in the presence of oxygen and nitrogen. When RNG is the targeted end product, oxygen (O2), nitrogen (N2), and CO2 can be considered diluents. The presence of O2 is of additional concern as it may result in mixtures that are above the methane flammability limit.

RNG is produced from biogas by removing contaminants and diluents (i.e., “upgrading”) to achieve the gas quality required for a particular application. Fossil NG pipelines specify limits on the amounts of contaminants and diluents (e.g., <3%–5% total inert gas content [i.e., CO2, N2, etc.], <0.2–0.4% O2, <5.7 Mg H2S m−3, etc.) and a range of acceptable values for the Wobbe Index (e.g., 48–52 MJ m−3) and gas energy content (e.g., 35–42 MJ m−3)7, 8 (see Supplementary Material Section S2). Note that pure methane has a gross calorific value of 38 MJ m−3.

Gasification is the primary thermochemical conversion process that can be used to synthesize RNG (sometimes call synthetic RNG or SRNG). Figure S3.1 (figures and tables numbered as SX.Y are found in the Supplementary Material Section X) presents a schematic diagram of the thermochemical RNG production process.9 Gasification is the partial oxidation of biomass (wood, bagasse, regionally available fiber materials, etc.) to form a combustible gas. The goal of the gasification process is to simultaneously maximize the solid fuel carbon conversion and the energy content of the product gas. Air, steam, oxygen, or mixtures of these gases can be used as oxidation agents. The gasification process occurs at temperatures ranging from 700 to 1200°C. When oxygen or air is used to create the heat needed to drive the thermochemical process, oxidizer is limited to ~30% of that needed to support complete combustion. Feedstocks for thermochemical gasification are typically required to have ≤10% moisture content (wet basis). Conversion of carbon present in the fuel should approach 95%. The product gas contains primarily carbon monoxide (CO), CO2, hydrogen (H2), and CH4. Particulate matter and other compounds will be present as contaminants and the latter may include higher hydrocarbons (C2+ and both permanent gases and condensable species), ammonia, hydrogen cyanide, hydrogen sulfide, carbonyl sulfide, thiophene, oxides of nitrogen, chlorides, and other inorganic species. Contaminants pose hazards to materials (e.g., catalysts, heat exchanges surfaces, etc.), human health, and/or the environment. To produce RNG from the product gas, contaminants must be reduced to acceptable levels, the ratio of CO, CO2, and H2 must be adjusted (gas conditioning), and then CO and CO2 are reacted with H2 to form additional CH4 (synthesis/methanation). The methane rich product gas from the synthesis step is upgraded to meet specifications required by the RNG offtaker.

3 LITERATURE REVIEW

Biomass contributes 9.3% of the world's primary energy supply.10 With policies that increase biomass and other renewable energies in developed regions (i.e., the Renewable Energy Directive),11 bioenergy, including RNG, is expected to play a key role in achieving low carbon energy supply.12-14

Consequently, biomass resource estimates, or assessments, are used to inform these and other policies as well as support developer's business plans. Large scale biomass resource assessments have been done including global,15, 16 regional, that is, for the European Union,17, 18 and for large countries.19-21 There are many general biomass resource assessments for smaller countries or districts, often called “case studies”.22-25 Biogas-specific resource case studies, like this investigation, have been produced throughout the world.13, 14, 26-36 Other assessments include temporal availability and geographic distribution of the resource.37, 38 Finally, case studies specific to island economies, as is this work, are in the literature.39-41

4 MATERIALS AND METHODS

This assessment of resources for RNG production in Hawaii included livestock manure, wastewater treatment plants (WWTP), landfill gas, food waste, and energy crops. The State of Hawaii includes the counties of Kauai, Honolulu, Maui, and Hawaii. Livestock manure resources were estimated from Hawaii State Department of Agriculture livestock statistics published at the county level.

The Hawaii State Department of Health, Environmental Health Administration includes a Wastewater Branch and a Clean Water Branch that regulate WWTPs. Data on WWTPs and their capacities were collected from both branches. Each county government includes offices and staff with responsibilities to operate WWTPs and to manage solid waste. Data and information was accessed from these sources and from private companies contracted as WWTP operators.

The US Environmental Protection Agency hosts the Facility Level Information on Greenhouse gas Tool (FLIGHT). FLIGHT provides information about GHG emissions from large facilities in the United States. These facilities are required to report annual GHG emissions data to EPA as part of the Greenhouse Gas Reporting Program. As required, all Hawaii landfills report landfill gas flow rates and methane concentrations under this program.

RNG production potentials from the conversion of renewable resources were estimated using published values from the peer reviewed literature and are cited below at the point of use. Biomass resources for biological and thermochemical conversion processes in Hawaii are summarized in the following sections.

4.1 Biomass resources for biological conversion

Biomass resources in Hawaii that could be used for RNG production via biochemical pathways include animal manure, biosolids/activated sludge at waste water treatment plants, and biogenic components of municipal solid waste (MSW) disposed in landfills.

4.1.1 Livestock manure

Inventories of hogs, cattle and calves, and poultry in Hawaii are summarized in Table 1. Data on the size and number of farms and the inventory of animals on farms can be used to identify opportunities where sufficient manure may be produced to justify onsite anaerobic digestion. Waste management may be a necessary component of a livestock production facility. The Environmental Protection Agency's National Pollutant Discharge Elimination System (NPDES) criteria to identify and classify animal feeding operations is provided in the Supplementary Material Section S4 along with extended data on the 2017 animal population from the 2019 USDA42 census of agriculture in Supplementary Material Section S5.

TABLE 1. Summary of livestock populations and farm sizes in Hawaii, 2017 data.42
Hawaii County Honolulu County Kauai County Maui County State
Farms Head Farms Head Farms Head Farms Head Farms Head
Hogs 93 2252 28 D 25 D 80 1831 226 >8568
Cattle 847 98,851 46 4984 132 15,004 193 19,091 1218 137,930
Poultry 410 >8000 97 (D) 48 >1060 211 (D) 766 228,912
  • Note: (D)—Withheld to avoid disclosing data for individual farms.

Hogs

Although it is not possible to arrive at a total number of hogs in the state from the data in Table 1, it is possible to estimate that the population is at least 8500 head. Using USDA estimates for hog manure production (70 kg average weight, 2.7 kg volatile solids days−1 500 kg animal unit−1, as-excreted basis)43 and methane production from anaerobic digestion (350 L kg−1 of volatile solids),44 the annual potential production of RNG from the Hawaii swine population is estimated to ~15,500 GJ year−1. Note that this is an estimate of potential only, and this value does not reflect what would occur in practice. Production scale (farm size and anaerobic digester [AD] volume), siting considerations, waste collection and management system design, operation, and maintenance all affect actual productivity.

Cattle

Cattle production in Hawaii is focused on beef production rather than dairy and is carried out largely on pasture. The number of animals across the state total ~138,000. Melrose et al45 reported pasture by island that totaled ~308,000 ha across the state. Using these data, average pasture stocking rates of ~2 ha per animal can be calculated. Although it is a generalization that may not reflect management practices of individual producers, the low stocking rate suggests that collecting beef cattle waste for RNG feedstock is not practical under current production practices.

Poultry

Poultry production in Hawaii is focused on chickens that produce eggs and layers accounted for 84% of the total state poultry population (228,912 birds).42 Based on the layer population of the state and a daily production value of 16 g volatile solids per animal per day, the annual manure resource relevant to anaerobic digestion is ~1140 Mg of volatile solids per year. The use of poultry manure in anaerobic digesters is limited by its high nitrogen content and low moisture content46 and these properties may encourage its use as fertilizer. Nonetheless, based on the same set of assumptions used above to estimate RNG potential for hog manure, the annual potential production of RNG from the Hawaii poultry population is estimated to ~15,000 GJ year−1. Note that Rodriguez-Verde et al46 determined that CH4 yield from digested poultry manure was ~45% of the yield from hog manure, but were able to achieve comparable yields by pretreating or blending the poultry waste. As such, attaining this estimated RNG potential in practice would require additional management compared to hog, wastewater, or food waste based systems described below. Production scale (farm size and AD volume), siting considerations, waste collection and management system design, operation, and maintenance all factor into actual productivity.

4.1.2 Wastewater treatment plants

Hawaii currently has ~190 WWTPs, including both public and private facilities serving communities or properties with multi-dwelling units. This does not include cesspools or septic tanks (on site disposal systems) serving individual properties which number more than 100,000 across the state.47 The number and scale (average daily flow) of WWTPs are summarized by county in Figure S6.1. Table 2 summarizes information on treatment plants that receive more than 3800 m3 of wastewater per day. Three facilities on Oahu (Sand Island, Honouliuli, and Kailua) receive volumes in excess of 60,000 m3 days−1. Sand Island, serving central Honolulu, is the largest and treats ~290,000 m3 days−1. Nine WWTPs treat between ~3800 and 18,900 m3 days−1. With the exception of East Honolulu and Schofield, all are public, county-owned facilities. Table 2 also summarizes available data on final sludge generation rate, biogas generation rate and use, methane content of biogas, and potential RNG production amounts. Gross statewide RNG potential from WWTPs (>3800 m3 days−1) is estimated to be ~200 TJ year−1.

TABLE 2. Salient characteristics of WWTPs in Hawaii receiving daily wastewater flows greater than 3785 m3 per day
Name County/ownership Wastewater receiveda (average m3 day−1) Anaerobic digester Biogas production (m3 day−1) Methane concentration (%) Methane production (m3 day−1) Methane production (TJ year−1) Biogas useb
Sand Island Honolulu/public 287,700 Yes 9,570 60 (assumed) 5,740 78 C, D
Honouliuli Honolulu/public 97,300 Yes 8,500 60 5,100 69 B, C, D
Kailua Honolulu/public 61,700 Yes 2,950b 60 (assumed) 1,770 b 24 b C, D
Waianae Honolulu/public 14,400 Yes 800 50–70 480 6.5 D
East Honolulu Honolulu/private 16,700 Yes 1,050 57 600 8.1 D
Schofield Honolulu/private 9,100 Yes 450 60 270 3.7 C, D
Lahaina Maui/public 15,900 No na na na na na
Wailuku-Kahului Maui/public 14,800 No na na na na na
Kihei Maui/public 13,600 No na na na na na
Hilo Hawaii/public 15,900 Yes 765c 60 (assumed) 456c 6.2c D
Kealakehe Hawaii/public 6,400 No na na na na na
Lihue Kauai/public 4,200 Yes 200c 60 (assumed) 120c 1.6c D
  • a Source: 47.
  • b B—RNG (Hawaii Gas), C—combusted for process heat (e.g., biosolids drying or digester heating), D—balance flared.
  • c Assumes 28.7 m3 CH4 per 1000 m3 WW based on the averaged operating data from Sand Island, Honouliuli, Waianae, East Honolulu, and Schofield WWTPs.

4.1.3 Landfill gas

The State of Hawaii has 15 landfills, six of which are closed and and a seventh that is open but no longer receiving waste (Table S7.1).48 Six landfills have gas collection systems in place and produce LFG ranging from 1,560 to 32,000 m3 days−1. In all cases, collected LFG is flared.

Five landfills in the state are identified by US EPA's Landfill Methane Outreach Program48 as energy project candidates; for additional information see Supplementary Material Section S7. Table 3 summarizes information relevant to RNG resources from the six MSW landfills in Hawaii that have LFG collection systems installed with corresponding historic annual methane production values presented in Figure 1. The Central Maui Landfill is the largest producer of LFG (227 TJ year−1 in 2018), has the highest methane concentration (52%), and has had an upward trend in production volume from 2010 to 2018, averaging a 9% annual increase. Waimanalo Gulch Landfill & Ash Monofill on Oahu produced slightly more than 28,300 m3 of LFG per day in 2019 with production potential of 187 TJ RNG per year. Note, however, its downward LFG production trend. The Kekaha Phases I&II landfill on Kauai produced ~17,800 m3 of LFG per day in 2019 presenting a potential production of 103 TJ RNG per year. The LFG collection system was installed at Kekaha50 and the upward trend in LFG production data may be due in part to improved management of the system over time.

TABLE 3. Estimate of LFG methane resource at landfills with collection systemsa
Landfill name CH4 in LFG (vol%)a CH4 volumea (million m3 year−1) CH4 energy (TJ year−1)
Central Maui Landfill 52 6.1 227
Kapaa and Kalaheo Sanitary Landfills 42.3 1.7 64
Kekaha Landfill/Phases I & II 42.9 2.8 103
Palailai Landfill 40.8 0.23 8.4
Waimanalo Gulch Landfill & Ash Monofill 47.3 5.0 187
West Hawaii Landfill/Puuanahulu 41.65 1.6 61
State Total - 17.5 651
  • a 2018 LFG methane concentration and volume data, source EPA GHG reporting program.49
Details are in the caption following the image
Annual methane production at Hawaii landfills with LFG systems installed

4.1.4 Food waste

Food waste includes kitchen trimmings, plate waste and uneaten prepared food from restaurants, cafeterias, and households as well as unsold and spoiled food from stores and distribution centers and loss and residues from food and beverage production and processing facilities.51 The City & County of Honolulu defines food waste as “all animal, vegetable, and beverage waste which attends or results from the storage, preparation, cooking, handling, selling or serving of food. The term shall not mean commercial cooking oil waste or commercial FOG waste” (see Supplementary Material Section S8 for the Honolulu ordinance details).52

The US generates approximately 57.1 million Mg of food waste per year (see Table S8.1) which represents one-third of the total food supply.51, 53-55 Management practices (or fate) include using food waste for animal feed (as appropriate), or feedstock for compost or anaerobic digestion processes; or sending it to landfill or combustion facilities. In the United States, 39–47 million Mg of food waste are landfilled or disposed in combustion facilities.51, 53-55

4.1.5 Food waste in Hawaii

Estimates for annual food waste generation in Hawaii range from 147,800 Mg in 1999 to 335,600 Mg in 2008 (Table S8.2).56-60 For Hawaii, per capita food waste estimates range from 111 to 240 kg ca−1 year−1 (Table S8.2) and averages 156 kg ca−1 year−1, significantly lower than the U.S. value, ~180 kg ca−1 year−1. Food waste management in the state currently includes animal feed (in-state hog farms and some export to the continental US), feedstock for in-state biodiesel production (yellow grease), home-based composting, and disposal to landfill or combustion (on Oahu).50, 57-59 Food waste currently landfilled in Hawaii is a potential resource for RNG (via anaerobic digestion). State wide, currently disposed food waste totals could support about 14.6 million m3 year−1 of methane production via anaerobic digestion (Table 4). See Supplementary Materials Section S9 for waste characterization data used for each county and Supplementary Materials Section S10 for information on recent legislation affecting location of new or modified waste facilities.

TABLE 4. County food waste disposal and associated methane potential via AD by county
2015 2019
Maui ISWMP (2008), OSWM (2016), OSWM (2020)61-63
Landfill Disposal (Mg, MSW including food waste) 166,132 202,552
Food Waste Disposal (Mg) 24,036 29,305
Food Waste Recovered for AD (Mg, assumes 50% recovery) 12,018 14,653
Potential CH4 production from AD (million m3 CH4 year−1)* 4.2 5.1
Potential CH4 production from AD (TJ CH4 year−1) 155 189
Kauai 2016 Waste Characterization (2008), OSWM (2016), OSWM (2020)62-65
Landfill Disposal (Mg, MSW including food waste) 73,921 83,518
Food Waste Disposal (Mg) 7,629 8,619
Food Waste Recovered for AD (Mg, assumes 50% recovery) 3,815 4,310
Potential CH4 production from AD (million m3 CH4 year−1)* 1.3 1.5
Potential CH4 production from AD (TJ CH4 year−1) 50 56
Hawaii County ISWMP & 2008 Waste Characterization, (2008), OSWM (2016), OSWM (2020)62, 63, 66
Landfill Disposal (Mg, MSW including food waste) 162,383 229,798
Food Waste Disposal (Mg) 26,468 37,457
Food Waste Recovered for AD (Mg, assumes 50% recovery) 13,234 18,729
Potential CH4 production from AD (million m3 CH4 year−1)* 4.6 6.5
Potential CH4 production from AD (TJ CH4 year−1) 171 242
Honolulu- City & County ISWMP & 2017 Waste Characterization67, 68
Landfill Disposal (Mg, MSW including food waste) 58,141 44,120
Food Waste Disposal (Mg) 11,691 8,872
Food Waste Recovered for AD (Mg, assumes 50% recovery) 5,846 4,436
Potential CH4 production from AD (million m3 CH4 year−1)* 2.0 1.5
Potential CH4 production from AD (TJ CH4 year−1) 75 57
Combined (Maui, Kauai, Hawaii, Honolulu)
Landfill Disposal (Mg, MSW including food waste) 460,577 559,989
Food Waste Recovered for AD (Mg, assumes 50% recovery) 34,912 42,127
Potential CH4 production from AD (million m3 CH4 year−1)* 12.1 14.6
Potential CH4 production from AD (TJ CH4 year−1) 451 543
  • * Assumes food waste is 70% moisture, volatile solids comprise 85% of total solids, and specific gas production of 346 m3 CH4 per tonne volatile solids.69, 70

4.2 Thermochemical RNG resources

RNG production using thermochemical gasification will rely on the availability of biomass fiber resources. These could include urban solid waste, agricultural or forestry residues, and purpose -grown energy crops. The latter, also referred to as dedicated feedstock supply systems, include fast growing grasses or trees that are cultivated for the sole purpose of supplying fiber to an energy conversion facility.

Whereas methane generation and RNG potential at WWTP's and landfills are outcomes of (i.e., depend on) the amounts of waste handled and management, an advantage of thermochemical production is that it can be scaled to fit the demand for RNG, within the limitation of available fiber resources. Fiber resources can be transported and combined to increase conversion facility capacity. A recent study71 evaluated thermochemical RNG production in California from a mixture of forest waste, demolition wood waste, and orchard residuals and can provide context for system scales. In summary, the facility design:
  • assumed operation for 7,884 h per year (90% availability);
  • required a biomass flow rate of 29.9 dry Mg h−1, 712 Mg day−1, 234,000 Mg year−1;
  • produced RNG with an energy content of 36.4 MJ m−3;
  • produced RNG at a rate of 82 million m3 year−1 (2,950 TJ year−1).

The biomass feedstock requirement, 234,000 Mg year−1, can be compared with recent fiber production in the Hawaii sugar industry. Hawaiian Commercial & Sugar Co. reported bagasse production of 267,600 Mg of dry fiber annually.72 Kinoshita et al's evaluation of a dedicated fiber production system on the island of Oahu as part of integrated resource planning exercises estimated production of 235,800 Mg of dry fiber annually on 4,860 ha.73 These comparisons indicate that a thermochemical gasification facility of the scale described in the GTI study is consistent with possible fiber resources in Hawaii. The 2,950 TJ of RNG per year produced in the GTI study is comparable to the 2870 TJ of annual utility gas sales in Hawaii. Thermochemical gasification plants of smaller scale could also be considered.

4.2.1 Urban solid waste fiber resources

Urban waste fiber resources include materials disposed as MSW and CDW. The fibrous and/or combustible portion of MSW include the drier, non-food biomass components of the waste stream (paper, cardboard, woody material, and green waste), textiles, and plastics (fossil or non-renewable carbon components).

Based on the same data for solid waste composition and disposal amounts used in the food waste discussion earlier, disposal and RNG potential from the fibrous/combustible portion of the MSW stream is shown for each county in Table 5. RNG potential from this resource ranges from 400 TJ per year on Oahu to 2,000 TJ per year on Hawaii. (see Supplementary Information Section S11 for a comprehensive table that includes component moisture and energy content, wet and dry disposal amounts and RNG potential).

TABLE 5. Annual landfilled, and RNG potential of, combustible components of MSW by county
Maui61-63 Kauai62-65 Hawaii62, 63, 66 Honolulu67, 68
Landfilled (Mg) RNG Potential* (TJ) Landfilled (Mg) RNG Potential* (TJ) Landfilled (Mg) RNG Potential* (TJ) Landfilled (Mg) RNG Potential* (TJ)
Non-food biomass components 100,814 760 39,254 401 109,154 1393 20,142 253
Plastics and textiles 37,026 580 12,611 317 25,048 612 5,841 148
Totals 137,840 1340 51,865 717 134,202 2,005 25,983 401
  • Abbreviation: RNG, renewable natural gas.
  • * RNG potential based on moisture, energy content, assumed 90% material recovery and preparation yield, and 60% conversion efficiency from.71, 74-76

CDW is disposed separately in the City & County of Honolulu. Through 2020, approximately 236,000 Mg year−1 (635 Mg days−1) of CDW was disposed at the PVT CDW landfill in Nanakuli. Bach et al reported that roughly 20% of the material sampled from a feedstock processing line was inert and the combustible remainder had an energy content of 18 MJ kg−1.77 Assuming 90% material recovery and preparation yield and 60% conversion efficiency,71, 74 the CDW material landfilled on Oahu could potentially produce up to 3000 TJ of RNG per year.

4.2.2 Agricultural and forestry residues

The change of Hawaii's land use for agriculture and commercial forestry from 1935 to present is summarized in Figure 2. Note that data are presented using a logarithmic scale. The reduced footprint of the two long time mainstays of Hawaii agriculture, sugarcane and pineapple, is readily apparent. The closure of Hawaiian Commercial & Sugar Co. in 2016 eliminated sugar cane acreage for large scale production of raw sugar. Current cultivation supports rum production on several islands and is estimated to be on the order of 400 ha in total. Current pineapple production services fresh markets and canning operations have ceased, leading to lower cultivated areas.

Details are in the caption following the image
A summary of the change of Hawaii's land use for agriculture and commercial forestry from 1935 to 2015.45

Between 6,000 and 7,300 ha of macadamia nuts have been harvested annually over the past 20 years with average gross production of nut-in-shell of ~22,500 Mg per year.78 Nut shells suitable for use as feedstock for thermochemical conversion would be expected to be ~13,600 Mg. Shells are commonly used as boiler fuel to provide electricity and supplemental heat for processing operations, thereby reducing their availability. Macadamia nut shells are a high quality biomass fuel, having both low moisture content and energy content of ~20 MJ kg−1, however their availability as fuel for thermochemical RNG production is limited.

The forest industry in Hawaii includes four sectors; eucalyptus, koa, sandalwood, and other species for local use (craft eucalyptus for flooring, kamani, milo, etc.). While commercial forestry area across the state was estimated at ~9,300 ha in 2015,45 actual harvesting for timber production that would be expected to generate forest residues (i.e., slash, composed of limbs and smaller diameter wood) is limited.79

4.2.3 Purpose-grown energy crops

Purpose-grown energy crops to support production of electricity and transportation fuels in Hawaii have been explored several times over the past 40 years.73, 80-90 These studies have typically considered fast growing trees (eucalypts or leucaena) or grasses (sugar cane, fiber cane, or banagrass) with the exception of one oil crop assessment.89 These include both statewide assessments and those focused on a specific location (infrastructure and environment). Interest was driven by the decline of the sugar industry and the state's dependence on imported petroleum; both of these themes remain timely.

The state's ~1.6 million ha are classified into land use districts and just less than half falls in the agricultural land use district. Based on geographic information system data, estimates of agricultural land in Hawaii are summarized by island in Table 6 including information on the type of land and slope.91 Land capability class (LCC) is one method to classify soils and provides an index (value of 1 through 8; lower values are favorable) of limitations for agricultural use. In general, LCCs in the range from 1 to 4 have increasing degrees of limitations (1 lower and 4 higher) but these limitations can be managed by the choice of plants and by adopting conservation practices. LCCs of 5 and 6 have greater limitations and are generally suitable for pasture, range or forestry.92 Slope data were derived from an interferometric synthetic aperture radar data set (InterMap Technologies Inc., Englewood, CO). Roughly 260,000 ha across the state are in LCCs 1–4 and have a slope of less than 20%. LCCs of 5 and 6 with slope less than 20% total ~72,800 ha. Slope is a consideration for erosion control and machinery operations.

TABLE 6. Summary of area (hectares) in the agricultural land use district in the State of Hawaii
Agricultural Land Use District (2015 data)
Island Total LCC 1–4 LCC 5–6 LCC 1–4 Slope ≤20% LCC 5–6 Slope ≤20%
Kauai 58,416 31,448 5,664 27,171 2,955
Oahu 48,882 17,771 2,074 16,836 896
Molokai 44,836 17,098 5,433 16,285 3,609
Lanai 18,054 8,837 741 8,521 590
Maui 95,194 41,089 22,252 35,428 11,618
Hawai‵i 478,878 190,042 67,853 156,233 54,357
Total 744,259 306,285 104,019 260,475 74,026
  • Abbreviation: LCC, land capability class.

Agricultural land in use as of 2015 is summarized in Table S12.1 based on the study conducted by Melrose et al.45 Pasture has the largest single footprint on the Hawaii agricultural landscape occupying more than 304,000 ha of the 728,000 ha in the agricultural land use district. Crop land is roughly 1/6th of this amount at ~50,600 ha. Figures S12.1–S12.3 show the (a) areas of the agricultural land use district with slope less than 20% and LCCs from 1 to 6, (b) 2015 agricultural land use,45 and (c) their difference, representing an estimate of agricultural lands with slope less than 20% and LCCs from 1 to 6 which is underutilized. Figure S12.3 indicates that ~101,000 ha of these underutilized lands lie in LCCs 1–4 while ~30,350 ha are in LCCs 5 and 6. Table 7 summarizes underutilized land resources by island. Note that recent events, such as the changes resulting from the 2016 closure and subsequent sale of Hawaiian Commercial & Sugar Co., are not reflected in these figures. Nonetheless, this information provides a starting point for assessing agricultural land resources that could support feedstock production for thermochemical RNG systems. As noted above, Kinoshita et al73 estimated that 4,860 ha of land with adequate water availability could produce 236,000 Mg of dry fiber per year based on assumptions of 48.2 Mg dry matter ha−1 year−1 and a harvest frequency of 8 months. Similarly, fiber production from trees at a mean annual growth increment of 22.4 Mg ha−1 year−1 and a harvest frequency of 4 to 5 years would require ~10,500 ha.83, 85

TABLE 7. Underutilized land resources (hectares) in Hawaii by island as shown in Figure S12.3
LCC 1–4 LCC 5 and 6
Kauai 10,924 1601
Oahu 7326 659
Molokai 8528 2283
Lanai 8495 590
Maui 11,937 2879
Hawaii 54,702 23,103
Total 101,913 31,115
  • Abbreviation: LCC, land capability class.

Comparing these production area requirements and the rudimentary assessment of underutilized land, it would appear that land resources would not limit feedstock production to either support a facility in its entirety or in part if feedstocks were combined with other fiber resources. This comparison does not address the availability of other factors of production needed for a successful agricultural enterprise or the political, social, cultural, or regulatory environments that would be equally important.

5 SUMMARY AND CONCLUSIONS

Feedstock resources for RNG production by biological and thermochemical conversion methods in Hawaii have been reviewed. Estimates of resources for biological production have the potential to support 1,390 TJ year−1 of RNG production statewide (Table 8). Similarly, estimates of the combustible portions of CDW and MSW have the potential to generate 7,470 TJ−1 of RNG production statewide. Honolulu has the largest resource base for these urban waste streams. Underutilized agricultural land resources in the state could support substantial RNG production from dedicated energy crops (260–520 GJ ha−1 year−1), although agronomic suitability of specific candidate energy crops would need to be evaluated and confirmed.

TABLE 8. RNG potential summary (TJ per year) for resources in Hawaii
Resource type Maui Kauai Hawaii Honolulu State total
Livestock manure * * * *
Wastewater treatment plants - 2.1 6.3 190 200
Landfill gas 227 104 60.9 260 652
Food waste portion of MSW 189 55.6 241 57.2 543
Combustible portion of MSW 1,339 721 1,997 402 4,460
CDW - - - 3,007 3,007
Agricultural and forestry residues
Energy crops § § § § §
Totals >1,755 >883 >2,311 >3,904 >8,863
  • Abbreviation: RNG, renewable natural gas.
  • a Insufficient number and size of animal feeding operations to justify methane production and recovery.
  • b Insufficient available agricultural residues and ongoing forestry harvesting residues.
  • c Underutilized agricultural land resources in the State could support substantial RNG production from dedicated energy crops (~260–520 GJ per hectare per year).
  • d Totals would be larger with implementation of energy crop-based RNG production.

These estimates of potential RNG feedstock resources and RNG product do not take into consideration factors including economics, accessibility of a resource, availability of complementary factors of production, or the political, social, cultural, or regulatory environment. These factors would need to be considered in order to assess viability. Location of resources and access to infrastructure needed to implement successful RNG production, transmission, and distribution would necessarily depend on site specific details. Market signals by consumers and utility companies and input from the broader stakeholder community will be instrumental in shaping RNG demand.

AUTHOR CONTRIBUTIONS

Scott Turn: Conceptualization (equal); data curation (equal); formal analysis (equal); investigation (equal); methodology (equal); writing – original draft (equal); writing – review and editing (equal). Robert B. Williams: Conceptualization (equal); data curation (equal); formal analysis (equal); investigation (equal); methodology (equal); visualization (equal); writing – original draft (equal); writing – review and editing (equal). Wai Ying Chan: Data curation (equal); formal analysis (equal); methodology (equal); visualization (equal).

DATA AVAILABILITY STATEMENT

Data available in article supplementary material.