Resources for renewable natural gas: A Hawaii case study

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.


| 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 À480 to 66 g CO 2 eq MJ À1 . 1,2 Fossil NG has net GHG emissions of about 70.1 g CO 2 eq MJ À1 . 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 À1 . 3 Details are provided in Supplementary Material Section S1.

| 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 (CO 2 ), and methane (CH 4 ). This conversion does not occur with 100% efficiency and some portion of the biogenic material will remain. CO 2 and CH 4 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, CO 2 and CH 4 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, CH 4 can account for up to 70% of the total gas volume with CO 2 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 (O 2 ), nitrogen (N 2 ), and CO 2 can be considered diluents. The presence of O 2 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., CO  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), CO 2 , hydrogen (H 2 ), and CH 4 . Particulate matter and other compounds will be present as contaminants and the latter may include higher hydrocarbons (C 2 + 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, CO 2 , and H 2 must be adjusted (gas conditioning), and then CO and CO 2 are reacted with H 2 to form additional CH 4 (synthesis/methanation). The methane rich product gas from the synthesis step is upgraded to meet specifications required by the RNG offtaker.

| 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][13][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][20][21] There are many general biomass resource assessments for smaller countries or districts, often called "case studies". [22][23][24][25] Biogas-specific resource case studies, like this investigation, have been produced throughout the world. 13,14,[26][27][28][29][30][31][32][33][34][35][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

| 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.

| Livestock manure
Inventories of hogs, cattle and calves, and poultry in Hawaii are summarized in Table 1

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 À1 500 kg animal unit À1 , as-excreted basis) 43 and methane production from anaerobic digestion (350 L kg À1 of volatile solids), 44

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 al 45 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 content 46 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 À1 . Note that Rodriguez-Verde et al 46 determined that CH 4 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.  in LFG production data may be due in part to improved management of the system over time.

| 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

| 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.

F I G U R E 1 Annual methane production at Hawaii landfills with LFG systems installed
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 study 71 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 À1 , 712 Mg day À1 , 234,000 Mg year À1 ; • produced RNG with an energy content of 36.4 MJ m À3 ; • produced RNG at a rate of 82 million m 3 year À1 (2,950 TJ year À1 ).
The biomass feedstock requirement, 234,000 Mg year À1 , 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

| 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.  Table 6 including information on the type of land and slope. 91   Agricultural land in use as of 2015 is summarized in and a harvest frequency of 4 to 5 years would require $10,500 ha. 83,85 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.

| 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 T A B L E 7 Underutilized land resources (hectares) in Hawaii by island as shown in Figure S12. 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. support 1,390 TJ year À1 of RNG production statewide (Table 8).
Similarly, estimates of the combustible portions of CDW and MSW have the potential to generate 7,470 TJ À1 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 À1 year À1 ), although agronomic suitability of specific candidate energy crops would need to be evaluated and confirmed.
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.

DATA AVAILABILITY STATEMENT
Data available in article supplementary material.