Environmental & Economic Impacts of Compressing RNG for use as CNG Vehicle Fuel
Biogas can have impurities removed and upgraded to render it suitable for use as a vehicle fuel. This analysis examines the environmental and economic costs associated with cleaning, upgrading, and compressing RNG for use as compressed natural gas (CNG) used to power vehicles, using a Unisons Solutions biogas CNG (BioCNG) upgrading system and fueling station for input flows of 50-200 standard cubic feet per minute and a Unisons Solutions hydrogen sulfide removal system, a Guild pressure swing adsorption separator, together with an ANGI compressed natural gas fueling station for a larger system with a biogas inflow of 1,600 standard cubic feet per minute.
Efficiency of Compressing RNG (on-site fuel)
Unison Solutionsâ€™biogas CNG upgrading systems are installed at various sites around California, including the Blue Line renewable natural gas facility located in South San Francisco and the CleanWorld biodigester facility located in Sacramento.
The smaller capacity system utilizes single-pass membrane technology to separate carbon dioxide from the methane component, resulting in around 70% of the methane being converted into CNG fuel. The larger capacity system, which utilizes the Guild pressure swing adsorption system, recovers a larger percentage of methane â€” typically 85% or more. This analysis is based on a methane recovery rate of 70% for smaller systems (biogas inputs of less than 200 standard cubic feet per minute) and 85% for the larger system (biogas input of 1600 standard cubic feet per minute). In both cases, the balance of the methane (30% and 15% respectively) is emitted from the system with the carbon dioxide in the tailgas, which cannot simply be released to the atmosphere untreated but must be combusted either in an engine or by flaring.
According to the EPA report, Evaluating the Air Quality, Climate & Economic Impacts of Biogas Management Technologies, for smaller systems (with biogas inflows of <200 standard cubic feet per minute) the efficiency of compressing RNG for CNG fuel based on a 70% methane recovery rate ranges from 1.3 MMBtu/hour with an output gas flow of 22.1 standard cubic feet per minute producing a gasoline gallon equivalent (GGE) of 241 GGE per day for a biogas flow input of 50 standard cubic feet per minute. For input flows of 100 and 200 standard cubic feet per minute RNG fuel product output increases to 44 and 88 standard cubic feet per minute respectively, producing a gasoline gallon equivalent of 482 and 963 GGE per day respectively. Based on an 85% methane recovery rate, the efficiency of the larger system with a biogas inflow rate of 1600 standard cubic feet per minute, produced an RNG fuel output of 860 standard cubic feet per minute or 9,360 GGE per day (See table 22, page 29 of the EPA report.
One important benefit of upgrading biogas for use as RNG fuel or for injecting into a natural gas pipeline is that emissions of on-site criteria pollutants tend to be significantly lower than those produced by combustion engines. However, criteria pollutants are produced when and where the final product (CNG) is used. The levels of these pollutants will vary depending on the ultimate use, which include, for example, domestic gas appliances, vehicle emissions, and natural gas power stations. Only on-site criteria pollutants were evaluated for this analysis.
Since the unrecovered methane discharged as a byproduct gas in the tailgas is a potent greenhouse gas, it needs to be processed to prevent it from escaping into the atmosphere. In certain cases it may be possible to combust the tailgas in a combustion or turbine engine to oxidize the methane, converting it into carbon dioxide. However, for this analysis, the assumption is that the methane component is destroyed by flaring, and is based on emission factors associated with flaring as a biogas management tool.
Emissions from tailgas flaring tend to range between 0.71-11.04 parts per million (ppm) for nitrogen oxides and, 0.59-9.13 ppm for carbon monoxide, 0.15-2.39 ppm for particulate matter, 0.08-1.21 ppm for volatile organic compounds, and 0.50-7.79 ppm for sulfur oxides (see table 23, page 30 of the EPA Report).
Greenhouse Gas Emissions
The greenhouse gas emission factors assume 1% of methane is lost through leakage during the upgrading process together with methane and nitrous oxide emissions discharged during tailgas flaring. The carbon dioxide emission factor takes the CO2 that is separated from the methane component during the upgrading process before being flared into account, together with carbon dioxide produced as any methane present in the tailgas is combusted (oxidized) during flaring and the 1% lost to leakage as mentioned above.
Emission factors for greenhouse gases produced when biogas is upgraded and compressed for use as CNG vehicle fuel are as follows: the average greenhouse gas emission factor for methane is 0.43 (85% methane recovery) - 0.44 (70% methane recovery) lbs CH4/MMBtu, while the average GHG emission factor for carbon dioxide is 88.3 (85% methane recovery) - 106.5 lbs CO2/MMBtu (70% methane recovery), and the average nitrous oxide emission factor is 0.00034-0.00070 lbs N2O/MMBtu.
Based on the emission factors above together with the global warming potential over 100 years (GWP100) of the greenhouse gas in question, the output-based greenhouse gas emissions (carbon dioxide equivalent) of methane when compressing RNG for use as CNG vehicle fuel is estimated to range from 18.9-590 lbs CH4/day for gas inflows of between 50-1600 SCFM respectively, with the global warming potential ranging between 642-20,100 lbs CO2eq/day depending on the flow input. For carbon dioxide, this ranges from 4,600-122,000 lbs CO2/day, and for nitrous oxide, this ranges from 0.003-0.47, with the global warming potential ranging from 9-140 CO2eq/day depending on the flow input. Emission factors for criteria pollutants and greenhouse gases when upgrading and compressing RNG for use as CNG vehicle fuel can be viewed in Table 24 and Figure 23 on page 31 of the EPA report).
Costs of Compressing RNG
Costs associated with upgrading and compressing biogas for use as CNG vehicle fuel were estimated using methane recovery efficiency data and installation/operating costs derived from financial reports of an existing Biogas CNG project.
Estimated total Levelized costs of energy range from $18.30-$9.34 /MMBtu for smaller input flows of 50-200 SCFM respectively (70% methane), to $4.02/MMBtu for larger input flows of 1600 SCFM (85% methane). A full breakdown of capital costs together with operating and maintenance costs for four different sized projects can be viewed in Table 25 and Figure 24 on page 32 of the EPA report.