Back to The Future!
Remember that great movie? Fueling a vehicle with food waste was a concept made famous by the movie Back to the Future in the 1980s. Almost 30 years later, what was once a futuristic idea has become a reality. Biogas, also known as renewable natural gas (RNG), produced at locations such as landfills or dairy farms, can supply gas to on-site fueling infrastructure for vehicles such as refuse haulers and dairy trucks.
Converting biogas from source-separated organics to compressed natural gas (CNG) is one of the most viable uses for the renewable natural gas (RNG) fuel.
In order for RNG to be used as vehicle fuel, it must first be refined, which requires investment in equipment. The actual fueling station using RNG is similar in cost to those connected to a utility pipeline, but the overall economic and environmental incentives behind biogas conversion make it an option not to be overlooked.
Drivers of Biogas Conversion
Many states, cities and other jurisdictions in North America have recently adopted or are phasing in mandates to divert more than 70 percent of their municipal solid waste from landfill disposal. Processing for traditional recyclables can only get the diversion rate to 50 percent or so. The new mandates for more diversion create a need to focus on organic waste (yard waste and food waste) which accounts for more than 25 percent of the waste stream and is still largely unrecycled in North America.
One of the options for organics is anaerobic digestion (AD), the treatment process that produces biogas. Biogas contains greater than 50 percent methane, and as such it can be extracted, cleaned and turned into compressed natural gas (CNG).
While biogas can be used for other purposes, including generating electricity, project-specific economic analyses to date indicate that conversion to CNG vehicle fuel is usually the highest return option for using biogas. Among the reasons is the fact that CNG from biogas is a clean, domestically produced alternative fuel. Compared with vehicles fueled by conventional diesel and gasoline, CNG vehicles can produce lower levels of some emissions. And because CNG fuel systems are completely sealed, CNG vehicles produce no evaporative emissions.
Other important drivers include incentives for renewable energy and greenhouse gas reduction. Biogas displaces fossil fuels when used as a fuel. Therefore biogas projects are eligible for the grants, loans and incentives available to reduce greenhouse gases (GHGs). While the U.S. federal government has struggled to implement regulations and incentives for GHG reduction, many states, Canadian provinces and independent foundations and agencies do offer relevant incentives, including some for renewable energy, that can make a significant economic difference. In fact most biogas projects coming online today benefit from one or more of these programs.
In addition, to accelerate the use of fuels derived from renewable sources, Congress established standards under the Energy Policy Act of 2005 designed to encourage the blending of renewable fuels into our nation’s motor vehicle fuel supply. This initial renewable fuels standard (RFS, referred to as RFS#1) mandated that a minimum of 4 billion gallons be used by 2006, rising to 7.5 billion gallons in 2012.
Congress strengthened the renewable fuels program under the Energy Independence and Security Act of 2007 (RFS#2) to include specific annual volume standards for total renewable fuel and also for specific renewable fuel categories of cellulosic biofuel, biomass-based diesel and advanced biofuel. This act also greatly expanded the biofuel mandate and extended the date through 2022.
The price of natural gas remains low relative to other fuel resources as a result of the use of horizontal drilling, fracking and other new natural gas extraction techniques. This has caused many truck fleet owners to begin converting large parts of their fleets to CNG-fueled vehicles. The demand for CNG compression and fueling systems has grown in tandem with this trend, which has not only reduced the price of these systems, but has brought modular systems into the marketplace providing greater reliability and cost certainty than in the past.
All of these factors add up to define the economics of biogas-to-energy projects. Others are spurring dairy and other livestock farmers to process manure, producing various products that can be used on the farm, such as animal bedding and fertilizer as well as biogas. Still other factors are driving an increasing number of wastewater treatment plants to install or retrofit digesters to accept feedstocks such as food wastes and fats, oils and grease in addition to sewage sludges.
Given the variety of factors that can affect any given project, it’s necessary to carefully examine all potential inputs and outputs to determine the optimal combination of feedstocks and outputs in evaluating economic viability. Indeed, several entrepreneurial merchant facilities have sprung up that combine all of the above feedstocks in the same digester and produce a variety of products, including CNG from biogas.
According to a 2014 U.S. Department of Agriculture study, more than 2,000 biogas production facilities of all types are today operating in the United States, and adequate feedstock is available to expand that number to 13,000 facilities that could generate a little more than 650 billion cubic feet per year of biogas that could produce the equivalent of 2.5 billion gallons of gasoline per year.
For the waste industry, the best candidates for biogas production are source-separated organics (SSO). These include wastes collected in municipal programs that require households and businesses to separate organic wastes. While only a few of these have been implemented, several jurisdictions, including the states of California and Massachusetts and the cities of Seattle; New York; San Francisco; Toronto; and Calgary, Alberta, have or are in the process of implementing requirements for businesses and/or residences to source-separate their organic wastes. (See the related article, “Pumped about biogas” at www.REWmag.com/rew0315-biogas-fuel.aspx.)
Options for Biogas Production
Biogas is produced by specialized bacteria that thrive in airless environments, which explains why the process is referred to as anaerobic digestion (AD). However feedstocks ultimately drive everything in digestion and biogas production. As such, a range of digester types are available today to do this.
Even more important is the fact that some AD types work better for some feedstocks than for others. In general, digester types are grouped according to the moisture-content of the feedstocks. Wet digesters are basically big tanks where very wet feedstocks (with a solids content of less than 20 percent) are pumped in and pumped out. Meanwhile, dry digesters handle materials that are stackable (greater than 40 percent solids), and the material is moved with a front-end loader. In dry digesters, liquids are percolated through the organic material rather than submerging or suspending them. In between these two types are high-solids digesters that handle slurries of between 20 percent and 40 percent solids. Dry digesters are potentially much more efficient, requiring less heat and energy input and producing much less wastewater, but they are designed for drier feedstocks.
Biogas yields from different feedstocks vary greatly. Even within a given type, various sources report a range of values. Rules of thumb for use in preliminary alternatives comparisons are listed in Table 1 below, covering the range from high to low. For project engineering and budgeting, samples of the actual feedstocks should be tested. A standard test called the biomethane potential (BMP) test is done in a laboratory mini-digester, and factored to adjust for non-laboratory conditions.
In order to use biogas as vehicle fuel, it must be cleaned up, compressed and stored. It’s important to understand that vehicle fuel doesn’t have to be cleaned to the same standards as fuel for pipeline injection. Standards include SAE J1616 and California Code of Regulations Title 3, Article 3 – Specifications for Alternative Motor Vehicle Fuels.
While pipeline companies may require biogas to be cleaned up to 98 percent or higher methane concentration, the California CNG standard only requires 88 percent methane, and vehicles can generally be operated efficiently at this level. This can make a large difference in the costs involved to clean and treat the gas to use it for fuel. Typically, removal or reduction of moisture, carbon dioxide, hydrogen sulfide, volatile organic compounds (VOCs) and siloxanes are required for vehicle use of biogas.
As mentioned, carbon dioxide, moisture and impurities all need to be removed from biogas to protect vehicle engines. This can consume as much as a third of the biogas produced for very small systems (on the order of 5,000 cubic feet per hour of biogas), to only a few percent for larger systems (20,000 cubic feet per hour of biogas and larger). These reductions and the resulting volume reduction for removal of carbon dioxide yield the ranges of CNG volumes listed in the table on page 25, as well as gallons of gasoline equivalent.
Biogas is typically compressed in two stages using reciprocating compressors. The initial compression may be up to an intermediate pressure level of 500 pounds per square inch gauge (psig) for processing. Final compression would be up to the CNG fuel storage pressure of 3,600 psig for a fast-fill refueling facility.
The cost of biogas production can vary from $0.60 to $1.00 per Gasoline Gallon Equivalent (GGE), which is about 5.66 pounds of CNG. However, digestion project costs are usually offset by other revenues, including tipping fees for the organic wastes and digestate that can be used as fertilizer. For this reason, every project must be analyzed based on its own circumstances.
The total cost of developing a CNG fueling station depends on a number of factors, including the fuel demand from the fleet and other users, the fleet’s applications and duty cycles, site conditions, the complexity of equipment installation and local permitting processes. As a result, costs can vary widely from one project to another.
For details on the various cost considerations for these facilities, see the related article, “Project development costs for CNG fueling stations” at www.REWmag.com/rew0315-cng-fueling-costs.aspx.
Biogas projects are inevitably complex because there are multiple inputs and multiple outputs. Feedstocks will probably come from multiple sources, and these supplies need to be assured through various forms of agreement with the generators. The big plus here is that unlike most energy projects, a biogas project owner actually gets paid to take the fuel, which is considered a waste for which the generator expects to pay for disposal. In addition to energy from biogas, there is usually at least one more revenue source from products of digestion, which can often be sold for fertilizer value or can be composted to produce saleable products. The multiple revenue sources provide robustness because market fluctuations for one won’t affect the others. But the complexity, as well as the relative lack of precedents, make careful planning and supplier commitments essential for these projects.
AD technology is in a state of rapid development because of the wide array of feedstocks that are now being considered for biogas production. Technology vendors are making claims that, however valid, have little field experience to back them up. Every biogas project today should include check-ins on the latest available technologies and data available from vendors, and compare not only data for that technology, but how its effects could ripple through the whole project, from requirements for feedstock collection and preprocessing through production, distribution and sale of all products that will result from the project.
1. Significant savings over gasoline and diesel
- 50-75% savings over current gas/diesel cost
- Cost-competitive to fossil-based CNG•
2. Local, green, renewable fuel source
- Up to 90% GHG reductions v. gas and diesel
- Renewable fuel and GHG credits
3. Control your future!
- Cost locked in for 15-20 years
- Hedge against traditional potential rising Natural Gas prices
About the Authors
Bob Wallace, MBA – Founder and a Principal of Phoenix-based WIH Resource Group Inc., has more than 27 years of experience in waste and recycling collections program management, alternative fuels, operational performance assessments, recycling/solid waste operations, landfills, planning and development. He can be reached by email at email@example.com.
Tom Kraemer is a principal technologist with CH2M Hill, Englewood, Colorado, and specializes in organic waste management. He can be reached at firstname.lastname@example.org.
Pumped About Biogas: Find out how the cities of Tacoma, Washington, and Sacramento are converting biogas from organics to CNG at www.REWmag.com/rew0315-biogas-fuel.aspx.
A Look at CNG Station Costs: For further reading on the project development costs for CNG fueling stations, visit www.REWmag.com/rew0315-cng-fueling-costs.aspx.
SOURCE: WIH Resource Group & US News & World Report
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