With crude oil prices staying high, the future is bright for projects that create renewable natural gas.
Paul Greene
Organic wastes have considerable potential for creating clean, green renewable energy. By recycling food scraps, yard wastes, sludges and commercial kitchen wastes valuable landfill space can be preserved. In spots in the U.S., disposal capacity is at a premium and there is momentum to restrict landfilling of organics and diverting them to higher uses.
The most common technique for generating energy from these wastes is called anaerobic digestion (AD). While quite established in Europe, AD is fairly new to the U.S. market for managing food wastes. The technique can be applied in two different approaches, wet digestion and dry digestion. A wet system runs at 3 to 10 percent solids slurry concentration in a mixed tank using wastes in a pumpable form. A dry system, on the other hand, digests wastes that are stackable where their solids levels are in the 15 to 30 percent range.
Gas Yield
In order to properly model digester performance, tip fee revenue and power generation revenue, it is important to start by characterizing the feedstocks available. The accepted industry standard test for calculating energy yield is called the Biomethane Potential Test. During this test a specific feedstock, or “substrate” as they are known, can be tested to show how much renewable gas, or biomethane, can be produced per ton of waste. Generally materials that are high in fats and proteins can give very high gas yield and produce more energy revenue.
Contamination
It is critical to understand the level of packaging and contamination as well as what type of package or bag the organic waste streams will be shipped in. Generally pre-consumer wastes such as cafeteria food waste and food industry processing waste are easier to manage versus post-consumer wastes as there is less contamination and packaging. Curbside collected green wastes can contain significant contamination and undesirable components. While chicken bones and peach pits, for example, are indeed considered food waste it doesn’t necessarily mean they are the best digester feed materials. While paper products can provide some buffering capacity in a digester they have poor gas yield and are preferably removed ahead of a digester.
Taking in expired and damaged food products that are in a package can represent a significant differentiator for an AD facility. Technologies that can break or slice open a plastic or glass package and remove the contents are more commonly applied and can be very efficient. Macerators, grinders and chopper pumps are widely applied to convert solid foods into more readily digestible forms.
Wet Systems
In order to provide proper operation conditions for the bacteria, wet AD systems typically run at 95 degrees F˚. This temperature level is considered a mesophilic condition and sometimes warmer, “thermophilic” conditions are roughly 140˚F. Detention times range from 20 to 40 days to maximize the amount of gas produced and the destruction conversion efficiencies. Some facilities will incorporate a short, hot heat treatment cycle of 170 F˚ for an hour in order to more thoroughly pasteurize the material if pathogen considerations are an issue in the resulting solids. “Free” heat is captured from the onsite engine generator and recirculated in a hot water loop through the tank.
Resulting liquids and solids from the wet digester have high nutrient levels and are considered good fertilizers. Food waste digesters that partner with dairy farms can dewater the solids and return them to animal barns for bedding materials and use the liquid nutrients on their crop fields. Facilities in more urban settings will need to evaluate criteria and costs for discharge of the liquid digestate fraction as the elevated organic levels (measured as Biochemical Oxygen Demand) nutrient loadings can prove costly to dispose of. These urban digesters can be candidates for their own wastewater processing system or for nutrient purification technologies and can look at recovered nutrients as revenue generators. Liquid digestate is dewatered in a centrifuge and resulting solids can be excellent feed material for further composting and sale of pathogen-free produced compost.
Dry System
Dry AD systems rely on being able to stack the wastes in a fixed pile. This can require a mix of 1:2 to 1:4 food waste to green waste to give waste piles needed structure and porosity. After a few days of being charged with wastes, the dry AD unit is sealed and the multi-week process of digesting the material begins. First, the pile is kept wet by spraying hot water on the stack. This liquid has been naturally seeded with innoculum bacteria over time and is collected at the bottom of the stack after its has percolated through the pile and leached out partly digested organics. Once collected, the material can be sent to a nearby mixed digester tank where biomethane generation occurs. Digester units are run at 95 F˚ and are heated by capturing the waste heat from the onsite biogas engine generators.
Wastes are commonly digested in a dry reactor for 3 to 4 weeks. After such time they can be removed and further composted nearby or, depending on the reactor design, kept in place and converted to an aerated static pile composting unit.
Power Generation
AD units produce roughly eight cubic feet of methane for every pound of organic matter (COD) they digest on average, depending on the waste mix. This methane comes out of the digester at 50 to 70 percent purity. From there many options for capturing the gases Btu value can be exploited.
Commonly, digester gas is first dried as it comes out of the digester fully saturated with water vapor. Drying can be done with drying filters or with refrigeration units. Sometimes removal of hydrogen sulfide gas is required by media filters or by biological units that convert the H2S to an innocuous form.
The raw digester gas is at a very low pressure, expressed in inches of water column. Pressurization is needed to get it up to pressure to filter it or send it to combustion or as vehicle fuel. As some utilities will pay a fair price of over 10 to 12 cents per kwh for biogas generated power, the use of either an engine generator or fuel cell can be practical to introduce power onto the grid. With the current softness in natural gas prices more vehicle fleet operators are looking at CNG fueling of their hauling fleet. This trend lends itself to easy adoption of purified biomethane from digester gas for fueling fleet vehicles.
Policy
In some spots across the U.S., government policies are in place to encourage development of AD projects. Recently, the American Biogas Council was created to develop more equitable policy treatment for anaerobic digester projects and to educate the market on the technology’s benefits. However, much work needs to be done. Historically, the renewable energy value of digesters has only been appreciated when these projects create electricity with their resulting biogas and the incentives have not been at parity with other renewable energy sources. With crude oil prices staying high, the future is bright for projects that create renewable natural gas.
Paul Greene is Vice President of O’Brien & Gere Engineers (Bridgewater, NJ), a top 100 Environmental Contractor and builder of Anaerobic Digesters. He is also a Director at the American Biogas Council and blogs at www.digester.com. Paul can be reached at [email protected].