With California’s Low Carbon Fuel Standards in place, manufacturers of ethanol and other biofuels are scavenging for means to reduce their carbon footprints. Among the various technologies that promise such a result is combined heat and power (CHP). CHP technology isn’t new, but it’s received attention as incentives to increase efficiency and lower carbon outputs intensify.
CHP has been around since the 1800s, when industries began to adopt electrical power. There were few centralized power stations, and utility operators charged high rates for their electricity. So, those who owned large buildings or industrial complexes often ran generators. These generators were steam-based, of course. But rather than venting the de-energized steam that had passed through the turbines, the owners would route the steam elsewhere in the complex and employ it for heat or hot water. The system thus ran on the principle of CHP.
The same idea can be applied to modern ethanol plants and other biofuel facilities. Operators may employ a CHP to produce electricity and feed steam to their dryer, increasing overall efficiency. However, fuel is key to the ability of CHP to impact a plant’s carbon output. While still beneficial, CHP’s impact is limited if it runs on natural gas. To be truly effective in reducing carbon emissions, it must utilize biomass or biogas, i.e., renewable natural gas (RNG).
RNG-Fired CHP
RNG-fueled power generation makes the most sense for plants that operate near a feedlot. The feedlot owners or the biofuel manufacturers may operate an anaerobic digestion (AD) unit to produce RNG in the form of methane. The methane, in turn, directly feeds a gas turbine CHP system. Anaerobic digesters are available in various sizes, with their output depending on the amount and type of waste they utilize.
Common sources of feedstock for AD include manure, barn wash water, feed waste, farm mortalities, slaughterhouse waste, sewer sludge, fats, oils, organic greases, and corn silage. AD operators usually sell the leftover material as compost or fertilizer.
Pictured is an anaerobic digestor located on a Great Britain dairy farm.
CHP operators may also obtain RNG through gasification (i.e., pyrolysis). In gasification, the system delivers feedstock to a chamber that heats the material in an oxygen-deprived environment. In these conditions, the material does not combust but converts into a gas. This synthetic gas, referred to as syngas, is refined into natural gas on site. The coproduct of gasification is biochar—the carbon remains of the feedstock. Those feedstocks vary widely, as gasifiers can consume many materials, including plastics, though any one gasifier requires a limited range of feedstocks to operate efficiently.
The advantage of gasification over AD is that a gasifier can quickly convert a high volume of material into natural gas. It also requires a smaller footprint than an AD facility with comparable output. For reference, compare the facilities pictured below. The two AD facilities require sprawling complexes and process less material per hour as the gasifier: 38 and 10.5 tons per hour compared to 64 tons per hour.
Pictured is the largest wet anaerobic digester in the US, which operates at 38 tons/hr. (335,000 tons/yr) in Perris, CA.
ZeroWaste Energy in San Jose, CA processes 250 tons of waste per day. The company claims it is the largest dry anaerobic digester in the world.
Aries Clean Technologies operates a downdraft gasifier in Lebanon, TN. This facility converts 64 tons of waste materials per hour into biogas and biochar.
At a large scale, neither technology is cheap, with costs in the millions of dollars. Small AD facilities produce comparatively little RNG, however. An anaerobic digester that utilizes the waste from 1,250 dairy cows may produce enough biogas for an electric output of only 255 kWh.[1] While that’s enough electricity to power several houses, it’s hardly a dent in what an ethanol plant requires to operate. Smaller biomass-powered CHP gasification systems are also available, with various electrical outputs up to hundreds of kWh. Either system—AD or gasification—provides the benefit of reduced carbon emissions, whatever the scale, and savings in efficiency.
As for CHP technology, two categories are available for RNG. The first, a gas turbine system. In this system, the RNG is used to heat air, which spins the turbines. The system then utilizes heat transfer equipment to move energy from the exhaust gasses to water. The second is a steam-turbine system, which sources steam via a gas-fired boiler. In this system, the low-pressure steam exiting the turbines gets transferred for use elsewhere in the facility.
Biomass powered CHP
Ethanol and other biofuel manufacturers may alternatively use biomass as a direct source of green fuel in a steam-based CHP system, burning it to heat the boiler. As with gasifiers, various biomass materials may be used to fuel the system, though, again, any one boiler requires consistent fuel characteristics. Stover can be one source of fuel, as can woody briquettes or woodchips.
Unless those who a biofuel plant already have access to RNG, it may be advantageous for them to choose a biomass-powered CHP because they won’t have the added cost of constructing a facility to generate gas. Another advantage is that boiler systems utilizing biomass can be built larger and with a higher energy output than those relying on RNG. The limiting factor for RNG is the amount of fuel available. Only so much RNG can be produced from one anaerobic digester or one gasifier, whereas the sources of biomass are plentiful.
Stover is likely the most plentiful feedstock for most ethanol and other biofuel manufacturers, but it’s not the only source, even in farm country. Energy crops are another source of biomass. Biofuel operators can contract local farmers to grow plants like miscanthus on marginal land. Other energy crops include hybrid poplars, hybrid willows, and switchgrass. If there does happen to be a sawmill nearby, operators may alternatively acquire their residuals in the form of woodchips, shavings, and sawdust.
Biomass-powered boilers do have downsides. Biomass is not as clean as RNG. It produces more pollution when burned, and even highly efficient systems produce copious amounts of ash. And that ash is the only coproduct—a lower-value material than biochar or fertilizer. Biomass also requires a dry storage environment and equipment designed the handle the material. Repurposed grain-handling equipment is not robust enough to handle biomass, which is abrasive and often acidic. Conveyors and other equipment made specifically for biomass deliver lower operational costs and prevent runaway maintenance expenses.
Determining which renewable CHP system makes sense for your plant ultimately comes down to what proves most economically feasible. In terms of carbon credits, however, there’s no going wrong. Any CHP system—even a conventionally powered one—lowers overall energy consumption. If you’re interested in biomass gasification or a direct-fire biomass CHP system, give us a ring. We know biomass and what it takes to design a system that performs reliably day in and day out. Contact us today.
[1] https://www.epa.gov/agstar/livestock-anaerobic-digester-database
