Digester methane is taking its place as a mainstream renewable fuel. Today’s fuel treatment and engine-generator technologies help make biogas a highly attractive and earth-friendly energy source for power generation and heating. As recently as 10 or 20 years ago, billions of cubic feet of methane from landfills and wastewater treatment plant anaerobic digesters was flared off and wasted or – even worse – simply released to the atmosphere. Today, this biogas has become a prized source of fuel, and the potential is significant. But while we’re using more biogas – many municipal wastewater treatment plants with capacities over 30 million gallons per day (113,500 m³ per day) now use it to generate power – we’re also making more of it. Growing numbers of wastewater treatment plants feed organic materials to their digesters to boost gas production, and stand-alone digesters that produce biogas from manure, food waste and other products are receiving serious consideration. With this in mind, the time is right for businesses that generate volumes of organic waste to evaluate the benefits of transforming that waste material into fuel.
The Business Case
A variety of industries can potentially benefit from biogas-to-energy projects. Feedstocks for anaerobic digesters can include silage from energy crops like corn and sweet sorghum, crop residues, green cuttings from landscape maintenance, food waste from restaurants and cafeterias, fats and oils, and wastes from food and beverage facilities like sugar plants, fruit and vegetable processors, and meat processing plants. Animal waste alone is among the lowest biogas producers – the recipe can be improved by co-digesting with fats and oils or vegetative material.
The analysis of benefits begins with estimating revenue. The base calculation assumes an estimated fuel flow and fuel quality, and an engine-generator of a given capacity operated continuously for one year. For example, a 20-cylinder generator set rated at 2,000 kW can generate USD112 per hour based on an electricity sale price of USD0.056 per kWh. Based on 8,760 hours per year, that unit theoretically could generate USD981,120 in revenue annually. Of course, no generator can operate around the clock for a year. Real revenue is determined by the theoretical revenue multiplied by capacity factor – the percent of its total potential output the units actually achieves.
It is also important to understand how tradeoffs between generator set capacity factor and generator set electrical efficiency affect revenue. Assume there are two 1,000 kW units with an electricity sale price of USD70 per MWh and a fuel production cost of USD2/MMBtu (USD70.63 /Nm³). Now assume that both units operate at 96 percent capacity, but that Unit A is 39 percent efficient while Unit B is 42 percent efficient. In that scenario, Unit B has a 2.2 percent net revenue advantage. Now for the same two units, assume that electrical efficiency is the same at 42 percent, but that Unit A’s capacity factor is 90 percent and Unit B’s is 96 percent. In this scenario, Unit B has a 6.25 percent revenue advantage.
Owning and operating expenses fall on the opposite side of the ledger, usually measured in cost per kilowatt-hour sold or produced. The kilowatt-hour figure needs to account for conditions that reduce generator output: parasitic loads, capacity factor, derates and others. In biogas applications, the major expenses are capital costs (including fuel treatment) and maintenance and repairs. The latter includes routine engine maintenance as well as periodic top-end, in-frame and major overhauls. Maintenance practices should not follow a fixed calendar schedule, but should be based on predictive indicators. Predictive maintenance can help extend service and overhaul intervals and reduce service costs by up to 15 percent.
“Predictive maintenance can help extend service and overhaul intervals and reduce service costs by up to 15 percent”
Caring for Fuel
The biggest variable affecting biogas project costs is fuel quality. Depending on its source, biogas contains a variety of impurities that can increase wear and shorten maintenance and service intervals. These impurities include: Hydrogen sulfide, Silicon, Siloxanes and Water. There are three basic ways to deal with fuel impurities. They can be used alone or in combination, depending on the fuel quality, ambient conditions, financial objectives and other considerations.
- Treat the Fuel: While effective at reducing contaminants in the fuel, fuel treatments do add to biogas-to-energy system capital costs, add parasitic loads to the system, and require additional maintenance materials and labor.
- Choose a ‘hardened’ engine: Some manufacturers offer engines with design features that “harden” components and systems against biogas fuel impurities. These units can operate for near-normal maintenance intervals with less intensive fuel treatment.
- Accelerate maintenance: In some cases, it may be more attractive to owners to defer on the installation costs associated with advanced fuel treatment systems, and accept the operating costs associated with shorter maintenance and overhaul intervals. This ‘pay me now or pay me later’ dilemma is often decided based on a developer’s assessment of risk and reward.
Dealing with fuel impurities means weighing the pros and cons of different engine technologies and fuel treatment systems as well as initial and long-term costs. For example, if the fuel quality falls within the limits prescribed for hardened low-energy-fuel engines, then that engine can be used with minimal or no fuel treatment equipment, likely saving hundreds of thousands of dollars in capital and fuel treatment operating costs. On the other hand, if the fuel impurities are such that extensive pretreatment is required regardless of the engine technology, then high-compression/high efficiency engines may be a prudent choice, as operating savings from the efficiency gains will help offset the costs of installing, operating, and maintaining the treatment system.
Knowing the gas
In choosing a strategy for dealing with impurities, there is no single right answer. The best approach for a given site depends on operating conditions, financial objectives and, above all, on biogas quality. Fuel needs to be analyzed over time to understand the level of impurities, the methane content and heating value, and how much those parameters vary over time – hourly, daily or seasonally. Trending of all these is important. It is common for biogas originating from anaerobic digestion to contain about 60 percent methane, 35 percent carbon dioxide, and the balance other gases. It is critical to understand the volumetric energy content in the fuel. For example, a lower-quality biogas with just 30 percent methane content means doubling the gas flow to deliver the same energy – and exposing the engine to twice the amounts of impurities. In addition, the fuel low heating value – the actual Btu/ft3 (MJ/Nm³) – influences the size of the fuel delivery system.
The fuel’s carbon dioxide content is also critical. Carbon dioxide affects flame speed in the cylinders, and the temperature must fall below a certain maximum level to keep exhaust and valve temperatures in a safe operating range and minimize maintenance. Another key variable is the ratio of methane to free inert gases, including carbon dioxide and free nitrogen (nitrogen not entering naturally from air in the fuel). Excessive inert gas content hinders fuel ignition, causing lean misfire and resulting in a loss of power and increased in exhaust emissions.
Siloxane content is a critical variable found primarily in biogas from landfill and wastewater treatment plants. High-compression ratio engines are often preferred in biogas applications for their greater efficiency, but they are also less tolerant of siloxane contamination. Sites using high-compression-ratio engines may have no choice but to add fuel siloxane treatment, and its initial cost and ongoing maintenance needs to be considered in the project financial profile.
“Biogas is now recognized as a valuable renewable fuel”
Application Example: Using Biogas in a Wastewater Treatment Facility
The Encina (Calif.) Water Pollution Control Facility uses biogas from its anaerobic digester to fuel four Cat® G3516 generator sets rated at a combined 3260 kW. On a typical day, the generator sets run mostly on the biofuel; one operates on natural gas for four to eight hours per day during peak rate times. Online since 2009, the system generates 12 million kWh per year, meeting about 71 percent of the Encina Wastewater Authority’s needs. The agency’s plan calls for the wastewater facility to produce 96 percent of its energy needs on site by 2020.
Biogas is now recognized as a valuable renewable fuel. Besides creating a new source of revenue, a biogas project can help further corporate commitments to carbon footprint reduction and sustainable operations. Businesses with potential to produce and use biogas are wise to explore the possibilities in detail.
Michael A. Devine
Gas Product Marketing Manager
Caterpillar Electric Power