George Wiltsee and Holly Emerson
BioCycle February 2004, Vol. 45, No. 2, p. 53
During the last three years, the first commercial microturbine projects using biogas at landfills, municipal wastewater treatment facilities, dairy and hog farms, and food processing plants have demonstrated several advantages of small gas turbine technology over small internal combustion engine technology. These initial microturbine projects have also provided some important “lessons learned”.
Landfills and wastewater treatment plants have incorporated cogeneration facilities to utilize their methane production for many years. Typical projects are several megawatts in size, and use large engine generators, small gas turbines, and/or boilers and steam turbine generators. In 1999-2000, when the first 30-75 kilowatt microturbines became available, several waste management agencies sponsored demonstration projects and tests. Success in these early demonstrations led to more than 20 commercial microturbine projects at landfills and wastewater treatment plants, ranging in size from 30 kilowatts to 1.5 megawatts. New microturbine projects are being planned and installed at an increasing pace in these applications.
A number of issues relating to manure management are currently facing operators of large dairy and hog farms, also known as Concentrated Animal Feeding Operations (CAFOs). The EPA estimates that the costs incurred from addressing these issues, as they relate to the updated 503 regulation, will result in the closing of many large CAFOs. In many cases, the solution involves the installation of an anaerobic digester for manure collection and processing, which can be costly. On-site generation of electricity and hot water can be an important element in making an anaerobic digester installation economically feasible. In most cases, project sizes are below 500 kilowatts. The majority of projects to date have used engine generator sets, but a few microturbine projects have been installed in CAFO applications.
Food and beverage processors represent another emerging opportunity for generation and use of biogas from biomass waste and wastewater streams. Many of these facilities send their effluents to municipal treatment plants. As the food processors grow, and as the municipal treatment plants strain to serve growing populations, pressures are increasing to move the treatment (or pretreatment) of these effluents into the processing facilities themselves. Use of anaerobic digesters and cogeneration systems provides opportunities for reduction of both wastewater treatment bills and energy bills at these facilities. A small number of microturbine projects have been initiated in these applications, and more are expected.
Microturbine systems are small gas turbine generation packages designed to provide on-site power and thermal energy, with low emissions and low maintenance requirements. The turbines use lean premix combustion systems for low NOx and CO emissions (less than nine parts per million by volume in the turbine exhaust, at an adjusted oxygen content of 15 percent). Actual turbine exhaust oxygen content is about 18 percent. The turbine engines are recuperated, which results in an efficiency of about 30 percent. Exhaust leaves the recuperator at 450°-600°F, depending on the specific microturbine model. The electrical output is at 480 volts and 60 cycles/second.
Thermal energy can be captured and used on-site in several ways, depending on the site details. Hot exhaust can be ducted directly into an air-to-water heat exchanger to generate hot water from the exhaust for a variety of possible uses. In some cases, the hot exhaust can also be used directly in the facility.
Microturbines were originally designed to burn natural gas and to be used in commercial and industrial cogeneration applications. They have now been adapted for use with biogas, flare gases, and other waste fuels.
The most significant financial benefit of the use of digester gas for on-site energy generation is usually the reduction in the facility’s electric bill. In some cases, revenues are derived from the sale of excess electricity to the utility company.
HOT WATER PRODUCTION
A by-product of the gas turbine combustion process is heat. This heat can be used directly from the exhaust or to produce hot water through the use of an air-to-fluid heat exchanger. The production and on-site use of both thermal and electrical energy is known as cogeneration or combined heat and power (CHP). Some of the uses for hot water and/or hot exhaust include:Digester heating; Heating of the facility workspaces; Absorption chiller power (air conditioning, refrigeration); and Thermal drying and pelletizing of biosolids for sale or land use.
COSTS, EMISSIONS AND ODOR CONTROLS
Project costs can often be reduced through the use of grants, loans and utility net metering programs. Various state and federal incentives are available for renewable energy, and for agricultural biogas projects in particular. Often these incentives can ensure the success of a project. In California, New York, Minnesota, Wisconsin and other states, biogas cogeneration projects have been assisted with grants and other financial incentives that have offset project costs by 30-50 percent.
The capture and beneficial use of the methane generated from the digester reduces greenhouse emissions. In addition, some fossil fuel-derived energy and carbon dioxide emissions are offset by the biogas recovery and use. A microturbine can offset its own weight in fossil fuel CO2 emissions every day that it operates on digester gas. Additionally, microturbines emit very low emissions of NOx and CO, especially when compared to the more commonly used IC engine generator.
Odors associated with concentrated animal waste are not only a nuisance; the foul-smelling air is also a potential liability to the farmer due to perceived health risks. Although an anaerobic digester installation can reduce these odorous compounds by up to 97 percent, the collection and temporary storage of manure can also result in unwanted odors. It is possible that these “excess” odors can be effectively eliminated by a microturbine.
Central Gas Manitoba and other partners conducted a collaborative study at the city of Winnipeg’s wastewater treatment plant to test the control of odors and noxious gases using microturbines. It was determined that the high turbine inlet temperature effectively destroyed 99 percent of hydrogen sulfide and 95 percent of total odors from a highly intense point source odor stream fed through the combustion air inlet of the microturbine.
MICROTURBINE TECHNOLOGY VERSUS SMALL IC ENGINE GENERATOR TECHNOLOGY
A number of manufacturers or packagers supply IC engine generators for use with digester gas. Models range in size from less than 50 kW to greater than 800 kW. Some models accept digester gas at atmospheric pressure; others require compression of the gas. Hot water can be recovered from jacket cooling and from exhaust heat exchangers.
Project developers and mechanics are very familiar with IC engine generators, and generally prefer to select this technology despite its drawbacks, which can include: Frequent oil changes and overhauls; Large number of moving parts and a resulting tendency to break down; Dedicated maintenance personnel; Noisy and dirty operation; and High emissions of NOx, CO, and other pollutants.
Microturbines, in contrast, are relatively new and unproven in these applications. The number of manufacturers is small, and model sizes range from 30 kW to 250 kW at present (see Table 1). Microturbines require digester gas to be compressed to about six atmospheres, and require the compressed gas to be dried. Hot water can be recovered from exhaust heat exchangers.
To date, a small number of project developers and plant operators have chosen to install microturbines for digester gas applications. In general, they are seeking the following benefits from this relatively immature technology: Infrequent oil changes and overhauls; Very small number of moving parts to potentially break down; Unmanned, remotely controlled operation; Quiet and clean operation; and Extremely low emissions of NOx, CO, and other pollutants.
George Wiltsee and Holly Emerson are with Ingersoll-Rand Energy Systems. This article is adapted from presentations made by the authors at two recent conferences – Anaerobic Digester Technology Applications in Animal Agriculture and the Third Annual BioCycle Conference on Renewable Energy From Organics Recycling.
LESSONS LEARNED IN EARLY BIOGAS MICROTURBINE PROJECTS
The first microturbine tested on digester gas was a Capstone 30 kW unit at the Palmdale wastewater treatment plant (Los Angeles County Sanitation Districts). The lead author of this report supervised that test, and subsequently supported the installation and operation of more than 100 microturbines in more than 20 projects that used digester gas or landfill gas as fuel. Some of these projects have operated quite successfully and have achieved up to 20,000 operating hours per turbine. Other projects experienced problems, primarily due to difficulties encountered in fuel conditioning. Some of these have been converted to successful projects after replacing or reworking the fuel conditioning equipment. The lessons learned from this experience are summarized as follows:
o Drying of compressed digester gas is very important. Otherwise, compounds in the condensate foul the microturbine fuel control valves and fuel injectors. Refrigerated dryers have generally proven to be effective and reliable.
o Some compressor failures have also been caused by condensate. Gas/liquid separation and the design of the compressor cooling system are important.o Materials of construction must address the corrosive properties of digester gas. Stainless steel is best. Carbon steel and yellow metals are not recommended.
o Siloxanes in sewage sludge digester gas and landfill gas convert to silica (ash) during combustion. This problem does not exist in agricultural or food processing digesters.
o When clean, dry fuel is supplied reliably to the microturbines, the systems operate continuously, with no attention required for months at a time.
In general, projects that utilize experienced biogas engineers and fuel conditioner designers are successful from the start. Projects completed by companies unfamiliar with biogas tend to produce serious “learning experiences.”