Biogas, a renewable energy source, is one of several benefits associated with anaerobic wastewater treatment. With rising fuel costs, options exist even if a low volume of biogas is being generated. However, the abundance of biogas utilization alternatives can often make the question of what do with your gas difficult to answer. If a system is properly investigated, planned, and executed, biogas can often offer cost savings or generate income for a facility. Biogas can be utilized to generate heat, electricity, and natural gas. Options to create these energy sources are outlined below.
Electricity and Heat (Cogeneration) Options
Cogeneration is the simultaneous production of usable thermal and electrical energy from biogas. The most common application is as fuel for internal combustion engines used to drive pumps, blowers, compressors, or electric generators. For greater energy efficiency, engine coolant and exhaust gases can be used as heat sources for a variety of purposes.
Internal Combustion (IC) Engines
An IC cogeneration unit is powered by a conventional internal combustion engine connected to a generator. The engine is generally modified to use the lower Btu value biogas. Heat is recovered from the engine water jacket and the exhaust pipe recovery jacket. IC units are available in a wide range of sizes, from 250 kW up to 3,000 kW. The biogas typically needs to be cleaned somewhat to reduce maintenance requirements. The primary fuel gas parameters of concern are sulfides, moisture, and siloxanes. IC units have electrical efficiencies of 36 to 40 percent. With heat recovery, the overall efficiency can be increased to 80 to 85 percent.
A microturbineis the small version of a gas turbine. The basic components of a microturbine are the compressor, turbine, and the generator. Microturbine units are generally available in relatively small sizes ranging from 30 kW to 250 kW. Heat is recovered from the compressor and turbine. The incoming biogas must be cleaned to a significant level, generally for sulfides, moisture, and siloxanes. The biogas required must possess high inlet pressure. Microturbines typically have electrical efficiencies between 30 to 34 percent.
Fuel cells have been around since the mid-1900s, and the space program has used them for decades. A fuel cell operates similar to a battery. It converts oxygen and hydrogen into electricity in the presence of an electrolyte. Biogas can be used as a fuel source in high-temperature, high-efficiency fuel cells designed for stationary applications. Heat is recovered from the hot air exhaust. Fuel cells are efficient, quiet, and have very low air emissions. Fuel cells have stringent fuel gas requirements, requiring a significant level of biogas cleaning. Commercial fuel cells are available in a range of sizes from 250 kW to 2,000 kW. Fuel cells have electrical efficiencies of 42 to 45 percent.
In contrast to traditional internal combustion engines that burn fuel and air in the cylinder, Stirling engines are external combustion engines. The Stirling engine uses external heat to expand the gas contained inside the cylinder and push against its pistons. The incoming biogas can be relatively dirty compared to some of the other technologies and can also be operated at low incoming fuel pressure. The Stirling engine is a good fit for small biogas producers as it is only available in sizes less than 100 kW. Stirling engines have electrical efficiencies between 29 to 31 percent.
Natural Gas Options
Biogas is often utilized for fuel if boilers already exist onsite. Boilers can be supplied with burners designed for biogas only or a combination of fuels. Little or no biogas cleaning is required for boiler use, reducing overall capital and maintenance costs. Biogas can also be cleaned to utility natural gas quality for use at one’s facility or to sell to the local utility. Natural gas utilities have stringent fuel gas requirements, requiring a significant level of biogas cleaning.
Biogas Cleaning Requirements
For biogas to be used as a fuel, most of the impurities must be removed because they can cause corrosion, deposit, and wear in the equipment. Substances that require special attention are hydrogen sulfide (H2S), moisture, and inerts (oxygen and carbon dioxide). The following table depicts some of the requirements for the use of biogas in the five options.
|Parameter||IC Engine||Microturbine||Fuel Cell||Stirling Engine||Natural Gas
|H2S (ppm)||< 1000||< 400||< 0.1||NA||< 4|
|Moisture (dewpoint)||< 60°F||< 40°F||< 40°F||NA||< 7 lbs per million scf|
|Inerts (oxygen)||NA||NA||< 5% by vol||NA||< 4%|
|Pressure (psig)||1-6||60-75||30||> 2||varies|
IC engines and fuel cells are often good fits for systems with moderate to high gas flows. IC engines offer long, successful track records, a variety of suppliers and good overall energy efficiency. Fuel cells have higher efficiency, but also have a high capital cost and biogas cleaning requirement.
Microturbines and Stirling engines, at this time, are better suited for small biogas flows. Natural gas line injection is only cost effective with very large volumes of biogas (> 1,000 cfm) due to the high quality of gas required. The utilization of biogas for boilers is often cost effective.
Hopefully this sheds light onto the options available for biogas utilization. Funding is available in many states for evaluating and analyzing waste-to-energy alternatives.