Miura America Co., Ltd. (Miura) manufactures boilers designed to operate on natural gas, propane, and #2 oil. Operating with alternative fuels such as biogas is at the sole discretion of the customer. Currently, there are no ASME pressure vessel codes or UL standards regulating the use of biogas in a boiler. Therefore, it is not possible to use biogas and maintain a UL listing from the factory; field approval may be necessary, depending on jurisdiction. However, there are clear OSHA guidelines for managing the harmful components of biogas. Customers who use biogas in a Miura boiler are solely responsible for its proper use.
LX series and low NOx (EXN and LX-SGN) boilers are currently incapable of running on biogas fuels. If a customer plans to burn biogas in the boiler(s), they should install EX series Miura boilers.
Fuel Requirements
For the boiler to function correctly and have the best chance of maintaining correct combustion, Customer should use biogas which complies with the following:
Fuel Composition and Stability
Fuel composition must be clearly defined and must not fluctuate, as specified below.
Hydrogen Sulfide (H2S) in the Work Environment
The concentration level of hydrogen sulfide (H2S) in Customer’s work environment should not exceed 10 parts per million (“ppm”). Although OSHA standard is not to exceed 20 ppm, Miura recommends 10 ppm for the better protection of human health.
Hydrogen Sulfide (H2S) and Corrosion
To prevent corrosion of boiler components in contact with hydrogen sulfide (H2S) before it is combusted, the concentration level of hydrogen sulfide (H2S) in the biogas should always be less than 100 ppm.
Methane (CH4)
Methane is the main combustion component in biogas. Natural gas usually contains more than 95% methane, but biogas contains much less. Miura boilers require ≥60% methane in the biogas to achieve complete and stable combustion. If methane content drops below 60%, the boiler will incompletely combust the fuel, causing the boiler to shut down.
Carbon Dioxide (CO2)
Carbon dioxide is usually the second largest component in biogas, and it does not contribute to combustion. Miura boilers require ≤40% carbon dioxide in the biogas to achieve complete and stable combustion. If carbon dioxide content rises above 40%, the boiler will incompletely combust the fuel, causing the boiler to shut down.
Fuel High Heating Value
Natural gas provides a high heating value of approximately 1000 BTU/SCF while biogas provides much less. Unlike natural gas, biogas usually contains small amounts of other gases such as ethane and propane which contribute to the heating value along with methane. The total high heating value of biogas must be ≥612 BTU/SCF to achieve complete and stable combustion. Additionally, the high heating value of the biogas should not fluctuate more than +/- 2.5%. If the total high heating value drops or the high heating value fluctuates too much, the boiler will incompletely combust the fuel, causing the boiler to shut down.
Siloxane
Biogas may contain siloxane which can lead to mineral deposits on surfaces and combustion trouble. If siloxane is present, a device must be installed to completely remove siloxane before it enters the boiler.
Extraneous Biogas Content
Moisture, liquids of any kind, foreign materials, and other gases not listed as allowed must be removed from the biogas before it enters the boiler.
Stabilized Pressure Supply
Biogas supply pressure must remain stable. A biogas holding tank is recommended. If the existing biogas supply pressure is not adequate, Customer should install and properly maintain a biogas booster pump to achieve the required pressure.
Pilot Gas Supply
The boiler’s pilot fuel must be LPG or natural gas.
Biogas Fuel Mixture Properties and Conditions
Verified by Customer. Final mixture should not fluctuate more than ±1.5%.
| Composition | Composition | Fluctuation of average value |
|---|---|---|
| Hydrogen Sulfide (H2S) | ≤100 ppm | |
| Carbon Monoxide (CO) | ≤500 ppm | |
| Methane (CH4) | ≥60% | ± 1.5% |
| Carbon Dioxide (CO2) | ≤40% | ± 1.5% |
| Total High Heating Value | ≥612 BTU/SCF | ± 2.5% |
| Siloxane | 0 ppm | |
| Supply Gas Pressure | Must be determined based on high heating value |
Fuel Management
The customer should:
- Consistently monitor and maintain stable fuel properties
- Maintain a stable fuel supply pressure
- Monitor for fuel leaks and maintain safety equipment
Monitoring
It is required by OSHA to monitor the concentration level of H2S in the work environment, and it is the customer’s responsibility to install a densitometer (concentration level sensor) and monitor the level so that the H2S concentration does not exceed 20 ppm. Miura recommends maintaining ≤10 ppm for the better protection of human health. Should the concentration ever exceed 10 ppm in the boiler room, Customer should turn off boilers and not allow personnel to enter the boiler room until the H2S issue is resolved.
Emissions
Miura has not conducted tests using any specific biogas mixture as a fuel for any boilers. Therefore, emissions levels cannot be guaranteed. It is expected that NOx emissions will be reduced compared to the standard emissions level for natural gas, but this cannot be confirmed without field data.
Customer Conditions
Please complete fuel composition data and return to Miura.
| Composition | Composition | |
|---|---|---|
| Hydrogen Sulfide (H2S) | ppm | |
| Carbon Monoxide (CO) | ppm | |
| Methane (CH4) | % | |
| Carbon Dioxide (CO2) | % | |
| Total High Heating Value | BTU/SCF | |
| Siloxane | ppm | |
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