Biogas Basics

Biogas is produced by anaerobic digestion of organic matter by bacteria and consists of mostly methane and carbon dioxide. However, exact composition depends on the process that created the gas. Biogas is less energy dense with a higher heating value typically around 600 BTU/SCF than typical fuels like natural gas or propane.

Miura Experience

The Miura Group has substantial experience but limited sales in North America due to the low cost and abundance of natural gas.

• Miura Japan has sold >150 biogas boilers
• Miura China has sold >100 biogas boilers and produces around 10/year
• Miura America has established designs with limited installations.

Biogas products are in active development and a new low-NOx biogas burner for use in EX-style boilers is currently in testing.

Miura America Products

Boiler Models

• EX(H) boilers with standard NOx burners are available for biogas (standard NOx mean <100ppm but >30ppm.)
• LX boilers and low-NOx EX boilers cannot operate on biogas due to their specific burner designs.

Boiler Options

• Economizer typically not supplied due to corrosion and clogging concerns, but special stainless options are available.
• Natural gas can be used as a backup fuel by using the dual gas train option, but combustion tuning is mandatory between fuel changes because of the shared air damper.
• Oil or propane fuels cannot be used without changing the main burner.

Challenges with Biogas

Fuel Quality

• Methane content varies from 45% to 75% by volume
• The remainder is mostly CO2
• HHV varies greatly
• Fuel supply can be subject to fluctuations in feedstock
• Fuel pressure is very low when produced and must be boosted

Solutions

• Mix with natural gas to raise and stabilize HHV
• Use gas pressure booster and regulator to maintain stable, adequate supply
  • Stable fuel supply is absolutely necessary to maintain proper combustion

Corrosion

• Biogas contains up to 2500 ppm sulfides
  • Compare to less than 7 ppm sulfides in natural gas
• Hydrogen sulfide (H2S) is highly corrosive
• H2S combusts to form sulfur dioxide (SO2), which reacts with water to become sulfuric acid (H2SO4), also highly corrosive

2H2S ＋ 3O2 → 2H2O ＋ 2SO2

2SO2 + O2 + 2H2O = 2H2SO4

Solutions

• Pre-treat biogas to reduce H2S to below 100 ppm in fuel
• Remove economizer or use special economizer design

Scaling

• Biogas often contains siloxanes
• Silicon-based compounds
• Builds up in the system
  • Reduces heat transfer
  • Restricts airflow and prohibits proper combustion

Solutions

• Pre-treat biogas to remove siloxane from fuel
  • Some biogas sources do not produce siloxane
• Remove economizer or use special economizer design

Safety

Hydrogen sulfide is hazardous to human health and OSHA limits allowable concentration in a work environments to at or below 20 ppm.

H2S gas detectors are required in the boiler room along with a gas train air purge system if H2S concentration > 20ppm

Miura Fuel Requirements

Miura biogas boilers require the following fuel composition

Miura Biogas Boiler Design Details

• Specialty main gas train with biogas rated components and larger pipe sizes for increased flow rates
• Special biogas burner
• Independent natural gas or propane pilot line
• Gas train air purge system with control box
• Increased blower size
• no economizer as standard but stainless-steel options are available.
  • Residual siloxane will clog the standard economizer
  • hydrogen sulfide will corrode the steel at the temperatures within the economizer.
  • Without economizer efficiency ~78% resulting in 12.8MMBTU/hr input required for 300BHP
    • >12.5MMBTU/hr may trigger additional jurisdictional rules not normally encountered.
    • 12.5MMBTU/hr = ~290 BHP to derate the boiler.

Required Auxiliary Equipment

The following equipment is required for biogas operation but is not available from Miura.

• Pre-treatment
  • H2S and siloxane scrubbers
  • Gas mixers
  • Pressure boosters
• H2S gas leak detectors