Energy Efficiency and Renewable Energy

Improve Boiler Combustion Efficiency

Combustion Efficiency

Operating the boiler with the optimum amount of excess air minimizes heat loss through the stack and improves combustion efficiency. Combustion efficiency measures how effectively the heat contained in the fuel is converted into usable heat. Stack temperature and the oxygen (or carbon dioxide) concentration in the flue gas are key indicators of combustion efficiency.

Assuming complete mixing, a precise or stoichiometric amount of air is required for a given quantity of fuel to react completely. In practice, combustion conditions are never ideal, so additional or “excess air” must be supplied to ensure complete fuel combustion.

The correct amount of excess air is determined by analyzing the oxygen or carbon dioxide concentration in the flue gas. Insufficient excess air leads to unburned fuel, soot, smoke, and carbon monoxide, while excess air causes heat loss due to increased flue gas flow and thus reduces the boiler’s overall fuel-to-steam conversion efficiency. The table relates stack measurement values to boiler performance.

Recommended Actions

Boilers often operate with more excess air than optimal. Monitor flue gas composition periodically and regularly optimize your boiler settings to keep excess air at optimal levels.

Consider online monitoring of oxygen levels in the flue gas to quickly identify energy-loss trends. This can provide early warning of control system malfunctions and enable data-driven decision-making.

Combustion Efficiency for Natural Gas

Well-designed natural gas-fired systems can achieve excess air levels of 10%. A commonly used rule of thumb is that every 15% reduction in excess air or every 40°F drop in flue gas temperature can increase boiler efficiency by 1%.

Example

A boiler operates 8,000 hours per year and consumes 500,000 million Btu (MMBtu) of natural gas annually while producing 45,000 lb of steam per hour at a pressure of 150 psig. Flue gas measurements show an excess air level of 44.9% when the temperature difference between the flue gas and combustion air is 400°F. According to the table, the boiler’s combustion efficiency is 78.2% (E1). By optimizing the boiler settings, the excess air level is reduced to 9.5%, and the temperature difference between the flue gas and combustion air is 300°F. The boiler’s combustion efficiency rises to 83.1% (E2). Assuming a fuel cost of $8.00/MMBtu, the annual savings are:

         Annual Savings    = Fuel Consumption × (1–E1/E2) × Fuel Cost
                                       = 29.482 MMBtu/year × $8.00/MMBtu
                                       = $235.856

Flue Gas Analyzers

The percentage of oxygen in the flue gas can be measured using low-cost gas absorption test kits. More expensive handheld and computer-based analyzers, ranging from $500 to $1,000, display oxygen percentage, flue gas temperature, and boiler efficiency. These instruments are a recommended investment for all boiler systems with annual fuel costs exceeding $50,000.

Oxygen Trim Systems

Consider using an online oxygen analyzer in systems where fuel composition varies significantly (for example, refinery gas, wood waste fuel, or multi-fuel boilers) or where steam flow varies considerably. An oxygen “trim” system feeds back to the burner controls, automatically minimizes excess combustion air, and optimizes the air-to-fuel ratio.

For more information on monitoring, download case studies on the following submetering applications from the AMO Publications Library:

• Solutia: Using Submetering to Drive Data-Based Energy Project Approval
• Nissan North America: How Submetering Changed the Way a Plant Does Business

You can also refer to the following guidebook on the Federal Energy Management Program (FEMP) website: www.femp.energy.gov

• Metering Best Practices: A Guide to Achieving Utility Resource Efficiency, Version 2.0

Adapted from an Energy TIPS information sheet originally published by the Georgia Tech Industrial Energy Extension Service.

Resources

The U.S. Department of Energy’s software tools, the Steam System Assessment Tool and Steam System Scoping Tool, can help you assess steam systems and identify potential system improvements. You can also consult Improving Steam System Performance: A Sourcebook for Industry for more information on steam system efficiency opportunities.

To access these resources and many others on industrial efficiency, as well as training information, visit the Advanced Manufacturing Office website at manufacturing.energy.gov.