Overview

Natural Gas and Coal Cofiring is a technology that uses gas burners inserted into the boiler in a way that allows both coal and natural gas to be burned at the same time. The main advantages are environmental and load control. Gas burners can be turned up and down very easily and quickly compared to coal, especially, stoker type coal boilers. Gas burners can also be used for pilot burners to start the flame, and for flame stabilization at low load factors.
Application

Large coal and bio-mass boilers with a need to improve emissions and/or control problems are good candidates for the addition of natural gas burners. GRI did a substantial amount of research and co-funded several pilot installations in the mid-1990s. Their work identified the gas cofiring advantages as falling into two general categories of Environmental and Operational. Because natural gas is more expensive than coal, it does not compete on a BTU basis; there must be some other need to justify a cofiring system.
Environmental Benefits

• Opacity Reduction, especially at start-up and turn-down
• NOx Reduction
• SOx Reduction

Operational Benefits

• Improved Turn-down and easier start-up
• Improved Load following
• Recovered Derate (if capacity was lost due to environmental or coal system limitations)
• Fuel Flexibility
• Standby Reserve (quicker response time)
• Safety (cleaner burning, less carbon/soot build-up)
• Reduced Maintenance
• Seasonal gas usage

Burner Sizing and Economics

Natural gas burners are typically sized for 30 - 50% of total boiler capacity, and generally use 2 or 4 burners located in corners of the fire box. The gas burners will typically fire at 5 - 15% of boiler capacity, depending on their design and needs of the boiler. For example, if used for opacity control, when the boiler is at high-fire, very little gas is needed; however, at start-up and low-fire conditions, a very high percentage of natural gas is required. Burners are measured in millions of BTUs and in MCF/Hour of usage. Annual loads can be substantial.

The burner cost is not substantially affected by its gas input rating. The total cost of the project is mostly affected by the total number of burners to be used and the retrofit cost of the fire box to install them. For example, if technically feasible, 2 burners rated at 75% of boiler capacity is less expensive than 4 burners with a total boiler capacity of 30%.

Payback is generally measured in months, not years, and is NOT on a fuel cost BTU basis. Since gas is likely to always be more expensive than coal or any other bio-mass fuel, payback costs are considered against the costs of a bag house or other emissions control system. In some cases, it may be the ONLY way the boiler can be fired and meet emissions requirements.

For more information see GTI's website www.cofire.com

Source: Energy TechPro