When considering modifying an existing coal-fired boiler to burn natural gas, it is usually in terms of a complete changeover of fuels. In some cases, it may not be possible to achieve 100% load capability on natural gas alone. Reasons may include: the supply of natural gas in the area is insufficient (and increasing the supply capability is not feasible or cost-prohibitive), or the supply may be curtailed at times. In these cases, co-firing of both coal and natural gas is an option. Natural gas would normally be used for boiler lightoff, initial warming, and low load operation; coal would then be used, either alone or in conjunction with natural gas, to achieve rated capacity.
While the addition of natural gas firing will eventually help the facility realize reduced fuel cost, it may take longer to reach the “break-even” point due to conversion and installation costs. Although many of the benefits of a complete changeover [of coal to natural gas] will not be fully realized, there are still significant merits to firing both fuels. There are also some unique control issues which must be considered when co-firing.
One of the primary benefits of converting to natural gas firing capability is emissions reduction and the need for most, if not all, emissions control equipment. In a co-firing environment, emissions control equipment must remain operational for the periods of time when coal will be burned. However, emissions during initial lightoff, where control can be difficult, is simplified during lightoff on natural gas. Further, co-firing units can typically achieve 50% or greater capacity firing natural gas alone. During these periods, existing emissions control systems can be bypassed.
Another benefit of converting to natural gas firing capability is a fuel cost and the associated savings of maintenance on coal handling equipment operation and maintenance. Again, in a co-firing environment, the coal burners must remain in service to bring the facility to rated load when the natural gas cap is reached. This means maintenance and operation of coal storage, conveying, crushing, and other handling equipment.
One benefit of natural gas conversion which remains fully-realized when co-firing is the improved low load capability. The enhanced “turn-down ratio” capability with natural gas makes extended operation at low-load conditions significantly easier to maintain, either through the use of the natural gas ignitors, natural gas main burners, or a combination of the two.
There are some unique controls considerations which also must be taken into account while co-firing coal and natural gas. Will all emissions control equipment be bypassed during natural gas-only operations? If not, what effect does the low-emission content of the natural gas exhaust have on the existing systems? For example, are there temperature considerations if the furnace exhaust is routed through the baghouse to prevent condensation? How will ignition of coal be handled – will the natural gas ignitors or natural gas main burners be close enough to the coal burners to achieve ignition? Are the existing flame scanners and other instrumentation capable of co-firing?
Just as when considering a full conversion from coal firing to natural gas firing, consideration to perform a conversion to co-firing capability is likely centered on long-term costs. As discussed previously, many of the cost-saving factors achieved with full conversion will not be fully realized when the co-firing capability is maintained. However, because it may not be possible to supply the facility with sufficient natural gas to achieve full load, co-firing can still be an effective method of reducing long term costs and emissions.
While design and construction of the conversion project are vital to success, choosing a company to develop a training and qualification program as well as operating and maintenance procedures is just as vital. Make sure that the company chosen to perform these aspects of the project has the expertise and experience necessary to develop and deliver an Operational Readiness Program that will meet those demands.
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