Modifying an existing coal-fired boiler to natural gas firing capability, either to replace or to supplement coal as the primary fuel source, is an extensive – and expensive – project. Although each project is different, the scope of the conversion will ultimately determine the installation costs. Capital costs can be offset by several factors. First, natural gas systems have fewer “moving parts” such as coal conveyors, crusher/dryers, etc. For this reason, the maintenance and manpower costs associated with the natural gas systems are less expensive than the coal systems. Second, projected fuel costs between coal and natural gas show that natural gas will remain cheaper per BTU than coal for the foreseeable future. Finally, by ranking higher in the merit order, a natural gas powered unit may be called upon more frequently to be brought online to meet demand, thereby increasing revenue.
There are numerous factors which must be considered when evaluating the possibility of a coal to natural gas conversion project, such as cost (both short term and long term), access to fuel supply, emissions, and impacts to power plant performance. Each factor must be evaluated to determine if a fuel conversion is feasible.
Initial cost is a large factor to be considered. Converting a coal fired boiler to natural gas requires extensive modifications to existing power plant equipment (including the internal structure of the boiler), the installation of new equipment, and the demolition (or retirement) of existing coal handling equipment. In some cases, existing plant equipment such as flame scanners may be able to be reused following the conversion. However, a careful evaluation of each system should be made to ensure that the equipment to be reused won’t have a negative impact on the performance of the power plant following conversion.
Additionally, the adequacy of the natural gas supply must be considered. Some sites may already have natural gas available, either to other units or as an ignition fuel source. In this case, the existing supply may be sufficient to meet the demand. However, if the existing supply is not sufficient, or there is no supply available, the addition of a new natural gas pipeline as well as the associated regulating and filtration equipment must be considered.
Firing natural gas causes a significant change to the boiler heat profile as compared to firing coal. Because firing natural gas nearly eliminates slagging and fouling of boiler tubes, the heat released by burning natural gas is absorbed lower in the furnace. This results in less heat available in the upper areas of the boiler for the superheater tubes (pendants), which results in lower superheat temperatures. Some boiler designs are able to operate despite this reduction in temperature; others require additional equipment, such as flue gas recirculation fans, to raise the overall boiler temperature to maintain superheat temperature and maintain boiler efficiency.
These initial costs must be balanced with the long term financial benefits of natural gas firing. Fuel costs are one of the primary considerations in this long term analysis. The direct cost savings of natural gas verses coal are obvious; others may not be as readily apparent. Transportation of fuel, fuel handling (coal pile management, conveying, drying, crushing, etc.) are all eliminated. The need for post-combustion controls, such as ash removal and chemical injection for emissions control may be completely eliminated from the power plant processes. These processes are both labor and maintenance intensive, and by elimination, significant cost savings to operations can be achieved.
Any new systems associated with the conversion will require modifications to the existing control systems, including changes to the burner management system to maintain boiler safety and to the combustion control system to maintain boiler load. Changes to power plant operating procedures, such as startup and shutdown, maintenance procedures, and qualification procedures for both present and future operators must also be considered.
While design and construction of the conversion project is 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.
