| Boiler technology


For example, coal pulverisers and ash handling equipment are not required when firing natural gas, and neither are SO2


scrubbers and particulate


reducing baghouses. The conversion to natural gas also will result in notable lower operating and maintenance costs for the facility. Maintenance, upkeep and consumable expense associated with pulverisers, ash handing equipment, scrubbers, and baghouses can be eliminated, and ongoing replacement component costs will be reduced as the conversion from coal to gas eliminates the severe erosion and corrosion conditions associated with coal firing and prolongs the life of the steam generator components. The regulatory environment and economic considerations involved in determining if a coal to natural gas conversion is appropriate are site- and location-dependent. Sites with a readily available or reasonably attainable natural gas supply and regulatory approval often have the necessary drivers in place to justify the conversion.


Performance considerations Coal is a widely variable fuel composed of differing percentages of carbon, volatile matter, ash, and moisture. These non-uniform characteristics are considered in the design of the steam generator, and associated equipment, and units designed for the same output conditions may be notably different due to the coal characteristics around which they were designed.


By comparison, natural gas is a much more consistent fuel, comprised predominantly of methane (CH4


).


While some generalisations can be made when comparing differences in the utilisation of coal and natural gas in a steam generator, each option still must be evaluated on a case-by-case basis, with the specific conditions of the site being considered.


Natural gas also has a clear advantage over coal in terms of lower CO2


and greenhouse


gas emissions due to its notable hydrogen component. Heat is generated from the burning of both the carbon and hydrogen components of the natural gas/methane, forming CO2


and water,


emissions in comparison to natural gas for the same heat output.


while the combustion of coal involves the burning of carbon nearly exclusively, resulting in higher CO2


However, the water formed in the burning of the hydrogen component of natural gas will vaporise in the furnace of the steam generator, and, as a result, there is a negative efficiency impact. While this efficiency impact is not significant enough to offset the carbon benefits in comparison to coal, the increased heat input while firing gas may have an impact on steam generator performance.


Another differentiator is that coal includes components that are not involved in the combustion process or the generation of heat – notably moisture and ash. The moisture in coal is not driven from the combustion process, but rather is one of the constituents of the fuel. Natural gas has no moisture component in the fuel, nor does it have any ash.


As a result, coal firing has a higher mass flow of flue gas than its natural gas counterpart, and


Natural gas combustion system for retrofitting to an existing coal boiler


Local control cabinet


Ignition transformer Gas igniter


XCL-S burner


Existing BMS cabinet


New BMS cabinet


Local burner/ igniter valve train


Natural gas header Ignition cooling air blower


this difference in mass flow may impact the heat transfer in the furnace and in the convection surfaces of the unit.


One means of offsetting this difference is the utilisation of flue gas recirculation – adding increased mass flow in the gas firing case to make it more comparable to the coal for which the unit was originally designed. Another benefit of this approach is lower NOx


emissions, as NOx


emissions from natural gas can be reduced via flue gas recirculation – a common method of reducing thermal NOx source of NOx


emissions, which are the primary when combusting natural gas.


Finally, with respect to the steam generating unit’s performance, a newly designed and supplied gas fired unit will very likely perform differently from an identically designed unit that has previously fired coal. These differences are attributable to the ash and slag accumulation on the unit’s steam generating component surfaces, and their impact on heat transfer. A unit that has been operating and firing on coal for prolonged period will not be as clean as a newly supplied unit.


As a first step in evaluating a coal to natural gas conversion, an experienced and capable engineer should be engaged to evaluate the performance impacts on the unit. Ideally, current operating performance data, the current fuel analysis, and emissions targets should be provided to the engineer, so a thorough unit evaluation can be conducted.


Scope of supply considerations There are both newly supplied components as well as areas of the existing steam generator that must be assessed for alterations in the typical coal to natural gas conversion.


With respect to newly supplied components, there will be a definitive need for components related to the natural gas fuel delivery system and combustion, or burner, hardware. With respect to combustion equipment, there are many variables to consider in defining the scope of a coal to natural gas conversion. Most coal fired units were designed for a pulveriser- out-of-service condition, and in the conversion to natural gas for wall-fired steam generators, the number and configuration of the burners should be assessed. In many instances, the total number of burners can be reduced, and the arrangement of the remaining burners can be evaluated for benefits to boiler performance – specifically furnace exit gas temperature. In addition, for both wall and tangentially fired units, emissions are a consideration, as it may be efficient to convert a top burner elevation to overfire air. If the existing burners are in good condition, with air vanes/burner tilts operable, it may not be necessary to completely replace burner hardware, and the addition of gas nozzles or elements can be considered to modify the existing combustion hardware. If the burners are in poor condition, then a full burner replacement may be prudent. Another critical aspect is the gas supply system to safely provide natural gas to the combustion equipment. These systems typically include pressure regulation and flow-control valves and double-block and bleed pressure relief and venting components.


When considering the scope related to altering existing components, the control system, or DCS, must be modified. Additional control system hardware (including input/output modules) may be required when coal will continue to be utilised, while a complete switch to natural gas


www.modernpowersystems.com | October 2024 | 13


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