Boiler technology |
may allow the reuse of existing hardware. Older control systems may require updates based on available hardware or requirements of the local jurisdictional authority.
In some instances, burner changes, newly supplied gas supply valve skids and DCS modifications will cover the scope required to allow gas firing. In other instances, efficiency impacts, flue gas mass flow changes, combustion and shifting heat absorption patterns, as well as emissions requirements and air system needs will result in steam generator performance limitations that may be addressed with additional scope. These changes and alterations may include: Air supply duct work: In pulverised coal units, some of the air needed for complete combustion of the fuel is provided via the primary air fans, which convey the coal through the pulveriser and to the burners. An evaluation of the existing forced draft fan against the air flow needs and pressure losses through the system post conversion must also be performed. In some instances, the primary air fans may be needed to supplement the existing secondary air fans to provide adequate air flow, and air supply duct scope must be addressed. Convection pass (superheat and/or reheat) materials and surface: An evaluation of the shifting heat absorption and flue gas mass
Main gas header for a coal to natural gas conversion project
flows may show that existing superheat and/ or reheat metals need to be upgraded to avoid over-stress conditions when operating with natural gas. In addition, the total surface area of these components can be evaluated. If a steam generator is underperforming with respect to
steam temperature, the conversion may also offer the opportunity to correct surfaces and restore turbine generator output. Flue gas recirculation (FGR) fans: Flue gas recirculation may be considered for maintaining gas mass flow and steam generator performance and/or for lower NOx
emissions.
Forced draft fans: In some instances, forced draft fans may need to be modified (tipped), or even replaced, to provide the required air flow and static pressures to the converted unit. In the evaluation of draft losses, the decommissioning of sulphur scrubbing equipment and particulate removal baghouses – items that will not be required when firing natural gas – should be considered.
Case by case evaluation In summary, the conversion of a coal-fired steam generating unit can be considered when an economic natural gas supply can be provided to the site, and when regulatory approvals allow. Many owners and operators of coal-fired steam generators are considering this scenario as
natural gas economics can be favourable and CO2 emissions per MW can be significantly reduced. While natural gas is a very consistent and homogenous fuel, coals are widely varied, as are the impacts relative to the design conditions of the originally supplied equipment. The performance differences between coal and natural gas combustion must be evaluated against the original design and operating characteristics of the exiting unit. Given the wide variability of coals and original unit design considerations, scope needs of the conversion from coal to natural gas must be evaluated on a case-by-case basis and may not only include combustion and natural gas supply components and control (DCS) modifications, but also air flue work, fan and boiler convection pass component metals and modifications.
Burner/igniter gas valve train for coal to gas conversion application, with local control panel
14 | October 2024|
www.modernpowersystems.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51
