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August 2014 · Energy-Tech Magazine
October 2008 Go to Page 1 2 3 4 5
Operating experience of a coal-fired boiler retrofit with an advanced hybrid of coal gasification for SO2 and NOx emissions control
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Figure 1. CCS Flow Diagram
Figure 1. CCS Flow Diagram

Introduction  
Utility and industrial boiler owners want to know, "How can my boiler stay competitive in today's environment with ever higher fuel-costs and new requirements to meet EPA's stringent air-quality emission regulations?"  
The recent operating experience of an advanced hybrid of coal-gasification retrofit on an industrial coal-fired boiler shows promise to reduce operating cost, improve boiler
efficiency, and control both SO2 and NOx emissions.
The project described here is not an Integrated Gasification Combined Cycle (IGCC) system, but a much simpler, lower-cost, air-blown, entrained-flow, coal gasification technology that replaces the boiler's existing burners. Final combustion of the gasified coal is completed in the boiler furnace with over-fire air (OFA). Called the Clean Combustion System™ (CCS), the CCS simply prevents formation of SO2 (sulfur dioxide) and NOx (nitrogen oxides) emissions during coal combustion. Such a boiler conversion can meet EPA's stringent new air quality regulations while burning the lower-cost coals. This hybrid of coal-gasification technology can make the conversion of gas, oil, and coal-fired boilers to coal-firing more practical, while also helping achieve lower operating cost and emissions compliance.

The Clean Combustion System  
The CCS concept evolved from a confluence of rocket-engine technology and advanced combustion modeling techniques, and from experience in coal gasification and wet-bottom (slag-tap) boiler design and operation.  
In conventional coal-fired boilers, the fuel is fully oxidized to carbon dioxide (CO2) and water (H2O). The nitrogen and sulfur in the coal also are both oxidized to the pollutants nitrogen oxides (NOx) and sulfur dioxide (SO2). In coal-gasification, firing coal with a limited amount of air (oxygen) entails partial oxidation (of the hydrocarbons); to create a hot gas whose major components are hydrogen (H2), carbon monoxide (CO), and nitrogen (N2). See Figure 1. CCS Flow Diagram.
Earlier field demonstrations of the CCS show that during controlled high temperature gasification conditions, the formation of both NOx and SO2 can be prevented.
The CCS operates much like a conventional pulverized coal-fired burner, using typical off-the-shelf hardware that is familiar to the operators. It fires pulverized coal with some limestone added to provide the calcium that captures the sulfur in the coal, developing a clean, hot, fuel-rich gas that enters the boiler furnace. Subsequent over-fire air (OFA) staged in the furnace completes the combustion with excess air, providing at least the same furnace steaming performance as before the retrofit.

Boiler retrofit - the CCS Stoker®
Phenix Limited, LLC, was contracted to design, engineer, and supply the equipment to install the Clean Combustion System (CCS-Stoker®) on a 1940s coal-fired stoker boiler. The client installed the equipment. The project's objective was to significantly lower the facility's energy cost by firing low-cost, high-sulfur coals with emissions control, in lieu of burning expensive compliance (low-sulfur) coals.  
The project was initiated in October 2005. Since this was an emissions control project, a permit to construct was received with waivers of NSPS (new source performance standards) and PSD (prevention of significant deterioration) and no NSR (new source review). The permit emissions limits were <0.9 # SO2/mmBtu and <0.25 # NOx/mmBtu. Compliance coals were to be fired until the unit was commissioned. Construction began in August 2006 and commissioning check-out commenced in January 2007.
The unit began regularly firing coal by April, with Phenix providing T&M consultant support. In May, several equipment modifications were made, and by the end of
June, operational testing at design loads began.
A cross section view of a typical stoker boiler is shown in Figure 2. Lump stoker grade coal is fed from the coal bunker to a hopper and metered by the stoker feeder onto the boiler's moving grate. The coal is suspended on the moving grate and burns with air fed from below until combustion is completed. The coal ash drops from the front of the grate into an ash pit below the grate.
The steps to a CCS-Stoker® retrofit are significant for this type of boiler, since the unit must be converted to a pulverized coal firing system. The CCS-Stoker® retrofit
modifications include:
  • Remove the existing coal stoker and ash hopper, and also brick-over the grate system.
  • Install a new CCS Burner and gasification chamber, firing through the stoker opening.
  • Add a new air preheat system and OFA distribution ducting.
  • Install a coal pulverizer and coal feed system.
  • Provide a powdered limestone silo and metering system.
  • Install a new bottom ash (and slag) collection system.
  • Replace the obsolete manual controls with a new redundant computerized burner management and combustion control system with operator interface screens.
  • The motor control center also was replaced with new equipment, as were much of the piping, wiring, and boiler instrumentation.

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