Retrofitting gas, oil and coal-fired boilers can reduce operating costs and meet EPA regulations
By Keith Moore, Phenix Limited
Coal remains the most stable low-cost source of energy, but its lower qualities challenge users to meet stringent environmental regulations. The first step in any modification or fuel switching of a boiler requires careful evaluation of the U.S. Clean Air Act Amendments (CAAA) “New Source Rules” (NSR), Prevention of Significant Deterioration (PSD) or New Source Performance Standards (NSPS). If a modification is judged by EPA to increase the plant’s emissions, the modification may result in a violation of the CAAA that can trigger fines, cause the plant to be shut down or be required to upgrade emissions controls with best available control technology. As a result, boiler owners are sometimes nervous of initiatives to reduce operating cost; for example, switch fuels, improve efficiency and even repair and maintain the boiler equipment.
Any opportunity to reduce operating costs must first show that the new emissions from the modified facility will be reduced and will meet state and CAAA environmental rules.
The trademarked Clean Combustion System (CCS) concept evolved from a confluence of advanced combustion modeling know-how, experience implicit in coal gasification and wet-bottom (slag-tap) boiler operation and design.
In conventional coal-fired boilers firing with excess air, the fuel is fully oxidized to carbon dioxide (CO2) and water (H2O). The nitrogen and sulfur in the coal are also both oxidized to the pollutants NOX and SO2. Whereas in coal-gasification, firing coal with a limited amount of air entails partial oxidation of the hydrocarbons to create a hot gas whose major components are hydrogen (H2), carbon monoxide (CO), nitrogen (N2) and some unburned carbon. Under certain gasification conditions, it has been shown that the formation of both NOX and SO2 can be prevented from forming.
The CCS is a high temperature, air-fed entrained flow gasifier that replaces the boiler’s existing burners. The CCS operates much like a conventional PC burner, using typical off-the-shelf hardware familiar to operators. It fires pulverized coal with some limestone added (the only “chemical” needed) to provide the necessary calcium to capture the sulfur in the coal and provide a clean, hot fuel-rich gas to the boiler furnace. Subsequent over-fire air (OFA) staging in the furnace completes the combustion with excess air, providing the same furnace performance as before the retrofit. Figure 1 shows the CCS process flow diagram.
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Commercially available fluidized bed combustor (FBC) designs have shown that sulfur capture in combustion is a well-established process. An FBC boiler burns coal with some limestone at rather low temperatures (<1,600 F) and long residence times (+2 seconds). The calcium from the limestone reacts with sulfur during combustion to form a solid CaSO4 compound, thereby meeting EPA’s SO2 emission regulations.
In contrast to an FBC, the CCS process operates to capture sulfur in a different combustion regime. The CCS gasifies pulverized coal at high temperatures wherein the combustion reactions occur in milliseconds. With limited or no oxygen present, the sulfur reacts with calcium to form a calcium sulfide (CaS) compound, which is a solid particle under these conditions, thus removing the sulfur from the gas stream. The high CCS temperatures also melt the coal ash, largely alumna (Al2O3) and silica (SiO2), which then combine with the solid CaS. The result is a “sulfur bearing glass” waste product. The melted ash separates from the gases and drains into a water-filled quench tank for disposal. The resulting hot combustion gases that flow into the boiler furnace are primarily nitrogen, carbon monoxide and hydrogen. Any coal fly-ash entering the boiler furnace tracks the gas temperatures and solidifies to a non-sticky fly-ash particle. Also, as sulfur was removed, SO3 in the stack gases is near zero.
The predominant source of NOX from coal-fired plants results from the oxidation of the nitrogen inherent in the coal (typically around1 percent) and is about 85 percent of the total NOX emissions from coal firing. The additional NOX is created from the high temperature (thermal) oxidation of the nitrogen in the air; which is a function of the time and temperature profile in the boiler furnace section.
The CCS gasification process simply prevents formation of NOX from fuel-bound nitrogen. Even though the temperatures are high, the NOX levels at the CCS gasifier exit are very low. The hot, “clean” fuel-rich gases then enter the boiler furnace where sufficient time is made for the gases to cool and generate steam. At these lower furnace temperatures, the thermal NOX formation is frozen, yet the gases are still hot enough to complete combustion of CO and H2. Over-fire air is then added to the furnace through appropriate ports to complete the combustion (with excess air to 3 percent O2). The clean hot gases, without the pollutants SO2 and NOX, continue through the boiler to generate steam, just as before the conversion. Note that the final combustion is quite efficient, with low CO in the gas or carbon in the fly ash (LOI <1 percent).
The high temperatures in the gasifier melt the ash products in the coal and drains from the gasifier to a water-filled trough and drag-link conveyor. In this form, the sulfur is bound tightly and cannot be released by oxidation with air or leached by water. The waste product is safe for landfill and also has potential value. This product sells (at around $3 a ton) to the metal working industry for use as shot grit. The fly-ash product is easily removed downstream of the boiler by a bag house or typical electrostatic precipitator, providing stack emissions particulate control. There is no wastewater discharge.
The CCS technology was conceived at Rockwell International and evolved from theoretical combustion modeling that predicted the feasibility of simultaneous control of NOX and SO2 from firing coal. This led to a progression of R&D programs from conceptual testing and pilot scale operation, to a “greenfield” demonstration at an industrial site in Alberta, Canada. Throughout this period, guidance, funding and peer review were provided by a group of utilities including Southern California Edison, Houston Light & Power, Niagara Mohawk, Wisconsin Public Service and TransAlta Utilities.
TransAlta Resources Investment Corp., the non-regulated subsidiary of TransAlta Utilities, initiated a field demonstration called the Low NOX/SOX - Coal Applications Pilot (LNS-CAP) Project at the ESSO Resources Cold Lake - Mahihkan site in Alberta heavy oil recovery facility.
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Figure 2, entitled “LNS-CAP - 3 T/hr Burner / Boiler Emissions”, shows the measured stack outlet data from this field demonstration. SO2 emissions reported were around 0.2 SO2 lb. /106 British thermal units (Btu). Excellent NOX control was also demonstrated, with an emission rate of around 0.15 lb. NOX /106 Btu. Carbon conversion was also good with carbon (LOI) measured in the fly ash collected in the baghouse less than 0.1 percent. No carbon is found in the slag.
As important, the facility was comparatively easy to operate with straightforward startup, turndown and shutdown. Late in the demonstration period, the facility ran with one operator per shift. It was noted that the coal-fired stack had lower emissions than the adjacent steam generator units firing “raw” natural gas.
The initial commercial application of the CCS is to update and retrofit an industrial stoker design boiler.
The steps to a CCS-Stoker retrofit are significant for this type of boiler, as the unit must be converted to pulverized coal firing. Custom engineering modifications include:
- Removing the existing coal stoker and ash hopper and brick-over the grate system
- Installing new CCS Burner and gasification chamber, firing through the stoker opening
- Adding a new air preheat system and air distribution ducting
- Installing a coal pulverizer and coal feed system
- Providing a metered flow of powdered limestone to the pulverized coal
- Installing a new bottom ash (and slag) collection system
- Replacing the manual controls with a new PLC - BMS, with necessary piping, wiring and instrumentation.
Figure 3 shows a drawing of the CCS-Stoker boiler, including the CCS gasification chamber, pulverizer and baghouse retrofit modifications.
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This project is now under construction with planned startup in early 2007. As an emissions reduction project, the plant has received a construction permit with waiver of PSD and NSPS requirements for the modifications. Emission reduction projects do not trigger CAAA NSR issues.
The CCS-Stoker retrofit is expected to reduce the facilities cost of energy by firing the lower-cost higher-sulfur coals with improved boiler efficiency. The limestone additive is estimated to add $1.00 per ton of coal fired to the operating cost. The CCS retrofit will also replace high maintenance cost items like the stoker grate, coal feeders and ash system. The annual maintenance requirements for the new CCS burners should consist of an annual outage for inspection of refractory surfaces, boiler internals and instrument calibration.
The primary application of the CCS technology is to retrofit older, smaller (less than 300 MW) coal-fired power plants that cannot afford the cost of FGD scrubbers and SCR/ammonia systems. Adding more CCS burners to a common gasification chamber allows straightforward scaleup to larger megawatt boilers. The CCS modifications to a typical pulverizer coal boiler (three components in all) are to replace the existing burners and burner wall with a fabricated CCS burner/gasifier section; add new, multiple, overfire air ports and ducting; and convert the furnace to “wet bottom” operation with a new slag/ash transport system. The CCS modification will continue to use the existing boilers pulverizer and air heater as is. Balance-of-plant modifications include the limestone additive and necessary equipment to feed the powdered limestone/coal to the CCS. Most of this equipment will fit within the existing boiler wind box and adjacent footprint. Ongoing CCS retrofit studies for power plant units will develop site-specific requirements of each, and confirm an engineering basis for efficient multipollutant control and over all retrofit design.
With the control of both SO2 and NOX and much of the fly-ash, the CCS can address EPA’s Clean Air Interstate Rules (CAIR) when firing most Eastern and Midwest bituminous coals. When firing Western low-sulfur Powder River Basin coals, the CCS has potential to qualify a new coal-fired power boiler with CAAA NSPS performance.
References: “Demonstration of TransAlta’s Low NOX/SOX Burner”; Frasier, W.L. and G. Elia, International Joint Power Generation Conference, San Diego, CA October 6 - 10, 1991.
Author: Keith Moore is president of Phenix Limited, LLC