Operating Cost Reductions in Stoker Boilers With Natural Gas Co-Firing

• Efficiency Improvements
  
• Relaxed Coal Specifications
  
• Opportunity Fuels
  
• Summer Loads/Turndown
  
• Derate Recovery
  

Overview

Natural gas co-firing is a technology that blends the most desirable characteristics and capabilities of natural gas with more difficult and problematic solid fuels like coal and wood wastes. Natural gas co-firing involves the use of a very small amount of natural gas, typically less than 10%, firing over stoker grates through special burners. The special co-firing burners that have been developed for this purpose are of a high-pressure drop, very turbulent design. The effect of the gas-firing zone is to burn out carbon particulate, lower excess air requirements, and smooth out solid fuel variability and excursions. This translates into lower operating costs and improved stoker operations even with natural gas fuel costs being more than the cost of coal on a BTU basis.

Burner companies throughout the world and especially in the United States have installed natural gas burners into solid fuel fired boilers, primarily for back up, for more than 50 years. This, however, is not co-firing. Co-firing is the special systems, equipment, and operational techniques that allow for natural gas to be burned simultaneously with other fuels.

Natural Gas Co-Firing

Starting in the mid 1980's, the gas industry began exploring whether auxiliary gas burners could offer significant benefits to boiler operators when used for sustained gas co-firing rather than solely for warm-up or standby duty. Early tests showed that warm-up or standby burners could not provide all of the turbulence and flame characteristics required to maximize co-firing benefits.

Consolidated Natural Gas Company (CNG), through one of their LDC's (Local Distribution Companies) East Ohio Gas, evaluated co-firing at Kent State University and at a power plant in Painesville, OH (the sites had chain grate and spreader stokers, respectively). These early projects were both equipped with single COEN DAZ burners. The Gas Research Institute (GRI) evaluated co-firing at a Vanderbilt University spreader stoker equipped with dual COEN DAZ burners. These early co-firing sites were the first generation of co-firing specific burners. These burners provided a broader range of benefits to the owners than warm-up burners.

The process of co-firing has come through a learning curve and progression just like a number of other technologies. The early projects described above used off-the-shelf single gas burners designed for 100% firing. These burners did displace some coal BTU input with natural gas and showed additional benefits of improved operational flexibility, increased efficiency, and reduced NO_x emissions and opacity.

Additional work was needed in both market development and burner /boiler engineering to deliver the full potential of co-firing benefits. To further develop co-firing practices, GRI initiated a series of demonstration projects with a team comprised of East Ohio Gas, Columbia Gas, The Industrial Center, Acurex Environmental, COEN Company, and CEC Consultants, Inc. to optimize co-firing equipment and practices, and to verify performance and economic benefits.

Co-firing today typically means two burners arranged for tangential offset opposed firing to maximize turbulence. The design of these burners creates a spinning flame as the burner fires over the grate. This turbulence makes for an incineration zone over the grate.

At the time of this paper, 15 stoker co-firing projects are operating or in a state of final procurement as shown in Table 1.

Table 1: Current Stoker Co-firing Projects

The most prominent economic drivers for co-firing projects have traditionally been environmentally based. Co-firing is one of the most cost effective means to avoid air pollution compliance upgrades with stoker boilers when using solid fuels. Co-firing's ability to reduce particulate is well documented through extensive testing at a number of the operational sites. The five most popular operating cost reduction drivers that have been experienced with co-firing include efficiency improvements, relaxed coal specifications, opportunity fuels, summer loads/turndown, and derate recovery. The balance of this paper provides examples of how stoker boiler owners and operators can take advantage of each of the primary drivers previously described.

Co-firing Operating Cost Reduction Scenarios

The growing population of successful natural gas co-firing with coal or wood projects has yielded important information on operating cost savings. The following scenarios are for a fictitious site created for example purposes. However, the information used has been obtained or inferred to be reasonable based on actual site performance data shown in Table 2.

Table 2: Coal Stoker Information

Description of Operating Cost Reduction Drivers

Efficiency Improvements

Co-firing with natural gas in stoker boilers has been shown to improve efficiency (heat rate) by 2.6% at Dover Power and Light (DP&L), OH and 3.0% at Oberlin College, OH. The three phenomena that contribute the most to this are carbon burnout, excess air reduction, and firebox heat transfer improvements.

Carbon burnout occurs as unburned carbon in airborne particulate comes off stoker grates and becomes consumed in the gas co-firing zone. DP&L had 33 - 35% measured reductions of carbon in fly ash.

Carbon burnout also helps to reduce quantities of ash that need disposal. Co-firing's more effective burnout of carbon also makes for less sooting and a cleaner fireside. This improves heat transfer between soot blows and also reduces soot blow frequency. When co-firing burners are deployed they also make for more effective mixing/ turbulence in the firebox area. This increases heat transfer in the firebox section of the boiler. Stack temperature reductions of 15-20°F have been experienced with co-firing.

Co-firing burners deliver air and oxygen for combustion (excess air) in a zone where over-fire air is typically introduced. This air helps to minimize the need for over-fire and excess air.

Sites have experienced an overall reduction in induced draft (I.D.) fan airflow requirements. The reduction in flow requirements and friction losses makes for horsepower savings at the I.D. fan.

Efficiency increases tend to almost offset the increased fuel costs associated with natural gas. These increases alone generally do not offer enough operating cost savings to make co-firing attractive demonstrated in Table 4.

Table 4: Efficiency 3.0% With Co-firing

Relaxed Coal Specifications

Co-firing with natural gas has been documented to give boilers a wider operability range. Operability for a stoker generally means the ability to perform without opacity (smoking) episodes and/or slagging. Co-firing also reduces sulfur dioxide, nitrogen oxide, and carbon monoxide emissions. This increased operability also means that a wider range of fuels can be successfully burned.

Co-firing experiences to date have shown the biggest coal specification change opportunity to be in the area of burning more fines. Coal fines (severely undersized particles) generally blow through stokers and cause back end (flue gas cleanup) problems. Co-fired boilers (the Dover, OH case especially) have been able to accept substantially more in the way of fines.

The potential for trying local/lower-priced coals may have merit with a co-fired stoker. This is especially helpful in a world in which stoker coal suppliers are becoming scarce.

The following economics shown in Table 5 apply to our example site for a scenario where coal costs are able to be reduced by 10% by accepting relaxed specifications.

Table 5: Relaxed Coal Specifications 10% Coal Cost Reduction

Opportunity Fuels

Co-firing's unique ability to incinerate carbon/volatiles over stoker grates makes for a unique opportunity to utilize waste fuels including biomasses (i.e. opportunity fuels). In cases where waste fuels are used, co-firing may make preparation (sizing/drying) less critical. This can especially improve the potential for more biomass use.

In cases where opportunity fuels have been considered but deemed not feasible (possibly solid wastes from food or furniture manufacturing) because of a need for baghouse upgrades or extensive back end cleanup, times may have just changed. A number of studies have identified co-firing as the lowest, first cost way to reduce particulate emissions for many operating/fuel scenarios.

The following economics apply to the example site for co-firing with 10% opportunity fuels. It was assumed for purposes of this paper that an opportunity fuel would be available at $.10/MMBTU's. In some cases, off-site disposal cost savings would credit handling/preparation costs for the opportunity fuels shown in Table 6.

Table 6: 10% Opportunity Fuels

Summer Loads/Turndown

Co-firing gives you the ability to turn your old, minimal turndown stoker boiler (typically four to one or less) into a 10 to one or more turndown gas boiler. This can be done almost instantaneously and only for as long as you need it to be that way.

Our firm has known of a number of cases where stokers can not get down to where they need to be for summer loads or during process load reductions (evenings and/ or weekends). The answer for some is to make the minimum amount of steam and just vent it. Still others discontinue stoker operations at the first sign of seasonally lower loads and run gas boilers. Co-firing allows for the turndown conditions to occur in a way that matches and follows loads.

Another issue to consider is the typical degradation of stoker boiler efficiency as loads drop. Operating on gas during these load conditions makes for more efficiency. This also helps to offset the natural gas cost premium. Cost savings can come from eliminating the need to make steam and then vent it. Savings can also come from eliminating the need to operate alternate equipment (standby boilers) just to handle what the stoker cannot.

Firing on gas during low loads and summer conditions also frees up labor and makes for less wear and tear on boiler auxiliaries. Additional electrical savings also occur from reduced coal/ash handling equipment operations. In some cases overtime for coal or ash handling system maintenance can also be minimized.

The potential for cost reductions available from eliminating summer or low load waste is demonstrated in Table 7.

Table 7: Ten to One Turndown Versus Four to One

Derate Recovery

Co-firing has been demonstrated to allow stoker boilers to once again operate near design steaming capacity. Steaming capacities can degrade over time for a number of reasons. In some cases, coal conditions from various suppliers have changed from design such that operating at full load makes for severe opacity (smoking).

Recovering this capability with co-firing makes it possible to avoid the use of alternate equipment (in some cases gas boilers) to regain this load. This means that more load can be provided at lower mixed fuel costs. Derate recovery also helps reduce costs where on-site power generation occurs. Our firm has seen a number of cases where derated stokers leave money on the table by not generating enough steam to meet turbine capacities. Co-firing helped Dover Light & Power and Boise Cascade increase peak steaming capacity, which made for an additional 1.5 MW of capacity at each site. The potential for co-firing cost reduction related to better use of mixed fuel capacity for the example site are shown in Table 8.

Table 8: 20,000 MMBTU's Increased Capability

Wood Waste Derate Recovery

Derate recovery is especially important to stoker operators burning wood wastes. Coal fired stokers usually get into capacity constraints related to particulate/emission restriction issues. When some coal stokers try to run at high loads they smoke due to changes in coal quality, a lack of forced draft and induced draft fan capacity, and/or less than optimal particulate removal equipment function.

Wood stoker capacity constraints are more an issue of wood wetness. The pulp, paper, and wood waste burning industry is now seeing more wet wood on a more regular basis. This is occurring due to the industry cutting younger trees that are greener and wetter as it preserves old growth tree stands. Also, timber is now transported over greater distances since local wastes are becoming scarcer. When this occurs wood waste fuels often get soaked by rain and snow in transit.

Most of the industry's wood-fired stoker boilers were designed to burn fuel with a maximum of 50% moisture. Today's wood wastes often contain even more water. This moisture reduces the boiler's efficiency and burdens its combustion air fans because more fuel and oxygen are required to burn off the water. Because of this, wood waste stokers can't make as much steam. In many cases this means production or power generation from topping turbines must be cut.

Wet wood also means lots of carbon monoxide (CO) gets generated, making both emissions and safety problems. Natural gas co-firing minimizes wet wood CO production.

The Boise Cascade plant in Emmett, ID recently applied co-firing very successfully for wet wood waste issues. The plant's natural gas co-firing system makes almost full rate steam and electrical power production possible even with wood wastes containing 55% water. The co-firing system has also improved carbon burnout and has dramatically reduced CO emissions. The incremental power output now available during wet wood conditions is worth as much as $2,000 per day.

Another successful wet wood co-firing installation is in place at the Washington WaterPower facility. This site can now reach near full output with wood that is over 50% moisture. Having gas as a back-up fuel has also meant improved plant reliability.

Conclusion

Operating cost benefits for the example site described above, using each of the five largest operating cost savings drivers, make for the following annual benefits: (8,000 hours total, 65% at full load, 35% at 15 MMBTU).

Table 9: Example Site Annual Benefits

It's very rare that all of these benefits would be available to any one site. However, it's also very rare that co-firing is implemented only because of operating cost benefits. In fact, most of the installed site drivers considered both environmental and operating cost benefits as primary motivators.

Site Economics

The cost of co-firing depends on many site-specific issues. However, installed costs have generally ranged between $250,000 and $400,000 per boiler. Most sites with only one or two of the significant operating cost drivers identified will find attractive investment returns with co-firing.

All sites present unique and specific circumstances. Careful detailed analyses need to be done before project commitments are made.

Co-firing stoker boilers with natural gas are now becoming a more general tool with substantial operating history. The technology has now evolved and operating practices are well defined. The industry expects the number of project sites to at least double by the end of 1999 with wood sites being a major growth sector.

About the Author: John R. Puskar, P.E. is president of CEC Consultants, Inc., in Cleveland, OH. He holds a bachelors degree in mechanical engineering from Youngstown State University and an MBA from the Weatherhead School at Case Western Reserve University.