By Richard F. Abrams, Babcock Power Environmental Inc. and Kevin Toupin, Riley Power Inc.
There is a need for today’s power plants to meet the growing demand for electricity while, at the same time, achieving efficient combustion, low emissions and no net CO2 releases into the environment. Biomass boilers equipped with new combustion techniques enhance efficiency, which results in lower heat rates. Combined with new, proven emissions control devices to significantly reduce NOX and CO emissions, the challenge of meeting higher energy demands will be met.
We will discuss new combustion advancements and present quantitative comparisons of the new units available today vs. the existing biomass boilers in operation. Advancements such as a new over-fire air (OFA) design and state-of-the-art stoker technology with grate oscillation/vibration are used to increase combustion efficiency. The furnace is designed to reduce flue gas laning along the walls and increase the mixing of fuel and air. Without impacting performance these improvements afford better fuel utilization, lower unburned carbon, lower CO emissions and the ability to handle a wide range of fuel moisture content.
We will also introduce a new system for the eduction of NOx emissions to levels hereby unheard of for biomass boilers. Emissions are controlled using a system called the “RSCR®”, which is a selective catalytic device applied to the “cold” gas (after the boiler and particulate removal equipment) prior to its discharge to the stack, achieving NOx reductions of greater than 80 percent. This system will be described in more detail in the July 3 issue of Power Engineering’s electronic newsletter.
Biomass Stoker Combustion
Historically, industrial biomass combustion systems utilized three types of stokers: water cooled stationary stoker (commonly referred to as pin-hole grates), traveling grate spreader stoker (TGSS) and water cooled vibrating/oscillating stoker. Biomass combustion technology has evolved from incineration of a nuisance waste fuel to combustion of a valuable fuel. With this biomass fuel evolution, the combustion systems have been continually upgraded for improved efficiency. Currently, environmental regulations have added further changes to the stoker designs, as discussed in this article. The resulting objectives of a modern biomass stoker combustion system include: maintaining an efficient, stable combustion process while supplying the desired boiler heat input with low emissions.
- Efficient Combustion: Produce efficient combustion with low carbon monoxide (CO) and low unburned carbon (UBC).
- Stable Combustion: Produce stable and consistent combustion to maintain consistent design parameters and boiler performance.
- Heat Input: Generate the heat input to produce the desired boiler steam flow, pressure and temperature.
- Low Emissions: Produce low carbon monoxide (CO), low unburned carbon (UBC) and low nitrogen oxides (NOX).
When optimizing the combustion system, it is important to understand that both the combustion systems and boiler systems are not stand-alone entities. Both systems are interlinked. To optimize the overall plant design, both the combustion and boiler systems need to be designed together.
Three T’s of Combustion
The age-old “Three T’s” of combustion-time/temperature/turbulence-apply to the design of an efficient biomass stoker combustion system. Some of the combustion design parameters that play an important roll in meeting the requirements of the “Three-T’s” on stoker units are fuel characteristics, fuel distribution, air distribution, fuel/air mixing, reduced air infiltration and furnace retention time.
Improvements of modern stoker combustion systems consist of both combustion and boiler components. The following lists recent system improvements: improved fuel feed control and distribution across the grate, improved combustion air distribution, advanced overfire air systems, reduced excess air requirements, lowered grate heat release rates (larger grate surfaces), increased furnace retention time, improved furnace arrangements to reduce flue gas laning along the walls and increase mixing of fuel and air, improved mixing of fuel and air by the use of grate oscillation/vibration and reduced air infiltration by new seal designs (air bypassing the combustion system). For more detail, refer to the boiler arrangement shown in Figure 1.
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Table 1 lays out a general performance summary and comparison of biomass past stoker designs verses modern stoker designs.
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The benefits of the modernized stoker system are exhibited in the combustion system stability, lower CO emissions and lower unburned carbon, all of which can be directly measured and the improvements evaluated. However, these improvements, as associated with all combustion processes, optimize the combustion process but do not reduce the NOX emissions. In fact, NOX tends to increase with combustion optimization. Methods for reducing NOX emissions consist of lowering excess air (approximately 5 percent reduction), adding flue gas recirculation (approximately 10 percent reduction), adding a SNCR (approximately 40 percent reduction) and/or adding a back-end RSCR (described in a Power Engineering magazine e-newsletter exclusive July 3).
The following lists the major equipment design upgrades associated with stoker combustion improvements.
Fuel Feed and Fuel Distribution
The objective is to uniformly distribute fuel on the grate surface to reduce fuel piling and maintain proper air/fuel ratios. This is achieved by both fuel feed and fuel distribution systems. Principal enhancements include:
- Improved pneumatic fuel distributor designs allowing for side and depth distribution adjustments. This is achieved by controlling the air pressure, air vanes and fuel trajectory plate angle.
- Improved control room excess oxygen measurement across the boiler width by the use of multiple oxygen sensors located at the economizer outlet. The excess oxygen profile across the unit is a direct indication of the combustion system fuel and air distribution. Analyzing the excess O2 profile across the unit, the operator can adjust the fuel and air distribution to optimize combustion.
- Improved control system capable of biasing individual feeder fuel flows to balance the fuel distribution on the grate.
Stoker Designs
The objective is to provide a surface to combust the larger fuel particles and to remove the ash and inorganic materials after combustion. Currently the most common stokers are the traveling grate spreader stokers, stationary water-cooled grates with steam cleaning, and water-cooled vibrating grates.
Principal enhancements include:
- Improved combustion air distribution by compartmentalizing the grate air plenum. This allows for control of the air flow to the grate sections for balancing or biasing air flow to the grate.
- Improved designs for increased grate clip pressure drop (increased back pressure) which improves the air distribution through the grate and reduces the influence of the fuel/ash bed thickness on air distribution.
- Improved grate clip metallurgy allowing for combustion air temperatures up to 600 F. Higher combustion air temperatures improve the drying of high moisture woods.
- Improved stoker seal designs to reduce leakage around the periphery of the stoker.
- Improved fuel and air mixing, reduced fuel piling and improved fuel distribution by using grate surface oscillation/vibration.
- Increased grate surface casting life by using water cooled grate surface. Water cooled surfaces allow the capability to vary air flow as required without overheating of the grate cast surface (i.e.: the surfaces do not require air flow for cooling) and also reduces maintenance costs.
- Design the grate surface area for a maximum of 850,000 btu/hr-ft2 (previous designs were at 1,000,000 and greater)
Furnace Designs
The objective is to combust the fuel and recover radiant heat generated by the combustion process. (Refer to Figure 2, which outlines the separate furnace zones and design objectives.)
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Principal enhancements include new furnace configurations to improve combustion efficiency (reduce CO and LOI) while firing a range of wood waste fuels. The objectives of the new furnace configurations:
- Establish a defined combustion zone.
- Reduce stratification along the side walls.
- Increase mixing and turbulence in the combustion zone.
- Increase residence time in the combustion zone and main furnace.
- Minimize char (unburned carbon) entrainment and char carryover.
- New furnace designs include flat wall, single arch and double arch designs all with multi level OFA systems.
Over-fire Air Designs
The objective is to complete the combustion occurring in suspension and reduce the unburned carbon furnace carryover. Principal enhancements include:
- Design changes to increase the turbulence and mixing of the OFA and fuel in the combustion process by improved nozzle penetration and optimized nozzle locations.
- Reduce flue gas stratification along the water walls.
- Control capability for varying the OFA flow to optimize combustion during fuel changes and at reduced loads.
- Design OFA flows for 50% total combustion air flow.
- Multiple OFA levels with individual level control dampers.
- Use of Computational Fluid Dynamics (CFD) modeling to optimize the OFA system design.
Cinder Reinjection Designs
The objective of this system is to reinject the char (unburned carbon) from the convection pass hoppers and/or dust collectors back to the combustion zone for reburning. Principal enhancements include improved sand separator designs with the development of rotary sand separators and vibrating sand separators for increased performance and reliability.
Editor’s Note: In an online exclusive, the July 3 issue of Power Engineering magazine’s e-newsletter (available at www.power-eng.com) continues this idea by featuring an article by Richard F. Abrams and Kevin Toupin discussing a new system for the eduction of NOx emissions to levels previously unheard of for biomass boilers. They describe how emissions are controlled using a system called the “RSCR”, which is a selective catalytic device applied to the “cold” gas (after the boiler and particulate removal equipment) prior to its discharge to the stack, achieving NOx reductions of greater than 80 percent. The authors will describe the design and overall performance of a typical biomass boiler plant using these new technologies. And they provide actual operating data on the RSCR, which was retrofitted to existing biomass-fired units.