Operating with an effective overfire air (OFA) system is key to optimizing the combustion process in a biomass boiler and reducing its carbon monoxide (CO) emissions. This post will discuss the basics of combustion optimization and OFA system design. Part 2 will focus on the measured reductions in CO from several of Jansen’s recent OFA upgrades.
To review, CO formation in a boiler furnace is caused by incomplete combustion, during which fuel carbon burns to CO rather than carbon dioxide (CO2). In an ideal combustion process, all the carbon would completely burn to form CO2 so CO emissions would be zero. But combustion in the real world is not ideal so CO emissions are a fact of life. Minimizing CO should be a top goal for good reasons. Economically it represents unburned fuel, reduced thermal efficiency and drives fuel expense higher. Environmentally, CO should be minimized to remain in compliance with any permit stack CO limits. Additionally, stack carbon dioxide (CO2) levels per pound of steam generation will also drop, thus reducing the boiler’s greenhouse gas footprint.
In a stoker-fired boiler, the two main air sources for grate fuel combustion are undergrate air (UGA) flowing from below, and OFA entering above the grate through dedicated ports or nozzles on the furnace walls. Both are equally important in the combustion process: UGA serves as the primary oxygen source for the combustion of fuel particles on the grate, while OFA burns out volatile gases released by the grate fuel along with any solid fuel particles lifted into suspension. Proper design of the OFA system is crucial. It must be sized to deliver adequate air (usually around 40% of the total air needed for combustion) and provide enough air jet velocity to penetrate deep into the furnace.
In a fluidized bed boiler, combustion air flows are similar to those of a stoker-fired boiler. In these units, primary air (PA) enters the furnace under the fuel bed, like UGA, and secondary air (SA) enters above the fuel injection ports like OFA does. The major differences are that PA is supplied through nozzles at pressures typically 20 inches water gauge (in. w.g.) or greater, compared to 1-2 in. w.g. for UGA, and that fluidized bed boilers generally have multiple SA ports on all four walls rather than just two walls. Regardless of boiler type, air supplied above the grate as OFA or SA serves to promote complete combustion in the lower furnace and is therefore an important tool in CO management.
Jansen’s OFA systems are specifically designed to achieve these goals. The system uses our patented High-energy Multi-range Combustion Air Nozzle™ featuring the large nozzle of the type illustrated in Figure 1. The nozzle geometry creates only a small pressure drop across the nozzle, allowing the air to penetrate deep into the furnace with high velocity and high momentum. The nozzles are arranged in an interlaced configuration on the boiler sidewalls as shown in Figure 2 with typically three to four nozzles per sidewall. In contrast, many older OFA systems on biomass boilers have multiple rows of small ports. The ports’ small size creates a high pressure drop, significantly reducing the velocity of each air jet and hence its ability to effectively contribute to fuel burnout.
Figure 1: Jansen Multi-Range OFA Nozzle
Figure 2: Jansen OFA nozzles on a biomass boiler
The engineers at Jansen are well-versed in applying basic principles to solutions for real-world challenges. Contact us to discuss ways we can use that knowledge to help you improve your boiler’s operation.
Authors:
Morgan Silverman – Process Engineer
Steve Campbell, P.E. – Senior Process Engineer
Samit Pethe – Manager, Process Technologies