Combustion Troubleshooting

Biomass boiler owners and operators are showing increased interest in evaluating the combustion performance of their equipment to improve its efficiency. The driving forces behind this interest are: (1) the rising cost of fossil fuels and the need to generate steam from cheaper sources; (2) stricter environmental regulations to reduce air pollutant emissions, requiring cleaner biomass combustion; (3) the general desire to operate equipment more efficiently and minimize wasted heat; and (4) the increasing demand for using renewable fuels to reduce carbon dioxide output from fossil fuels.

Biomass fuels should be differentiated from the broader category of renewable fuels. Some people define renewable fuel as any non-fossil fuel, including municipal solid waste and tire-derived fuel. However, my definition of biomass fuels only includes various types of waste wood (bark, hogged fuel, sawdust, wood clippings, chips, pellets, construction and demolition wood) and agricultural wastes from harvesting and processing (shells, husks, pits). Black and red spent liquors that are burned in chemical recovery boilers in the pulp industry can be counted as biomass fuel because their fuel value derives from lignin and hemicellulose that are dissolved from wood chips during the pulping process. Waste sludge from water treatment plants, which consists of organic material, is also regarded as biomass fuel, although its high moisture content prevents combustion in the absence of other fuels.

Biomass boilers come in a variety of sizes. The larger boilers burning biomass fuels (alone or co-fired with fossil fuel) are conventional units with a steaming range of about 20 tons/hour to over 250 tons/hour, producing high-pressure steam at elevated temperature for electrical power generation and/or cogeneration of process steam. The two most common technologies for biomass combustion are stoker grates and fluidized beds, with stoker technology far more prevalent in North America. Industries using these biomass boilers include pulp and paper and other forest products, as well as independent power producers. In recent years, utilities have begun to look more into converting their fossil fuel-fired boilers to burn biomass. Here, I focus on the combustion of solid biomass on a stoker grate.

Biomass Combustion Goals
Most biomass fuels have a relatively high moisture content, typically 30 to 55%, but possibly in excess of 60%. The fuel value stems from the carbon and hydrogen content. Combustion consists of reactions between fuel components and oxygen from air and fuel, releasing heat and light. The “Three T’s” must be met for the proper burning of any fuel: Time, Temperature, and Turbulence. Sufficient reaction and retention time is needed to complete the oxidation/combustion reactions, sufficient heat must be present to start and sustain combustion, and adequate mixing of fuel components and combustion air is necessary.

Fig. 1. Combustion occurs in three stages.

Furnace puffing is a symptom of poor combustion.

Biomass combustion occurs in three stages: drying, volatiles release and burning, and char combustion (Fig. 1). The drying stage requires heat to evaporate water, with the rate of drying depending on particle size and temperature. Next, pyrolysis gases (carbon monoxide, hydrogen, carbon dioxide, water, and volatile organic compounds) are released, and oxygen is needed for volatiles combustion. By weight, biomass contains about 70% volatiles and 30% fixed carbon. Finally, char combustion requires oxygen, releases heat, reduces particle size, and leaves residual ash. Combustion problems can be deduced from not meeting all of the Three T’s and can occur in any of the combustion stages.

In pursuit of continuous and reliable operation, the owner/operator typically has the following goals in mind for the boiler:

Meeting these goals simultaneously is often more difficult to achieve for biomass fuels than traditional fossil fuels. Specific operational goals and priorities will vary from boiler to boiler and may change over time. However, factors affecting fuel costs and emissions performance will always have a high priority.

Symptoms Indicating Problems
In attempting to meet operational and economic goals, the owner/operator may face challenges because of combustion problems. These problems are evidenced by one or more symptoms (Fig. 2):

The resulting net effect of these combustion problems is a reduction in the biomass burning rate and, depending on steam demand from the boiler, an increased need for fossil fuel co-firing.

Fig. 2. Graphical illustration of (a) poor combustion and (b) good combustion conditions.

Engineering Evaluation
After identifying the symptoms, the task at hand is to troubleshoot the boiler’s operational performance and determine the root causes of combustion problems. A boiler engineering evaluation characterizes the unit, diagnoses the problem, establishes the unit’s strengths and limitations, and determines the appropriate measures needed to overcome the limitations and meet the goals of the owner/operator. Key steps in the evaluation are:

The outcome of an engineering evaluation is a clear understanding and path forward to remedy the combustion problems.

Typical Root Causes
To meet the Three T’s, the root cause(s) of combustion problems usually are related to inadequacies in the fuel and air delivery (aside from limitations in fuel supply and assuming the biomass is delivered at proper particle size and distribution and acceptable moisture content). The design and operational performance of (1) the fuel distributors, (2) the stoker grate, and (3) combustion air delivery parameters are three areas that have a significant effect on a boiler’s biomass combustion capacity and performance. A further breakdown of these factors shows:

Fuel Distributors:

Stoker Grate:

Combustion Air Delivery:

Effective fuel and air delivery hardware and proper operational parameters provide for these desirable combustion conditions:

Fig. 3. Elements of an overfire air supply upgrade.

Upgrading Combustion Systems
Combustion problems can be eliminated, and relative trouble-free combustion can be achieved, by upgrading some or all of the hardware components or operational parameters in the three major areas listed above (e.g., Fig. 3). For example, since the late 1990s, Jansen Combustion and Boiler Technologies has completed combustion system upgrades on 50 solid biomass-fired boilers. These units were manufactured by a variety of original equipment manufacturers, with installation dates ranging from the mid-1950s to early 1990s. Upgraded units have experienced the following benefits:

Arie Verloop is vice president of technology and client relations for Jansen Combustion and Boiler Technologies Inc. He is a licensed professional engineer, specializing in chemical recovery and biomass boiler processes. Since joining Jansen in 1980, he has conducted and developed engineering evaluation projects for more than 150 biomass boilers and has led many boiler operations training seminars and workshops. Since its inception in 1976, Jansen has conducted performance evaluations of over 300 industrial biomass boilers worldwide. This article was written with contributions from Jansen’s process engineering and mechanical design groups. www.jansenboiler.com