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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.


February 24, 2010
By Arie Verloopa

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.

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Fig. 1. Combustion occurs in three stages. 

PhotoA  
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:

Attain uninterrupted and stable operation;
Meet regulatory limitations for emissions of air pollutants such as
carbon monoxide, volatile organic compounds, nitrogen oxides (NOx), and
particulate matter;
Optimize fuel economy, often by displacing fossil fuels with less expensive biomass;
Maximize thermal efficiency and heat recovery.

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):

Fuel piling on the grate and unburned fuel coming off the grate;
Frequent furnace puffing caused by unbalanced mixing of air and combustible gases;
Elevated levels of ash carryover and high unburned carbon content in
the fly ash, including sparks and embers. This may lead to accelerated
erosion of pressure parts, ducting, induced draft (ID) fan, etc. It can
cause fires in the boiler’s downstream equipment;
Delayed combustion and high gas temperatures in the upper furnace. This
may lead to overheating of superheater tubing and high gas temperatures
exiting the boiler, sometimes limiting the ID fan capacity; 
High excess air or oxygen in flue gas resulting in a loss of thermal efficiency;
•  Increased stack concentrations of air pollutants. 

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.

picture-1 
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:

Collecting operational data and information to characterize the design and operation of the unit;
Analyzing combustion to quantify material and heat flows and establish thermal efficiency and fuel economy;
Computational fluid dynamics modelling to simulate combustion process parameters and heat transfer characteristics;
Analyzing steam/water circulation to resolve potential problems;
Defining conceptual modifications to remedy the combustion problems and limitations;
Quantifying potential benefits of upgrading the boiler’s combustion system;
Analyzing heat transfer to identify opportunities to improve efficiency and/or steam parameters. 

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:

Number and width of the fuel spouts
Fuel delivery method: mechanical or air swept
Degree of flexibility to control fuel trajectory onto the grate

Stoker Grate:

•  Type of grate (pinhole, travelling, vibrating, reciprocating) 
•  Dimensions of the grate 
•  Air distribution under the grate 
•  Mechanical repair condition 

Combustion Air Delivery:

•  Quantity, temperature, and pressure of undergrate air (UGA) and overfire air (OFA) 
•  Flexibility to control UGA to different grate areas
•  Number, size, and location of OFA ports/nozzles 
•  Ratio of OFA to total air flow and control flexibility.

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

•  Even and balanced distribution of fuel on the grate with flexibility to control fuel trajectory; 
•  Limited UGA quantities to minimize lift-off of fine fuel particles from the grate; 
•  Preheated combustion air, particularly when burning fuels with high moisture content; 
•  Effective OFA supply, with high individual air jet momentum, leading to
proper mixing of air with combustible gases coming off the grate. 

Fig3
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:

Increased biomass burning capacity, typically by 5 to 40%, depending on boiler size and particular goals; 
Improved ability to handle biomass/mixed wood and sludge with a wide range of moisture content; 
Reduced or eliminated need for fossil fuel co-firing; 
Improved thermal efficiency by reducing excess air, flue gas temperatures in the stack, and unburned carbon in the ash; 
Reduced carryover of fly ash and other inert material to minimize the abrasive effects of erosion on pressure parts, ducting, and ID fan; 
Reduced stack emissions of carbon monoxide, NOx, and particulate matter, with an average reduction of 41% for carbon monoxide and 6% for NOx.

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


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