Effects of Inconsistent or Improper Fuel for Biomass Systems

Biomass is a plant-based product used as fuel to generate heat and electricity. It is an eco-friendly renewable alternative to traditional forms of energy, such as fossil fuels. Biomass includes wood products such as chips, logs, pellets, and other forestry materials such as limbs and brush. 

Biomass-fired combustion systems are useful in many wood product manufacturing and power generation processes. These systems are commonly used for hot gas generation and indirect heating applications. Some applications include:

  • Rotary Dryers – Flue Gas Generation from Combustion
  • OSB, Belt Dryers, ORC, and Misc Secondary Users – Heating Thermal Oil 
  • Steam Turbines – Steam Boiler

Sigma Thermal is a leading provider of modern furnace systems fired by biomass. Our engineering experts provide logistics, troubleshooting, custom maintenance, and related equipment for world-leading industrial contractors. Learn more about the importance of fuel moisture, recommended fuel specifications, and proper inspections for biomass fuel systems.

Importance of Fuel Moisture

Fuel moisture is the water content present in the fuel, represented as a percentage. Moisture content matters in solid fuels because it influences burning behaviors. If the biomass has too much water moisture, it may not burn all the fuel or as easily and produces no energy or less useful heat energy relative to unit mass.  It can also cause excessive amounts of carbon monoxide which comes from incomplete combustion. If the biomass is too dry, it can reach ash fusion temperatures, foul the heat recovery equipment, or pose an explosion risk.

Because of these challenges, water content of the biomass must fall within a certain range to be most effective. Sigma Thermal’s reciprocating grate furnaces operate ideally with fuel moisture between 35% to 55%. It is important to size the grate floor of the furnace properly according to the maximum percentage of moisture the plant will be burning.

Equipment operators must also understand the roles of primary air zones, secondary air, and flue gas recirculation (FGR). Primary air is the amount of air used in the lower furnace to initiate combustion and controls the quantity and speed of fuel burned. Secondary air enables mixing of combustion gases and fresh air, completes the combustion, and allows the fuel to burn completely, which means combusting all of the carbon in the fuel making the combustion process more efficient.

The amount of secondary air you need depends on the firing rate and the moisture in the fuel. If the fuel is dry, more secondary air is needed to quench the combustion temperature and vice versa. If the fuel contains higher moisture, then less secondary air is needed. FGR is a technique that significantly reduces nitrogen oxide (NOx) emissions by recirculating flue gasses into the combustion chamber. This technique lowers flue gas temperature and oxygen content in the combustion mixture.

Moisture coming into the system must be consistent; operators should be aware of drastic fuel moisture swings, as these extreme changes can cause loss of control of the combustion inside the furnace and make the overall fuel usage go up while actual conversion of carbon-to-carbon dioxide goes down. This would mean the process is less efficient and not all of the fuel gets combusted.

Wet vs Dry Fuel

Wet Fuel Dry Fuel
Lower Heating Value Higher Heating Value
Must Burn More Mass Burn Less Mass
Need more Drying time Need Thicker Bed
Need Less Combustion Air & Secondary Air More Secondary Air or Recirc Air for Cooling
Lower Combustion Temperature and Potential for more CO  Watch for Glassing


Particle Size Consistency

Particle size consistency is another crucial factor in biomass combustion systems. For 100% bark fuel, particles should not exceed 6 inches, and particles in 90% bark content should be no more than 4 inches. The sizes vary further depending on different types of fuel content. Regardless of the type of biomass, fines—very small, fine particles—pose a variety of issues, including:

  • Ash carryover
  • Buildup and glassing
  • Bed becomes difficult to maintain
  • High flame temperatures

The wrong balance of primary and secondary air can also aggravate problems arising from particle size inconsistencies. For example, too much primary air can cause high velocity through the grates, which increases carryover of fines. Not enough primary air can result in unburned fuel being dumped off the grates and other production inefficiencies.

The presence of excess fines usually means moisture content is too low, usually under 40%. To resolve the issue of too many fines, operators should first decrease the flow of primary air and then increase secondary air or recirculate to the quench chamber. Under-fire air dampers in zones 1 and 2—the main drying chambers—should be reduced in 3-5% increments but should not dip lower than 10%. The fire line will slowly move down the grate. If the bed is thin, operators can then increase fuel feed rate.

Recommended Fuel Specifications

Sigma Thermal’s reciprocating grate furnaces are designed to burn biomass with high ash content, low heating value, and high moisture content. The design allows finite combustion control that minimizes emissions and uses fuels with unique fuel particle sizes, chemical compositions, and moisture content. Larger percentages of fines can cause high flame temperatures, large volumes of unburned carbon in flue gas streams, glassing, and difficulties maintaining bed thickness. 

In fuel analysis testing, there are important factors to consider, including: 

  • Heating value
  • Moisture content
  • Fuel particle size distribution
  • Ash content
  • Nitrogen content
  • Sulfur content
  • Carbon and hydrogen content
  • Ash fusion temperature

Different world regions have varying standards for biomass furnaces, so check your local regulations before implementing a system in your facility.

Furnace Inspection

Furnace inspection involves checking the condition of all essential components, such as grate bars, refractory, moving frame roller tracks, emergency stack, fans, and dampers. It also includes cleaning, testing, and lubricating any moving parts.

The following are signs of an improper operation: 

  • Cracked bars: These indicate rapid temperature changes and rapid contraction of the alloy steel, typically from a water
  • Warped bars: Warped bars result from high flame temperatures and oxygen concentrations.
  • Holes in bar nose: This damage is due to wear and high temperatures.
  • Refractory damage: Large amounts of glassed material in the upper furnace are a sign that the operating temperature in the furnace has been too high. In extreme cases, the refractory anchors can overheat and conduct heat back to the furnace casing, where the anchor is welded to the wall.    This will lead to refractory failure.

Biomass Fuel Systems from Sigma Thermal

Sigma Thermal provides complete biomass fuel system solutions. We understand what it takes to design, engineer, and manufacture efficient, high-performance systems. Our products include indirect process bath heaters, direct-fired process heaters, thermal fluid heating systems, electric process heaters, biomass-fired energy systems, and more. Contact us for more information, or request a quote to start your solution today.  

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