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# Effects of Inconsistent or Improper Fuel for Biomass Systems
Posted by Sigma Thermal on August 25, 2022 8:50 am | Leave a Comment
Biomass, derived from plant-based products, serves as a renewable energy source to produce heat and electricity. It represents an environmentally friendly alternative to traditional fossil fuels. Commonly used biomass includes wood products like chips, logs, pellets, along with forestry residues such as branches and brushwood.
Biomass-powered combustion systems play a vital role in various industrial processes, particularly in wood product manufacturing and power generation. These systems are widely employed for generating hot gases and providing indirect heating. Examples of their applications include:
- Rotary Dryers: Generating flue gas for combustion.
- OSB, Belt Dryers, ORC, and Other Secondary Users: Heating thermal oil.
- Steam Turbines: Supporting steam boilers.
Sigma Thermal specializes in offering advanced furnace systems fueled by biomass. Our team of engineering experts provides comprehensive support, including logistics, troubleshooting, custom maintenance, and related equipment for top-tier industrial contractors worldwide. Explore more about the significance of fuel moisture levels, recommended fuel specifications, and proper inspection procedures for biomass fuel systems.
## Importance of Fuel Moisture
Fuel moisture refers to the water content present in the fuel, expressed as a percentage. The level of moisture significantly impacts the combustion behavior of solid fuels. When biomass contains excessive water, it can hinder complete combustion, resulting in reduced energy output or lower heat efficiency relative to the fuel's mass. Moreover, high moisture content can lead to increased carbon monoxide emissions due to incomplete burning. Conversely, if the biomass is too dry, it might reach ash fusion temperatures, causing fouling in heat recovery equipment or posing explosion risks.
Given these challenges, the water content of biomass must remain within a specific range to ensure optimal performance. Sigma Thermal’s reciprocating grate furnaces operate best when fuel moisture falls between 35% and 55%. It’s critical to adjust the grate floor of the furnace accordingly based on the maximum moisture percentage the plant anticipates processing.
Operators should also grasp the roles of primary air zones, secondary air, and flue gas recirculation (FGR). Primary air supports combustion initiation at the lower furnace, controlling the amount and rate of fuel burned. Secondary air enhances gas mixing with fresh air, completing the combustion process and ensuring thorough burning of the fuel, thereby improving combustion efficiency.
The amount of secondary air required depends on the firing rate and the moisture content of the fuel. For drier fuel, more secondary air is necessary to cool the combustion temperature, whereas wetter fuel requires less secondary air. FGR is a technique that significantly reduces nitrogen oxide (NOx) emissions by reintroducing flue gases into the combustion chamber. This approach lowers the flue gas temperature and decreases oxygen concentration in the combustion mixture.
Consistency in incoming moisture levels is key. Operators must be mindful of significant fluctuations in fuel moisture, as extreme variations can disrupt combustion control within the furnace. Such changes can increase fuel consumption while reducing the carbon-to-carbon dioxide conversion efficiency, making the process less efficient and potentially leaving some fuel unburned.
## 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 | More Secondary Air |
| Lower Combustion Temp | Watch for Glassing |
## Particle Size Consistency
Consistent particle size is another critical factor in biomass combustion systems. For 100% bark fuel, particles should not exceed 6 inches, and for 90% bark content, particles should be no larger than 4 inches. Sizes vary depending on the type of fuel. Regardless of the biomass type, fines—tiny particles—can cause several issues, including ash carryover, buildup, glassing, difficulty maintaining bed thickness, and high flame temperatures.
An imbalance in primary and secondary air exacerbates problems caused by inconsistent particle sizes. Excessive primary air can increase fines carryover due to high velocities through the grates. Insufficient primary air results in unburned fuel being discharged from the grates, affecting productivity.
The presence of excess fines generally indicates low moisture content, usually below 40%. To address this issue, operators should initially reduce 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 decreased incrementally by 3-5%, but they shouldn’t drop below 10%. As the fire line moves down the grate, if the bed is thin, the fuel feed rate can be increased.
## 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 ensures finite combustion control, minimizing emissions and effectively utilizing fuels with varied particle sizes, chemical compositions, and moisture levels. Higher percentages of fines can lead to high flame temperatures, large volumes of unburned carbon in flue gas streams, glassing, and challenges in maintaining bed thickness.
In fuel analysis testing, key factors to consider include:
- Heating value
- Moisture content
- Particle size distribution
- Ash content
- Nitrogen content
- Sulfur content
- Carbon and hydrogen content
- Ash fusion temperature
Regional standards for biomass furnaces vary globally, so always consult local regulations before installing a system in your facility.
## Furnace Inspection
Furnace inspection involves examining the condition of essential components like grate bars, refractory, moving frame roller tracks, emergency stack, fans, and dampers. It also includes cleaning, testing, and lubricating any moving parts.
Signs of improper operation include:
- Cracked bars: Indicate rapid temperature changes, often due to water exposure.
- Warped bars: Result from high flame temperatures and oxygen concentrations.
- Holes in bar noses: Caused by wear and high temperatures.
- Refractory damage: Large amounts of glassed material in the upper furnace suggest excessively high operating temperatures. In severe cases, overheated refractory anchors can conduct heat back to the furnace casing, causing refractory failure.
## Biomass Fuel Systems from Sigma Thermal
Sigma Thermal offers comprehensive biomass fuel system solutions. Our expertise covers designing, engineering, and manufacturing high-efficiency, high-performance systems. Our product lineup includes indirect process bath heaters, direct-fired process heaters, thermal fluid heating systems, electric process heaters, biomass-fired energy systems, and more.
For more information or to request a quote, contact us today to start your solution.
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June 01, 2025