# Effects of Inconsistent or Improper Fuel for Biomass Systems
Posted by Sigma Thermal on August 25, 2022 8:50 am | Leave a Comment
Biomass refers to plant-based materials used as fuel to produce heat and electricity. As an environmentally friendly and renewable alternative to traditional energy sources like fossil fuels, biomass offers significant advantages. It includes wood products such as chips, logs, pellets, as well as forestry residues like branches and brushwood.
Biomass-fired combustion systems play a vital role in various industries, including wood product manufacturing and power generation. These systems are widely used for generating hot gases and indirect heating processes. Applications range from rotary dryers for flue gas production to belt dryers, organic Rankine cycles (ORCs), and other thermal oil heating systems. Additionally, steam turbines and boilers powered by biomass help meet energy demands efficiently.
Sigma Thermal is a leading provider of advanced furnace systems fueled by biomass. Our team of engineering experts specializes in logistics, troubleshooting, custom maintenance, and related equipment tailored for industrial contractors worldwide. Discover more about the critical aspects of fuel moisture, recommended fuel specifications, and proper inspections for biomass systems.
## Importance of Fuel Moisture
Fuel moisture represents the water content present in the fuel, expressed as a percentage. The moisture level significantly affects how biomass burns. When the moisture content is too high, the fuel may not burn completely or efficiently, resulting in reduced heat output and increased carbon monoxide emissions due to incomplete combustion. Conversely, excessively dry biomass can reach ash fusion temperatures, potentially damaging heat recovery equipment or posing explosion risks.
To optimize performance, biomass moisture levels should fall within an ideal range. Sigma Thermal’s reciprocating grate furnaces operate optimally with fuel moisture between 35% and 55%. Proper sizing of the grate floor is essential based on the maximum moisture content expected in the plant. Operators must also understand the roles of primary air zones, secondary air, and flue gas recirculation (FGR).
Primary air supports initial combustion in the lower furnace and regulates the rate and intensity of fuel burning. Secondary air enhances mixing of combustion gases with fresh air, ensuring complete combustion and maximizing efficiency. The required amount of secondary air depends on both the firing rate and the moisture content of the fuel. Higher moisture levels necessitate less secondary air, whereas drier fuels require more to cool the combustion process. FGR, which reintroduces flue gases into the combustion chamber, helps reduce nitrogen oxide (NOx) emissions by lowering flue gas temperatures and oxygen content.
Consistent moisture levels are crucial. Drastic fluctuations in fuel moisture can disrupt combustion control within the furnace, increasing fuel consumption while reducing the effective conversion of carbon into carbon dioxide. Such inefficiencies result in wasted resources and lower overall system performance.
## Wet vs Dry Fuel
| **Wet Fuel** | **Dry Fuel** |
|--------------------------|-------------------------|
| Lower Heating Value | Higher Heating Value |
| Requires More Mass | Burns Less Mass |
| Longer Drying Time Needed| Needs Thicker Bed |
| Less Combustion Air Needed| Requires Recirculated Air for Cooling |
| Lower Combustion Temp & Potential for CO | Watch for Glassing |
## Particle Size Consistency
Particle size consistency is another critical factor in biomass combustion systems. For 100% bark fuel, particles should not exceed 6 inches, while those containing 90% bark should be limited to 4 inches. The acceptable particle sizes vary depending on the type of biomass. However, regardless of the fuel composition, fine particles—those smaller than 1 millimeter—can create several operational issues, including ash carryover, buildup, glassing, difficulty maintaining bed thickness, and high flame temperatures.
The balance of primary and secondary air plays a significant role in managing these challenges. Excessive primary air can increase fines carryover due to higher velocities through the grates. Insufficient primary air leads to unburned fuel accumulation and inefficiencies.
When fines become excessive, it often indicates low moisture content, usually below 40%. To mitigate this problem, operators should first reduce the flow of primary air and then increase secondary air or recycle it to the quench chamber. Adjustments to under-fire air dampers in zones 1 and 2—where primary drying occurs—should be made incrementally, but they should never drop below 10%. Gradually, the fireline will descend along the grate. If the bed is too thin, the fuel feed rate can be increased accordingly.
## Recommended Fuel Specifications
Sigma Thermal’s reciprocating grate furnaces are engineered to handle biomass with high ash content, low heating value, and high moisture levels. Their design facilitates precise combustion control, minimizing emissions even when dealing with diverse fuel characteristics. High percentages of fines can cause elevated flame temperatures, significant unburned carbon in exhaust gases, glassing, and challenges in maintaining bed thickness.
In analyzing fuel quality, several key factors must be considered: heating value, moisture content, particle size distribution, ash content, nitrogen and sulfur levels, and carbon-hydrogen ratios. Additionally, the ash fusion temperature is critical for understanding potential operational risks.
Regulatory standards for biomass furnaces vary across regions. Always consult local guidelines before implementing a system in your facility.
## Furnace Inspection
Regular inspections are essential to ensure optimal furnace performance. These checks involve evaluating the condition of critical components such as grate bars, refractory materials, moving frame roller tracks, emergency stacks, fans, and dampers. Cleaning, testing, and lubrication of all moving parts are integral parts of this routine.
Signs of improper operation include cracked or warped grate bars, holes in bar noses, and refractory damage. Cracks in bars typically stem from rapid temperature changes causing thermal stress on alloy steel. Warping arises from high flame temperatures and oxygen concentrations. Holes in bar noses result from wear and extreme heat exposure. Refractory damage indicates excessive furnace temperatures, sometimes severe enough to melt glassy substances. In extreme cases, overheated refractory anchors can transfer heat back to the furnace casing, leading to catastrophic failure.
## Biomass Fuel Systems from Sigma Thermal
At Sigma Thermal, we deliver comprehensive biomass fuel system solutions. Our expertise spans design, engineering, and manufacturing of high-efficiency, performance-driven 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.
If you’re seeking reliable biomass solutions tailored to your needs, contact us today. Alternatively, request a quote to kickstart your project. Together, let’s build a sustainable future.
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