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Optimize Biomass fuel combustion process

The main reason for purchasing a biomass-fueled steam boiler is because biomass fuel is a cheaper alternative to coal, oil, and gas, shortening the payback period.

Many years ago, all boilers used coal, but as coal prices have risen and environmental pollution issues have become more serious, biomass fuels have gradually gained market share due to their cost advantages and environmental friendliness.

Technology has taken biomass-fired boilers a long way in recent years. Systems can be built to run automatically with an electro-mechanical fuel supply system, controlled by frequency converters, and even ash removal can become automatic.

However, even with modern technology, cost savings must still be achieved when using biomass fuels. When operated correctly, these boilers can run continuously, only stopping when there is a scheduled shutdown.

To take advantage of the benefits of biomass-fired boilers, it is necessary to understand some principles of fuel flow and combustion. Here are some tips to optimize the operation of biomass-fired boilers.

Schematic diagram of a fluidized bed boiler
Schematic diagram of a fluidized bed boiler

1. Continuous and Uniform Fuel Supply is Key

A boiler system lives and dies (so to speak) by the fuel flow into the furnace. The most problematic boilers are those with poor control over the uniformity and consistency of the fuel flow. This principle is even more important in applications with fluctuating loads. The system must exploit the ability to control the fuel input because even a minor interruption in the input can cause disturbances in the load. Fuel dosing into the boiler must match the load requirements; otherwise, the process will be unbalanced. It can be helpful to consider fuel as a component, along with air during combustion, to generate energy.

With biomass fuels, there are almost always various particle sizes. Due to the difference in fuel size, fuel supply must keep the fuel stream constantly mixed to prevent different sizes from separating. If segregation occurs, the fuel bed in the furnace will be uneven, and the combustion process will tend toward certain specific areas. Homogeneous fuel density will create a larger burning surface area and prevent hot spots and dead air zones in the furnace.

Using frequency-controlled screw feeders is an effective way to precisely control the feed rate and maintain a consistent mixture of fuel sizes.

Fuel screw feeder into the furnaceFuel screw feeder into the furnace

The screw feeder is much more effective than a conveyor belt for fuel supply. The geometry of the screw allows for much more accurate forward movement speed prediction. With conveyor belts, material buildup often occurs, leading to uneven fuel layers on the furnace grate, making the combustion process inefficient. The fuel screw feeder also helps create a separation between the furnace and the external environment. The sealed feeder path system helps reduce excess air passing through the fuel feed path into the combustion chamber and also reduces the occurrence of backfire.

2. Primary Air: Less is More

Typically, biomass fuel systems use too much primary air, resulting in insufficient fuel in the furnace. When this air/fuel ratio is unbalanced, premature combustion occurs, not only reducing potential efficiency but also potentially damaging the furnace.

When biomass fuel burns, it undergoes a progression. First, moisture in the fuel evaporates. Once dry, the fuel starts releasing volatiles. When enough air is introduced, the volatiles ignite and release energy. This process continues until only carbon remains to burn completely. Finally, ash is produced and disposed of.

Looking at the fuel bed in the furnace, ideally, no glowing coals should be visible. In fact, it shouldn't even look like the fuel bed is burning. With sufficient air, the fuel bed will appear "smoky," but what's actually happening is heat and air reacting with the fuel and releasing fuel volatiles. If too much primary air is used, the volatiles will escape and burn simultaneously, releasing heat onto the furnace grate instead of at the top of the furnace, where the heat transfer process begins. This premature combustion can quickly reduce the life of the grate as well as decrease heat transfer capability and even carry off ash/dust particles in the exhaust gas.

Volatile gases being released from the fuel
Image of volatile gases being released from the fuel

Be careful not to reduce the amount of primary air too low, causing the boiler to lose combustion. This can be dangerous because the system may react by increasing the feed rate, pushing more fuel out, and filling the furnace. If these gases ignite suddenly, a boiler explosion may occur, causing damage to the boiler equipment as well as anyone nearby. The best way to ensure the proper amount of air is to have a control system regulating the input air along with the fuel input. Different fuel types may require different ratios. Keep records of which situations keep the fuel bed best on the grate and the appropriate amount of air to burn the fuel completely at the appropriate production rate.