The fundamental justification behind purchasing a solid-fueled boiler is that the fuel used is a cheaper alternative to oil and gas, which can lead to faster return on investment. Years ago, all boilers were solid fueled. However, the convenience of oil and gas piping along with declining fuel prices, rising labor costs and strict EPA regulations have raised questions about which type of boiler will save more money.
Technology has brought solid-fueled boilers a long way in recent years. Systems can be built to run automatically with electromechanical fuel-feed systems, variable-frequency controls and even automatic de-ash machinery. Nonetheless, even with modern engineering, savings in a solid-fueled boiler must be earned. When operated correctly, these boilers can run continuously, stopping only for scheduled shutdown procedures.
To take advantage of the benefits of a solid-fueled boiler, a few principles on fuel flow and combustion must be understood. Here are three tips on optimizing the operation of a solid-fueled boiler.
1. Continuous, Uniform Fuel Feed Required
A boiler system lives and dies — so to speak — by the flow of fuel into the furnace (figure 1). Boilers with the most problems are those that do not have good control over how uniform and consistent fuel flow is.
Uniform fuel feed is even more important in applications with frequent load fluctuations. The system must master control of the fuel input because even a slight supply interruption can cause a disturbance in the load. The metering of fuel into the boiler must match the load requirements or the process will not be in equilibrium. It may help to think of the fuel as an ingredient that — along with the under-fire and over-fire air (discussed later) — produces the energy.
With solid fuels, there is almost always an array of particle sizes. Because of this disparity of fuel size, the fuel metering must keep the flow in constant turbulence so the different sizes will not separate themselves. If a trend of separation occurs, the furnace bed will not be uniform, and the burning will be biased toward certain areas. A uniform consistency of fuel will produce a larger surface area of burning and prevent hot spots and dead air zones within the furnace.
Implementing a fuel transfer and metering system that uses augers to transfer fuel is an effective way to precisely control feed rates and maintain a consistent mixture of fuel sizes.
2. Under-Fire Air: Less Is More
In furnaces, under-fire air gasifies the solid fuel while over-fire air burns the gases. More often than not, solid-fuel systems use too much under-fire air and, consequently, do not have an adequate fuel pile in the furnace. When this air-to-fuel ratio is unbalanced, combustion occurs prematurely, which reduces efficiency potential and can induce furnace damage.
As solid fuel burns, it goes through a progression. First, any moisture within the fuel evaporates. Once dry, the fuel will start to release volatile gases. As more air is introduced, the gases ignite and release energy. This process will continue until just the carbon is left to burn out. Finally, the ashes are released and remain to be discarded.
When looking at the fuel pile within the furnace, there should be no grates visible. In fact, it should not even seem like the pile is burning. When adequate amounts of air are used, the fuel pile will appear to be smoking; in fact, the heat and air are reacting with the fuel and releasing the fuel’s volatile gases. If too much under-fire air is used, the volatile gases will release and combust at the same time, releasing heat on the furnace floor instead of in the upper section of the furnace where heat transfer will begin to occur. This premature combustion can rapidly reduce the life of the grates, impair heat transfer and even entrain ash and dust particles within the flue gases.
Be careful not to reduce the under-fire air so low that the boiler loses combustion. This can be dangerous because a system may react by ramping the fan rates, which will cause more of the fuel to volatize and fill the furnace. If these gases were suddenly sparked, there could be dangerous blowback and damage to the boiler equipment as well as anyone within close proximity.
The best way to ensure an appropriate amount of air is to have a control system that throttles the air input along with the fuel input. For certain kinds of fuels, different ratios will need to be used. Keep records of which scenarios keep the best fuel pile on the grates and enough air to volatilize the fuel at a rate to keep up with production.
3. Dial In the Over-Fire Air
Once the heated fuel reacts with the under-fire air and volatile gases are released, over-fire air is used to violently mix with the gases and cause them to combust. This releases heat to be transferred through the boiler heating surfaces and into the water within the vessel.
The goal is to achieve stoichiometric combustion, where every available fuel molecule released is matched by an oxygen molecule from the fan, resulting in a flue-gas analysis that reveals no carbon monoxide and no oxygen. This perfect mixing is only possible in laboratory environments; however, there are ways to attain very efficient combustion within a boiler environment.
If there is a lack of over-fire air, large quantities of carbon monoxide and other combustibles will travel through the system and out of the stack. This waste of fuel results in heat loss and decreases efficiency. An over-abundance of combustion air results in heat loss absorbed by the excess air, also decreasing efficiency.
The goal is to find a “sweet spot” for the over-fire air. In the same way that the under-fire air should modulate with the fuel feed rate, the amount of over-fire air should be solely dependent upon the amount of oxygen in the stack. Lower amounts of oxygen indicate more efficient combustion. Take stack readings to see the correlation between the carbon monoxide and oxygen levels to determine the best oxygen setting for a particular boiler system.
In conclusion, understanding how a solid-fuel boiler works boils down to understanding the fuel and the combustion process as well as the equipment that controls how the fuel is burned. Improper operation can lead to unwanted maintenance time and frustration for the boiler owner. When operated properly, however, solid-fuel boilers can be reliable, consistent and cost effective.