Biomass combustion systems in the wood based panel industry
Biomass combustion systems are an integral part of wood based panel plants. Almost every new plant is equipped with reciprocating grate-firing system. The main reason is the heat demand of the dryer, the hot press and the refiner. The second reason is the need to get rid of the biomass waste generated with the panel production: bark, sander dust, wet and dry fines or over sizes. Sometimes oil or gas burners are also used due to lower investment costs. But in this case only sander dust can be disposed of.
Other alternatives are fluidized bed firing systems or traveling grates. Both are quite seldom in the wood based panel industry. Reciprocating grates are the common choice, able to deal with a great variety of fuel moistures and sizes.
A typical reciprocating grate firing system consists of a fuel storage on the ground with mechanical conveyors to bring the fuel to a fuel feeding chute. The fuel chute is a little steel bunker located at the beginning of the reciprocating grate. The steel bunker has to be high enough to allow an even fuel distribution over the complete width or has to be equipped with suitable spreading devices inside.
Picture 1: Fuel chute with two entries for biomass. In the lower part four hydraulic cylinders for the fuel pushers will be installed.
Picture 2: Upper part of a fuel chute. The wider the grate and thus the chute, the higher the chute has to be manufactured to allow an even fuel distribution over the complete width.
The fuel level within the chute has to be monitored. A minimum fuel level before the opening to the combustion chamber is important to prevent backfire from the refractory lined combustion chamber to the chute. Apart from the level monitoring, some fuel chutes are equipped with temperature monitoring, water-cooled double casing and extinguishing systems to prevent backfire and overheating of the chute.
From this bunker the needed fuel quantity is dosed to the top of the grate and thus to the beginning of the combustion, by means of hydraulic pushers. The grate can be compared to a stair. Every second row of grate bars can move back- and forward (reciprocating) leading to a movement of the fuel from the top to the bottom where the ashes fall into a wet ash conveyor.
Picture 3: Two reciprocating grate sections ready for loading.
Over the length of the grate are several speed sections, allowing an individual speed control of the movement of the grate bars in each section. From below the grate bars, air is induced that passes between the gaps to the combustion chamber to provide the needed oxygen and to cool the grate bars. The amount of combustion air can also be individually adjusted in different air zones.
Picture 4: Gaps in between the grate bars for primary combustion air supply.
The combustion capacity of a grate is determined by the fuel moisture, the grate size and some overall combustion settings. It is clear that a bigger grate surface allows a bigger combustion capacity or a longer retention time of the fuel and thus a more complete burn out before dropping in to the wet ash conveyor. Sander dust and other dry fines normally are injected above the grate and not feed with the bulky, wet fuel.
The energy plant design is determined by the type of consumers. A biomass combustion system aligned with a flash tube dryer for MDF fibers is different from a system designed for a rotary drum dryer for wood flakes. Flash tube dryers operate with an inlet temperature in the range of 150 to 190 °C and an outlet temperature in the range of 60 to 80 °C. The exhaust is completely released to the atmosphere. The hot gas from the combustion has to be brought to the dryer mixing chamber.
Rotary drum dryers operate with an inlet temperature in the range of 450 to 500 °C and an outlet temperature in the range of 90 to 120 °C. The flue gas at the exhaust is recirculated to the dryer mixing chamber and possibly also to the combustion chamber. The dryer fan induces the under pressure in the energy plant.
The different dryer types lead to different design conditions of the energy plants. In the case of flash tube dryers the overall process efficiency (reflected in the fuel consumption) will be lower with high fuel moisture. The combustion temperature can be low without affecting the process efficiency, thus flue gas recirculation is not needed for the efficiency considerations. For rotary dryers the moisture of the fuel and the moisture of the flue gas is irrelevant where as the oxygen content in the flue gas should be as low as possible. The oxygen content in the flue gas is an indicator of the amount of flue gas or dryer exhaust gas recirculation and efficiency of the process.