Introduction
Boilers are used to turn the energy originating from a combustion process to usable heat and/or power. If electrical power is required, the boiler will produce steam (or an organic vapour if the working fluid is not water) which will undergo a thermodynamic cycle and power a turbine coupled to a generator. If only heat is required, steam (organic vapour) generation is not necessary and hot water production will be sufficient. This is the case when it comes to small-scale heating. Therefore, only hot water boilers will be dealt with here.
Two main boiler technologies are currently available on the market: watertube and firetube boilers. In watertube boilers, the water is heated by circulating in tubes around which the hot combustion gases flow. In firetube boilers, the hot combustion gases flow through tubes immersed in a tank filled (or partly filled if steam generation is required) with water.
Very large boilers with high ratings usually involve high water temperatures and pressures. Given these operating conditions, watertube designs are more convenient since only the water tubes should be designed to withstand such high pressures and temperatures. At lower power, firetubes designs are usually cheaper. If the output of the boiler does not exceed 20 MW and the pressure 20 bars which is the case for this feasibility study, a firetube boiler is the most adapted technology.
Description of the firetube boiler technology
1.The Furnace
In a firetube boiler, the combustion takes place in the furnace. This furnace, usually cylindrical, can either be covered with refractory material like ceramic (dryback furnace) or be in contact with the boiler's water (wetback furnace) which significantly increases the heat-exchange surface. The end of the furnace is called the reversal chamber since the hot gases make a U-turn and are fed into the first tube-pass. At the end of the reversal chamber, the gas' temperature must be sufficiently low to avoid excessive thermal stress on the tubes. The reversal chamber may be equipped with a drain to collect the water condensing on its sides (the hot combustion gas usually contain a fraction of water vapour). Even though the furnace and the reversal chamber only represent 10% of the exchange surface, they account for 40-50% of the heat exchange (mainly through radiation) given the very high gas temperatures. Some biomass boilers include two furnaces to make sure that the combustion is as complete as possible. In this case, a secondary air supply must be included in the design of the boiler.
2. The Tube Passes
The first tube-pass is entirely immersed in the water and goes from one end of the boiler to the other. Depending on the boiler's design there might be a second tube-pass (also fully immersed) in which case the boiler is called a "three pass" model (since the furnace is counted as the first pass). The diameter of the tubes has an important impact on the heat recovery performance. Clearly, for the same overall cross-section, a number of narrow tubes will be more efficient than one large tube since the heat exchange surface will be much greater. However, this multi-tube layout will be more expensive and small tubes are more likely to be blocked so maintenance costs will also be greater. The heat-exchange takes place mainly through a convective process with a limiting heat resistance on the dry side of the tubes.
3. Combustion Gas Circulation
Firetube boilers represent a significant resistance to gas flow: the combustion gas circulation is made possible by the use of a fan. Typically, pressure losses are low in the furnace given its large cross section but are significant in the reversal chambers (the gas undergoes a U-turn) and in the tube passes (large gas velocity). The so-called "draught loss" must be calculated (many correlations are available in the literature) and the fan power deduced.
4. The Water Tank
Contrary to firetube boilers designed for steam generation where a steam disengagement surface must be provided, the water tank of hot water firetube boilers is completely filled with water.
Typical firetube boiler
1. Furnace
2. Reversal chamber
3. Second tube pass
4. Front smoke box
5. Third tube pass
6. Gas outlet
