The principle of operation is based on the proven technology of burning wood fuel in cyclone-type furnaces on the grate with rotating rolls (Wellons, Inc. know-how). A wide range of steady operation (20-100% of the rated load); low-temperature combustion of fuel; minimal emissions of harmful substances; utilization of a variety of fuels/biomass with up to 65% moisture content; high unit steam output capacity of the plant to 120 MTH – these are the main advantages of our boilers. The reliability of the chosen technology has been confirmed by 377 boilers successfully operating in North America and Russia.
Engineers and service personnel operating the boilers mentioned the simplicity of their design, reliability; virtual exclusion of boiler house operation cost; excellent technical solutions implemented in the design of furnaces, ash handling systems; low metal consumption; manufacturing quality, etc. Example of JSC Solikamskbumprom shows that with proper handling of the boiler, its maintenance cost is minimal: after 15 years of daily use, no replacement of grate rolls has been required; the original refractory lining of furnaces and boiler, installed prior to its commissioning still remains intact!
WELLONS offers the boilers for various applications withsteam output capacity from 10 MTH to 120 MTH, including: technological saturated steam boilers (up to 21 bar and above); high pressure superheated steam power boilers (up to 65 bar and higher/560°C).
Cell-type furnace system.. The main distinction of Wellons systems from both the traditional shearing grates, and fluid-bed boilers comprises in a design feature providing for installation of one to six furnaces varying in size in one boiler. A wide selection of furnace dimensions allows us to construct boilers that satisfy most of the customers' requirements. Due to the fact that each furnace is equipped with individual power supply system, the impact of changes in fuel composition, whether in fractional makeup or humidity (fuel calorific capacity), is MINIMAL! The large number of furnaces (up to 6 pcs.) for one boiler prevents a simultaneous sharp drop in the thermal power of all furnaces in case of change in the percentage of fuel mixture components or increase of firewood humidity. This solution ensures process stability by timely change of combustion modes in other furnaces, leaving unchanged the total amount of heat required to produce steam. At the same time, the qualitative steam specification remains unchanged as well (because of the constant amount of heat). This control mechanism persists in case of lowering the fuel humidity and increase of its calorific efficiency. Timely change of combustion mode does not allow the combustion temperature to increase and prevents the premature failure of the lining (lining life reaches 10 years and more).
The furnace design. The figure below shows the principle schematic of an individual furnace design. It can be seen that Wellons furnaces employ a three-zone scheme of firing low-calorie wood fuel featuring its pre-gasification and subsequent burning of the gasification products in a boiler afterburning zone. The primary air is fed directly to the combustion zone, thus ensuring intense burning process of the fuel particles and high heat transfer. Tuyere holes in the furnace refractory lining, arranged tangentially at a certain angle (both vertically and horizontally), provide the primary air swirl within the furnace and ensure the maximum heat transfer, and the highest efficiency of the furnace and the boiler as a whole. Formation of vortex flows of combustion products and fuel particles inside the furnace ensures the completeness of combustion and prevents the carryover of unburned fuel particles and fine ash fractions to the convective section of the boiler. Air dampers maintain the exact proportion of air and fuel at various combustion modes and boiler loads. Precise air supply regulation allows to minimize the formation of slag by controlling the combustion temperature. The temperature in the furnace combustion zone is 870°C to 1040°C. Combustion gases leaving the furnace cell, enter a large chamber where the final combustion takes place to burn CO, sublimation products and unburned fuel.
The boiler lining.The furnace of the boiler itself is lined with super duty, high-alumina refractories (Al2O3content >82%), with a load softening point equal to 1650-1700°C. Moreover, the use of these materials ensures resistance to high temperatures throughout the entire boiler system in the event of a sharp increase in the fuel calorific value (for example, due to a decrease in fuel humidity, or a change in its qualitative composition). Quite a large quantity of fire-resistant material (up to 30 tons per furnace) with a large heat capacity enables accumulation of a large amount of heat to be utilized for additional fuel drying in case of a sharp increase in its moisture content, or decrease of calorific value. The advantage of monolithic refractory lining: maximum strength of the entire structure, absence of masonry joints (e.g. there are up to 19 meters of horizontal and vertical joints per 1 m² of brickwork refractory, whereas 1 m² of monolithic refractory has no seams at all (0.00 meter)! Application of this type of refractories provides the lining service life of 7-10 years or more.
Fire grate.Photography, given by way of example, shows the condition of the fire grate after ten years of continuous use. Such durability of grate rolls is achieved through the use of Wellons, Inc. know-how characterized in that the grate design provides 100% cooling of knobbed rolls working surface with water. The temperature of the rolls working surface is only limited by cooling water temperature not exceeding 130°C.
Burning of fuel occurs directly on a bed of ash which not only provides protection of grate rolls against high temperature, but, consisting of inert material, creates a zone of direct heat exchange between the hot bed ash and wet fuel. This process is accomplished due to passing hot air with temperature of 200°C supplied under pressure. The result – fast drying and combustion of wood fuel.
With appropriate quality of the wood fuel, slag formation is minimal. Moreover, gear shaft grates while rotating in the opposite direction (towards each other), perform the function of a toothed roll crusher, destroying possible slag formation and removing them to ash removal section.
Boiler water-tube section.A certain design feature of the boiler water-tube system (screen headers are installed at a height of over four meters above the grate level) allows to avoid a sudden heat removal from the fuel by water tube system, whereas this heat is necessary to ensure the drying process in the event of a sharp increase of firewood dampness. The process inevitably occurs in other systems, such as boilers with shearing grates and fluidized bed boilers, and adversely effects their efficiency and steam parameters. In boilers other than those of Wellons, Inc., the water-tube system is located in close proximity to the surface of the burning fuel that is the reason of the above mentioned process, clearly visible when the fuel moisture content is high (low calorific capacity).
The boiler water-tube section is designed for the necessary boiler performance and the required steam parameters (up to 65 bar and above). The large volume of the combustion/flue gas afterburner chamber allows to minimize emissions of harmful combustion products and large-size ash particles due to low gas velocities. The dependability of the boiler system is achieved by using boiler steel grades, including alloyed steel, with ASME certificated quality.