Wellons technology vs Step grates

  WELLONS is the only company manufacturing wood-fired boilers with a furnace grate and substantial  unit output — up to 120 MTH of high-pressure steam. These boilers require no auxiliary fuel (fuel oil or fuel gas) for operation. In the article "OPERATION PRINCIPLE AND DESIGN FEATURES OF WELLONS BOILERS" we focused on the description of technological features and advantages of our systems where a patented technology of wood fuel being burned on rotary grates is used (WELLONS, INC. know-how).

  In this review, we want to draw your attention to a number of Wellons technology advantages versus different designs of wood combustion using step grates. These are as follows:

1. GRATES:

Wellons:​

  • Lifetime of rotary grates reaches 12–16 years or more and is equal to boiler lifetime! The listed photographs show that in 16 years of heavy operation (joint combustion of sludge and bark waste — fuel ash up to 27 %) the rotary grates are in excellent condition and do not require any further activities related to repair works. Such durability of the rotary grate service life is due not only to the high-alloy steel used, but to the 100 % cooling of toothed grate roll working surface with water (Wellons, Inc know-how). The temperature of the roll working surface is only limited by cooling water temperature not exceeding 130 °C.
  • A small number of grate rolls (12 grate rolls in one cell instead of 750 rotary grates in a step grate system!) significantly improves the reliability of the furnace system and the boiler as a whole.
  • Toothed grate rolls rotate in the opposite direction with specified speed (1 revolution per 45 minutes on average — the rotation speed is selected automatically depending on fuel quality). Rolls can rotate in either direction by 360°. The special teeth ensure operation of any pair of rolls as a toothed roll crusher, enabling the fragmentation of possible slag formations. The maximum ash particles size removed by ash path is 10 mm, which guarantees uninterruptible, reliable operation of the ash path.
  • Toothed grate rolls are arranged in the grate at a certain intervals, which allows trapping large ash particles and also prevents mechanical wear due to the absence of friction between metal surfaces of grate rolls, which increases the service life of the entire system (up to 12–16 years or more without replacement and repairs).

Step  grates:

  • The service life of step grates, even made of high-alloy steels or cast iron, does not exceed 2–3 years due to the grate’s structure, which does not provide water cooling of grates. The grate’s surface at the metal/fire interface is cooled only by the primary air. Grate temperature is very high, at least 600–800 °C, and in some cases can reach high values up to 1,000 °C. Only the beams the grates are mounted on are water cooled.
  • Grates are subjected to wear, not only due to high temperatures, but also because of frictional force. Step grate structure provides the movement of movable grate on a fixed grate's working surface, which also limits grate service life within 2–3 years, even with a stable fuel combustion mode (when bark waste is used it is virtually impossible — unstable humidity, caloric efficiency, fractional composition, etc.).  
  • A large number of grate bars in a step grate (up to 750 pcs in a low capacity boiler) increases the risk of the boiler stoppage due to the failure of one or two grate bars as a result of their burnout, gulling, grate lifting and other similar causes. Because of these defects, unburned fuel falls into the under-grate space where it burns increasing the temperature, which in its turn affects bearing bars and ash system; also, unburned fuel can move through the ash path into the ash receiving bin. As a result, boiler efficiency drops sharply.
  • Possible slag formations, sometimes of a large size (up to 500 mm or more), are moved due to the grates rotation in the "cold" part of the grates  and disposed into the ash removal section. Regardless of the ash conveyor type used (chain, scraper, and screw conveyors), large slag pieces cause frequent breakdowns and failure of an ash handling system, and consequently shutdowns of the boiler equipment in general.   
  • OPEX is significantly higher by at least 25–30 %.

2. BOILER REFRACTORY:

Wellons:

  • The cell of the boiler itself is lined with super duty, high-alumina refractory (Al2O3 content > 82 %), with a load softening point equal to 1,650–1,700 °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).
  • The advantage of monolithic refractory: 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)! Use of this type of refractory provides its service life of 7–10 years or more.
  • High refractory maintainability. The example shows a quick and fairly easy recovery of the fuel chute after more than 7 years of operation (local repair) using plastic refractory concrete. It does not require highly skilled refractory installers. 

Step grates:

  • The refractory design may vary depending on the boiler structure, but in most of the modern boilers, particularly in the fire-tube boilers, refractory design is complex, especially in furnace arrangement. This complex structure is needed for the multipass dust and gas ducting to ensure required fuel combustion mode and to increase boiler efficiency as a whole. In the above boiler scheme and the boiler cell photograph it is shown that the main element of a refractory structure is a roof made of refractory bricks. This roof is very important technologically, and the high efficiency of its structure and castable installation quality is required. This is especially important in the later stages of operation. The photographs show how the quality of refractory bricks installation can affect the roof construction in general.
  • Low maintainability, long repair period, the need to engage highly qualified specialists or specialized companies (the high cost of refractory works).

3. BOILER DESIGN FEATURES:

Wellons:

  • Cell-type furnace system. The main distinction of Wellons systems from both the traditional step grates and fluidized bed boilers is a design feature providing for installation of one to six furnaces in one boiler. Due to the fact that each furnace is equipped with individual fuel supply system, the impact of changes in fuel composition, whether in fractional makeup or humidity (fuel calorific capacity), is MINIMAL!
  • Wellons boiler is specially designed for burning of wood waste with high moisture content. Although the boiler has up to six cells, depending on its capacity, combustion products afterburner chamber must be of a large volume, which is needed for the combustion products expansion, gas velocity minimization, complete fuel combustion, minimization of harmful emissions of dust particles as well as CO, NOX into the air (see the boiler drawing on the Wellons boiler figure).

Step grate:

  • These systems are designed primarily for low-calorie fossil fuels (coal, lignite, peat, etc.) with stable calorific value and moisture content. When comparing the fossil fuel with wood fuel (bark and wood waste) these rates can vary widely, therefore the boilers with cells equipped with step grates are very sensitive to changes in these parameters. Fuel moisture content/calorific valuesignificantly impact the boiler operation as a whole, namely the boiler steam output, steam parameters, etc.
  • When using the boiler part as of "fire-tube boiler" version the maintainability of the pipe part is greatly reduced. To continue the boiler operation the fire-tube of the boiler drum must be plugged, which gradually reduces the boiler efficiency.

 

  We are always pleased to introduce your staff to Wellons boilers operation. All the professionals that operate our boilers, point out the simplicity of design, reliability, possibility of incinerating/recycling sludge deposits with high moisture content (up to 70 %), extremely low operation cost, excellent technical solutions from the design of furnaces to ash handling systems, low metal intensity of the structure, workmanship, etc. Example of OJSC "Solikamskbumprom" shows a lack of large expenditures on the operational needs of the boiler itself — after 15 years of heavy operation no replacement of grates are required; the original refractory of furnaces and boilers, installed at the boiler commissioning still remains intact.

  We hope this brief information will allow you to better understand the technology of burning all kinds of waste wood in our systems compared to step grates’ technology and use a more balanced approach to choosing a boiler system for your projects. By choosing Wellons boiler you insure yourself against the disadvantages of other systems.


Wellons boiler

 

Boiler with shearing grate