What Are the Advantages and Disadvantages of Biomass Reciprocating Grates?

The advantages of a biomass reciprocating grate lie in fuel tolerance and its ability to turn and break up the bed; the drawbacks lie in wear, sealing and maintenance. Whether it fits depends on fuel quality and operating pattern.

Advantages

• Wide fuel tolerance: it handles high-moisture, long-fibre, irregular, low-calorific, high-ash agricultural and forestry biomass such as sawdust, straw, bark, rice husk and bagasse, coping with inconsistent mixed fuel and covering the weakness of a chain grate, which clinkers and smoulders on such fuel.
• Built-in turning and clinker breaking: the moving bars shift back and forth, continuously stirring the bed and breaking the clinkered hard shell, improving permeability, reducing matting and smouldering, raising burnout and lowering carbon in the ash.
• Zoned precision air distribution: separate air boxes allow the drying, main combustion and burnout zones to be set independently, matching biomass that first loses water and then releases large amounts of volatile matter, which favours complete combustion and lower emissions.
• Flexible operating adjustment: stroke, frequency and feed speed can be changed to alter fuel travel speed and turning intensity, suiting heating and drying duties with large load swings.
• Sound combustion sequence: the inclined stepped structure carries the fuel through drying, ignition, combustion and burnout with slag removal, matching the slow drying and high volatile content of biomass.

Disadvantages

• Heavy wear, shorter life and more maintenance: moving and fixed bars slide against each other continuously, and high-temperature corrosion by alkali metals, chlorides and sulphur plus repeated thermal cycling wear, crack and jam bars, pins and push rods, requiring frequent repair and parts replacement.
• Poor sealing: the dynamic sliding gap between fixed and moving bars makes side sealing difficult, so slag and air leak and local flame flashback occurs, lowering boiler efficiency and burning out side bars.
• Fuel limitations: it is unsuitable for pure fine powder, which flashes back and overheats the grate locally, and for fuel with much hard impurity or very high calorific value, which causes slag jams and overheating damage.
• Complex structure and higher cost: the reciprocating drive and multi-box air system cost more initially than a conventional chain grate, and the drive can fail and jam.
• Less steady than an integral chain grate: alternating reciprocating loads cause slag jams and uneven wear, so it is not suited to very large boilers running long at full load.