BFB boiler (bubbling fluidized bed boiler) is mostly small and medium-sized industrial boiler. It has greater advantages than CFB boiler (circulating fluidized bed boiler) when burning biomass and other wastes. Biomass pellet fuel is less difficult to supply, which can meet long-term normal operation of small-capacity biomass industrial boiler. The fuel is biomass pellets, mainly wood chip mixed with compressed agricultural and forestry crop stalks.

BFB Boiler Design Parameters
Rated evaporation capacity 10t/h

Outlet steam pressure 1.25MPa

Outlet steam temperature 193.3°C

Feed water temperature 104°C

Inlet air temperature 25°C

Exhaust gas temperature 150°C

Specific gravity 0.9～1.1t/m3

Particle diameter 8～10mm

Particle length <100mm

Heating value of 12141kJ/kg

BFB Boiler Advantage Over CFB Boiler
(1) The concentration and heat capacity of materials in the boiling bed are very large. The new fuel into the furnace only accounts for 1-3% of the hot bed material. The huge heat capacity can make the new fuel quickly catch fire;

(2) BFB can burn a wider range of fuels, including many fuels with lower heating value, and also are suitable for mixed combustion of multiple fuels;

(3) The heat transfer coefficient is large, which strengthens the overall heat transfer effect;

(4) The original dust concentration of the outlet flue gas is lower;

(5) BFB boiler start-stop and operation is easier, and the load adjustment range is large;

(6) BFB boiler has a simple structure, small floor space, low steel consumption, no cyclone separator, refeeder and high-pressure fan.

BFB Boiler Structure Design
1. Overall Structure

This BFB boiler is a natural circulation water tube boiler, with double drums arranged horizontally. Main heating surface is water-cooled wall, flue duct, convection tube bundle, economizer and primary & secondary air preheater. The furnace adopts suspended structure, surrounded by membrane water walls.

The frame adopts all-steel structure, 7-degree earthquake intensity and indoor layout design. Both sides are platform and ladder for operation and maintenance.

The BFB boiler uses under-bed hot flue gas ignition, and the combustion air is divided into primary air and secondary air. The distribution ratio of primary and secondary air is 7:3.

2. Combustion System and Flue Gas Flow

2.1 Ignition and Air Distribution Device

The ignition fuel is diesel oil. When igniting and starting the boiler, the temperature of hot air in the water-cooled air chamber shall be strictly controlled to ensure that it does not exceed 800°C to avoid burning the hood. The water-cooled air chamber is composed of front wall water-cooled wall pipe and water-cooled walls. The upper part of water-cooled air chamber has a mushroom-shaped hood.

2.2 Furnace Combustion Chamber

The cross section of water wall is rectangular, the cross-sectional area is 5.8m2, furnace height is 9m, and effective area of air distribution plate is 2.8m2. The top of furnace is front water wall elbow. The outlet of furnace is on the upper part of rear water wall, with a height of about 1.5m.

3 Steam-water Cycle

Feed water enters the economizer in the tail flue duct and then flows into the upper drum. The boiler water enters the lower header through the distributed downcomer, flows through the membrane water wall and returns to the upper drum. Wall enclosure tubes on both sides are respectively connected with upper and lower drums through headers. The convection tube bundle is welded to upper and lower drums.