After Gutenberg

Sometimes referred to as a PCFB (Pressurized Circulating Fluidized Bed) system, a CFB (Circulating Fluidised Bed) boiler is an example of fluidized bed combustion. The system shown above is for the gasification of biosolids, a.k.a., sewage sludge.

The Biopact team recently reported on plans to build a 350MW biomass fueled thermoelectric power plant in Wales.

The biofuel from the fuel storage area will be transferred to a CFB boiler by means of an enclosed conveyor belt system from one of three fuel blending silos. The CFB boiler will raise steam for a single steam turbine. Exhaust steam from this turbine will be condensed by means of a dry air cooled condenser and will therefore require no water for cooling purposes. Condensed steam will then be recirculated back into the CFB boiler.

“UK approves world’s biggest biomass plant“

The following description comes from “High Efficiency Pressurized Fluid Bed Systems” by Berman and Dille (see DOE report ANL/FE-81-65 prepared by Argonne National Laboratories under contract W-31109-Eng-38 printed in April, 1982).

• A typical system is comprised of a single vertical refractory lined pressure cylinder and a set of cyclones (primary and secondary). Solids entrained by the combustion gases enter the primary cyclones through horizontal ducting. Primary solids separation occurs in the first stage cyclone at about 95 percent efficiency. Gases leaving the first stage cyclone are directed to the second stage for further solids separation.

• Solids collected in the primary cyclone flow by gravity through a heat transfer section with horizontal tube bundles below the barrel of the cyclone. The solids are then collected in a conventional hopper and returned to the furnace through a siphon duct arrangement.

• A second stage of cyclone provides further cleanup. Air leaving this cyclone is finally cleaned as solids are let down through a lock hopper system. This ash contains little or no carbon and, after cooling, the ash is sent to waste disposal.

• Cyclone theory shows that performance is affected less by pressure, and more by particle size, inlet velocity, and barrel diameter. Solids separated in the cyclone barrel will rain down over horizontal heat transfer tube bundles.

• Combustion air from the high pressure gas turbine is split into primary and secondary streams. Primary air is injected into the bottom of the furnace under the air distributor. Secondary air is injected into the furnace above the primary combustion zone. Excess air (30 percent) is used to establish total air flow and gas turbine output in the system.

• It is expected that pressurized operation will improve reaction kinetics. The furnace height should be more than adequate to achieve high combustion efficiency (99 percent). The circulating fluidized bed also allows the use of staged combustion to reduce the NOx formation and improve combustion efficiency.

• Inventory for the process is controlled by a solids diverter valve. Solids are discharged through the valve to the solids cooler to keep the furnace pressure drop constant. Additional solids are removed from the system in the secondary cyclone and final gas cleanup system.

The experimental Fluidized Bed Combustion System at Penn State University has been integrated with a 2 million Btu/h watertube boiler to recover heat from the flue gas. A negative pressure is maintained in the boiler by adjusting the speed of the induced draft fan to avoid gas/particulate leakage. Flue gases are cooled below 500°F in an economizer prior to passing through a baghouse. The baghouse contains sixteen 5 inches diameter by 96 inches high-temperature filters sewn with felted P84 fibers.

According to Japanese Advanced Environmental Equipment, some of the features of a CFB incinerator are:

(1) Stable Combustion

Inside the Circulating Fluidized Bed Incinerator, a large volume of heat circulates, with sand as its heating medium. As a result, the temperature inside the incinerator is uniform, temperature control (combustion control) is easy, and stable combustion can be achieved.

(2) Low Energy Consumption

The blower required to circulate the sand operates with a low discharge pressure. As a result, blower power can be reduced to 60-70% of that used by Bubbling Fluidized Bed Incinerators.

(3) Heat Recovery Inside Incinerator

An external heat exchanger (a fluidized-bed heat exchanger) is used, permitting heat recovery inside the incinerator. An external heat exchanger also allows the amount of heat recovered to be freely controlled.

(4) High-Intensity Combustion

In the Circulating Fluidized Bed Incinerator’s superficial velocity range of 4.0-8.0 m/s, the relative speed (slip speed) of gas and particles is maximized, as is their contact efficiency. As a result, high-intensity combustion is possible, and incinerator size has been reduced, saving on space.