Circulating Fluidized Bed : Circulating Fluidized Bed M.Usman
(08-chem-02-B) University of Engineering & Technology, Lahore Oldest power station utilizing circular fluidized bed Technology, in Lünen, Germany : Oldest power station utilizing circular fluidized bed Technology, in Lünen, Germany Chemical Engineering Department, Uet Lahore Fluidized Bed : Fluidized Bed A fluidized bed is a bed of solid particles which are set into motion by blowing a gas stream upward through the bed at a sufficient velocity to suspend the particles. Chemical Engineering Department, Uet Lahore Bed Appearance : Bed Appearance The bed appears like a boiling liquid. Chemical Engineering Department, Uet Lahore Fluidization??? : Fluidization??? The fluidization occurs when the drag force on the particles in the bed due to the upward flowing gas just equals the weight of the bed. Chemical Engineering Department, Uet Lahore History : History During the 70s and also in 80s,, it appeared that conventional pulverized coal-fired power plants had reached a plateau in terms of thermal efficiency. The efficiency levels achieved were of the order of 40 percent in the US and the UK.The corresponding figures for India, however, were lower at 36 to 37 percent.
An alternative technology, Fluidized Bed Combustion (FBC), was developed to raise the efficiency levels. In this technology, high pressure air is blown through finely ground coal. The particles become entrained in the air and form a floating or fluidized bed. This bed behaves like a fluid in which the constituent particles move to and fro and collide with one another. Chemical Engineering Department, Uet Lahore Types : Types Chemical Engineering Department, Uet Lahore Types : Types Bubbling FB Bubbling Beds have low Fluidization velocities to prevent solids from being elutriated
The Bubbling Bed can achieve 70-90 % removal. Circulating FB Circulating Beds employ high velocities to actually promote Elutriation.
The Circulating Bed can remove 90-95 % of the Sulphur content from the coal Chemical Engineering Department, Uet Lahore Types (FB Boilers) : Types (FB Boilers) Bubbling fluidized bed (BFB) 2. Circulating fluidized bed (CFB) Chemical Engineering Department, Uet Lahore Quality of fluidization : Quality of fluidization Chemical Engineering Department, Uet Lahore Explanation : Explanation In a CFB combustion process, crushed coal is mixed with limestone and fired in a process resembling a boiling fluid. The limestone removes the sulfur and converts it into an environmentally-benign powder that is removed with the ash. Chemical Engineering Department, Uet Lahore Fluidized-bed combustion : Fluidized-bed combustion Circulating Fluidized Bed Boiler
In a CFB boiler furnace the gas velocity is sufficiently high to blow all the solids out of the furnace.
The majority of the solids leaving the furnace is captured by a gas–solid separator, and is re-circulated back to the base of the furnace.
The primary combustion air (usually sub-stoichiometric in amount) is injected through the floor or grate of the furnace
The secondary air is injected from the sides at a certain height above the furnace floor.
Fuel is fed into the lower section of the furnace, where it burns to generate heat.
A fraction of the combustion heat is absorbed by water- or steam-cooled surfaces located in the furnace, and the rest is absorbed in the convective section located further downstream, known as the Back-pass. Chemical Engineering Department, Uet Lahore Fluidized Bed Combustion Detail : Fluidized Bed Combustion Detail http://envfor.nic.in/cpcb/newsletter/coal/ccombs.html Chemical Engineering Department, Uet Lahore Circulating Fluidized Bed Combustion : Circulating Fluidized Bed Combustion http://envfor.nic.in/cpcb/newsletter/coal/ccombs.html Chemical Engineering Department, Uet Lahore Advantages : Advantages Fluidized bed can burn a variety of fuels-coal as well other non-conventional fuels like biomass, petro-coke, coal cleaning waste and wood. This bed contains only around 5 percent coal or fuel. The rest of the bed is primarily an inert material such as ash or sand.
Fluidized bed boilers are capable of burning a wide range of fuels cleanly, including biomass fuels such as wood waste. Chemical Engineering Department, Uet Lahore Advantages : Advantages The Temperature in FBC is around 800-900°C compared with 1300-1500°C in pulverized coal combustion (PCC).
Low Temperature helps minimize the production of NOx (nitrogen oxides).
With the addition of a sorbent into the bed (mostly limestone), much of the so2 formed can be captured.
The other advantages of FBC are
Ability to burn low calorific values (as low as 1,800 kcal/kg)
Production of ash which is less erosive.
Moreover, in FBC, oil support is needed for 20-30 percent of the load versus 40-60 percent in PCC.
FBC based plants also have lower capital costs compared to PCC based plants. The capital costs could be 8-15 percent lower. Chemical Engineering Department, Uet Lahore CFBC Vs. PC Fired Boiler : CFBC Vs. PC Fired Boiler Unlike conventional PC-fired boiler, the CFBC boiler is capable of burning fuel with volatile content as low as 8 to 9 percent (e.g. anthracite coke, petroleum etc. with minimal carbon loss).
Fuels with low ash-melting temperature such as wood, and bio-mass have been proved to be feedstock's in CFBC due to the low operating temperature of 850-900°C. CFBC boiler is not bound by the tight restrictions on ash content either.
It can effectively burn fuels with ash content up to 70 %.
CFBC can successfully burn agricultural wastes, urban waste, wood, bio-mass, etc which are the low melting temperature as fuels.
The low furnace temperature precludes the production of "thermal NOX" which appears above a temperature of 1200 to 1300° C. Besides, in a CFBC boiler, the lower bed is operated at near sub-stoichiometric conditions to minimize the oxidation of "fuel-bound nitrogen". The remainder of the combustion air is added higher up in the furnace to complete the combustion. With the staged-combustion about 90 percent of fuel-bound nitrogen is converted to elemental nitrogen ( N2) as main product. Chemical Engineering Department, Uet Lahore Applications of CFBC : Applications of CFBC The first CFBC power plant of 110 MW at Nuclu. Colorado, USA is operating since 1990.
Several such CFBC power plants are operating in Germany, UK, Canada and Japan using various kinds of coal and bio-mass fuels.
The largest CFBC power plant is the 250 MW unit in Gardane, France, commissioned in 1996.
Presently, 350 MW units are being constructed in Canada and Japan.
CFBC is a mature Technology with more than 300 CFBC boilers in operation world wide ranging from 5 MW to 250 MW.
With lime stone addition, 90 % of the sulfur emission can be retained. With staged combustion and with relatively low combustion temperature of 850 / 900° C, NO2 formation is about 300 to 400 mg/Nm3 only against 500 to 1000 mg/Nm3 in conventional PF fired boilers. Chemical Engineering Department, Uet Lahore