23-01-2013, 04:51 PM
CFBC & PFBC Technology
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Introduction
Circulating Fluidised Bed Combustion (CFBC) and Pressurised Fluidised Bed
Combustion (PFBC) are advanced coal combustion technologies available now a
day to improve overall cycle efficiency of the system, for generation of electricity.
Fluidised Bed Combustion
During the seventies and also in eighties, it appeared that conventional
pulverised 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, Fluidised 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 fluidised bed. This bed behaves like a fluid in which the constituent particles
move to and fro and collide with one another.
Fluidised bed can burn a variety of fuels-coal as well other non-conventional
fuels like biomass, petro-coke, and 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.
Circulating Fluidised Bed Combustion (CFBC)
Circulating Fluidised Bed Combustion Process:
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 850-9000 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 percent.
For fuels with high moisture content and low heating value such as biomass,
municipal wastes, paper & pulp industry wastes, sludge etc. and small capacities,
bubbling fluidised bed technology is recommended. The circulating fluidised bed
technology is considered suitable for waste fuels with a high percentage of noncombustibles
(heating value 5-35MJ/kg).
The Circulating Fluidised Bed Combustion technology is environmentally benign.
The process employs a Circulating fluidised bed combustor that operates at a
temperature of around 800-9000C. The fuel(crushed coal) along with the
sorbent(limestone) is fed to the lower furnace where it is kept suspended and
burnt in an upward flow of combustion air. The sorbent is fed to facilitate capture
of sulfur from the coal in the bed itself resulting in consequent low sulfur
emission. The combustion air is fed in two stages - Primary air direct through the
combustor and Secondary air, way up the combustor above the fuel feed point.
Improved Combustion Efficiency
The long residence time in the furnace resulting from
collection/recirculation of solid particulate via the cyclone, plus the
vigorous solids/gas contact in the furnace caused by the fluidisation
airflow, result in high combustion efficiency, even with difficult-to-burn
fuels. 98-99% carbon burnout has been achieved. The very high internal &
external re-circulating rates of solids result in uniform temperatures
throughout the combustor.
• In-situ Pollution Control & Reduced Emissions
Desulfurisation is accomplished in the combustion zone itself by addition
of limestone in the bed. SO2 removal efficiency of 95% and higher has
been demonstrated along with good sorbent utilisation. Low furnace
temperature plus staging of air feed to the furnace produce very low NOx
emissions. Chlorine & Fluorine are largely retained in ash.
Fuel Feed System
Fuel feed system is either pneumatic or wet type. Normally coal is fed as coal -
water mixture as they have demonstrated to burn more evenly. The optimum
system design depends upon ash and sulfur content in coal. For fuels with low
ash contents, coal-water mixture has found favours since large quantities of
water are needed for coals with high ash, which affects its efficiency. The fuel is
fed in the form of coal-water paste with 25% water by weight. The fuel feed size
is lower than 0.75 in.
Sorbent Feed System
Sorbents are not combustibles and are generally fed either continuously or
intermittent. In the latter case, lockhoppers are used. The sorbent is crushed to
around 3 mm top size, dries and fed in lock hoppers.
Technological Advancements
In addition to external particle recirculation CFB, internal recirculation CFB has
been developed. Internal recirculation CFB uses U-Beam separators installed in
the flue gas exit path to collect and recycle the solids directly to the bottom of the
furnace. U-beams are a staggered array of stainless steel channels in the
furnace exit plane which capture most of the solids suspended in the flue gas. In
addition, the multi cyclone dust collector captures finer solids which pass through
the U-beam and recycles them to the lower furnace in a controlled manner. The
regulation of this secondary recycle system offers furnace temperature control
resulting in improved boiler performance. The manufacture claim to achieve
99.8% particle collection efficiency for the two-stage particle separation system.
The IR-CFB operates at low flue gas velocities of 8 m/s as compared to 27 m/s
with external recirculation CFB. This reduces erosion problems in the furnace
which are a major cause for maintenance problems in CFBC. This design uses
significantly less amount of refractory due to elimination of hot gas cyclone path.
This IR-CFB technology is exclusively patented by Babcock & Wilcox.
Status of PFBC Technology Development
The first demonstration plant of capacity of 130 MWe (+224 MW, co-generation)
has been operating in Stockholm, Sweden since 1991 meeting all the stringent
environmental conditions. Another demonstration plant of 80 MWe capacity is
operating in Escatron, Spain using 36% ash black lignite. The third
demonstration plant of 70 MWe at TIDD station, OHIO, USA was shut down in
1994 after a eight year demonstration period in which a large amount of useful
data and experience were obtained. A 70 MWe demo plant has been operated at
Wakamatsu from 1993 to 1996.
Presently, a 350 MWe PFBC power plant is planned in Japan and another is on
order in USA (to be operated at SPORN). UK has gathered a large amount of
data on a 80 MWe PFBC plant in Grimethrope during its operation from 1980-
1992 and is now offering commercial PFBC plants and developing second
generation PFBC. ABB-Sweden is the leading international manufacturer which
has supplied the first three demonstration plants in the world and is now offering
300 MWe units plants. In India, BHEL-Hyderabad has been operating a 400 mm
PFBC for the last eight years and has collected useful research data. IIT Madras
has a 300 mm diameter research facility built with NSF (USA) grant. A proposal
by BHEL for a 60 MWe PFBC plant is under consideration with the Government
of India.