The present invention relates to a Circulating Fluidized Bed Combustion system for high marcasite content lignite.
Background:
Lignite is one of the major source of energy for utility and industrial boilers but the plant load factors (PLF) of lignite fired boilers is very much impaired due to the presence of marcasite (FeS2-Ferric sulphide) in lignite which is usually present in the range of 0.5% - 2.0%. Because of low softening temperature of marcasite around 850 Deg.C, the ash fuses at normal boiler operating temperature (between 1200 Deg.C and 1400Deg.C), causing heavy slagging and clinkering. Due to separate factors that govern the origin of marcasite, it is highly difficult to predict the occurrence of marcasite in lignite seams. Its size and concentration are varying widely and hence it becomes difficult to separate it out during handling.
Hence, selection and development of a suitable combustion system has become an immense and long felt need to reduce or mitigate slagging so as increases the plant load factor.
Current state of art;
In conventional pulverized coal fired boilers, lignite is fired at high temperature between 1300 and 1500 DegC. The marcasite fuses at high temperatures and deposits on the waterwall, evaporator and super heaters. They are removed periodically using wall deslaggers and soot blower. At times large lumps of slag / deposits, removed from the water wall fall down and damage the water wall tubes at lower elevation which affect the entire boiler operation. The heat transfer area may not get fully cleaned despite wall deslagging and soot blowing.
Consequently, the heat transfer to boiler tubes reduces, resulting in poor generation of steam and lower steam temperatures.
Limitations of current state of art design :
1. It is well known that the combustion temperature of any fuel in
pulverized fuel fired boilers is between 1300 Deg.C and 1500 Deg.C (The
fuel lignite is pulverized to less than 200 micron size in mills and
pneumatically admitted to the furnace).Due to its high hardness of marcasite
(Hard groove Index between 5 and 7.5), lot of difficulties are experienced
while pulverizing the same.
2. The slag, associated with low melting marcasite, deposits on the water
cooled enclosure is cleaned periodically using deslagger/soot blowers. The
soot blowers are installed along the height of combustor, super heaters and
evaporators.
3. The heat transfer to water cooled enclosure is reduced due to deposition
of slag on the surfaces. Consequently, the steam generation and steam
temperature are reduced.
4. The slag falls down while cleaning the water wall surfaces maintained
throughout the boiler from bottom to top of the boiler, hit the tubes
enclosing the boiler and damage them. At times, the tube puncture occurs
due to heavy impact on the tubes which impair the availability of boiler.
An object of the present invention is to provide an unproved from of a circulating fluidized bed combustion (CFBC) system wherein the slag either do not form or do not affect the performance of the system.
Another object of the present invention is to avoid pulverizing the high hardness marcasite with lignite. In the pulverized fuel fired boilers, the mills, at times, are damaged due to crushing of marcasite with lignite.
A further object of the present invention is to minimize the installation of deslagger/ soot blower since the formation of the slag is very much reduced. A still further object of the present invention is to minimize S02 liberation.
The marcasite, (Ferric sulphide - FeS2) release sulphur-di-oxide during
combustion and the same can be absorbed by addition of limestone.
Thus the proposed innovation is a novel method to use low temperature and
higher hardness content marcasite with lignite in circulating fluidized bed
combustion boilers without sacrificing the availability of boiler.
According to the present invention there is provided a circulating fluidized
bed combustion boiler installment for high marcasite content lignite
characterized by a fluidized bed combustion for receiving a feed of
marcasite rich lignite at its lower end being fed by a variable speed
conveyors, an air distributor provided at the bottom of the said combustor
containing plurality of air nozzles with bed drain arrangement, a fluidized
bed ash coolers for draining out coarse bed materials and soot blowers
provided at selected locations for removal of slags from different sections of
the combustor.
Description of the invention :
The proposed invention 'Circulating Fluidized Bed Combustion system for
high marcasite content lignite' is characterized by the following parameters
and features.
1. Air distributor.
2. Bed material drain arrangement.
3. Refractory material lining and its height on water wall enclosure.
4. Appropriate location of soot blowers.
5. Change in boiler layout.
The present invention will be described in detail with reference to the accompanying drawing where Figure 1 shows the schematic arrangement of the CFBC system.
The CFBC system of the present invention is provided with an air distributor (1) at the bottom boundary containing plurality of air nozzle with bed drain arrangement (2). The lower zone of CFBC, combustor is lined with refractory (3) at a height above water wall formed by feeding water from a feed pump and fed to the entire water wall formed by tubes maintained throughout from bottom to top of the combustor boiler to avoid erosion of water wall tubes and wherein marcasite rich lignite is fed by variable speed conveyors (4). The unique special range of operating parameters as proposed by this invention is given below. The values hi bracket correspond to operation of normal conventional CFBC system:
a) Fluidizing velocity = 3.0-6.0 m/sec (6.Om.sec)
b) Operating temperature = 700-880 Deg.C (850 Deg.C)
c) Desired particle suspension
density at higher elevation of
CFBC combustor = 20-30 kg/m3
d) Desired wind box pressure = between 1,200 mmwc and
2,000 mmwc
e) Drawing of bed material based on operating parameters like box pressure, suspension
density of particles, combustor temperatures at select locations, etc to flush out higher
size particles from fluidized bed.
The saturated steam generated in water wall is separated and admitted to steam super heaters.
The coarse bed material is drained, through fluidized bed ash coolers (5) and discharged to a silo. The soot blowers (6) are installed at select locations to remove slag from various sections. The unburnt fuel, ash and fluidized bed material are separated from flue gas in the water/steam cooled or refractory lined cyclone (7) and returned to the combustor for reburing through a seal pot (8).
The circulating fluidized bed combustion (CFBC) system for marcasite containing lignite, operates as per above details, can be effective upto 2.0% of marcasite by weight (usual range of marcasite is between 0.5%). This invention is, thus as effective technique for utilizing high marcasite content lignite.
The invention as described hereinabove is only by way of an example. Any modification or development thereof such is well within the knowledge of a skilled person is intended to be covered within the scope of this invention.

WE CLAIM :
1. A circulating fluidized bed combustion boiler installation for high marcasite content lignite characterized by a fluidized bed combustion for receiving a feed of marcasite. rich lignite at its lower end (3) being fed by a variable speed conveyors (4), an air distributor (1) provided at the bottom of the said combustor containing plurality of air nozzles with bed drain arrangement (2), a fluidized bed ash coolers (5) for draining out coarse bed materials and soot blowers provided at selected locations for removal of slags from different sections of the combustor.
2. The installation as claimed in claim 1, wherein lower part of the said
combustor is lined with refractory material (3) over and above water cooled
tube enclosure to avoid erosion of water wall.
3. The installation as claimed in claim 1, wherein the material blown from
the combustor are collected in hot cyclone (7) and returned to the combustor
through a seal pot (8).
4. The installation as claimed in the preceding claims, wherein the
operating parameters maintained to be effective upto 2% of marcasite by
weight of lignite are fluidizing velocity - 3.0-6.0 m/sec, operating
temperature-700-880°C, particle suspension density at higher elevation of
CFBC combustor-20-30 kg/m3 and wind box pressure between 1,200 mmwc
and 2,000 mmwc.
5. A circulating fluidized bed combustion boiler -installation far high
marcasite content lignite as herein described and illustrated.