FIELD OF THE INVENTION
The present invention generally relates to coal feeding device for feeding raw coal to pulverizes in coal-fired boilers corresponding to demand including computation of the amount of coal fed to the pulverizes over a period of time. More particularly, the present invention relates to a system for detection of fire inside coal feeding devices employed in coal-fired boilers in thermal power plants.
BACKGROUND OF THE INVENTION
Coal is one of the most significant natural resources in the world. The abundance, wide distribution, and versatility of coal make it an important source of energy for the present and future. More electricity is produced from coal than any other fuel source. For fossil-fuel power plants, the single, largest, operating cost is fuel. The cost of fuel accounts for about 50-60 percent of the cost of electricity generation; therefore, obtaining lower priced fuel is often a major objective. The design and performance of thermal power plants and the cost of power generation are influenced by coal properties. Often power plants are unable to pre-determine the quality of coal that they will be receiving from the coal-suppliers.
There are different ranks of coals including anthracite bituminous, sub-bituminous and lignite. Within each type, there are a variety of grades. No two mines produce the same quality of coal. The coal quality varies substantially between different types of coal including outputs from individual mines (lot-to-lot) in the same geographic region.

From boiler operational standpoints, the following coal properties are most influencing :
(a) heating value;
(b) moisture;
(c) volatile matter;
Heating value
An important property, which indicates the useful energy content of a coal and thereby its value as a fuel, is its calorific value (also known as heating value), which is defined as the amount of heat evolved when a unit weight of the fuel is burnt completely and the combustion products cooled to a standard temperature of 298 K.
Moisture
Coal contains a significant amount of moisture. The coal must be dried before it is burnt, usually within the pulverizes. The amount of hot air required for drying the coal is directly related to the moisture content. If the coal has too much moisture, the pulverizes may not have the capacity to dry it properly.
Volatile matter
Volatile matter in coal refers to 'those products, exclusive of moisture, given off by the fuel as gas or vapour'. The components of volatile matter are usually a mixture of hydrocarbons, carbon monoxide, carbon dioxide, tar and water. The composition of the volatile matter of coal is different for different ranks of coal.

The content of volatile matter in coal varies from 2 percent to 40 percent (dry mineral matter free basis) and it has a very large impact on the ignitability and burn-out characteristics of solid fuels. High volatile coals (volatile matter >32%) are easier to combust but they are more susceptible for self-heating or spontaneous ignition in coal storage yard.
In a power plant, raw coal stored in bunkers is caused to pass parallely through a number of Coal feeding devices and then to the pulverizes. The coal feeding devices deliver coal in proportion to the fuel demand generated from the combustion control system provided in a boiler. The power plants are now operating in a highly competitive regime and also not linked to any particular source for the coal. Hence according to availability, at times the power plants burn good grade coals and burn other types during the remaining period. In brief, burning of different grades of coal may result in choking or accumulation in the coal feeding device which may lead to fire hazards. In a coal-fired boilers, coal continuously flows from the bunker to the pulverizes through a coal feeding device. Inside the coal feeding device, transportation of coal from inlet to outlet of the feeding device is taken care by a conveyer belt. Hot primary air is supplied to the pulverizes to carry the powered coal to the burners located in the boiler furnace. To avoid entry of hot primary air from the pulverizes to the feeder, seal air at positive pressure is provided to the coal feeding device. This arrangement prevents entry of hot primary air from the pulverizes to the coal feeing device. In case of failure of the seal air provided to the coal feeding device, the hot primary air enters into the coal feeding device and raises the internal temperature. If this is not monitored and detected, this results in exposure of various electrical and electronic components as well as the rubber based components mounted inside the coal feeding device to high temperature. This leads to damage of the components and subsequent tripping of the coal feeding device. The tripping of

the feeding device may result in loss or reduction in power generation for the power plant. Also this results in loss of revenue since the damaged components need to be replaced before subsequent operation.
The Existing coal feeding devices are not provided with means for continuously monitoring the internal temperature of the devices to detect in advance a possibility of fire. Thus, failure to early-defection of fire, causes damage of various electrical components namely, motors, limit switches, load walls of the coal feeding devices.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose a system to continuously monitor the temperature inside a coal feeding device in coal- fired boilers for early detection of fire.
Another object of the invention is to propose a system to continuously monitor the temperature inside a coal feeding device in coal- fired boilers for early detection of fire, which is enabled to monitor the temperature at different locations inside the coal feeding devices through multiple probes / sensors.
A further object of the invention is to propose a system to continuously monitor the temperature inside a coal feeding device in coal- fired boilers for early detection of fire, which is provided with early-warning means whenever the temperature inside the coal feeding device exceeds a threshold limit.
A still further object of the invention is to propose a system to continuously monitor the temperature inside a coal feeding device in coal- fired boilers for

early detection of fire, which is equipped with monitoring means operable under harsh and under positive pressure condition in coal dust loaded environment.
Yet another object of the invention is to propose a system to continuously monitor the temperature inside a coal feeding device in coal- fired boilers for early detection of fire, which allows an operator for easy troubleshooting and maintenance.
SUMMARY OF THE INVENTION
According to the present invention, at least one fibre optics based temperature probe is provided to monitor the temperature inside the coal feeding device. A sensor is attached to one end of the optic fibre and provided with Teflon sheath to withstand the dusty environment inside the coal feeing device. The probe can be of variable lengths depending on the sensor location. The other end of the probe fiber is routed through a wall feed-through connector of the device. An outdoor extension cable is connected to the other side of the feed-through connector, which carries the signal from the probe to an apparatus where the data are processed. The apparatus with multi-channel facility is mounted inside a remote control panel of the feeding device. The sensor is subjected to excitation by a light pulse, generated by a Light Emitting Diode (LED) at a pre-selected wavelength, which produces hundreds of pulses per second. The output signal from the apparatus is outputted through a man-machine interface mounted on the door of the remote control panel. The man-machine interface provides necessary temperature indication as well as alarm for the operator.
The advantages of this method includes:
1. No calibration or long-term drift: The optics bases temperature monitoring

system is not subjected to long term drift and therefore do not require periodic calibration.
2. Since the temperature sensor is passive in nature, reliable measurements can be made at any time as long as the fiber remains intact.
3. The probe can be of variable lengths (2 to 4 m) depending on the sensor location.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - Shows a schematic diagram of a monitoring system for coal feeding devices according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
As shown in Figure - 1, the system consists of a coal feeding device; a remote control panel; a local control panel; and associated cables.
The Remote control panel (1) mounts a controller module and other interface electronic modules. A Man-Machine interface (2) is mounted on the door of the remote control panel (1). The local control panel (3) is mounted on the Coal feeding device (CFD). The coal feeding device (CFD) consists of a motor (4) with an eddy current clutch (5), and a gear reducer (6) to drive a conveyor belt (not shown). The temperature probe (8) is mounted on both the sides of the coal feeding device (CFD). The interconnections between remote control panel (1) and local control panel (3) are made by screened cable (7). The controller maintains the feed rate accurately as per fuel demand signal from the

combustion control, by regulating the current supplied to the eddy current clutch (5). The regulation of the clutch current varies the speed of the belt conveyor (not shown). The man-machine Interface (MMI) provided on the door of the remote control panel (1) is configured corresponding to operational parameters of the feeding device (CFD). A multi- channel apparatus (9) containing the electronics for optics, data processing means and communication means for each channel is housed inside the Remote control panel (1). The apparatus (9) is suitable to receive one to four channels from the temperature probes (8) mounted on the coal feeding device (CFD).

WE CLAIM
1. A system to continuously monitor temperature inside a coal feeding device
in coal- fired boilers for early detection of fire, comprising :
- at least one fibre optics based temperature probe / sensor (8) mounted inside the coal feeding device (CFD) to monitor temperature at different locations inside the coal feeding device;
- a Man-Machine interface (2) mounted on the door of a remote control panel (1), and connected to the coal feeding device;
- the coal feeding device having a motor (4), an eddy current clutch (5), and a
gear reducer (6) to drive a conveyor belt through which coal being fed from a bunker to the pulverizer;
- screened cable (7) for interconnection between the remote control panel
(1) and a local control panel (3), wherein an apparatus (9) is provided on
the remote control panel (1) to determine the temperature inside the coal
feeding device (CFD) based on the signal output from the probe / sensor
(8), and wherein the apparatus (9) comprises a signal processor to
process the signal received from the sensor (8), and allow automated
control of relays of a distributed control system (DCS) of the boiler.
2. The system as claimed in claim 1, wherein coal includes all types of coal
and biomass.

3. The system as claimed in claim 1, wherein said man-machine interface (2) further comprise diagnostic features for quick remedial action in case of possible fire hazards.
4. The system as claimed in claim 1, wherein said apparatus further comprises a direct interface with the Distributed Control System (DSC) of the coal-fired boiler.

ABSTRACT

The present invention relates a propose a system to continuously monitor the temperature inside a coal feeding device in coal- fired boilers for early detection of fire comprising of one or many fibre optics based temperature probes / sensors (8) mounted inside the coal feeing device (CFD) to monitor temperature at different location inside the coal feeding device; a Man-Machine interface (2) mounted on the door of a remote control panel (1). Operably connected to the coal feeding device; the feeding device having a motor (4), an eddy current clutch (5), and a gear reducer (6) to drive a conveyor belt through which coal being fed from a bunker to the pulveriser; screened cable (7) for interconnection between the remote control panel (1) and a local control panel (3), wherein an apparatus is provided on the remote control panel (1) to determine the temperature based on the signal output from the probes / sensors (8), and wherein the apparatus comprises a signal processor to process the signal received from the sensor (8), and allow automated control of relays and direct interface with the distributed control system (DCS) provided for the boiler.