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Fuel and Air Gas Circuit
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Suction Mill Air Supply System
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Oil Pumping, Heating and Recirculation
System
Oil is ignited by expelling the oil through a small nozzle ( a pressure jet) or by
compressed air or steam, this process is called atomization. The fuel is circulated to theburners via a recirculation system, this process is referred to as spill-back.
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Gas-firing systems
Protection against leakage into the furnace through the fuel-supply valves is achieved
by use of double-block-and-bleed valve assemblies which provide a secure sealbetween the gas inlet and the furnace. Before a burner is ignited both block valves are
closed and the vent is opened. In this condition any gas which may occupy the volume
betwwen the two block valves is vented to a safe placeand it can never develop
enough pressure to leak past the second block valve. When start-up of the burner is
required, a sequence of operations opens the block valves in such a way thet gas isadmitted to the burner and ignited safely.
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Waste to Energy Plant based on
Bubbling Fluidized Bed Boiler
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Burner Management System
Safe operation of burner and igniters
Flame Scanner
Sequential operation of fuel firing system (safe start up, shut down)
Purging done for a stipulated time by the FD & ID Fans ensuring that acertain volume of clear ambient air (In a coal fired boiler the flow rate >
40% of the full-load volumetric air flow) has passed through the furnace.
Boiler firing is initiated, ignition is successful operation of igniter or pilot
Flame scanners (ensure presence of flame inside furnace): electronicdevices operating in close proximity to high-energy spark ignition systems,and in conditions of extreme heat and dirt. The should provide reliable
indication of flame. Flame failure trip a protection against accumulation of unburnt fuel which
could lead to explosion
On shut-down of a burner, unburned fuel is cleared from the pipework bya procedure called scavaging.
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Oil Pumping, Heating and Recirculation
System
Control System Includes
Controlling the temperature of the fuel
The pressure of the atomizing medium The equalization of the fuel pressure at
various levels on the burner front
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Combustion Control
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Heat Losses in a Furnace
These are,
Heat wasted in the exhaust gases
Heat content of the fuel which is due tounburnt carbon
Heat lost by radiation from hot surfaces in the
boiler and its pipework
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Heat Losses in a Furnace
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Effect of Tramp Air or Setting Leakage
If the tramp air or setting leakage amount isincluded in the total air being supplied to thefurnace, and if that total is apportioned to the
total amount of fuel being fired, the implicationis that some burners (at least) will be deprivedof the air they need for combustion of their fuel.In other words, the correct amount of air is
being provided in total, but it is going to placeswhere it is not available for the combustionprocess.
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Simple Parallel Control
The easiest way of maintaining a relationship between fuel flow andair flow is to use a single actuator to position a fuel-control valve andair-control damper in parallel with each other.
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Flow Ratio ControlIts a closed loop controller with measured value of fuel and air flow linked to
controllers for tracking each other. A gain block or amplifier in one of theflow-signal lines is used to adjust the ratio between the two flows. As the gainof this block is changed, it alters the slope of the fuel-flow / air-flowcharacteristic, changing the amount of excess air that is present at each flow.
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Flow Ratio Control
As the gain (g) of the block is changed, it alters the slope of fuel-flow / air-flow characteristic, changing theamount of excess air that is present in each flow. When the gain is fixed , the amount of excess air is thesame for all flows. However some air inevitably leaks into the furnace, with the result that the amount ofexcess air is proportionally greater at low flows than high flows.
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Flow Ratio Control
The excess air line curves hyperbolically upwards at lowflows. Practically the air flow is slightly greater than thetheoretical air as per the stoichiometric ratio.
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Flow Ratio Control
In a fuel lead system , when the loaddemand changes, the fuel flow is adjusted first
and the controller than adjusts the air flow to
match the fuel flow. In air lead system , when the load demand
changes, the air flow is adjusted first and the
controller than adjusts the fuel flow to matchthe air flow.
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Comparing the fuel-lead and air-
lead approaches Fuel-lead version provides better response to load changes, as it
does not depend on the slow response air supply system.
The air lead system is slow as the draught plant is slow to react.
This can lead to fuel rich conditions on load increases and fuel leanconditions on load decrease.
Fuel-rich condition can cause explosion. Too much excess air can lead to back-end corrosion of boiler
structure and undesirable stack emissions.
Hazard in air-lead system is slow as the load increases.
As the load decreases however the slow responding air system is
prone to create fuel reach conditions, however the hazard is lesser. The failure of air flow measurement transmitters also is risky.
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Basic Cross Limited Control System
FUEL FLOW
AIR CONTROL VALVEFUEL VALVE
AIR FLOW
MASTER
DEMAND
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GAS ANALYSIS FOR FUEL/ AIR RATIO
OXYGEN TRIMMING OF FUEL- AIR RATIO
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GAS ANALYSIS FOR FUEL/ AIR RATIO
OXYGEN TRIMMING OF FUEL- AIR RATIO
The oxygen content of air is 21% by volume.
A given change in oxygen represents approximately five times that change in excess air.
It is the air flow being controlled.
The gain of the controller should be set at a kick-off low value(0.25) and a proportional
band of 400%.The time constants of the fuel/air/flue-gas system are long, hence the integral term of
oxygen controller tends to be long.
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The set value of oxygen controller is trimmed
by the output of the carbon monoxide
controller.
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Carbon in ash measurement
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Multiple Burner System
Oil and gas burners can be operated by
maintaining a defined relationship between
the fuel pressure and the differential pressure
pressure across the burner air register (ratherthan proper flow measurements), but even
with such economics the capital costs are high
and the payback low. The need to provide amodulating actuator for each air register adds
further cost.
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Setting the demand for the
steam generator
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Nature of the demand
Steam is generated in boiler by burning fuel.
Boiler Fuel Master demand is set as per the
steam demand
BFM sets the burner firing rate, air flow, feed
water flow
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