combustion control1

8/12/2019 Combustion Control1

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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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