Improving boiler efficiency

C.2.1 Management and control of boiler load

The simplest type of boiler load control is when the boilers operate independently on and off load according to a fixed set-point, such as water temperature or steam pressure. This is inefficient because:

n There is no overall assessment of the site heat demand so more boilers may be fired than is actually necessary.

Appendix C – BOILERS AND FIRED HEATERS

n With only one high firing rate the boiler may be on load for short periods meaning that standing heat losses when the boiler is off will be a high proportion of the total energy input. Air can be drawn through the boiler when it is off, resulting in increased heat losses through the flue.

n The short firing times will also result in high heat losses from the air purge of the combustion space that must take place before each firing.

n Boiler load controls can be improved by at least using high and low level firing rates. The boilers will only go to high fire if the heat demand from the site is sufficient. Otherwise the boilers remain at low fire, reducing fuel demand. After a pre-determined time, if the process load does not call for additional heat the burner will switch off until the demand returns.

n When operating existing boilers their number and size must be carefully reviewed so that all those in actual use operate as closely as possible to their designed ratings. Decisions are easier if the existing or likely future load pattern is known on an annual, monthly and daily basis. On existing plant a load survey will also reveal opportunities to reduce peaks in demand.

n Boilers in-use should be operated as continuously as possible, the maximum firing rate being adjusted to match the maximum demand for heat.

This is preferable to frequent on-off operation of a boiler fired at too high a rate. On-off operation aggravates heat losses to cold purging air.

n The wasteful practices of over firing and blowing safety valves can be reduced if increases and decreases in steam demand can be anticipated by using steam flow to initiate changes in firing rates rather than (or in addition to) steam pressure. When heavy and sudden demands for steam are unavoidable large-capacity ‘thermal storage’ boilers or some other thermal accumulator should be considered as an aid to steadier firing conditions.

n Sequence controllers can be a useful method of managing automatic boiler plant. The controller fires the best combination of boilers to meet the particular demand, avoiding the use of excess capacity.

n Sparging a standby boiler with steam generated by more efficiently operating boilers is one option of keeping an otherwise idling boiler up to temperature and pressure. Electrical savings can be made as there will be no need to operate the boiler fans.

n The temptation to make economies by reducing the operating pressure of steam boilers should be resisted. The dryness of steam at the point of use may well suffer and distribution mains may be inadequate. It is better practice to reduce pressure near the point of use to match the optimum steam pressure required for the equipment being supplied.

The following actions should be considered to ensure efficient boiler operation:

n Before increasing boiler capacity seek every opportunity to reduce demand, smooth the load and increase thermal efficiency.

n Try to improve load factor and efficiency by operating boilers as closely as possible to their design ratings. Select plant which will enable you to do this throughout the year.

n Evaluate the need for standby plant or spare capacity.

n Avoid unnecessary on-off operation of boilers by reducing maximum firing rates so that they just meet the maximum demands made on them.

n Check that purging is not excessive, but do not risk plant safety by purging insufficiently.

Appendix C – BOILERS AND FIRED HEATERS

n Try to anticipate sudden changes in demand by the use of appropriate controls. When heavy demands cannot be avoided use some form of thermal accumulator to smooth firing rates.

n Isolate off-load boilers with flue dampers.

n Operate all boilers at their designed pressures.

C.2.2 Isolating flue dampers

When a boiler goes into stand-by mode there is a continuous flow of air through the boiler to the flue due to natural convection resulting in heat transfer from the water and equipment to the cold incoming air. This heat is lost from the boiler to the chimney and can be significant where boilers are put on stand-by regularly due to process load changes. The function of a shut-off damper is to restrict airflow through the flue and prevent heat loss from the boiler when on stand-by. Dampers are particularly suited to situations where intermittent capacity is needed, and where it is necessary to operate a boiler in stand-by mode and cycle it to keep the required pressure/temperature conditions.

C.2.3 Optimising conventional combustion control

The ratio of air to fuel supplied to the burner is not necessarily constant but may need to be varied according to the firing level in the boiler. The oil or gas fuel supply to the boiler is controlled by a valve and the air supply controlled by a damper (or 2 dampers if there are separate primary and secondary supplies). The valve and damper are usually linked mechanically to enable the damper setting to be matched the valve setting using a characteristic cam. This essentially works as an articulated lever with a travelling fixed point. The locus of the fixed points can be varied to give the correct damper setting for a given valve setting.

The cam is a pre-set ratio control. The usual procedure is to set up the cam coarsely on the basis of previous experience and then, by carrying out a series of flue gas analyses over the full firing range, to finely tune the settings to optimum efficiency. The efficiency of the boiler can be optimised extremely precisely in this way but the cam suffers the same disadvantages as any other pre-set control; i.e. without frequent checking it will continue to control the same way whilst the optimum operating point of the burner drifts. This could be for a variety of reasons, including:

n The characteristics of the fuel, including viscosity, temperature and calorific value, could change.

n The burners wear, become damaged or dirty.

n The cam and its linkage could wear or stick.

The pre-set control will ignore these factors and the combustion efficiency of the boiler will be compromised. How serious the consequences of this drift will be depends on individual circumstances, with an older plant the drift maybe more rapid than with newer equipment. Burners are precision

engineered and need to be regularly serviced and maintained.

Appendix C – BOILERS AND FIRED HEATERS

In order to ensure efficient combustion it is important regularly monitor the flue gas conditions over the firing range of the boiler and reset the burner gas air-fuel ratio as necessary.

C.2.4 Oxygen trim

Oxygen trim control allows boiler plant combustion to be more tightly controlled, producing significant fuel savings. The principal difference between oxygen trim and conventional control is that there is a live measurement of the oxygen content of the flue-gas stream giving a good indication of the real time combustion efficiency. After measurement of the oxygen content, a signal from the control unit alters the amount of combustion air (via damper or fan-speed control) to maintain optimum combustion conditions throughout the range of firing rates at which the burner can operate. Oxygen-trim controls can be added to a conventional combustion control system or can form an integral part of a digital control system. They can take account of changes in air density such as when ambient conditions (for example, air temperature) alter or when airflow is restricted through filters. In addition changes to the fuel conditions, such as calorific value, can be accounted for.

It is most important that the boiler plant flue system is air tight. Air leakage into the flue could produce a false signal at the oxygen sensor, compromising the control logic.

The equipment requires regular inspection, cleaning and calibration, particularly of the probe as it operates in arduous conditions in the stack.

C.2.5 Optimising heat transfer

For efficient boiler operation heat must be transferred from the hot products of combustion produced by the fuel to the boiler fluid, usually water,

effectively. The boiler plate or tube may well have a layer of iron oxide and soot covering the fire side and a layer of scale covering the water side. Both layers will be poor conductors of heat.

In practice the temperature differences across the soot and scale layers and the stagnant surface films cannot be known with any accuracy but the bulk gas and water temperatures will be known. As the heat transfer surfaces become dirty, the heat transfer to the working fluid is reduced and this results in a rise in the flue gas temperature. A 15^oC rise in flue gas temperature indicates an increase in fuel consumption of around 1%.

Monitoring the flue gas temperature at a given boiler load regularly (say once a week) gives a measure of the gradual fouling of heat transfer surfaces and provides a guide as to how frequently the tubes should be cleaned.

A regular cleaning regime on both the fire and water sides of the boiler heat transfer surfaces is important to maintain boiler efficiency.

Appendix C – BOILERS AND FIRED HEATERS

C.2.6 Blow down control

The concentration of dissolved impurities in boiler water is expressed as the ‘Total Dissolved Solids’ or TDS. As water is evaporated in a boiler the concentration of impurities in the boiler will rise. To control this problem it is important to blow down the boiler and introduce fresh boiler feed water to reduce the overall TDS level in the boiler. It is important to set the maximum allowable TDS as high as possible to minimise blow down rates while protecting the steam system. If the desired TDS is set too low blow down rates will be excessive wasting boiler fuel. Maximum TDS values for shell boilers are typically in the range of 2,000 to 3,500 ppm.