Fuel for boilers

CoalCoal is the generic term given to a family of solid fuels with a high carbon content.

Types:1.Peat2.Lignite or brown coals3.Bituminous4.Semi bituminous5.AnthraciteThe bituminous and anthracite used as boiler fuel.The use of lump coal to fire shell boilers is in decline. Reasons:• Availability and cost - Demand for heat occurring.• Speed of response to changing loads - Stoking of coal into the boiler.Ignition of the coal.Steam being generated to satisfy the demand.Ash - Ash is produced when coal is burned.

* Stoking equipment

* Emissions -

During the combustion process:Sulphur will combine with oxygen (O2) from the air to form SO2 or SO3. Hydrogen (H) from the fuel will combine with oxygen (O2) from the air to form water (H2O). After the combustion process is completed, the SO3 will combine with the water (H2O) to produce sulphuric acid (H2SO4),

Approximately 8 kg of steam can be produced from burning 1 kg of coal.

OilClass D - Diesel or gas oil.Class E - Light fuel oil.Class F - Medium fuel oil.Class G - Heavy fuel oil.

The advantages of oil over coal include:A shorter response time between demand and the required amount of steam being generated. The boiler could therefore be smaller, radiating less heat to the environment, with a consequent improvement in efficiency.Due to smaller size the boiler occupies less production space.Mechanical stokers eliminated, reducing maintenance workload.Oil contains only traces of ash, virtually eliminating the problem of ash handling and disposal.The difficulties encountered with receiving, storing and handling coal were eliminated.Approximately 15 kg of steam can be produced from 1 kg of oil, or 14 kg of steam from 1 litre of oil.

GasGas is a form of boiler fuel that is easy to burn, with very little excess air. Fuel gases are available in two different forms:* Natural gas - This is gas that has been produced (naturally) underground. and contains a high proportion of methane.* Liquefied petroleum gases (LPG) - These are gases that are produced from petroleum refining and are then stored under pressure in a liquid state until used. The most common forms of LPG are propane and butane.

The advantages of gas firing over oil firing include:Storage of fuel is not an issue; gas is piped right into the boiler house.Only a trace of sulphur is present in natural gas, meaning that the amount of sulphuric acid in the flue gas is virtually zero.

Approximately 42 kg of steam can be produced from 1 Therm of gas (equivalent to 105.5 MJ) for a 10 bar g boiler, with an overall operating efficiency of 80%.

Which fuel to use?

* Cost of fuel* Cost of firing equipment * Security of supply* Fuel storage* Boiler design* Boiler types

Shell BoilersShell boilers may be defined as those boilers in which the heat transfer surfaces are all contained within a steel shell. Shell boilers may also be referred to as 'fire tube' or 'smoke tube' boilers because the products of combustion pass through the boiler tubes, which in turn transfer heat to the surrounding boiler water.

Shell boiler –(a) Dry and (b) Wet and dry back configuration

It is important to note that the combustion gases should be cooled to at least 420°C for plain steel boilers and470°C for alloy steel boilers before entering the reversal chamber.

Different types of shell boilers

Lancashire boiler

Economic boiler (two-pass, dry back)

Economic boiler (Three-pass, wet back)

Packaged boiler

Comparison of 5 000 kg / h boilers

Four-pass boilers

Four-pass units are potentially the most thermally efficient, but fuel type and operating conditions may prevent their use.

When this type of unit is fired at low demand with heavy fuel oil or coal, the heat transfer from the combustion gases can be very large. As a result, the exit flue gas temperature can fall below the acid dew point, causing corrosion of the flues and chimney and possibly of the boiler itself.

The four-pass boiler unit is also subject to higher thermal stresses, especially if large load swings suddenly occur; these can lead to stress cracks or failures within the boiler structure. For these reasons, four-pass boilers are unusual.

Reverse flame / thimble boiler

Heat transfer from the furnace tube

Pressure limitationA thicker plate, which has the structural strength to withstand the forces generated by pressure in the boiler.A thinner plate, which has the ability to transfer heat more quickly.For any one boiler, if the heat transfer rate is increased, the maximum allowable boiler pressure is reduced.

This leads to a practical pressure limit for shell boilers of around 27 bar.

Water-tube Boilers

As the tube diameter is significantly smaller, much higher pressures can be tolerated for the same stress.

Water-tube boilers are used in power station applications that require:•A high steam output (up to 500 kg/s).•High pressure steam (up to 160 bar).•Superheated steam (up to 550°C).

'thermosiphoning' Water-tube boiler sections

The furnace or radiant section

Heat transfer in the furnace or radiant section

Heat transfer in the convection section

Water-tube boiler designation

Alternative water-tube boiler layouts

Longitudinal drum boiler

Longitudinal drum boiler

Typical capacities for longitudinal drum boilers range from 2 250 kg/h to 36 000 kg/h.

Cross drum boiler

Typical capacities for a cross drum boiler range from 700 kg/ h to 240 000 kg/h.

Bent tube or Stirling boiler

Advantages of water-tube boilers:

•They have a small water content, and therefore respond rapidly to load change and heat input.•The small diameter tubes and steam drum mean that much higher steam pressures can be tolerated, and up to 160 bar may be used in power stations.• The design may include many burners in any of the walls, giving horizontal, or vertical firing options, and the facility of control of temperature in various parts of the boiler. This is particularly important if the boiler has an integral superheater, and the temperature of the superheated steam needs to be controlled.

Disadvantages of water-tube boilers:• They are not as simple to make in the packaged form as shell boilers.• The option of multiple burners may give flexibility, but more burners used in power stations makes complex control systems necessary.

Combined heat and power (CHP) plant

Miscellaneous Boiler Types, Economizers and Superheaters

In many applications:

•The amount of steam required is too small to warrant a shell boiler, i.e. Less

than 1000 kg / h.

•The small process requiring steam operates on a day shift only, meaning

that the plant would be started every morning and shut down every night.

•The capital cost of a conventional shell boiler would adversely affect the

economic viability of the process.

•The level of expertise on site, as far as boilers are concerned, is not as high

as would be required on a larger steam system.

Coil boiler

Vertical tubeless packaged steam boiler

Various models are available with outputs in the range 50 to 1000 kg/h, and pressures up to 10 bar g. Boiler heights vary typically from 1.7 m to 2.4 m for outputs of about 100 kg/h to 1 000 kg/h respectively.

Economizers

Superheaters

Three types of boiler ratings are commonly used:

Boiler ratings

• 'From and at' rating.• kW rating.• Boiler horsepower (BoHP).

'From and at' rating

The amount of steam in kg/h which the boiler can create 'from and at 100°C', at atmospheric pressure. Each kilogram of steam would then have received 2257 kJ of heat from the boiler.

'From and at' graph

Where:A = Specific enthalpy of evaporation at atmospheric pressure.B = Specific enthalpy of steam at operating pressure.C = Specific enthalpy of water at feed-water temperature.Note: These values are all from steam tables.

kW rating

Boiler horsepower (BoHP)

In New Zealand, boiler horsepower is a function of the heat transfer area in the boiler, and a boiler horsepower relates to 17 ft² of heating surface,

USA and Australia

In the USA and Australia the readily accepted definition of a boiler horsepower is the amount of energy required to evaporate 34.5 lb of water at 212°F atmospheric conditions.