Introduction

An overview of boiler regulations, with an evaluation of fuel types and comparisons.

#Fuel for boilers

The three most common types of fuel used in steam boilers, are coal, oil, and gas. However, industrial or commercial waste is also used in certain boilers, along with electricity for electrode boilers. Coal Coal is the generic term given to a family of solid fuels with a high carbon content. There are several types of coal within this family, each relating to the stages of coal formation and the amount of carbon content. These stages are:

• Peat.
• Lignite or brown coals.
• Bituminous.
• Semi bituminous.
• Anthracite. The bituminous and anthracite types tend to be used as boiler fuel. In the UK, the use of lump coal to fire shell boilers is in decline. There are a number of reasons for this including: Availability and cost - With many coal seams becoming exhausted, smaller quantities of coal are produced in the UK than formerly, and its decline must be expected to continue. Speed of response to changing loads - With lump coal, there is a substantial time lag between:
• Demand for heat occurring.
• Stoking of coal into the boiler.
• Ignition of the coal.
• Steam being generated to satisfy the demand. To overcome this delay, boilers designed for coal firing need to contain more water at saturation temperature to provide the reserve of energy to cover this time lag. This, in turn, means that the boilers are bigger, and hence more expensive in purchase cost, and occupy more valuable product manufacturing space. Ash - Ash is produced when coal is burned. The ash may be awkward to remove, usually involving manual intervention and a reduction in the amount of steam available whilst de-ashing takes place. The ash must then be disposed of, which in itself may be costly. Stoking equipment - A number of different arrangements exist including stepper stokers, sprinklers and chain-grate stokers. The common theme is that they all need substantial maintenance. Emissions- Coal contains an average of 1.5% sulphur (S) by weight, but this level may be as high as 3% depending upon where the coal was mined. 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), which can condense in the flue causing corrosion if the correct flue temperatures are not maintained. Alternatively, it is carried over into the atmosphere with the flue gases. This sulphuric acid is brought back to earth with rain, causing:
• Damage to the fabric of buildings.
• Distress and damage to plants and vegetation. The ash produced by coal is light, and a proportion will inevitably be carried over with the exhaust gases, into the stack and expelled as particulate matter to the environment. Coal, however, is still used to fire many of the very large water-tube boilers found in power stations. Because of the large scale of these operations, it becomes economic to develop solutions to the problems mentioned above, and there may also be governmental pressure to use domestically produced fuels, for national security of electrical supply.
• The coal used in power stations is milled to a very fine powder, generally referred to as 'pulverised fuel', and usually abbreviated to 'pf'.
• The small particle size of pf means that its surface area-to-volume ratio is greatly increased, making combustion very rapid, and overcoming the rate of response problem encountered when using lump coal.
• The small particle size also means that pf flows very easily, almost like a liquid, and is introduced into the boiler furnace through burners, eliminating the stokers used with lump coal.
• To further enhance the flexibility and turndown of the boiler, there may be 30+ pf burners around the walls and roof of the boiler, each of which may be controlled independently to increase or decrease the heat in a particular area of the furnace. For example, to control the temperature of the steam leaving the superheater. With regard to the quality of the gases released into the atmosphere:
• The boiler gases will be directed through an electrostatic precipitator where electrically charged plates attract ash and other particles, removing them from the gas stream.
• The sulphurous material will be removed in a gas scrubber.
• The final emission to the environment is of a high quality. Approximately 8 kg of steam can be produced from burning 1 kg of coal. Oil Oil for boiler fuel is created from the residue produced from crude petroleum after it has been distilled to produce lighter oils like gasoline, paraffin, kerosene, diesel or gas oil. Various grades are available, each being suitable for different boiler ratings; the grades are as follows:
• Class D - Diesel or gas oil.
• Class E - Light fuel oil.
• Class F - Medium fuel oil.
• Class G - Heavy fuel oil. Oil began to challenge coal as the preferred boiler fuel in the UK during the 1950s. This came about in part from the then Ministry of Fuel and Power's sponsorship of research into improving boiler plant. The advantages of oil over coal include:
• A shorter response time between demand and the required amount of steam being generated.
• This meant that less energy had to be stored in the boiler water. The boiler could therefore be smaller, radiating less heat to the environment, with a consequent improvement in efficiency.
• The smaller size also meant that the boiler occupied less production space.
• Mechanical stokers were 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. Gas Gas 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. It is used in its natural state, (except for the removal of impurities), 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. In the late 1960s the availability of natural gas (such as from the North Sea) led to further developments in boilers. 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%. Waste as the primary fuel There are two aspects to this: Waste material - Here, waste is burned to produce heat, which is used to generate steam. The motives may include the safe and proper disposal of hazardous material. A hospital would be a good example:
• In these circumstances, it may be that proper and complete combustion of the waste material is difficult, requiring sophisticated burners, control of air ratios and monitoring of emissions, especially particulate matter. The cost of this disposal may be high, and only some of the cost is recovered by using the heat generated to produce steam. However, the overall economics of the scheme, taking into consideration the cost of disposing of the waste by other means, may be attractive.
• Using waste as a fuel may involve the economic utilisation of the combustible waste from a process. Examples include the bark stripped from wood in paper plants, stalks (bagasse) in sugar cane plants and sometimes even litter from a chicken farm. The combustion process will again be fairly sophisticated, but the overall economics of the cost of waste disposal and generation of steam for other applications on site, can make such schemes attractive. Waste heat - here, hot gases from a process, such as a smelting furnace, may be directed through a boiler with the objective of improving plant efficiency. Systems of this type vary in their level of sophistication depending upon the demand for steam within the plant. If there is no process demand for steam, the steam may be superheated and then used for electrical generation. This type of technology is becoming popular in Combined Heat and Power (CHP) plants:
• A gas turbine drives an alternator to produce electricity.
• The hot (typically 500 °C) turbine exhaust gases are directed to a boiler, which produces saturated steam for use on the plant. Very high efficiencies are available with this type of plant. Other benefits may include either security of electrical supply on site, or the ability to sell the electricity at a premium to the national electricity supplier.

#Security of supply

What are the consequences of having no steam available for the plant ? Gas, for example, may be available at advantageous rates, provided an interruptible supply can be accepted. This means that the gas company will supply fuel while they have a surplus. However, should demand for fuel approach the limits of supply, perhaps due to seasonal variation, then supply may be cut, maybe at very short notice. As an alternative, boiler users may elect to specify dual fuel burners which may be fired on gas when it is available at the lower tariff, but have the facility to switch to oil firing when gas is not available. The dual fuel facility is obviously a more expensive capital option, and the likelihood of gas not being available may be small. However, the cost of plant downtime due to the nonavailability of steam is usually significantly greater than the additional cost.

#Fuel storage

This is not an issue when using a mains gas supply, except where a dual fuel system is used. However it becomes progressively more of an issue if bottled gas, light oils, heavy oils and solid fuels are used. The issues include:

• How much is to be stored, and where.
• How to safely store highly combustible materials.
• How much it costs to maintain the temperature of heavy oils so that they are at a suitable viscosity for the equipment.
• How to measure the fuel usage rate accurately.
• Allowance for storage losses.

#Boiler design

The boiler manufacturer must be aware of the fuel to be used when designing a boiler. This is because different fuels produce different flame temperatures and combustion characteristics. For example:

• Oil produces a luminous flame, and a large proportion of the heat is transferred by radiation within the furnace.
• Gas produces a transparent blue flame, and a lower proportion of heat is transferred by radiation within the furnace. On a boiler designed only for use with oil, a change of fuel to gas may result in higher temperature gases entering the first pass of fire-tubes, causing additional thermal stresses, and leading to early boiler failure.

#Boiler types

The objectives of a boiler are:

• To release the energy in the fuel as efficiently as possible.
• To transfer the released energy to the water, and to generate steam as efficiently as possible.
• To separate the steam from the water ready for export to the plant, where the energy can be transferred to the process as efficiently as possible. A number of different boiler types have been developed to suit the various steam applications.