An overview of boiler regulations, with an evaluation of fuel types and comparisons.
This Boiler House Block of the Steam and Condensate Loop will concentrate on the design and contents of the boiler house, and the applications within it. A well designed, operated and maintained boiler house is the heart of an efficient steam plant.
However, a number of obstacles can prevent this ideal. The boiler house and its contents are sometimes viewed as little more than a necessary inconvenience and even in today’s energy conscious environment, accurate steam flow measurement and the correct allocation of costs to the various users, is not universal. This can mean that efficiency improvements and cost-saving projects related to the boiler house may be difficult to justify to the end user.
In many cases, the boiler house and the availability of steam are the responsibility of the Engineering Manager, consequently any efficiency problems are seen to be his.
It is important to remember that the steam boiler is a pressurised vessel containing scalding hot water and steam at more than 100°C, and its design and operation are covered by a number of complex standards and regulations.
For the reasons listed above, the user must confirm national and local and current legislation.
The objective of this Module is to provide the designer, operator, and maintainer of the boiler house with an insight into the considerations required in the development of the boiler and its associated equipment.
Modern steam boilers come in all sizes to suit both large and small applications. Generally, where more than one boiler is required to meet the demand, it becomes economically viable to house the boiler plant in a centralised location, as installation and operating costs can be significantly lower than with decentralised plant.
For example, centralisation offers the following benefits over the use of dispersed, smaller 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 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:
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:
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:
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:
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.
With regard to the quality of the gases released into the atmosphere:
Approximately 8 kg of steam can be produced from burning 1 kg of coal.
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:
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:
Approximately 15 kg of steam can be produced from 1 kg of oil, or 14 kg of steam from 1 litre of oil.
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:
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:
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:
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:
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.
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.
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:
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.
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.
The objectives of a boiler are:
A number of different boiler types have been developed to suit the various steam applications.