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Furnace Size Varies for Gas, Oil and Coal Firing

written by: Dr V T Sathyanathan edited by: LamarStonecypher

Furnace sizing is a very important for a successful boiler design. The variation in boiler furnace

size can upset the performance drastically. Oil and gas fired units have more uniform sizes, butthe coal fired units have large variation. Chemical recovery boiler furnace is sized for drying

fuel.

Boilers designed to generate steam for both industrial and power generation applications usefuels like coal, oil and gas.Fuel characteristics have a great impact on boiler design. The furnace

designed in a boiler must be so sized that the furnace outlet temperature is predicted and

achieved within reasonable limits. The furnace outlet temperature is the basis for further heattransfer surface design. It is seen that the furnace size varies considerably between coal fired

boilers and oil and gas. The variation between oil and gas fired boiler furnace size is to a smaller

extent only.

Design criteria for boilers

To understand why the furnace size varies between different fuels, we have to have a broad idea

about the design criteria for boilers.

o First we should know the energy input level, this will depend on steam flow, feed watertemperature, pressure and temperature of steam, and an assumed efficiency of boiler

based on experience.

o The energy absorption level needed in boiler and other heat transfer system will have tobe defined or worked out.

o Based on the fuel given, for design calculate the fuel quantity, air and flue gas flowrequired.

o Now determine the size and shape of furnace which will require knowledge of burnersize, other combustion systems, emission requirements, ash handling, furnace outlet

temperature limitation, etc.

o Design superheaters, reheaters, economizers, and air heaters so that the second pass canbe finalized. Care must be taken to size the heat transfer areas such that any requirement

due to fouling and the erosive nature of fuel is addressed.

o Design the desuperheating requirement, location and number of equipment for cleaningof pressure parts.

o Boiler enclosures, supports, piping, expansion guides, and movements are all to bedesigned and checked.

o The code to which the boiler is designed is very important and has to be adhered.

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Coal fired boiler

The coal fired boiler furnace size is generally higher in volume by 20 to 35 %, depending on coaltypes, when compared to an oil fired boiler. The factors that increase the furnace size are many.

oThe reactivity of coal is much lower than oil and hence need more residence time.

o Even though the adiabatic flame temperature is higher than oil, the practically obtainedflame temperature is much lower around 1700 degree Celsius.

o The lower flame temperature needs more radiant surface to be provided.o The ash in coal needs to be addressed and hence the rule of thumb is to keep the furnace

outlet temperature lower than initial deformation temperature of ash by 50 degree

Celsius.

o The ash deposits on the walls reduce the effectiveness of heat transfer and this is lower inthe case of coal fired boilers.

o As it is not possible to have an accurate analytical solution for heat transfer in a steamgenerator furnace, this leads to conservative sizing by designers.

oThere are three furnace heat loading adopted by designers, the EPRS (Effective ProjectedRadiant Surface), the plan area and the volumetric. By virtue of experience and field

results from boilers, these loadings are much lower for coal fired boilers.

o Being a solid fuel, the burner size and numbers go up when compared to oil or gasburners; this factor also influence the furnace size to the higher side.

o The bottom ash handling requirement increase the size of coal fired boiler. Normallyabout 20 % ash is collected as bottom ash in coal fired boiler. The oil and gas firedboilers have nearly flat furnace bottoms.

It is to be remembered that among coal fired boilers themselves, the furnace sizes will varyconsiderably depending on the age of the coal, ash characteristics, the reactivity, etc.

Oil fired boiler furnace

Oil flame emissivity is higher than coal flame and so results in higher furnace absorption. Thefurnace heat loadings can also be maintained much higher due to near zero % ash. The very low

% of oil ash needs to be addressed from point of view of high temperature corrosion in heat

transfer surface. As a very general rule the volume of oil fired boiler furnace will have only 65 %of volume of a coal fired boiler furnace.

Gas fired boiler furnace

When compared to oil, gas is a much more reactive and cleaner fuel. This allows still higher heat

loadings in the furnace, higher second pass gas velocity, etc. All these factors reduce the furnace

size further and result in the lowest furnace size for a similar capacity boiler.

Just to give an idea, a size comparison between coal, oil and gas fired boiler furnace is shown.This is a very rough comparison and is for a specific nature of coal.

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Coal Analysis for Boiler Designers

written by: Dr V T Sathyanathan edited by: Lamar Stonecypher updated: 7/3/2009

Hydrocarbon fuels are the major source of energy for power and process steam generation, and

coal takes a major share in this. Boiler furnace design will depend more on fuel characteristics,and further heat transfer surface sizing will depend on furnace outlet temperature.

The world's thermal power is mainly dependent on coal as its fuel. When designing a boiler, fuelanalysis plays a major role. The performance of the boiler, and ultimately the entire unit, can

change considerably if the coal being used is substantially different from that for which the

boiler was designed.

What is fuel?

o Any combination of organics and inorganic material which during chemical reaction ortransformation gives out large amount of heat is called fuel

o Fuel can be hydrocarbon fuel and non-hydrocarbono Industrial fuels have heat values from as low as 500 kcal/kg to as high as 11000 kcals/kg

Heat generated by fuel is used a boiler to generate steam for process, power generation, and avariety of other applications. The chemical characteristics of the fuel decide many aspects of

boiler design. The boiler designed for gas fuel will have the smallest furnace size, and the boiler

designed for coal will have the biggest size.

Why is coal different?

o All fuels are hydrocarbonso Gas and oil have defined hydrocarbons and structure, which means C & H in fuel does

not vary much

o Coal is a heterogeneous fuel and has only an assumed structureC & H vary highlyo C & H in Coal can be only be known if you do an ultimate analysis, and the way in which

these hydrocarbon behave can be different from one coal to the other. This will depend

on its reactivity and formation.

o Formation of coal has vast impact on boiler design; the Gondwana coals are differentfrom American and European. (Coal formation theories).

Why is consistency in hydrocarbon important for boiler

o It starts from combustion air calculation; the carbon hydrogen ratio decides the quantityof combustion air.

o Flame temperature is dictated by the chemical composition of the fuel, and this changesthe furnace behavior.

o The completion of combustion is another very important factor in boiler design. This willdepend on how the hydrocarbon rings are formed and bonded. If the coal burns slowly

requiring more residence time then the SH and RH (superheater and reheater) behavior

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changes when compared to the fast burning coal. This will lead to large injection in SH

and/or RH which will reduce the life of the tubes and also reduce boiler efficiency.

o The amount of unburned material in fly ash and bottom ash of the boiler can change, and,if it increases will increase fuel requirement, maybe only marginally.

o Lack of consistency can lead to loss of the capability to generate when the ash levelincreases beyond a threshold of the fuel preparation system capacity. This can also leadto limitations on the fuel handling system.

Major effects of fuel on boiler design

o A wide range of coal properties can lead to oversized pulverizing/firing equipment whichwill in turn lead to a limitation on turn down and a possible increase in auxiliary powerconsumption. This will also increase the furnace size and the SH and RH will need to be

sized properly, which can be difficult at times, to get the rated outlet temperature.

o Oversizing of pulverizers and air heaters to handle occasionally high moisture coals willrequire excessive tempering air when handling normal moisture coals. Ultimately all

these will result in higher gas outlet temperature, reducing the boiler efficiency.o The behavior of the ash during the combustion process and managing the coal-ash are the

major issues in boiler design. Deposition and slagging, heat transfer upsets, and fouling inconvection passages are some of the major issues to be dealt during operation of the

boiler.

Steam generator performance is greatly influenced by the coal/ash properties. Changes in coal

property result in efficiency variation and the designer may not be able to demonstrate the

guarantees. This can also result in changes in power consumption, load limitations, boiler

availability loss due to slagging and fouling, tube failure increases, and plant heat rate reduction.There are known and unknowns with coal during combustion. The known are the coal

Proximate, Ultimate, Calorific Value, HGI, and Ash analysis. The unknown are Petrographicanalysis, Reactivity, Slagging and fouling analysis, etc. All unknown become known only afterthe combustion of coal in the boiler furnace and at the cost of boiler performance.