Basic Design Calculations for Flue Gas Stack Design for a Diesel Genset in India

Basic Design Calculations for Flue Gas Stack Design for a Diesel Genset in India Written by: Suvo • Edited by: Lamar Stonecypher Updated Jul 2, 2010 This article will talk about the basic design calculation of flue gas steel stack or chimney design of a diesel genset. A flue gas stack or chimney is typically a vertical tubular structure used for ejecting exhaust flue gas to the atmosphere. You can see the chimney or flue gas stack in thermal power plants, diesel gensets, kilns, and many other plants, where gases evolving from the combustion process need to be exhausted. The design calculation for flue gas stack varies from application to application. Here in this article we will discuss the basic design criterion of diesel engine-driven genset flue gas stacks. See below how to calculate the diameter and height of the flue gas stack of a diesel genset:

Calculate Flue Gas Stack Height - Calculate the specific fuel consumption of your diesel genset. Say it is X kg. Per hour. - Find out the percentage of sulphur content in the diesel you are using. Say it is P%. - Now, you have to calculate the sulphur dioxide (SO2) percentage in flue gas. Since the atomic weight of SO2 is double the atomic weight of sulphur, the percentage of the SO2 in flue gas is 2P %. - The height of the flue gas stack (in meters) according to SO2 emission can be calculated as: Height (H) = (X*2P)/100…………….Eqn. 1.1 - Now, you have to check out the recommended minimum chimney height by the Central Pollution Control Board (CPCB). In case the height calculated from the Eqn. 1.1 is higher than the recommended height by CPCB then you go ahead with the calculated height or else you have to stick to the CPCB recommended height.

Calculate Flue Gas Stack Diameter - Calculate the exhaust gas quantity. Say it is Y kg per hour. - Select the flue gas velocity you want to keep inside the stack. Say Z meters per second (Recommended flue gas velocity inside the stack is 16 to 20 m/sec as per IS: 6533). - Diameter (in mm) of the flue gas stack can be calculated as: Diameter (D) = [(4*Y)/ (3.142*Z)]0.5 .......Eqn.1.2

Conclusion The design of the steel stack or chimney is important from the diesel genset performance as well as air pollution point of view. While doing the flue gas steel stack design calculations you should consider the design formulas and local pollution control norms.

The Chimney or Stack Effect Explained

Written by: johnzactruba • Edited by: Lamar Stonecypher Updated Feb 3, 2010 The natural phenomena of the density difference between a hot and cold air column that creates a natural flow through a chimney is called the Chimney effect. Learn more about this in this article. You can see the tall flue gas stacks in all the power plants. The function of the stack is to disperse the hot gases, emissions and particulates that leave the boiler to a great height. At these heights the pollutants disperse in a very large area so that ground level concentrations are within permissible levels not harmful for humans or vegetation. Chimneys where in use from the times of the Roman Empire. Chimneys and fireplaces are a common household item in countries with a cold climate. It does the dual function of removing the hot gases out of the house at the same time bringing in fresh air to the fireplace for combustion. Flue gas stacks higher than 250 meters are common nowadays for larger power plants. The tallest stack currently is 420 meters in Kazakhstan. Many factors like terrain, dispersion pattern, plume heights, adjacent tall structures, and population density determine the height of the stack. There is a natural phenomena associated with the chimney or the flue gas stack. This is the natural flow of air up the chimney. This is called the ‘chimney or the stack effect’. This effect is found not only in chimneys but also in tall buildings.

What is the Chimney (or Stock) Effect? The gas temperature inside the flue gas stack is around 140 ° C. The outside ambient air temperature is around say 30° C. Consider this as two air columns connected at the bottom. The high density and heavier cold air will be always pushing the low density and lighter hot gases up. This causes the natural flow of gases up the flue gas stack. This pressure difference that pushes the hot gas up the flue gas stack or the chimney is the 'chimney or stack effect'. You can feel the effect if you stand near the doors or openings at the bottom of a stack or at open door of an elevator shaft. Depending on the height it can be gentle draught or heavy suction. This is the chimney or stack effect. In numerical terms this can be represented as Chimney effect = 353 x Chimney Height x [1/ Stack gas temperature – 1/ Ambient Temperature] Where Chimney effect is in mm of water column. Chimney height is in mteres. Temperatures are in ° Kelvin. For a thermal power plant with a stack height of 250 meters the effect could be around 77 mm of water column. In thermal power plants the stack effect aids the Induced draft fans in removing the hot flue gases from the furnace and dispersing them at the top of the stack. In tall buildings this effect could create problems for the airconditioning system. In deserts where the outside temperatures are higher than the cool interior of the buildings the effect will be in the reverse.

A flue gas stack is a vertical pipe, channel or similar structure through which combustion product gases called "flue gases" are exhausted to the outside air. Flue gas stacks are sometimes referred to as a "smokestacks". Flue gases are produced when coal, fuel oil, natural gas, wood or any other fuel is combusted in an industrial furnace or boiler, a steam-generator in a fossil fuel power plant or other large combustion device. Flue gas is usually composed of carbon dioxide (CO2) and water vapor as well as nitrogen and excess oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides. The flue gas stacks are often quite tall, up to 400 meters (1300 feet) or more, so as to disperse the exhaust pollutants over a greater area and thereby reduce the ground-level concentration of the pollutants to comply with governmental air pollution control regulations. When the flue gases are exhausted from stoves, ovens, fireplaces, or other small sources within residential abodes, restaurants, hotels, or other public buildings and small commercial enterprises, their flue gas stacks are referred to as "chimneys".

Contents [hide] • • • • •

1 History 2 Flue gas stack draft (or draught) 3 The flue gas flow rate induced by the draft 4 Other items of interest 5 References

History The first industrial chimneys were built in the mid-17th century when it was first understood how they could improve the combustion of a furnace by increasing the draft (draught) of air into the combustion zone.[1] As such, they played an important part in the development of reverberatory furnaces and a coal-based metallurgical industry, one of the key sectors of the early Industrial Revolution. Most 18th century industrial chimneys (now commonly referred to as flue gas stacks) were built into the walls of the furnace much like a domestic chimney. The first free-standing industrial chimneys were probably those erected at the end of the long condensing chimneys associated with smelting lead. The powerful association between industrial chimneys and the characteristic smoke-filled landscapes of the industrial revolution was due the universal application of the steam engine for most manufacturing processes. The chimney is part of a steam-generating boiler, and its evolution is closely linked to increases in the power of the steam engine. The chimneys of Thomas Newcomen’s steam engine were incorporated into the walls of the engine house. The taller, freestanding industrial chimneys that appeared in the early 19th century were related to the changes in boiler design associated with James Watt’s "double-powered" engines, and they continued to grow in stature throughout the Victorian period. Decorative embellishments are a feature of many industrial chimneys from the 1860s. The invention of fan-assisted draft (draught) in the early 20th century removed the industrial chimney's original function, that of drawing air into the steam-generating boilers or other furnaces. With the replacement of the steam engine as a prime mover, first by diesel engines and then by electric motors, the early industrial chimneys began to disappear from the industrial landscape. Building materials changed from stone and brick to steel and later reinforced concrete, and the height of the industrial chimney was determined by the need to disperse combustion flue gases to comply with governmental air pollution control regulations.

(PD) Image: Milton Beychok

Outside and inside pressure difference causes draft in a flue gas stack. Black arrows indicate inlet air and flue gas flow.

Flue gas stack draft (or draught) The combustion flue gases inside the flue gas stacks are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called "natural draft (or draught)", "natural ventilation", "chimney effect", or "stack effect". The taller the stack, the more draft (or draught) is created. The theoretical draft equation below provides an approximation of the pressure difference, ΔP, (between the bottom and the top of the flue gas stack) that is created by the draft:[2][3]

where: ΔP = available pressure difference, in Pa C = 0.0342 a = absolute atmospheric pressure, in Pa h = height of the flue gas stack, in m To = absolute outside air temperature, in K Ti = absolute average temperature of the flue gas inside the stack, in K The above equation is an approximation because it assumes that the molecular weight of the flue gas and the outside air are equal and that the pressure drop through the flue gas stack is quite small. Both assumptions are fairly good but not exactly accurate.

The flue gas flow rate induced by the draft As a "first guess" approximation, the following equation can be used to estimate the flue gas flow rate induced by the draft of a flue gas stack. The equation assumes that the molecular weight of the flue gas and the outside air are equal and that the frictional resistance and heat losses are negligible:[4][5]

where: Q = flue gas flow rate, in m³/s A = cross-sectional area of chimney, in m² (assuming it has a constant cross-section) C = discharge coefficient (usually taken to be from 0.65 to 0.70) g = gravitational acceleration at sea level, 9.807 m/s² H = height of chimney, in m Ti = absolute average temperature of the flue gas in the stack, in K To = absolute outside air temperature, in K Designing chimneys and stacks to provide the correct amount of natural draft involves a great many factors such as: • • • • • • •

The height and diameter of the stack. The desired amount of excess combustion air needed to assure complete combustion. The temperature of the flue gases leaving the combustion zone. The composition of the combustion flue gas, which determines the flue gas density. The frictional resistance to the flow of the flue gases through the chimney or stack, which will vary with the materials used to construct the chimney or stack. The heat loss from the flue gases as they flow through the chimney or stack. The local atmospheric pressure of the ambient air, which is determined by the local elevation above sea level.

The calculation of many of the above design factors requires trial-and-error reiterative methods. Governmental agencies in most countries have specific codes which govern how such design calculations must be performed. Many non-governmental organizations also have codes governing the design of chimneys and stacks (notably, the ASME codes).

Other items of interest It should be noted that not all fuel-burning industrial equipment rely upon natural draft. Many such equipment items use large fans or blowers to accomplish the same objectives, namely: the flow of combustion air into the combustion chamber and the flow of the hot flue gas out of the chimney or stack. A great many power plants are equipped with facilities for the removal of sulfur dioxide via flue gas desulfurization as well as removal of nitrogen oxides (NOx) via selective catalytic reduction, exhaust gas recirculation, thermal deNOx, or low NOx burners. In the United States and a number of other countries, atmospheric dispersion modeling[6] studies are required to determine the flue gas stack height needed to comply with the local air pollution regulations. The United States also limits the maximum height of a flue gas stack to what is known as the "Good Engineering Practice (GEP)" stack height.[7][8] In the case of existing flue gas stacks that exceed the GEP stack height, any air pollution dispersion modeling studies for such stacks must use the GEP stack height rather than the actual stack height.

References 1. 2. 3. 4. 5. 6. 7.

8.

 

↑ Douet, James (1988). Going up in Smoke:The History of the Industrial Chimney, Victorian Society, London, England. Victorian Society Casework Reports ↑ Everett B. Woodruff, Herbert B. Lammers and Thomas B. Lammers (2004). Steam Plant Operation, 8th Edition. McGraw-Hill Professional. ISBN 0-07-141846-6. ↑ Perry, R.H. and Green, Don W. (1984). Perry's Chemical Engineers' Handbook, 6th Edition (page 9-72). McGraw-Hill Book Company. ISBN 0-07-049479-7. ↑ Natural Ventilation Andy Walker, June 2008, National Renewable Energy Laboratory, U.S. Department of Energy from the National Institute of Building Sciences' website. ↑ Natural Ventilation Lecture 3 ↑ Beychok, Milton R. (2005). Fundamentals Of Stack Gas Dispersion, 4th Edition. author-published. ISBN 0-9644588-0-2. ↑ Guideline for Determination of Good Engineering Practice Stack Height (Technical Support Document for the Stack Height Regulations), Revised (1985), EPA Publication No. EPA–450/4–80–023R, U.S. Environmental Protection Agency (NTIS No. PB 85–225241) ↑ Lawson, Jr., R.E. and W.H. Snyder (1983). Determination of Good Engineering Practice Stack Height: A Demonstration Study for a Power Plant, EPA Publication No. EPA–600/3–83–024. U.S. Environmental Protection Agency (NTIS No. PB 83–207407)

 

Atmospheric Dispersion Equation Formulas Calculator Air Pollution Control - Stacks |     Solving  for  effective  stack  height.       Inputs:   physical  stack  height  (hphysical)  

 

plume  rise  (Δh)  

   

Conversions:    

physical  stack  height  (hphysical)  =  0      =  0    meter   plume  rise  (Δh)  =  0      =  0    meter    

Solution:   effective  stack  height  (H)  =  HAS  NOT  BEEN  CALCULATED       Other  Units:      

Change  Equation   Select  an  equation  to  solve  for  a  different  unknown     Gaussian  plume  dispersion  model  developed  by  Pasquill   plume   contaminant   concentration   at  a  point  in   space

plume   contaminant   concentration   at  ground  level contaminant   concentration   at  ground  level   along   the  plume   centerline contaminant   concentration   at  ground  level   along   the  plume   centerline   when  the   emission   source  is   at  ground  level   wind  speed  at  elevation  from  known  wind  speed  and  elevation   wind  speed  at  elevation

weather  station  wind  speed

elevation

weather  station  elevation

stability  parameter

  effective  stack  height   effective  stack  height physical  stack  height plume  rise   plume  rise  for  superadiabatic  conditions   plume  rise

stack  gas  exit  speed

stack  diameter

average  wind  speed

stack  heat  emission  rate   plume  rise  for  neutral  stability  conditions   plume  rise

stack  gas  exit  speed

stack  diameter

average  wind  speed

stack  heat  emission  rate   plume  rise  for  subadiabatic  conditions   plume  rise

stack  gas  exit  speed

stack  diameter

average  wind  speed

stack  heat  emission  rate   Where     C  =  downwind  concentration Q  =  pollution  source  emission  rate u  =  average  wind  speed   σy  =  y  direction  plume  standard  deviation σz  =  z  direction  plume  standard  deviation

 

x  =  position  in  the  x  direction  or  downwind  direction y  =  position  in  the  y  direction z  =  position  in  the  z  direction H  =  effective  stack  height

References  -­‐  Books:     1)  P.  Aarne  Vesilind,  J.  Jeffrey  Peirce  and  Ruth  F.  Weiner.  1994.  Environmental  Engineering.   Butterworth  Heinemann.  3rd  ed.