August 10, 2013

Boiler Efficiency - Parameters affecting it

Energy savings does not always imply efficiency improvement in boilers therefore, we need to understand the factors which are affecting it. So basically one should know all those parameters that influence efficiency.

For this one should know how to calculate the energy savings from parameters that do not influence thermal efficiency.

For very clear understanding between an increase in steam generation and improvement in efficiency, I have always emphasized that process engineer should be very clear about his options, definitions, & actual process happening due to modification proposed in the name of energy efficiency.

Recall the general efficiency equation of a boiler

(1) Eff = Absorbed Heat / Energy Input

The equation says nothing about the fact that not all adsorbed heat is useful heat. For instance the blow down is certainly “adsorbed” heat but rarely “useful”. In other words the energy in the blow down will be lost to ambient but any change in the blow down rate will not affect the system efficiency.

Equation (1) was converted into another equation

(2) Eff = 1 - Losses / Energy Input

It was shown that equation (1) and equation (2) are equal and should give the same result.

The energy input to the system is in the most simple case the energy of the fuel and the enthalpy of the combustion air. However one may add to the ener0gy input the steam necessary to atomize the fuel, or the electricity needed to power all electric motors of the boiler. In large power plant boilers it is especially important to draw a system boundary and prepare a list of all energy flows that enter and leave the system boundary.

Similarly the sum of losses depends on what we call a loss. Some obvious losses are the energy in the stack gas, the radiation and convection loss, and the refuse loss. However blow down is not considered a loss and therefore excluded from the sum. In fact the norms state no blow down is allowed during efficiency testing.

Using the efficiency simulator one will notice that system efficiency does not change at all if the following parameters are changed

• The steam pressure
• The steam temperature
• The blow down fraction
• The percentage of condensate return
• The condensate return temperature

The above parameters do not enter equation (2) and consequently the system efficiency will not change.

Nevertheless lowering the steam pressure or temperature or increasing the condensate return and temperature will certainly save fuel. There is absolutely nothing wrong with the definition of efficiency except the fact that we may save fuel by not changing the system efficiency at all.

Whenever Ps , Ts, xBD, xcon,Tcon, change the steam output changes as well, but the efficiency stays the same. Consequently we cannot calculate the fuel savings by the equation

(3) Fuel Saving % = ( Eff new - Eff old ) / Eff new

Another peculiarity are the savings one may achieve by preheating the air or the feedwater. The efficiency definition does not provide for entering the temperature of preheated air or feedwater, because preheating devices such as an economizer and air preheater are inside the system boundary. In other words preheating of the combustion air and the feedwater is taken into account through lowering the stack gas temperature.

So basically, the major purpose of writing it to again clarify that fuel saving does not necessarily mean the increase in efficiency. Currently I have worked an oil water emulsion technique which makes nano particle mix of fuel saving 3-5% fuel but there is no change in the efficiency of the system as none of the boiler parameter is changing. This makes saving just by using water as fuel.

So be Careful.

This Article is reproduced from the paper of GTZ from BEE India website.

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Deep said...

I agree with you Sir.. This post is eccentric yet very logical..I can not defeat your logic... as far as Eq. ! & Eq. 2 are used.. These ver the very fundamental equations derived carefully from acute common sense..
However In process Industry .I have a very bad experience.. in convincing some so called high profile professionals to understand thinks through such a simplistic but wise way.. In 2010 in 1 of my interview in Tata Group.. People discussed me how one can improve boiler efficiency .. They wanted me to say that by changing any of these parameters..• The steam pressure
• The steam temperature
• The blow down fraction
• The percentage of condensate return
• The condensate return temperature one can improve efficiency.. but I did not agree to it.. I tried to convince that to fuel saving concept you said.. but some times it's hard to change a fixed mind-set.. or mindset filled with common error in thinking called Rationalization.. I was not selected.. Now I feel it was good. as I know that organization & it's working in a better light.. They did not even understand absolute combustion.

Anyway thank you Sir.. I find a lot many people trying to befool companies by doing this or doing that without making any effort to improve absorbed energy part..

Techkasamba Agarwal said...

I think we cant change somebody if they feel so. A very high level technical director who is famous in India is actually putting isolation valves for flow control from a continuous decanter.

Due to my current situation I could not even react to it which I had been doing earlier strongly.

So sometimes if our charts are not good we have to leave it to others.

Anyway you have not shared your details for my study though it is your choice.

Techkasamba Agarwal said...

Missed one point did you agreed for percentage of condensate return???

Deep said...

Condensate return was not they asked.. they wanted me to convince with them.. on points that decreasing a blow-down will actually increase efficiency for upto 5%.. but by definition.. at the time of estimation too from BEE formula laos.. Continuous as well as Intermittent blow-down is to be closed or kept away from efficiency calculations.. as it's for healthy running of boiler to reduce TDS or any bottom settled salts.. CBD is continuous.. they are integrated in boiler system for different purpose..

Recently they have recruited a person from IGL.. K.A. who himself admitted to P.Y. that in interview when he was asked he cudn't tell the full form of HAZOP.. in IGL the Hazop level who knows better than you Sir..

I once again appreciate your post on hazop for continuous operation we take 7 guide words.. you provide 9 or 11 guide words.. exhaustively that include batch as well as continuous operations.. i an interview of a consultancy in gurgaon I mentioned your guide words and made a very good impression on recruiters.. the result is on hold as they told me because of their certain project is on hold..

Techkasamba Agarwal said...

I cud understand KA but not P in PY better to send on my mail

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