Question regarding an efficiency formula and CO levels 2

[MAEC]

Greetings List:

I have on my hands what some consider to be the bible of sugar cane milling process... LA SUCRERIE DE CANNES by E. Hugot.

It shows an interesting equation regarding boiler efficiency from the point of view of available heat delivered to the steam.

The equation is: eff = Mv/VCS where Mv is the ammount of heat transfered to the steam and VCS the upper calorific value of bagasse. If you consider VCS, there is one approach: considering ashes and sucrose content in bagasse which leads to what we can call a downgraded VCS.

Regarding Mv, he uses an expression which involves bagasse water content, which is quite high, and heat loss in the gases. The result is multiplied by some factors which leads to a supposed real condition of the process, incluiding non burned solids, radiation and poor combustion.

I just tried it on an spreadsheet and the result was a 46% efficiency of a 2 year old boiler!!!! Are efficiencies THAT low??... i know for a fact that whole cycle efficiencies are low but a new equipment???

This is kinda retorical but the thing is that if I´m right then Hugot is wrong!!! Are there some Thermodynamic professors out there who could give me a hand on this??

I know a better approach which consists in an indirect efficiency by considering heat losses in the system, which is the one the germans use. Do you have another approach??

By the way, now that i´m into this, does anyone have any equation to calculate CO levels theoretically? I have CO2, Ar, N2, H2O and H2 levels... but i have no CO... i do know that the same volume of CO2 corresponds to the same volume of CO and that you just need to find the proportions of those.

 

[49078]

I do not think you can calculate theoretical CO because it is essentially an inefficiency. I believe that when you do your calculations for CO2, H2O exhaust that you assume complete combustion and therefore no CO as all the Carbon goes to CO2.

But, I have seen a formula to give an estimate with a combustion analyzer. Our test numbers were sometimes close to this figure, but we found it fluctuated with how the system and eventually stopped using the figure. This also had a similar formula for calculating CO2. I looked for it this afternoon but was unable to find it.

 

[quark]

One reason for low efficiency may be that you are using gross calorific value of the fuel. You should only use this if your boiler is condensing type. Non condensing boiler efficiency is generally calculated with net calorific value.

You can use indirect method to check the boiler efficiency but the more appropriate and correct one is the simple ratio of heat of steam produced/ combustion heat of the fuel.

There is no theoretical calculation for assessing CO content. This depends upon extra fresh air quantity, flame quality, proper functioning of the diffuser and atomization of the fuel etc.

The following link presents you with a paper on boiler efficiency calculation equations

http://www.cleaver-brooks.com/Efficiency1.html

I recommend you to visit the two threads,

Thread404-100205
Thread391-111964

 

[MAEC]

49078

Thank you for the reply. I do have one methodology to get an aprox. CO value which is Ostwald Diagram. The problem is that i have to make the curves for each fuel i want to analyze. In combustion theory, you see that CO2 volume is the same as CO volume, they "share" that volume i mean. So Ostwald gives you a way to extrapolate the proportions of both gases... i won´t focus on CO since an O2 and CO2 analysis may get me close to the real big picture.

quark

I checked the links, although they are mostly about fire tube boilers using a simple fuel. The theoretical considerations are the same but the problem comes when you analyze bagasse. The variability of bagasse composition is such a huge problem. Also the humidity content. There are two interesting reason why to use net or gross calorific value:

1. if you use the lower calorific value, you are taking into consideration the heat losses due to evaporation of water formed by oxidation of hidrogen present in the bagasse and evaporation of the water content of the bagasse.
2. if you use higher calorific value you don´t take into account this.

The problem with bagasse is that if you have it all dry, you will get like 8,200 - 8,400 Btu/lb. But a typical water content of 50 to 53% and ash content and sucrose residuals lowers this value to about 30 to 40 % of it´s original value or perhaps even lower.

My boiler is a fuel cell oven, 4 ovens, superheated steam and we heat up air... we have this interesting design where heated air makes a bag around the lower oven refractory cell and then enters the furnace. We don´t use traveling, nor vibrating nor pinch hole grate since what we need is a thermal reservoir in the system since cane supplyes vary if weather condition changes. Way too much variables in sugar industry!!!!

What I´m thinking is that I will calculate ALL efficiencies I can, order them and compare them. All of them give you an idea of what is happening. So i think it is better to use lower calorific value since water content, ash content and sucrose content is not something that the boiler did... it receives the bagasse that way so i think the best approach is to use LCV as a heat input. WHAT DO YOU THINK ABOUT THIS?

Best Regards

Manuel

 

[UmeshMathur]

MAEC:

The CO increases strongly as excess O2 reduces. However, in general, there are innumerable burner design and operational criteria that affect performance. Poorly maintenance of burner tips in a liquid-fired boiler, for example, will reasult in marked increases in stack CO level.

 

[mauner]

Basic thermal efficiency for boilers using a fossile fuel is approximately 80%.

Excess O2 and temperature is measured to determine efficiency assuming a fuel type (stoichiometric combustion)

Another way to look at it is how much heat is going up the stack divided by the heat of combustion. Since you don't know the exact value then you need a different approach.

A large portion of wasted heat is due to heating nitrogen (80% of air) up through the combustion process. If you have excess O2 then you have excess N2. If you measure your exhaust temperature and combustion intake air flowrate then you can estimate how much heat is lost. The specific heat capacity for the exhaust mix will have to be determined based on an average composition. It is your choice to include the latent heat of vaporization for steam in the exhaust.

Thermal efficiency can be calculated by heat transfer to steam, Qsteam, and heat lost to the exhaust, Qexhaust:

Eff = Qsteam/(Qsteam + Qexhaust)