Finding thermodynamic values for compounds other than gases and pure substances

[longucdavis]

Hi everyone,

Long time checking up on these forums, first time posting.

I've never been really comfortable with most thermodynamic definitions despite trying for the past few years to be around them. Recently, I was given the opportunity to perform what I though was going to be a "simple" mass energy, and exergy, analysis of various theoretical pre-treatment processes.

I have a system of say 4 unit operations in which i can define the Temperature and Pressure at the entrance and exit of each. I can also state any mass gains or losses based on unit operation assumptions (say a washing operation is performed, resulting is some faction loss of impurities and some faction gained of water). All I'd like to do is define the mass, energy, and exergy at each state before and after each unit operation. From there I can play with thermodynamic equations (i think). Problem is, getting values for enthalpy, and entropy, at these steps. My system is a biomass processing system; being such, I can not (or haven't figured out however to) look at the back of my thermo-books, or the NIST webbook, for thermodynamic values of interest.

How do I find values for Energy, enthaply, and exergy of real world compounds such as biomass, biomass ash (based on composition ideally).

When I search the literature I continually find HHV and adiabatic flame temperature derivations. It has been suggested to me that I look at the HHV for energy content; but use of HHV only accounts for the biomass energy (dry ash free), along with estimates for loss in HHV base on moisture content and a really rough approximation of ash energy. HHV seem appropriate for combustion system, but i'm not comfortable with it for a pre-treatment process.

This is technically all i would need for the energy content at each stage (ignoring my distrust of these results). How do i proceed to exergy at each step (with out knowledge of a entropy value for biomass or ash).


I feel as though i'm missing some major concept in thermodynamics here as I cant seem to find values for what i need, yet there piles of papers quoting results of these type analysis. Ironically, I perform mass and energy balances quite often; but those are all based entirely off of calorimetry heat value results and theoretical HHVs.

I think i'll be best able to clarify in confusion based on specific responses.

Thank you for all of your help,

-Michael

 

[davefitz]

biomass is not a working fluid, per se, so why in the world would you need the entropy or exergy values? Are you really going to feed wood chips into a gas turbine? I'd really like to see the results of that test. I think it is fair to say that the most likely commercial operations with biomass would be gasification, combustion, and torrefication, and there may be some other operations that involve bacterial digestion that could be considered ( the exergy and entropy effects of bacterial digestion would be interesting to consider, though). I am at a loss how to consider biomass as a working fluid.

The largest variables in Biomass are the water content and ash content, so I think your calculations would be simplified if you used LHV and not HHV.

"Whom the gods would destroy, they first make mad "