Steam Boiler Sizing 1

[Basil_sa]

Hi,

I am a new engineer in my first job and I have been asked to source a steam generator for a biogas power plant. Can anyone give me some pointers on how to calculate the electrical output achievable from the generator? I think I have calculated the boiler capacity correctly - I used the mass flow rate of fuel to the boiler and the HHV of the fuel to calculate the energy available for heating, then assumed a combustion efficiency of 90% and a thermal efficiency of 90% to calculate the rate of energy transferred to the water. By using the latent heat of vaporisation at 1.25MPa I found that I would need a 70 tonne/hr boiler at this pressure.

Would it be better to get a boiler that operates at higher pressure?
What temperature does the steam typically enter the turbine after being superheated?
What temperature and pressure will the steam typically be at after passing through the turbine?

I think they are the main details I need to know to estimate the electrical output available... please correct me if I am wrong.

Thanks in advance,

Basil

 

[LittleInch]

Well in global terms you just need to look at the overall thermal efficiency of the system.

Basic burner types with little waste heat recovery are somewhere about 30 to 35%.

Some of your assumptions seem rather optimistic to me.

The best Combined cycle waste heat recovery systems are close to 50%.

This look like quite a small system with no heat recovery from the burnt gas so I would go for 35% as starters.

Unless you have a good use for the low grade heat coming out of the generators, I'm surprised you're not just burning this inside an engine.

if this is a real project then you really should be getting some more help from your company and also try doing a bit of research.

then ask some vendors or search some vendors websites for guides and information.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

I agree with LittleInch .... There are other important considerations

As a new engineer, you seem to be yet another victim of non-technical "MBA psychosis"....IMHO.

These simpleton MBAs waste a lot of time, and kill a lot of enthusiasm.

Ten years ago I performed a study on 30 landfill gas sites, evaluating the most economic engineering solution among, IC engines, gas turbines and steam cycle generation.

The results of my study were:

- Except for only a few sites where MASSIVE amounts of landfill gas were available, gas turbine cycles (Brayton cycle) made NO SENSE and only the smallest gas turbines were suitable. Gas turbines require frequent sophisticated maintenance which is expensive. Capital costs of GTs were high, but not as high as a steam turbine cycle ... A few sites with gas turbines exist at landfills today

- There were NO SITES where a steam turbine cycle is a good solution because of the massive capital investment, and the need for an expensive full time staff and EVEN MORE MASSIVE amounts of gas needed for fuel. Furthermore, the availability of pure water was always a huge problem. To the best of my knowledge, there are no steam turbine/landfill gas installations in the US today.

- Generating electricity using internal combustion engines ALWAYS makes the most sense in low-BTU gas applications. Vendors have DECADES of experience making these engines and the industry is very mature. Over 90% of landfill gas power generation use IC engines. Operation of IC engines is automatic in most installations

You are on a fools errand and your MBA boss should perform a little research before assigning things like this to his staff ....

MJCronin
Sr. Process Engineer

 

[LittleInch]

MJC,

I don't think that MBA's are the root of ALL evil like you seem to ( ), but totally agree with your synopsis and some great information.

Your 70 tonnes equals about 50MW? Of steam. That's a lot of biogas.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

https://www.clarke-energy.com/wp-content/uploads/ETS_E_T2_update13_rz.pdf

MJCronin
Sr. Process Engineer

 

[Basil_sa]

Thanks for all the responses!

My apologies, I realise I've described the fuel completely wrong by trying to simplify my question. The fuel is actually pyro-gas - the volatiles produced from pyrolysis consisting mainly of CO, CO2, Methane and tar (which is majority phenol). In response to LittleInch, the reason I was looking into steam generators is because I was worried the fuel properties would be too variable and cause too much fouling for an IC engine due to the high tar content. Any ideas whether that would actually be the case? Here are some more technical details:
mass flow of fuel = 7990 kg/hr
HHV of fuel = 20MJ/kg
Energy available from fuel = 176,728 MJ/hr
Fuel mass fractions -
H2 - 0.012
CO - 0.44
CO2 - 0.25
CH4 - 0.03
Phenol - 0.26

In response to MJCronin, yes my boss has thrown me in the deep end! It's a very small start up company so he doesn't have any expert engineers at his disposal. With the more accurate details on the fuel, do you still recommend I forget steam generators and look into IC engines? If the steam generators are still worth looking into, do you advise that a pressure of greater than 1.25MPa would be better? I still don't quite understand the pros/cons of operating at a higher pressure.

Sorry again for the inaccurate details on the fuel and thanks for the information!

Basil

 

[LittleInch]

Ok, so assuming this is pyrogas from biomass?

I agree if you have a variable and high tar like substances and pretty low CV values ( half that of methane), you probably want to burn it in a fired heater / boiler. The burner people can burn virtually anything with low emmissions of they get a good composition. IC engines are a bit more picky.

I think you need to reformat your question and open a different post in the boiler and pressure vessel forum.

something along the line of:

I have pyro gas at (give details above) which I wish to generate electricity from via a boiler. (50MW thermal input)

what are the issues associated with higher boiler pressure vs efficiency vs cost for this size of unit.??

It will help if you know how you're dealing with the condenser heat output and if you have other uses for low pressure saturated steam (heating something?).

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

If there is water vapor in this phenol laden gas, it will be too corrosive for IC engines. Burners, internal linings, any economiser / convection banks, and supports for steam lines within the boiler may also need to be resistant to corrosion from wet phenol. With this corrosive gas, heat recovery may also be limited to some safe distance away from the water dewpoint of the flue gas - presume this is the basis for the available heat recovery of 177e3 MJ/hr from this stream. Phenol doesnt burn easily, so you may need some fuel gas enrichment of this low btu feedstream to get high flame temperatures to maintain good combustion.

 

[Basil_sa]

Thanks again for the great info! I will make another post in the boiler and pressure vessel forums as you said. In response to georgeverghese, the water vapour content in the gas (previous to burning) should be very low as the biomass will be dried prior to pyrolysis. There'll be a fair amount of water vapour in the flue gas but hopefully the phenol levels will be low enough in the flue for acidity not to be a problem? In regards to the fuel gas enrichment I will bear that in mind, we will soon carry out testing on the feedstock and the pyrolysis products so we'll have a clearer idea then.
Cheers,
Basil

 

[georgeverghese]

If the gas is dried prior to pyrolysis, then you may not need to worry about acidic corrosion from phenol vapors. But that means you may need to place high reliability on the performance of the drier, and the boiler should be shutdown and purged out within a few hours if water vapor breakthrough into the boiler occurs. Unless this gas is clean, does not poison the drier dessicant and is solids free at the inlet to the drier, this may be a risky proposition.

 

[georgeverghese]

Info I read on google tells me 4 vinyl phenol is indeed produced from the fast pyrolysis of bamboo. However, the melting point of 4VP is some 75degC, and its vapor pressure at room temp is very low. So if this fuel gas is at ambient temp at the drier and downstream, most of the phenols would have dropped out of the vapor phase going to the burners. If this observation is correct, the gas composition for the feedstream of fuel gas to the boiler you have published is incorrect.

 

[Basil_sa]

Hey Georgeverghese. The gas itself is not being dried in the process, the biomass producing the gas is being dried so the gas being produced should be of low enough acidity providing the dryers are functioning effectively. In regards to the condensation of the phenols - the gas leaves the pyrolysis reactor at about 700C and we plan to keep it above 300C to prevent condensation of tars, so the gas should still have a significant phenol content by the time it gets combusted.

 

[georgeverghese]

Okay, we see now some of what the process scheme is here - you would still have some residual water bound in the bamboo biomass after solids drying in this case, I think. At > 300degC, phenol can be quite corrosive even with trace amounts of water vapor, I would guess. So this would then not be your "off the shelf" type boiler. Moreover, trace quantities of the unburnt high melting point phenols would condense on the outside surfaces of the cooler sections of the boiler internal tube runs, gradually reducing heat transfer duty.

 

[Basil_sa]

Ok sure that makes sense. I will have a browse for boilers with high corrosion resistance. Do you happen to know anything about choosing the optimum boiler operating pressure for the process? That is what I am struggling with most.
Thanks,
Basil

 

[georgeverghese]

Though I cant quote definite values or ranges for steam generation pressure, the following trends would be obvious:
a) Higher steam temperatures = higher power generation recovery eff - this is a direct consequence from Carnot cycle behaviour
b) Higher steam generation pressure requires purer BFW - lower dissolved O2,lower silica, lower TDS

Running at 12.5barg seems okay from a CAPEX point of view given the low Btu content of this feedgas

Standard practice is to superheat the feed steam to the turbine to the extent that exit conditions are still in the superheated condition. There is some detail on steam turbine performance and selection in Perry's Chem Engg Handbook, which also tells you what are standard industry pressures / temp for feed to turbines in power generation application. For the simpler turbines, isentropic eff is about 70% at BEP. The exit pressure from the turbine will depend on what you plan to use as the cooling medium ( air or some type of water) for the surface condensor. This exit pressure has some influence on power extraction.

 

[Basil_sa]

Thanks so much that info has helped me a lot! I will have a look at that book as well.
Cheers,
Basil

 

[LittleInch]

Just note the point about the temperature of steam exiting the turbine.

It's still pretty hot and has a lot of energy, but making use if it is difficult. Hence the loss of overall efficiency if you need to just dump this heat from the condensors.

Also looking back up, I think your thermal efficiency of 90% is way too high. The best condensing boilers can barely match that and you will need to keep a fairly high exit temperature so that you don't condense anything inside the boiler. You might be lucky to get 60-70%.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Basil_sa]

Hey LittleInch,
is it no good using the exiting steam to heat the boiler feed water? Thanks for the pointers on the thermal efficiency... think I was fooled by some of the Chinese manufacturers claiming >90% efficiency on their boilers.
Cheers,
Basil

 

[georgeverghese]

My experience on fired heater thermal design, which could be extrapolated to steam boilers too I suspect, is, based on LHV of the fuel gas, assuming 100% combustion eff:
a) Without convection bank 50-60%
b) With convection bank 70-80%

 

[Basil_sa]

Awesome thanks!