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Effect of Low Condenser Vacuum at Biomass Power Plant

11/26/2020 5:56 AM

I am consulting for a biomass power plant and design condenser pressure is 175mbar absolute pressure. The plant has a 3 stage steam ejector for air removal with a capacity of 15kg/h of air removal and ejector design suction pressure of 2" Hg. Even under 70% load conditions, the condenser pressure is about 600mbar. I am proposing adding a liquid ring vacuum pump to improve condenser pressure to specification level and also to increase turbine efficiency. This power plant is now burning about 2.5MT of biomass per MWh of electricity produced so there seems a lot is wrong with this plant. The cooling tower is not functioning well and we are suggesting modifications to improve heat rejection but even we lowered condenser cooling water inlet temperature we have not seen any improvement in fuel consumption. Would you agree that the most likely cause of this is the very high pressure in the condenser now? Also if we go from 600mbar absolute to 175mbar absolute, what would the expect improvement in efficiency be? Thank you in advance for the help!

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#1

Re: Effect of low condenser vacuum at biomass power plant

11/26/2020 6:16 AM

Sounds like you are dumping too much air into the burner and feeding too much fuel...Do you have a stack gas analyzer to set the parameters properly...?

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#2
In reply to #1

Re: Effect of low condenser vacuum at biomass power plant

11/26/2020 6:22 AM

My question is related to the steam loop not the burning parameters which is under a different control system. My gut feeling is that the condenser has more air ingress than the steam ejector systems can remove. In that case my question was twofold: 1) is adding a liquid ring vacuum pump a good solution for this, and 2) what would be the expected improvement in efficiency if we lowered the condenser vacuum level from the current 600-650 mbar absolute, to 175 mbar absolute?

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#3

Re: Effect of low condenser vacuum at biomass power plant

11/26/2020 6:30 AM

According to Mayhew & Rogers, "Thermodynamics and Transport properties of Fluids", the saturation temperature for steam/water at <...175mbar absolute...> is 56.5degC [interpolated calculation], and that of <...600mbar absolute...> is 86degC [interpolated calculation]. On that basis, either the <...condenser...> temperature is too high by 29.5degC, indicating either a cooling problem, or there is air leaking into it and raising the pressure and therefore the temperature, or a combination of the two. That is the first problem to solve irrespective of any <...efficiency...> calculations at this time.

Cooling tower performance is influenced by a range of factors including ambient temperature and wet bulb temperature. Performance of the <...condenser...> is influenced affected by the incoming temperature of the coolant and its flowrate. It is not possible to reduce the temperature of the circulating cooling water below the wet bulb temperature at any moment without introducing an intervening fridge plant to produce chilled water, which is uncommon on thermal power installations.

Mismanagement of evaporative cooling towers is commonplace, despite bio-hazards becoming present by so doing. A biofilm inside the <...condenser...> will also significantly affect performance.

It would be helpful to reveal to the forum whereabouts on the globe this plant is located as this may influence further responses below.

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#4

Re: Effect of low condenser vacuum at biomass power plant

11/26/2020 6:37 AM

I would suggest making ice and using the ice to lower the inlet air temperature to the turbine.

You will get a net gain from this method because it uses all three modes of latent heat harvesting to cool the air and increase air density into the turbine.

Utilities use this method to improve efficiency.

Does your boiler use forced draft and induced draft fans,with ID air being preheated by exhaust gas?

The FD fan forces preheated air,usually 600F+ injected under the fuel to increase efficiency.

You may also want to investigate condensing boilers that recover the heat from condensation .

Biomass has a lot of moisture that is vaporized and goes up the stack;Condensing boilers recover this heat.

However,single condensing boilers may not have sufficient capacity to handle your demand.It might require multiple units.

I have not been in this field for many years,and I am sure much progress has been made in this area.

Check the CO2 content of the exhaust gas to determine efficiency.

I do not know the best level for biomass,but for other fossil fuels it ranges from 10% to 14%. You will need excess air to achieve this level.

There are simple chemical methods to do this,as well as electronic methods.

Also monitor CO levels,as this can indicate a cracked heat exchanger,which can be very dangerous,even at low levels.

Be especially alert if CO level is increasing,it is a warning that a crack is getting larger.

A typical small scale biomass boiler,such as wood pellets, can achieve around 90% efficiency.

A typical wood boiler requires about 1 ton per MWh of electricity produced,but your mileage may vary according to efficiency.

Good luck.

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#6
In reply to #4

Re: Effect of low condenser vacuum at biomass power plant

11/26/2020 6:58 AM

The plant is 8MW nominal output so its not something we have in our backyard. Sorry if I didn't make this clear. Its not a direct fired turbine. I seem to have caused some misunderstandings. I am a consultant for a large industrial 8MW biomass power plant and there is a biomass boiler which creates steam, the steam drives a turbine in a closed Rankine cycle. The condenser rated pressure is 175mbar absolute and current is 600mbar. I am assuming that excess air ingress and/or lack of vacuum capacity is the main cause. Looking for some pointers from my seniors in the field!

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#11
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Re: Effect of low condenser vacuum at biomass power plant

12/02/2020 7:21 AM

The last thing one wants in a <...steam turbine...> is <...air...> that is simply going along for the ride.

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#5

Re: Effect of low condenser vacuum at biomass power plant

11/26/2020 6:51 AM

It would also be helpful to review as-commissioned and recent maintenance records to discover when the drop-off in performance began, leading to pointers to the cause(s).

It would also be worth talking through the problem with the Original Equipment Manufacturer(s) as these people will have come across this problem before. Make some calls.

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#7

Re: Effect of low condenser vacuum at biomass power plant

11/26/2020 11:54 PM

Condenser vacuum in a steam system is a result of the volume loss going from vapor to liquid. Liquid ring pump compressant would condense steam, but certainly not efficiently. Once you have removed the air from the system, it should not take much to keep the air out, unless you have huge air leaks.

You should be able to do a simple heat balance around the turbine to find out what the problem is. High fuel consumption should not be linked to turbine performance. Steam flow, inlet temperature, extraction flow if any, and hot well temperature will give you the necessary parameters to determine turbine mechanical output. Condenser vacuum is a result or indicator of the supply factors. It has been my experience that poor turbine performance after commissioning is due to deviation from design, usually low extraction flow due to process changes. Low extraction flow results in excess steam flow to the condenser, which was not designed to accept full inlet flow.

Once you have determined why the condenser is too small, then you will have to take steps to improve the capacity. Cooler water or higher hot well flow would be the practical first step, or increase the surface area. I expect that the electric consumption of your vacuum pump, plus the extra cooling of the compressant would exceed the net improvement of your generator.

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#8

Re: Effect of Low Condenser Vacuum at Biomass Power Plant

11/27/2020 3:48 PM

What is the calorific value of the biomass? Currently you're getting about 1.5MJe/kg biomass. Depending on the CV, that might not be too bad.

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#9
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Re: Effect of Low Condenser Vacuum at Biomass Power Plant

11/30/2020 4:20 AM

The Fuel energy content is 8,594 kJ/kg, moisture is 45-50%

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#10
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Re: Effect of Low Condenser Vacuum at Biomass Power Plant

11/30/2020 7:40 AM

I make that 16.7% efficiency, electric out/fuel in. Doesn't sound brilliant for a reasonably big machine, but I'm no expert.

You haven't given the steam pressure, but my guess is you could estimate the improvement achievable by lowering the condenser pressure as the change (600 - 175)mbar/steam pressure. So if steam pressure = say 10 bara, that's 425/10^4 ~ 4%.

Sounds to me like the air ingress is much too high for a closed cycle, and needs investigating.

The minimum condenser pressure is determined by the cooling water temperature. 175mbar vapour pressure is at ~ 57°C

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#12
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Re: Effect of Low Condenser Vacuum at Biomass Power Plant

12/02/2020 7:39 AM

How do I differentiate between too much air ingress (=lack of vacuum pump capacity) and lack of cooling from the cooling tower? The turbine inlet is about 38-40 bar and about 480C, and the outlet is 0.6 bar (no temperature given). Hot well temperature is about 85C.

The cooling tower flows 2680m3/h and temp in is about 48C and out is about 40C. Approach temperature is about 10C. I am addressing plugged fill in the CT now.

In other words is condenser pressure high because of lack of cooling, or is there a lack of cooling because the pressure is too high ( vacuum too low)?

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#13
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Re: Effect of Low Condenser Vacuum at Biomass Power Plant

12/02/2020 11:38 AM

It is not possible to tell from here, because:

  • The forum cannot see the equipment
  • The forum cannot see the as-commissioned documentation and compare it with today's data
  • The forum cannot see the maintenance records since the commissioning date of the plant

The <... addressing plugged fill in the CT now...> is an important first step. Doing this suggests cooling tower [CT] mismanagement is a major contributor to the problem.

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#14
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Re: Effect of Low Condenser Vacuum at Biomass Power Plant

12/02/2020 12:22 PM

The heat removed by the cooling tower is ~ 25MW, which is about what you'd expect for a turbine output power something over 8MW.

As I said before, I'm no expert, but wouldn't expect a LR vacuum pump to be needed. I assume some steam is blown down, enough to remove the air which has leaked in, that steam being condensed and the condensate returned to the boiler. And if the air leakage is enough to cause a problem it needs addressing.

48°C gives vapour pressure 112mb, so on the face of it 175mb should be doable.

Maybe somebody who knows more than me about these systems can comment.

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#15
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Re: Effect of Low Condenser Vacuum at Biomass Power Plant

12/02/2020 12:23 PM

The real parameter to look at is the inlet steam flow, and compare that with the design. None of those other parameters really matter, they are the result of your problem. If you are stuffing more pounds of steam into the machine than it is rated for, then you will get this problem. You also need to measure extraction flow, so you know how much is going to the condenser.

It’s unlikely you have so much air to affect condenser operation, lack of cooling is the cause of low vacuum, but low vacuum is a symptom of another problem.

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