Effect of Low Condenser Vacuum at Biomass Power Plant

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...?

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.

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.

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!

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

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.

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.

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

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

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)?

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.

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.

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.