Heat loss from a plastic tank

[the original hellbilly]

Hi all,

New member here looking for some help with a job I have for a customer.

I have had a chat with a local pig farmer. He has a biodigester on his farm that processes the effluent from his piggery.

Currently he is just flaring the gas to get rid of it, he would like to make use of the heat instead of wasting it.

He is feeding his pigs a porridge of grain and minerals that is watered down to the point where it can then be pumped around his sheds to feed troughs. We have discussed heating the water so that on cold days the pigs won't have near frozen porridge, this will help increase yield as the pigs won't be using body heat to warm their food.

I plan to build a stainless steel heat exchanger that is connected to a 10 000 l polyethylene water tank by a closed loop. This will use the gas to heat water and convection currents will circulate water into and out of the tank.

I am trying to figure out the math's for this system. I want to know what sort of temperature increase we can expect over a 24 hour period, taking into account the heat losses that will be constantly occurring.

Some stats are listed below:
Biogas generated in 24 hours: 210 cubic metres

Ambient air temperature: -5 degrees C at night in winter to +30 degrees C in summer

Water temperature before heating: 10-20 degrees C

Every 24 hour period will see the 10 000l tank completely used and refilled

If any one can provide me some advice on how to work this out, that would be much appreciated. If I left any details out please let me know.

 

[EdStainless]

To start with forget efficiency and the mechanics of this.
How much heat will that amount of gas generate?
How much will that heat 10kl?
If it is only a few degrees, then forget about it.
If it is a practical amount, then we can talk about how you would actually do it.

 

[georgeverghese]

Biogas usually has a good amount of CO2 in it, which lowers the heating value of the biogas since CO2 is an incombustible inert. So to work out the heating value, and if you have enough biogas for this operation, what is the CO2 content of this biogas?

So you are heating the extracted water phase ( only ) of this porridge mix ?

Presumably, you have 2 steps to this heating operation:
a)Heating up say 9kL of water from 10degC to say 40-50degC in say 2-3hours
b)Maintain the tank at 40-50degC while it is being drawn down from 9kL to min level in the remaining 21-22hours

So during step (a), you wont have hot water to blend into the porridge, assuming you have only 1 tank.

 

[LittleInch]

Cv of biomass is prob about 30 MJ per m3.

So 210 / day gives you about 6,000 MJ or 70 kW heat.

Heat required per C water per kg is 4.1 KJ

So 10000kg is 41 MJ per deg C.

So you seem to have more than enough energy to heat up the water to at least 60C

Even with losses and inefficiency you should be OK. 10000l in a square tank will lose only a small amount of heat.

Depends how quickly you want to heat the water.

 

[MintJulep]

-5 degrees C at night in winter to +30 degrees C in summer

But it seems that you'll only be able to use this part of the year.

So the payback likely won't be very good. Is there another thing that the gas could be used for all year?

 

[LittleInch]

There are plenty of ready made boilers and systems like this. https://shawrenewables.co.uk/biogas-boilers/

Using that heat can also make your bio digestor more efficient.

 

[the original hellbilly]

Hi all,

Thanks for the replies.

I'll clarify a couple of things here.

The water will flow into the 10 000l tank where I intend to heat it, from there it will go into a ribbon mixer and be mixed with the feed before being pumped off to the troughs. So yes, we will be heating the extracted water.

Currently the gas is just being flared off (burnt like a busen burner from a pipe coming out of the digester), so it is all being wasted. If we can make any use of it at all, that will be a gain.

Potentially there are other uses we could have for the gas. Where we have chosen to start is with the easiest, which is warming the water for the feed.

Little inch, would you be able to show the math's for that calculation? I did a similar one and got 102 degrees C after 24 hours of heating, but that had no allowance for continous heat losses. I can't figure out how to factor in the heat losses.

Thanks guys

 

[georgeverghese]

Ask local heater manufacturers to make you an API 12K style water bath heater. A forced draft unit will be safer than an induced draft. Heating bundle should be retractable so you can scrub off scale and dirt off the heating surfaces. Run this heater on a continuous basis rather than fill - drain every day. May be better yet if you run this heater (so heater duty = 43kW inc. 5% losses at heater + 5% storage tank - both insulated) for just 12hours a day to fill the tank for a whole days' consumption, if gas LHV = 30MJ/sm3

 

[goutam_freelance]

I understand that you are interested in estimating the heat loss from the polyethylene water tank. Here are some tips:


You can estimate the actual parameters suitably and add the heat loss to other heat loads.

 

[LittleInch]

That sounds about right. You will be amazed at how effective even 50mm of insulation is. In terms of energy loss this grows over time as the delta T increases but with about 70kW of heat available you should be OK at 50C.

 

[georgeverghese]

Typical thermal eff for an API12K water bath heater would be about 60%, so fuel gas demand, at 43kW heat transfer duty, would be 43/0.6 = 72kW.
Both heater and tank would have to be insulated - 2-3inches of hot ins should make heat loss to ambient minimal.

 

[the original hellbilly]

Once again, thanks all for the replies. I haven't given this too much attention over the weekend as I had to pull a couple of 12hr shifts to fix a broken pelletizer. Once that job is finished I'll sit down and go over some of this.

What I am presuming is that the temperature if Graphed, would show a steep rise to start with, then it would flatten out and plateau. I want to know at what temperature will it plateau.

Thanks

 

[LittleInch]

It will level off when heat in equals heat out.

But there are many variables, not least of which is wind and where this tank is located.

Inside a building or shelter and the convective heat transfer from goutam is very high and more more 25 W/m2/K. Outside in a stiff breeze and it could be more.

Inside a shelter then the air could heat up and reduce heat flow.

Any insulation will be important.

Efficiency of your gas boiler could be up to 90% or as low as 60%

It could plateau at more than 100 then you're into boiling or a pressure vessel.

Lots of people make boilers for bio gas. Just buy one.