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Biomass Calculations help required....please

 
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ecopower



Joined: 01 May 2012
Posts: 3

PostPosted: Tue May 01, 2012 4:43 pm    Post subject: Biomass Calculations help required....please Reply with quote

Hi

I am currently trying to calculate

(a) The estimated mass of fuel required in a year in Kg and

(b) the estimated hourly rate of fuel consumption in kg at the appliances rated power output.

The annual Heat load of the building in question is 185kW

We are looking to supply the full heat load (100%) via Biomass

The specified fuel type is wood pellets with a calorific value of 4.8kWh/kg and a bulk density 650kg/m3.

The manufacturers specified efficiency at rated output is 90.7%

We have to use the formulas provided in the MIS3002 document page 11 as we are applying for MCS certification and cannot use any other (until we are certified that is).

Any help would be gratefully appreciated.

Thanks

M
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adam2
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Joined: 02 Jul 2007
Posts: 7241
Location: North Somerset

PostPosted: Tue May 01, 2012 4:51 pm    Post subject: Reply with quote

You will need to determine the annual heating demand in KWH as a first step.
The stated 185KW is a rate of useing fuel or energy, not a total consumed.
It MIGHT mean a boiler rated at 185KW but that does not tell us for how many hours a year it is required.

If the anuall demand is 185KWH then that is very small indeed and most unlikely to justify a boiler.
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mobbsey



Joined: 24 Nov 2005
Posts: 2243
Location: Banbury

PostPosted: Tue May 01, 2012 8:23 pm    Post subject: Reply with quote

What's the existing fuel source? E.g., there would be a big difference in heat delivered between electric, oil and gas.

If you're using pellets it doesn't make a great deal of difference anyway. So much energy and carbon is used in their production that it's not going to make a great deal of difference to the carbon footprint -- certainly not so different compared to if you took that same money and insulated/modified the building you'd probably save more.
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Little John



Joined: 08 Mar 2008
Posts: 7038
Location: UK

PostPosted: Wed May 02, 2012 1:40 am    Post subject: Re: Biomass Calculations help required....please Reply with quote

ecopower wrote:
Hi

I am currently trying to calculate

(a) The estimated mass of fuel required in a year in Kg and

(b) the estimated hourly rate of fuel consumption in kg at the appliances rated power output.

The annual Heat load of the building in question is 185kW

We are looking to supply the full heat load (100%) via Biomass

The specified fuel type is wood pellets with a calorific value of 4.8kWh/kg and a bulk density 650kg/m3.

The manufacturers specified efficiency at rated output is 90.7%

We have to use the formulas provided in the MIS3002 document page 11 as we are applying for MCS certification and cannot use any other (until we are certified that is).

Any help would be gratefully appreciated.

Thanks

M
Any wood, by mass, has exactly the same thermal energy embodied within it as any other woods. I know that sounds counterintuitive, but it's true.

On the other hand, by volume, the thermal energy of different woods varies dramatically. This is because different woods have different mass by volume.

The way to test the above would be to take 1kg of absolutely dry willow and 1kg of absolutely dry ash and burn them under controlled conditions such that you could get an accurate measure of the total thermal energy output when they were fully burned. If you did, you would find their thermal outputs were identical.
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ecopower



Joined: 01 May 2012
Posts: 3

PostPosted: Wed May 02, 2012 8:37 am    Post subject: Reply with quote

adam2 wrote:
You will need to determine the annual heating demand in KWH as a first step.
The stated 185KW is a rate of useing fuel or energy, not a total consumed.
It MIGHT mean a boiler rated at 185KW but that does not tell us for how many hours a year it is required.

If the anuall demand is 185KWH then that is very small indeed and most unlikely to justify a boiler.


Hi adam2

The building is currently using 498,037kWh of gas per year, am I right in thinking that if we install a 200kW pellet boiler then our peak hours will be a smudge under 2500 hours and the estimated mass of fuel consumed per annum would be 148 tonnes ?

Thank you for your help
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adam2
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Joined: 02 Jul 2007
Posts: 7241
Location: North Somerset

PostPosted: Wed May 02, 2012 9:12 am    Post subject: Reply with quote

If you are currently consuming 498,037 KWH of gas, then if other conditions remain the same, you will need about 498, 000 KWH of heat from wood burning.
No great accuracy can be claimed since the efficiency of both the gas boiler and the wood pellet boiler are rather variable depending on load and duty cycle.
Remember also that heating demand can vary 30% or more from one year to the next owing to the very variable weather.

To supply 498,037 KWH from wood with a calorific value of 4.8KWH per kilo will require about 103 tons of wood.
This makes no allowance for losses, but presumably the qouted figure for gas burnt refers to energy at the gas meter, and not to energy output from the boiler. In that case one may estimate that the losses in gas boiler and wood boiler are broadly similar.

In practice the wood pellet boiler may be less eficient than hoped for, especialy at part load.

It would be prudent to allow for greater fual consumption than calclulated, for the following reasons
1)the wood burner may be less efficient than hoped for
2) the figure for gas consumption might have been unusually low and not typical
3) the wood might have a slightly lower calorific value than hoped for.
4) some of the wood pellets may be lost or wasted in handling, or used for other purposes.

Wood pellet boilers do not have a great record of reliability and I would strongly recomend that the gas boiler be reatained as a standby facility.
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woodburner



Joined: 06 Apr 2009
Posts: 4025

PostPosted: Wed May 02, 2012 9:23 am    Post subject: Reply with quote

adam2 wrote:

Wood pellet boilers do not have a great record of reliability and I would strongly recomend that the gas boiler be reatained as a standby facility.


Having a fossil fuel backup is normal practice for large installations,

Wood pellet boilers are a poor choice for the reasons Mobbsey stated as well. They require a highly processed fuel, and the market is not developed so there will be logistics issues in ensuring a continuous supply.
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ecopower



Joined: 01 May 2012
Posts: 3

PostPosted: Wed May 02, 2012 12:48 pm    Post subject: Reply with quote

Thanks adam2 & woodburner

I have grasped it now and we are now seriously considering keeping the gas boiler in place as a back up.

Thanks again for your help

Very Happy
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