Jump to content

openspaceman

Member
  • Content Count

    5,605
  • Joined

  • Last visited

Posts posted by openspaceman


  1. On 26/05/2020 at 12:47, aesmith said:

    Assuming you get 5kWh per kg of dry wood, the direct loss due to water is only around 3.3% for wood at 20% m/c, or just over 4% for 25.

    I used 5.17kWh/kg for dry wood and my calculation  gave 4.29% loss in steam at 300C.for 20% and 5.36%

    at 25%.

     

    I would like to know if people using flue temperature gauges actually see temperatures as high as 300C as it seems a bit high to me. Flue temperatures have to be above 100C at the chimney exit  to avoid condensation  with wetter wood but it would take a bit of working out what the dew point would be for dry wood, it would be less than 100 because the partial pressure of the water vapour  would  remain above the partial pressures of all the other combustion gases down somewhere below 100C. Of cause the chimney would be absorbing some heat and depressing the temperature of the column of flue gases, more so if it is not insulated. I use this as a feature, so the chimneybreast's thermal mass keeps the house warm after the fire is out.


  2. 9 hours ago, aesmith said:

    One cube of firewood will create the same amount of heat when it's burned whether it has 20% or 25% moisture content.  The difference will be whether that extra water stops it burning properly by reducing the temperature or in other ways. 

    You've got it

    Quote

     

    Some heat will be lost turning the water into water vapour but the science of that is beyond me at the moment.  It's apparently not just as simple as each kg of water using 0.6 kWh of energy to turn into vapour, the actual value depend on the temperature at which it happens.

    Actually the latent heat of water does vary with temperature but your 0.6kWh holds good for the approximation we are looking at.  When you burn wood it changes from a solid with associated moisture  to ash (a solid but very small part of the original mass) and gases/vapours that go up the chimney at whatever temperature it leaves the stove, this mix of gases carries away heat from the wood that never gives you benefit as heat in the room. If the moisture content of the wood is high then you lose that 0.6kWh for every kg of moisture that goes up the chimney plus the additional amount of sensible heat it contains at temperatures above 100C. I guess most stoves run a flue temperature of 150-350C so this sensible heat of the excess water is also considerable.

     

    As you say wet wood is harder to burn cleanly, mostly because it quenches the burn, this shows up as smoke and particulates, Products of Incomplete Combustion, so some of the potential energy of the wood also leaves as PICs. Worse still is the business of excess air; because wood is a complex mix of substances that burn in stages we have to add more air than  is what is needed to combine with the fuel to ensure enough oxygen meets the fuel to burn it out. Good wood stoves will use 1.5-2 times the ideal (stoichiometric) amount of air but this all has to go up the chimney at the flue temperature. Thus the massflow of products up the chimney consists of excess air, excess moisture plus the products of combustion (ideally steam and carbon dioxide, even burning bone dry wood produces water)). Worse still is wet wood tends to need more excess air than dry wood, hence increased sensible heat losses from the high massflow as well as extra steam.

     

    Bear in mind you need a stoichiometric mass of air about 6 times the mass of the burning dry wood because only 21% of the air is oxygen, the rest is nitrogen which has a free ride through the fire but still carries away heat up the flue, increase that with excess air and you see what a heat robber it can be.

    Quote

     

      I am also not sure whether "bound" moisture when wood is below fibre saturation takes additional energy to separate it from the wood.  So although I intended to do a rough calculation, I've had to admit defeat.

     

    I think it does but it is a tiny effect only of consequence when discussing how much moisture is regained as dry wood picks up moisture again as the humidity increases in the winter.

    • Like 1

  3. 12 hours ago, kevinjohnsonmbe said:

    Which is best VfM? 
     

    Despite all the calc’s and variables, surely it’s a simple matter of cost. 

    Yes, the thing is there is much wringing of hands and wailing about using wood to dry wood but the physics of it is all the same , energy is energy whether we use solar heat for the drying or intercept it first on its way to gaining entropy and store it in a tree.

     

    The bigger argument is whether our value system, money and wealth, are good measures by which we make decisions. If not we need to come up with another one that functions as well as this.

     

    @gdh has put some numbers on it  and it's simple to work out the difference in energy available in 25 or 20% mc wood but the rest is so many variables. One would need to know the parameters of the stove and the flue temperature to decide on which was better for which stove. From the supply side one would need to know the cost of covered space, seasonality of labour, cost of capital equipment etc. In practice a person makes simple decisions to run a business and one thing leads to another, if another business makes different decisions and as a result out competes the first then it is more successful, that's what drives capitalism, not basing decisions on what's good for the environment, that's what regulations do to keep everything acceptable to most of we also rans.

    • Like 2

  4. 2 hours ago, Stubby said:

    I suppose the act of getting up of the sofa every 5 mins to chuck another bit of kiln dried on the stove would keep you warm anyway 👍

    My apologies I snipped badly and somehow it mixed up the attribution, I don't know exactly how but it is something to do with the quote selection feature when I highlight the specific bit I am replying to.


  5. 1 hour ago, Stubby said:

    However what is never, ever mentioned is the fact that although kiln dried firewood, that has been cooked in a large oven, may produce a marginally higher kWh / kg than naturally air dried firewood, kiln dried firewood burns away far faster than air dried, so in reality burns for less time per Kg !

    This fact completely changes the economics of burning kiln dried.

     

    Drying does not increase the amount of heat from a given log, it has just  driven off the moisture and made it lighter but releases about the same energy. If the piece of wood had not been dried some of its energy would be used to volatilise the moisture first, so the heat is not available to the room but goes up the chimney as steam.

     

    The more significant effect is than in turning the moisture to steam the fire gets cooler and burns less completely, hence more pollution.

     

    The bit about burning too quickly is rather spurious because in burning quickly it gives out heat quicker, if it's giving out more heat than required it's normal to turn down the air control to compensate.

     

    @Stere the late Tom Reed, a combustion chemist from america, produced similar results to that graph, I have tried to explain the reasons in the past. Firstly the flame on a wood fire is largely not premixed like petrol:air in a saw or gas:air in a gas burner, it is diffuse, the hot wood gases meet incoming air and diffuse into each other at the flame:air interface. The oxygen in the air reacts differentially to strip hydrogen from the gases and these carbon bearing remains glow yellow in the flame. If the conditions are good the hot carbon meets fresh air and burns out, if not it exits as soot.

     

    A premixed fuel to air has the gases and air intimate and the burn is simultaneous so no carbon left to glow and the flame is blue, this is how a gasifier burns a flame.

     

    We all know that if you throw petrol on a fire it doesn't burn blue and the carbon cannot burn out because the air is not able to supply  enough oxygen, so we get black sooty smoke aka particulates.

     

    When air dry wood burns  the heat from the fire raises the surface temperature of the new log, firstly to 100C as any surface moisture is driven off and then the wood pyrolyses evolving the woodgas and leaving char on the surface, the woodgas burns higher in the fire and oxygen combines with the char, burning it away and the next layer in repeats the process.

     

    You will see the moisture in the wood controls how the wood burns away. The less moisture the quicker the log can pyrolyse and burn because volatilising the moisture takes some energy which has to be supplied from the burning surface area

     

     

    Pyrolysis is slightly exothermic above 330C and below 440C  so if there is little or no moisture present to rob heat a chain reaction can occur in the wood, the wood can quickly pyrolyse throughout with no extra energy from outside once the temperature is high enough to initiate pyrolysis. The wood then evolves more woodgas that oxygen can burn and like the petrol analogy soot is given off.

    • Like 2
    • Thanks 1

  6. 30 minutes ago, mr kipling said:

    When we ran utilities across some land we owned but planned to sell we had to have a legal agreement drawn up. It may have been called a way leave agreement, it was a few yrs ago but solicitor insisted on it as the owners of the new house we built may need access to their services running through their neighbours driveway

    Generally where the utility is to supply more than one other property the utility will require an easement, this gives them more rights than a wayleave and will prevent any building or other works which might impede the utility on the land, it is irrevocable.

     

    As the properties are so recently sold there should be details of what rights went with the properties, rights can be quite specific

    • Like 1

  7. 3 minutes ago, Rob D said:

    So we are told - again could be true and may not be true - we will likely never know.

     

     

    It's interesting how different peer groups have differing takes on subjects. This forum is populated with a majority of entrepreneurial types who are spurred on by success, this leads us to tend to be right of centre, question our masters and be sceptical.

     

    I can look back of a number of political events, from vietnam era and onward and with hindsight and occam's razor I note that the most likely reason for things to have happened are simple and most conspiracy theories are propounded by people out to make a buck, or sell a news story.

    • Like 1

  8. 44 minutes ago, Rob D said:

    So then why are all the population locked down? [I have heard the nonsense reasons for this - they must be nonsense as otherwise why is the lock down being lifted now?? It goes against the original reasons for the lockdown. If the lockdown was to stop the massive surge of cases on the NHS - then in theory it needs to be kept in place until the 'vaccine']

    Well of course the population isn't very locked down  and there is enough contact out there such that there are still 2000 new cases being found each day. It points to there being some super carriers out there and certain situations, dense housing, large multi occupancy household etc. meaning that containing the disease in these situations is not possible.

     

    Everything also points to increased viral load leading to much more severe effect of the disease, so repeated exposure is likely more damaging.. I'm just grateful I don't live in a high rise, commute using public transport or aeroplanes  or have to meet indoors with may people in my normal life, let alone now.

     

    The thing is legislation has to be one size fits all so we all suffer.

    • Like 2

  9. 12 hours ago, difflock said:

    Wor daughter told the mother that people need to be fit to survive being put on a ventilator

    If you think about it general anaesthetics are avoided for most older people, especially those with cardiovascular problems, my mate died on the operating table at 62 because his heart gave out, so as an induced coma is necessary for a ventilator to be used...

     

    I'm confident they have learned a lot about treating this virus from avoiding ventilators to treating "sticky blood" and we don't know how the disease has triggered the most fatality, whether it's blood clots, cytokine storm, induced coma etc.

     

    There are plenty of 50 somethings on this forum that think they will not react badly to the disease, and statistically they are likely right but even Boris at 53 was dependant on good ICU care to get through it.

    • Like 3

  10. 49 minutes ago, kevinjohnsonmbe said:

    It’s not me (honest Guv(no really, it’s not.))

     

    Its a community wood nearby. They were considerate enough to put a warning post on local FB page but have had a number of:

     

    ”you have to inform fire brigade”

    Always a good idea to save them wasting their time

    49 minutes ago, kevinjohnsonmbe said:

    ”I’m asthmatic”

     

    etc, etc posts. 

     

    Just wondered what others had experienced. I’ve used D7 for burning arisings before but this is coppice managed woodland. 
     

     

    If it's an earth burn or ring kiln it's exempt from environmental permitting as less than 1MW thermal input if not dark smoke there is no statutory nuisance, still a polluting way to produce charcoal though.

    • Like 1

  11. 36 minutes ago, kevinjohnsonmbe said:

    Any of you wise charcoal makers know if an EA exemption is required for making charcoal on the site where the timber has been cut?

     

    Im familiar with the D7 for burning green waste generated on site, but is there something similar if you’re making charcoal??

    EA exemptions deal with waste, EA position statement said virgin timber is not waste.

     

    Are you using waste to make charcoal?

     

    Clean air act  refers to dark smoke, are you emitting dark smoke?

     

    Combustion generally requires an environmental permit. There was an exemption for charcoal making prior to 2002 but I'm not sure about that now.

     

    Of course traditional charcoal making is polluting and emits greenhouse gases


  12. 1 hour ago, Jv20 said:

    Hi all,

     

    this is a young red oak tree that was purchased from a nursery and planted in December 2020. In the last 2 months the leaf buds turned into leaves and the tree looked quite healthy, but over the last week the leaves look like they are dying - curling up, turning brown (see pics). The bark of the tree doesn't appear to have any deformities.

     

    Does anyone have any advice on how to manage this? (All leaves are within reach of pruning if required)

     

    Thanks in advance!

    20200519_203409.jpg

    20200519_203447.jpg

    20200519_203527.jpg

    20200519_203351.jpg

    probably frost damage, where are you? The warm dry spring has caught a lot of plants out.

     

    The yellow leaves may indicate a mineral deficiency, keep it watered and wait and see.

    • Like 1

  13. 15 minutes ago, trigger_andy said:

    By the time you've paid someone to fix it you're genuinely better off just cutting your losses and buying a new saw.

    Yes or overcoming the OCD tendency and leave it empty of oil after use if the chain lubricates ok.

    15 minutes ago, trigger_andy said:

     

    As Peatff says, a wee Husky or a wee Stihl.

    Actually for just a m3 or so of logs a year an Einhell will do it, the one I acquired of is ok, although the consumer oregon bar leaves a lot to be desired, @Tony-1976 is welcome to try it and have it

    • Like 1

  14. 2 minutes ago, Rob D said:

    Since lockdown how much more are you drinking?

    I haven't drunk alcohol for nearly three years now, it has saved me over 20 quid a week and I lost a kilo.

     

    I did get a bottle of white wine for lending a neighbour an SDS drill last week and cooked some chicken in it, I made the mistake of drinking two glasses, couldn't get to sleep.

     

    lockdown hasn't affected me too much, I got stopped from volunteering work which was a couple of days a month and only got to do a couple of tractor repairs as felling/winching work was put on hold but the boss has worked with his son and others through.

About

Arbtalk.co.uk is a hub for the arboriculture industry in the UK.  
If you're just starting out and you need business, equipment, tech or training support you're in the right place.  If you've done it, made it, got a van load of oily t-shirts and have decided to give something back by sharing your knowledge or wisdom,  then you're welcome too.
If you would like to contribute to making this industry more effective and safe then welcome.
Just like a living tree, it'll always be a work in progress.
Please have a look around, sign up, share and contribute the best you have.

See you inside.

The Arbtalk Team

Follow us

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.