Making Briquettes From Garden Waste
As well as becoming something of a bore on the subject of log burners and thermal store technology following our challenging entry into the sustainable heating world late last year, I find I have yet another character flaw to add to the burgeoning list; I have become fuel obsessive.
Coming from a long line of near clinical pyromaniacs, we were never that far from a fire, and there were few pleasures to rival the big bonfire which inevitably followed a day clearing the garden, raking leaves or digging out brambles. All of my work clothes have scorch marks, and my favourite fleece is more hole than fabric. But I find I can no longer enjoy it; basking in the intense eyebrow curling heat, holding the obligatory stick to prod the embers, I find myself thinking “what a waste.”
My thoughts have turned increasingly to finding a way of heating the house using the materials we currently shove on bonfires. We have a small patch of woodland which we manage for fuel, but the bulk of our timber is bought in as trunks, which get logged on site. The main unexploited resources (i.e. tending to end up on the bonfire) seem to be:
Having bumped into TilHills large scale brash baling activities in the forest plantations at Coed y Brenin, and thinking what a cracking idea it was, I have been told in no uncertain terms that I can't have one of the machines here. However, taking a leaf out of their book, I made a jig to bundle small branches and twigs that would be impractical to handle individually. These faggots are then cut to boiler size using the chainsaw and stacked in the normal log-piles to season.
I've long had my eye on autumn leaves; they must account for a significant percentage of a trees annual biomass output, they feature heavily in bonfire activity, and I spend too much time clearing them up! Early this spring, during a rare dry period, we gathered dry leaves, spread them out on a flat area of the woods, and mashed them with the lawn mower. The resultant flakes were stored in a ton sand bag in the shed while I tried to work out how best to use them in the boiler. I first tried to burn them loose, then packed into cardboard boxes, but in both instances the flakes tended to spread around and choke the fire. It is the same story with sawdust, which we also produce in significant amounts with our logging activity.
Having drawn a blank (and with large stores of sawdust and mashed leaves taking up space in the shed) I wondered whether packing the stuff into sturdy briquettes would address the choking problem. A quick survey of the net turned up lots of mega-bucks high-pressure extrusion hardware for industrial briquette manufacture, but nothing much for the small scale operator.
That was until I happened across the Legacy Foundation, an organisation working internationally to develop briquette making as a fuel source in poor and developing areas where wood is a scarce commodity. They have a range of resources available in an e-book format, and I eventually got hold of a construction manual and a user manual for the briquette press from www.onetoremember.co.uk (The books were initially out of stock, which has somewhat undermined my understanding of what an e-book is).
The briquette press is by far the most intriguing and iconic piece of the set up. Given the Legacy Foundation aims, the press is a very low-tech affair requiring easy to find materials and basic wood-working skills, but is well designed and very effective. With the help of power tools I knocked mine up in a couple of days, and the fact that it is made from salvaged timber helps heighten the sense of “something for nothing”. On the down side, it is large, and against expectation, it seems to actually be the least important part of the process.
The briquettes hold together via natural processes akin to felt or paper making. In essence, materials are processed to free up natural fibres, mixed together as a slurry in water, and when the water is pressed out, the fibres lock together and hold their shape. Simple! Consequently success pivots on the choice, processing and blending of ingredients.
We have played around with various materials including leaf mulch, hay, sawdust and chainsaw chips, but always using paper pulp as the binding fibre. With enough effort, all paper based products and cardboard can be pulped, but for speed and ease, we tend to stick to paper. The most common so far has been a mix of paper:chainsaw chips 2:3 which is quick and easy to produce, but as we get further into autumn the leaf mulch may well take over. Drying times are a couple of months.
When burning, the briquettes glow rather than flare, and so need to be used with other fuel types, but work well in a wood burning stove as well as the wood boiler.
One Year On: answers to the big questions I’m sure you are dying to ask!
After a year of preparing and burning briquettes, just what has been learnt?
First and foremost, there is no getting around the fact that the process remains a messy and labour-intensive one, albeit strangely satisfying. As a means of producing fuel, it would be inefficient compared to, say, the same time spent splitting logs, but of course the whole process is designed for when something like that is not an easy or affordable option. What we are doing is essentially a recycling project, using waste products we produce on site (by cutting logs!), so the time and cost that would be consumed by disposing of these products in other ways need also to be factored in.
We have continued to experiment with different recipes, and the mix has slowly been modified, adapted and improved to use more of the waste product and less paper (since this is the hardest bit to process). With fine material such as saw-dust we have managed a 5:1 sawdust:paper mix, but the coarser the material the more paper is required to hold the briquettes together. Chainsaw chips work at around 4:1, and dry leaf mulch as low as 3:1. Leaf mulch is greatly improved by an overnight soak which makes the fragments less springy and so fit together more easily. Chainsaw chips work fine from dry. Wood-chipper chips seem to be too large for effective briquette making, whilst coal dust is too fine. By mixing and blending with other products, we have managed to use both in our production. The coal-dust is particularly messy to work with!
It seems that the briquettes burn best with a high wood:paper ratio. The lower the paper content the less ash seems to be produced, although leaf mulch briquettes still produce quite a lot. As the briquettes are burnt along with other fuels, it is not clear whether there is any effect on chimney tarring one way or the other. How these briquettes stack up against the commercially available pressure-extruded products in terms of burning properties is another unknown quantity. My guess is that the high pressure extrusion would lead to a denser product which would therefore burn longer.
An unexpected application of the dried briquette is the disposal of waste domestic kitchen cooking oil. By their fibrous nature, the briquettes are highly absorbent and will soak up warm oil easily, thus providing an easy solution for stuff that can’t go down the drain. These get put straight into the burner box ready for the next fire since the dogs see them as something of a stick and meat flavoured lolly if left unattended.