Ways to clean up wood burning
As I sit here writing, my nearest rural neighbours are warming their houses with wood stoves. They are all great neighbours; considerate, caring people, willing to help out at the drop of a hat. The only drawback is that their wood stoves (recently purchased ‘high efficiency’ Pacific Energy models) are more often than not spewing blue-grey smoke into the air. Both my son and I experience asthma when subject to this kind of particulate matter air pollution. So, on a night like tonight, like most every night the wind blows those toxins our way, we just have to stay indoors. This situation sucks, badly.
I love cooking and warming with fire; so much so that I make my living building masonry heaters and rocket stoves. I also serve on the technical advisory committee for the Masonry Heating Association of North America (mha-net.org) where we spend a lot of time generating and analyzing testing data on wood stove efficiency. This base of experience has helped me form a perspective on the wood stove-pollution issue that I feel impelled to share with y’all.
Any wood stove sold today in Canada or the U.S. must be certified by either the Canadian Standards Association or the U.S. Environmental Protection Agency. The criteria for certification are that the appliance must emit no more than 2.5 grams of particulate matter (PM) per hour, averaged over a 24 hour burning/heating cycle. This is determined via a standardized laboratory testing procedure involving very specific ‘crib’ loading and lighting techniques involving dry sticks of 2x4. At 2.5g/h the combustion process is about 75 per cent efficient. This is the point at which the exhaust is actually clear and free of odour.
The trouble is, laboratory testing and real world operation are two different situations. And here in the valley we see new ‘high efficiency’ woodstoves spewing smoke on regular basis. Under tightly controlled laboratory conditions things go fine; but under real world firing conditions many things can (and often do) go wrong causing the fire to crash into a smoulder. Those same stoves that achieved PM 2.5 g/hr in the lab are actually averaging closer to PM 7.5 g/hr in the real world. Common reasons for this trouble are as follows:
1. Damp or poorly-seasoned wood. Nothing kills a fire more than water. And the energy that a fire must put into evaporating water is energy lost from the combustion process. I once heard a firewood supplier advise a client to leave some of the wood out in the rain, so it doesn’t burn too fast. This rationale is completely whacked. A decent fuel storage shed with good ventilation is a must.
2. Fuel above the flame. No offence, but the trusted tradition of lighting from the bottom is a** backwards. With a small flame warming a pile of fuel above it, the gasses being released from the wood are already downstream from the flame, heading for the chimney. Yeah, the fuel load might eventually catch enough to get the stove’s reburn process up to opereating temperature. But the combustion process can just as easily sputter and spew for hours without ever reaching said threshold.
3. Reloading on top of coals. Same problem as above really — especially when we don’t ramp the draft back up for long enough to balance out the smoke-flame equation. Even when we can see some flames the whole smouldering process can just be chugging along.
I know a lot of decent, environmentally conscious people making these and other common mistakes as they attempt to heat with wood. I have also approached all of my nearby farming neighbours, explaining how best to clean up their burns. Most have taken the advice to heart, but still struggle to get things burning clean. Fact of the matter is, the more we try to slow a fire down, the more it is likely to crash into a smoulder, and any number of common operator errors can cause this to happen.
So, what to do then? Some concerned citizens propose banning wood fire altogether, as was done up in Courtenay a few years ago. But to my mind that would be throwing all kinds of babies out with the bath water. A fire can be made to burn clean by honouring the ‘three Ts’ of: ‘time’ (draft not too fast, not too slow); ‘temperature’ (the hotter the better); and ‘turbulence’ (optimized mixing of fuel-volatiles and oxygen into the flame). Masonry heaters achieve this routinely by operating at a fixed high burn rate. Pellet stoves also consistently burn clean by effectively regulating the fuel-air-flame ratios. The wood smoke pollution problem is not actually from wood fire per se, but from wood smoke not being consumed by the fire.
Another solution might be to beef up air-pollution bylaw enforcement. Perhaps make repeat offenders attend some sort of training program in which they actually have to read their owner’s manual… Personally, I think shame and blame is a very low-level motivator, and most of us have enough of it in our lives already. So maybe something more proactive, like having new stove buyers write an operator’s test? Not a cure-all, but perhaps part of a comprehensive strategy.
Another option — and I think this is the most pertinent one — will be to create incentives for slow-burn stove manufacturers to remedy the flaws in their products. This can be achieved with automatic draft controls (so the combustion stays above a smoulder) and/or supplementary harvesting mass (so they can burn faster and store the heat). But for that to work there will need to be some kind of actual pressure applied. Sales are good these days. And tooling up to make any change is always costly.
For now, the only way to make sure these flawed slow-burn wood stoves burn clean is to help operators develop healthier wood burning habits. The important principle here is making sure there is always flame above the fuel. The simplest way to achieve this is with a top down fire. To do this we load big pieces on the bottom; midsize in the middle; and kindling on top with a little nest of tinder at its centre. Think of how a candle functions, with the radiant heat from the flame drawing the fuel up into it. This process of burning from the top down will make any wood stove run cleaner — even the old beaters.
Whatever the case, continuing to sully our skies with wood smoke is not an option.
Pat Amos
Glenora