There is a lot of talk and publicity around biomass these days. Much of the discussion centers around ethanol production or utility scale woodchip-fired electrical generation. What, however, do we mean when we are talking about residential biomass? Well, for the most part, we are talking about good old firewood! The second most common biomass used by homeowners is wood pellets. Firewood and pellets are most commonly burned in free-standing stoves, but can also be burned in central heating boilers or furnaces (boilers produce hot water and furnaces produce hot air).
If you are in a cold climate, as most readers of this publication are, and you are considering an alternative energy source for your home, a central biomass boiler is well worth considering. A boiler is more versatile than a furnace in that it can easily be tied into most any type of heating distribution system (hot water baseboard, radiant floor, forced hot air, etc.) and can easily be made to produce domestic hot water too. As well, no other alternative energy source has the ability to practically, and cost effectively, produce one hundred percent of your space heating and domestic hot water needs.
Now I know that is a provocative thing to say and before you fire up emails to the editor, give me a chance to explain the statement. The more high profile alternative energy technologies of photovoltaics, solar thermal and small scale wind have improved significantly over the last generation and the current state and federal incentives make all of these well worth considering. However, no one would suggest that the electricity produced from PV or a residential scale wind turbine should be used for the production of domestic hot water or space heating, and in cold climate regions like New England, the vast majority of our household energy needs are for space and water heating. Likewise, while solar thermal is a great way to produce a high percentage of your domestic hot water, a typical residential scale collector array will not gather enough energy to provide a significant amount of space heating in anything but the smallest, most energy efficient home. If you were to tell me that a very well insulated, passive solar home, with excellent solar gain, radiant floor heat and an oversized solar thermal array COULD be heated with solar only, I would certainly agree with you. However, most people who are considering alternative energy are looking for options for the less than optimized home they already live in and that is where a wood or pellet boiler really shines.
But doesn’t burning wood create pollution? Well, it certainly can. During the woodstove boom of the 1970’s a haze of wood smoke lying in the valleys on a cold winter morning was a common sight. Today the countryside is dotted with outdoor wood boilers slowly spewing smoke whether winter or summer. By 1990 the US EPA had developed rules regulating the emissions of woodstoves and today’s appliances are very efficient and clean burning. In the last few years the EPA has developed additional standards that are improving the performance of outdoor boilers. Within the next couple of years the EPA will have in place long overdue, new, stricter emissions regulations that will apply to all solid fuel burners. Welcome news to be sure, but what do we do in the meantime?
Wood pellet burning appliances are a good option for many people. Bagged fuel for a freestanding stove, or even automatically feeding fuel from a bulk bin of loose pellets to a boiler, makes fuel handling much easier than firewood. Also, because of the very consistent nature of the fuel (homogeneous and very dry), it is easy to design a clean burning, highly efficient pellet stove or boiler. So why burn anything else? In rural areas, firewood is often significantly less expensive than wood pellets, and the appliances are normally less expensive too. Further, many people have access to their own woodlot further reducing fuel costs. By being able to cut your own, or buy from a neighbor, firewood offers independence and a measure of self-sufficiency that any fuel that must be processed and delivered does not. Make no mistake though, firewood is a lifestyle choice and requires a pretty big commitment of time and effort. Pellets offer a huge convenience and long burn times, and if you are in a more suburban area, pellets and firewood may be comparably priced, so for many people pellets are the right choice.
As I mentioned above, pollution from pellet burning appliances is not a big concern. Firewood, however, can be a concern. Some of the pollution created by wood burning today is simply a result of poor practice and some the result of inherently inefficient appliances like the traditional outdoor wood boiler. There are ways to burn wood efficiently and cleanly, and there are appliances that operate on those same principles.
The first thing to do to maximize the efficiency of wood burning is to make sure that the firewood is dry. A year of stacked, covered, outdoor storage should yield a supply of mixed hardwood with a moisture content of around 20%. It takes a tremendous amount of energy to turn water into steam. For example, firewood at 20% moisture content has approximately twice the energy available to heat your home as green firewood. As well, wet wood tends to smolder producing smoke and creosote; think of the last time you tried to burn a brush pile or camp fire when the wood was too wet. Regardless of what you burn it in, dry firewood will provide more heat and create less smoke and creosote than wet wood.
In addition to WHAT you burn, it is also important to consider HOW you burn. In order to burn wood most efficiently and most cleanly, it must be burned hot and fast. You may have experienced the smoking brush fire mentioned above and observed how the smoke disappears when the wind comes up and the fire really roars. As this experienced suggests, in addition to dry wood, the key to an efficient wood fire is lots of air. Large amounts of air assure a high primary burn rate and, very importantly, sufficient excess air (oxygen) to allow for complete secondary combustion. Secondary combustion is the burning of the smoke and creosote vapor (together referred to as wood gas) that is not consumed in the primary combustion process taking place immediately in and around the fuel load. Modern woodstoves accomplish secondary combustion by introducing fresh air into the top of the firebox. Next time you get a chance, look up at the top of the inside of the firebox of a running woodstove and you will see this secondary combustion in action. For the ultimate in clean burning and high efficiency, firewood can be burned in a downdraft gasification boiler.
Instead of relying on the natural draft of a woodstove or traditional wood boiler to move air through the boiler, a gasification boiler relies on a motorized fan to move air through the boiler (remember the wind in the brush pile). Additionally, the secondary combustion happens in a separate, refractory lined, burn chamber. The wood gas that is produced in the primary firebox is forced by the fan down into this second chamber. As this hot, smoky, oxygen-starved gas is forced into the secondary combustion chamber, a stream of fresh combustion air is injected into it and the wood gas lights off like a torch. This torch-like flame fires into the refractory lined chamber where temperatures may reach as high as 2000 degrees Fahrenheit. Only at these high temperatures can all the smoke and creosote left over from the primary fire be consumed. As a result of this very complete combustion, not only does a gasification boiler dramatically reduce emissions, but instead of sending all the energy that is contained in the creosote up the chimney, it is used to heat your home. In other words, with a downdraft gasification boiler, not only do you burn much more cleanly, but you also burn less wood.
So, now we have loaded our downdraft gasification wood boiler with nice dry firewood, the fan is forcing air through the primary and secondary combustion chambers, and we have achieved the hot, fast, clean –burning, efficient fire we are looking for. But wait, what to do with all that heat? Sure, the house will use some of that heat, but what about when the house is up to temperature and the thermostats are not calling for heat? With a fossil fuel boiler, when the thermostats stop calling for heat, the temperature of the water in the boiler rises until it hits a set point where the burner simply shuts off. When there is a new call from the room thermostat, heat is pumped away from the boiler and the boiler water temperature begins to drop. Once the boiler temperature falls to a low set point, the burner turns on and the boiler is brought back up to temperature. The very same thing happens with a pellet boiler, however, with a wood boiler you cannot simply turn off a wood fire. So, instead of turning off a burner when the high temperature set point is reached in a gasification boiler, we simply turn off the fan that moves combustion air through the boiler. This does not completely extinguish the fire, but it does dampen it down to a very low burn rate. It is completely acceptable to have a wood boiler sit in this idle mode for short periods of time between calls for heat; however, you do not want to allow excessive idling. Idling is the opposite of the hot, fast fire we are looking for and produces significant amounts of smoke and creosote. When a gasification boiler is in idle mode, this smoke and creosote is contained in the primary firebox (it does not go up the chimney), but that smoke and creosote vapor will condense and build up on the inside of the firebox. Excessive amounts of creosote are corrosive and will prematurely rot your boiler from the inside out.
There are two ways to avoid this build up of excessive creosote on the inside of the primary firebox: one is to only burn the boiler when there is sufficient load (cold outside) – say from Thanksgiving to the first of March. This does a good job of minimizing idling and you simply burn fossil fuel in the transition seasons of spring and fall.
Another way to address the idling issue is to present the boiler with an artificial load by means of a large volume of thermal storage. Thermal storage takes the form of a tank, or tanks, typically holding 500 – 1000 gallons of water. When the boiler is running at maximum output and efficiency and the house no longer calls for heat, instead of shutting off the fan and going into idle mode, the boiler is able to continue to burn hot and fast with the extra heat being generated dumped into the thermal storage. Think of these tanks as large heat batteries. Now, when the wood boiler runs out of wood and goes cold, the heating system recognizes that and starts to pull stored heat from the tanks. During the spring and fall you may only have to burn your wood boiler once every day or two to heat your home. In the summer, you may only need to run the wood boiler once or twice a week to produce and store enough energy to heat your domestic hot water.
Adding thermal storage will improve the operation of any wood boiler, so even if you already have a non-gasifying traditional wood boiler and would like to increase its efficiency, make it easier to use, use it more of the year AND reduce the amount of wood you burn and the amount of creosote you are producing, you may want to consider adding storage to your existing system. Then when the current wood boiler needs to be replaced, you are all set up to install a modern gasifying boiler in an optimized configuration.
Another benefit of storage is that it can provide a convenient and cost effective way to integrate solar hot water with a wood boiler heating system. The two systems are brought together at the thermal storage tank giving you what some might describe as the ultimate home heating system.
One thing that is important to make clear is that if your wood boiler has gone cold AND your thermal storage has been depleted, your back-up fossil fuel boiler will kick in automatically. If you are running a wood boiler without thermal storage, then the fossil fuel boiler will take over as soon as the wood boiler has gone cold. The system is set up in such a way that you do not need to worry about switching between systems, it is all done automatically.
Many of us feel an urgent need to pursue alternative means of providing energy for our homes. Personal and national independence, pollution and carbon output reduction and reducing costs are all powerful motivators. The choices can be bewildering; I hope this article has been helpful in enabling you to evaluate the residential biomass option. Additional online resources you may wish to consult are:
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