Explanation between traditional wood burning and blue flame combustion so called Aldehyde combustion |
| There is a difference between modern and traditional wood burning. We have earlier showed the environmental- and effciancy differences between modern technology and the old technology. In this case we will try to explain what it is that makes this difference so big and at the same time describe the advantages with the modern technology. Even though the blue flame technology has existed since the mid 80's there are many people who do not understand how the technology actually works. To understand the difference you have to go back to elementary combustion science. What is actually happening when you fire wood? |
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This is how it burns
It is actually not the log itself that is burning. But it is in the main the combustible gases that the wood emits that are combustibles. You can assume that approx. 90 percent of the energy content of the wood are emitted as gas. It is only 10 percent that makes glow. Combustion technically there are big differences between glow - and gas combustion. |
Glow combustion
A glow combustion is an absolute coal combustion, where sporadic carbon atomes, via carbon monoxide (CO), are combusted to carbon dioxide (CO2). The heat is mainly emmited via radiant heat from the glow bed to surrounding colder surfaces. A radiation can never pass "around a corner", which means that the convection surfaces of a boiler become less important. If you could build a boiler with absolute glow combustion the convection parts could be significantly reduced. |
Coke boilers
The first generation wood boilers were actually constructed for coke-heating. Coke is created when you heat up coal and utilize the volatile gases in the form of town gas. What is left is still a combustible - but a solid fuel low on gas - coke. In the 30's and 40's, coke was a relatively cheap fuel since coke was a waste product from an extensive town gas production. Boilers were constructed for coke burning - and you even presented the size of a boiler in m2 heating surface instead of in effect. This because it is the surface of the hearth that can be hit by the radiant heat of the glow bed and decides how much energy that the water jacketeering of the boiler can assimilate. A size estimation that then has been kept until our days. For glow combustion and radiant heat, the over combustion principal is preferable. The coke boilers therefore always have over combustion, and relatively short flue gas ways. Since the volatile hydrocarbons have already left the fuel, it is mainly a pure coal combustion that ocurrs. From an environmental perspective it is almost impossible to create unburned hydrocarbons in the form of PAH in an - absolute glow combustion. The emissions of unburnt that can accure is in the main CO, that is both transparent and odurfree. |
The war years
During the war years with problems in getting imported coal home, these boilers more and more started to be fired with wood. Because of this change from coke to wood fuel, defects in the construction of the boiler were revealed. Since the wood, unlike coke, is a fuel rich in gas, these first wood burned boilers, ment for glow combustion, came to work very badly. But since the market started to demand good wood boilers boilers more suitable for gas combustion were developed. |
The 40's with good technology
Already during the end of the 1940's the first absolute wood boilers were constructed, for example the Storebro-boiler, Aquator among others, that were built with under combustion and had a gas combustion chamber to better handle burning wood gases that fast were removed from the hearth. There are examples of wood burning technology from the early 1950's that almost could be compared with the modern technology of today. The wood-aga stove is such an example. After the war the oil burning rapidly came to "take over" the heating. The development of the wood burning technology that had started rapidly came in the shade and the experiences were forgotten. |
Gas combustion
A gas combustion is a much more complicated combustion technology. The pyrolysis gas created at the gasification is not only one gas but consists of a mixture of maybe up to about forty different substances/gases. Where every substance has its specific combustion technical peculiarities. At combustion other new substances are then created with basis from i.a. the access of air, temperature and turbulence. Some substances are volatiles and only exist for a short time during the combustion, then they are "out", and other substances exist during the whole firing cycle. It is therefore impossible to in advance exactly be able to decide what substances that will occur in the gas combustion. |
The importance of the temperature
To at least create theoretical conditions to reach a complete combustion, a combustion temperature over the ignition temperature is needed for the gases that are most difficult to ignite. At combustion of hydrocarbon compounds that temperature is approx. 850 °C. For the heavy hydrocarbons (PAH) many substances are around a temperature of 800 °C. That means that the flame needs to be able to reach at least 850 °C if you should reach good results from an environmental perspective. Volatile hydrocarbons (VOC) have in most cases ignition temperatures around 500- 600 °C and therefore should combustion technically not make any bigger problems at the combustion. |
Temperature
A high temperature also accelerates the combustion reactions in time. This means that more gases have time to burn in a shorter time - in other words, the effect increases. On principle this means that you should strive to reach the highest combustion temperature as possible. The higher temperature the more margin to the level where you jeopardize the combustion result. But there is also an upper temperature limit you have to observe. At temperatures around 1 050-1 100 °C the airborne nitrogen is ignited and creates undesired nitric oxide (NOx). This means that an ideal combustion temperature should be at a stable level of 900-1 000 °C to reach optimal preformance. |
Influence of the combustion result
Normally a wood flame burns with a white and clear, shining flame. There are burning coal particles that give the flame its lighting power. These coal particles also create soot if you cool down the flame. A red flame has glowing coal particles and is colder than a white flame. A wood flame consists of several pyrolyse gases that in a chain of combustion reactions create new substances that then are burned to new substances and so on. At last all carbon atoms should have created carbon dioxide (CO2) and all hydrogen atoms created water steam (H2O) if the combustion has been "complete". This chain reaction can easilly create different substances that then create other substances, and the principle continues for a relatively long time with kept high temperature. Something that can easilly favour the creation of undesired nitric oxides. |
Turbulence and water
Through an extreme turbulence and access to water steam the combustion principle can radically change. From burning with a bright shining white flame you can get the flame to burn with a totally transparent blue flame - so called blue flame technology. The flame no longer consists of coal particles but of more volatile hydrocarbon compounds, so called aldehydes. Aldehydes arre burned directly to carbon dioxide and water steam without taking the "back road" via a coal particle. You have obtained an almost soot free flame. |
Soot or not
The difference between a coal combustion and an aldehyde combustion could simply be compared to the difference between wood burning and LP-gas combustion. If you for example boil coffee over an open fire, coal (soot) will through the cooling to condense on the pot. If the same coffee pot is placed on a LP-gas flame the coffee can be boiled without depositing any soot. |
Recirculation
The technology has been known within the oil burning since the late 1960's. By recirculateing the combustion gases into the flame, the water steam is utilized (that i.a. been created at the combustion) as a kind of ”cracker” to break the long and heavy hydrocarbon chains to shorter molecules. Instead of a long molecule that is burned in a traditional way you get several shorter parts that can be burned at the same time. The result is both a faster combustion process, which decreases the creation of nitric oxides, and a combustion of more volatile substances that can be ignited at lower temperatures and that are "too short" to create a long row undesired substances. |
Blue flame boilers
Around 1985-86 the first wood boilers with blue flame technology came out on the market. For example Italian Unical and Danish HS Tarm. They utilized a fan to create an extreme turbulence and a specially constructed combustion chamber to burn the gases. Since there is water in the fuel and that water steam is created at the combustion now all conditions had been created for absolute blue flame technology and a more soot free combustion. At the same time you got a more stable preformance and lower emissions of unburned heavier hydrocarbons. The fan boiler technology therefore rapidy became the absolute market dominating wood burning technology all over Europe. |
Forgiving technology
The blue flame technology meant radically improved conditions to fire wood in more controlled ways. You became less sensitive to wood quality, outer conditions like draft, weather etc. The products became more forgiving to the misstakes of the fireman. During the late 1980's you started to look at emissions of volatile hydrocarbons (VOC). These hydrocarbons are gaseous and does not condence to soot and tar. This means that they have not been included in the environmental requirements set up for the firing equipment. VOC is - except for health effects - oxidant creating and together with sun light it creates ozone close to the ground that then is damaging growing crops. |
Reduces emissions
The blue flame technology reduced the emissions of heavy hydrocarbons (PAH) with about 99 percent compared with older traditional technology. The reduction of light hydrocarbons (VOC) in the same boilers, "only" became about 70 percent. Something that was rapidly noticed and led to discussions about new environmental requirements. Combustion technically it is at first sight strange that the combustion of heavy hydrocarbons is better than for the lighter substances. The heavier hydrocarbons need a higher combustion temperature than the lighter. The reason to that condition can be found in that you with blue flame technology created a soot free flame where you prioritized a short dwell time to not favour the creation of nitric oxide. The combustion chamber simply became too small and/or the dwell time too short for the flame to have time to burn all hydrocarbons. And since the volatile hydrocarbons can not condense to soot you simply did not know that there was unburned left after the flame. The reason to that these substances are not burned is that the gas creation after the flame is too thin for a continuous combustion. An effective end combustion of the light hydrocarbons therefore implies help from a longer dwell time in a flame to be completely burned. |
Recirculating blue flame technology
First out with recirculating blue flame technology was Italian Mescoli. You constructed a blue flame boiler with a prepressurized combustion space. The combustion cup was constructed from a heat resistant steel and enlargered so that the dwell time for the gas combustion increased. To prepressure and keep the flame in the combustion zone, a small border was put on the outlet. Hereby an over pressure was created inside the combustion chamger. An over pressure that may not spread to the wood hearth since it there could then easilly cause flue gas puffs. To avoid this the burner cup was perforated with hundreds of small holes that tolerate a certain over pressure but can then start to release the gases through the holes. To not increase the creation of nitric oxides, the space around the combustion chamber is water jacketed and the radiant heat can leave from the steel cup to the water jacketeering. Thereby the temperature of the flame goes down and the nitric oxide creation can be held back despite a longer dwell time. This boiler construction then came to be a model for a great number of wood boilers on the market. Today, Sweden has about ten models that you can derive to modern recirculating blue flame technology. Among the them we find Swedish-made boilers from Effectapannan, CTC/Bentone and Calmarpannan. |
Installation and handling
Despite the fact that the wood boilers of today have a very good preformance and are user-friendly there is no guarantee that the fireman will reach these good preformances. Of course it is more difficult to fail with a modern boiler, but it is still so that it is the installation and handling of the fireman that decide the final result. If the accumulator tank is under dimensioned or incorrectly connected it does not help the the preformance of the boiler is good. If the fireman is uninformed and ignorant when it comes to how the installation operates the result will be thereafter. That means that all wood burning even with modern technology can cause disturbances and complaints from neighbours and other people living around. If the wood burning in the future should be able to become completely accepted as fuel for urbanized areas, either the knowledge of the fireman has to increase or we need installations that can sense and can compensate the misstakes of the fireman. Maybe the solution is a combination of both. |
Driving licence for wood
It might time to implement some kind of wood burning certificate where the fireman has to document and prove that he has the basic knowledge required before he gets permition to install his wood burning. A procedure similar to the system the hunters have in their hunting certificate. |
Controlled combustion
On the equipment side we have started to get wood boilers that via a lambda probe sense what the flue gases look like, and that then from these values go in and adjust the primary - and secondary air so that the best combustion result is obtained regardless wood quality, mounting etc. We have gotten the first wood boilers taking charge over the fireman and automatically make sure you fire in the best way possible. In Europe today, there are about 10 boilers with lambda probe control. But the technology is expensive and the consumers hardly want to invest extra in a wood boiler to get the highest efficiency and environmental preformance possible as long as the wood price often only consist of your own use of labour and regulatory requirements are missing. |
Conclusion
With this we have tried to show that today's wood burning technology has the conditions to work well even in urbanized areas. It is important to establish that there is a big difference between the traditional technology and the technology offered today. And we have tried to show that is no less important that we demand that the fireman has knowledge than the manufacturer. With the right moder technology we are convinced of that even the traditional wood burning has a positive future. The wood burning is a big asset in the change to a long-term and persistent energy system. |
Pellet Combustion
Bio-energy
