Explanation between traditional wood burning and blue flame combustion (the so called Aldehyde combustion):

There is a difference between modern and traditional wood burning. We have earlier showed both the environmental and efficiency differences between the modern wood combustion technology and the old wood combustion technology. In this case we will now try to explain what it is that makes this difference so big, whilst at the same time try to 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?
This is how it burns
It is not actually the log itself that is burning, but it is in the main the combustible gases emitted from the wood which combust. You can assume that approximately 90 percent of the energy content of the wood is emitted as gas. It is only actually 10 percent which makes the glow. Regarding the combustion itself however, technically there are big differences between the glow and gas combustion.
Glow combustion
Glow combustion is absolute carbon combustion, whereby sporadic carbon atoms, via carbon monoxide (CO), are combusted to carbon dioxide (CO2). The heat produced is mainly emitted via radiant heat from the glow bed to surrounding colder surfaces. Heat 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 solid fuel low on gas content and called coke. In the 1930'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 is because it is the surface of the hearth that can be hit by the radiant heat of the glow bed which then decides how much energy that the water jacket of the boiler can assimilate. This size estimation has continued to been kept until our current times. 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 discharges. Since the volatile hydrocarbons have already left the fuel, it is mainly a pure coal combustion which occurs. From an environmental perspective it is almost impossible to create unburned hydrocarbons in the form of PAH within total glow combustion. The emissions of unburnt hydrocarbons that can occur are in the main Carbon Monoxide (CO), which is both transparent and odour-free.
The war years
During the war years and subsequent problems in getting imported coal home, within Sweden for example these boilers then became more and more fired with wood. Due to this change from coke to wood fuel, defects in the construction of the boiler were then revealed. Since the wood, unlike coke, is a fuel rich in gas, these first wood burned boilers were really meant for glow combustion and came to function very badly. This then led to the homeowner starting to demand good wood boilers and so boilers more suitable for gas combustion were subsequently developed.
The 40's with good technology
Already during the end of the 1940's the first absolute wood boilers were constructed. These included for example the Storebro-boiler, Aquator among others. These were built using the under combustion technique and had a gas combustion chamber to better handle the burning wood gases that fast were removed from the hearth. There are actually examples of wood burning technology from the early 1950's that almost could be compared with the modern technology of today. The Swedish wood ‘aga’ stove is such an example. After the war oil burning rapidly came to "take over" as the primary method of heating. The development of the wood burning technology that had started rapidly entered into the shade and the experiences learnt were forgotten.
Gas combustion
Gas combustion is a much more complicated combustion technology. The pyrolysis gases created during gasification are not only one gas, but consist of a mixture of possibly up to about forty different substances/gases. Each and every one of these substances also has their own specific combustion technical characteristics. During combustion these new substances are then created dependent on the varying amounts of air, temperature and turbulence. Some substances are volatile and only exist for a short time during the combustion before then disappearing, whilst other substances exist during the whole firing cycle. It is therefore impossible in advance to know exactly what substances which will occur in the gas combustion process.
The importance of the temperature
If attempting to create the right theoretical conditions to reach a complete combustion, a combustion temperature over and above the ignition temperature is needed for the gases that are the most difficult to ignite. The combustion of hydrocarbon compounds typically occurs at a temperature of approximately 850 °C. For the heavier hydrocarbons (PAH), many substances are combusted around a temperature of 800 °C. This then 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 of around 500-600°C and therefore technically they should not cause any big problems during wood combustion.
A higher temperature also reduces the time it takes for combustion reactions to occur. This then means that more gases have time to burn in a shorter period of time and in other words, the effect then increases. In principle then, this means that you should strive to reach the highest combustion temperature as possible. The higher the temperature the greater the ability you then have to control the combustion result. However, there is also an upper temperature limit which you have to observe. At temperatures around 1050-1100°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 between 900-1000°C to reach optimal performance.
Influence of the combustion result
Normally a wood flame burns with a white and clear, shining flame. These are burning coal particles that give the flame its lighting power. These coal particles also create soot if you then cool down the flame. A red flame has glowing coal particles and is colder than a white flame. A wood flame consists of several pyrolysis gases which in a chain of combustion reactions then create new substances that then are burned to new substances and so on. Finally all carbon atoms should have created carbon dioxide (CO2) and all hydrogen atoms should have created water vapour (H2O) if the combustion has been "complete". This chain reaction can however easily create different substances which then create other substances, and this process can continue for a relatively long time with a kept high temperature. These are then the conditions which can then favour the creation of undesired nitrous oxides.
Turbulence and water
Through extreme turbulence and access to water vapours the combustion principle can radically change. From burning with a bright shining white flame you can then get the flame to burn with a totally transparent blue flame – the so called blue flame technology. The flame no longer consists of coal particles but more of volatile hydrocarbon compounds, so called aldehydes. Aldehydes are burned directly with carbon dioxide and water steam without taking the "back road" via a coal particle. You have then obtained an almost soot free flame.
Soot or not
The difference between coal combustion and 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, during the cooling, 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.
This technology has been well known within oil burning since the late 1960's. By recirculating the combustion gases into the flame, the water steam is then utilized (that is created during the combustion), acting almost like a hammer to break the long and heavy hydrocarbon chains into shorter molecules. Instead of a long molecule that is burned in a traditional way you then get several shorter parts which can be burned at the same time. The result is both a faster combustion process, which decreases the creation of nitrous oxides, and a combustion of more volatile substances that can be ignited at lower temperatures and that are "too short" to create a longer chain of undesired substances.
Blue flame boilers
Around 1985-86 the first wood boilers with blue flame technology came out onto the market. Examples include the Italian Unical and the Danish HS Tarm boilers. 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 during the combustion, now all the right 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 very soon then 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. We became less sensitive to wood quality and outside conditions like draft and the weather etc. During the late 1980's the wood heating industry started to look more at the emissions of volatile hydrocarbons (VOC). These hydrocarbons are gaseous and do not condense down to soot and tar. This means that they have not been included in the environmental requirements set up for the firing equipment. VOC’s are , additional to producing negative health effects, also oxidant creating and together with sun light they can create ozone close to the ground, which can be damaging when growing crops.
Reduced emissions
The blue flame technology reduced the emissions of heavy hydrocarbons (PAH) by about 99 percent, compared with older traditional technology. The reduction of light hydrocarbons (VOC) used in the same boilers only reduced by about 70 percent. This was something that was very soon noticed and subsequently led to discussions about new environmental requirements. At first sight it seemed strange that when using the blue flame technology, that the combustion of heavy hydrocarbons was better than for the lighter substances. This was since heavier hydrocarbons need a higher combustion temperature than the lighter hydrocarbons. The reason for this condition can be found in that when using the blue flame technology you create a soot free flame where you create a shorter burn time which doesn’t then encourage the creation of nitrous oxide. The combustion chamber simply became too small and/or the burn-time too short for the flame to have time to burn all of the hydrocarbons. Since the volatile hydrocarbons can not condense to soot, you simply did not know whether there was any unburned hydrocarbons left after the flame had died down. The reason then that these substances are not burned is that the gas creation after the flame is too thin for a continuous combustion. An effective complete combustion of the light hydrocarbons therefore also implies that a longer burn time with a flame is required to make sure that all substances are completely burned.
Recirculating blue flame technology
One of the first to use the recirculating blue flame technology was the Italian manufacturer Mescoli. They constructed a blue flame boiler with a pre-pressurized combustion zone. The combustion cup was constructed from heat resistant steel and enlarged so that the burn time for the gas combustion was then increased. To pre-pressurize and keep the flame within the combustion zone, a small border was put onto the outlet. This then created an over pressure inside the combustion chamber. This over-pressure should not then spread to the wood hearth since it could then easily cause flue gas puffs. To avoid this, the burner cup was perforated with hundreds of small holes that could tolerate certain over pressure but also start to release the gases through the holes. To then not increase the creation of nitrous oxides, the space around the combustion chamber is water jacketed and the radiant heat can then leave from the steel cup and onto the water jacket. As a result the temperature of the flame then goes down and the nitrous oxide creation can be held back despite a longer burn time. This type of boiler construction then came to be a model produced by a great number of European wood boilers on the market. Today, Sweden has for example about ten models that you can see incorporating this modern re-circulating blue flame technology. Amongst them we find Swedish-made boilers from Effecta Energy Solutions, Ariterm, CTC/Bentone and Calmar.
Installation and handling
Despite the fact that the wood boilers of today achieve very good performance and are user-friendly, there is no guarantee that the fireman will experience these good performances. Of course it is more difficult to fail with a modern boiler, but it still remains that it is the installation and handling of the system itself which the fireman will decide whether they are effective and most importantly, safe. If the accumulator tank is incorrectly sized or incorrectly connected, it does not help the performance of the boiler atall. That said, it remains the case that with all wood burning, even when using modern technology, you can still cause disturbances and complaints from neighbours and other people living around you. If wood burning in the future should be more recognized and become more completely accepted as a fuel for urbanized areas (which we hope that it does!), either the knowledge of the end user has to increase, or we need installers who can sense and then rectify the mistakes of the end user. Maybe the solution is a combination of both.
Controlled combustion
More and more with wood boiler systems we are starting to get wood boilers which, via a lambda sensor, are then able to sense what the flue gases are doing. From these values received by the lammda sensor the operator is then able go in and adjust the primary and secondary air, so that the best combustion result is then obtained, regardless of wood quality or mounting of the system etc. We have now got wood boilers which can take charge of the operator and automatically ensure that they are being used in the most effective way possible. Within Europe today, there are more than 10 boilers with lambda probe controls. However, the technology is expensive and the consumers hardly want to invest extra in a wood boiler to get the highest efficiency and environmental performance if the price of wood they pay for is just for their own use and regulatory requirements are missing.
With this information provided we have tried to show that today's wood burning technology has the ability to work well even in urbanized areas. It is important to recognize that there is a big difference between the old traditional wood burning technology and the modern wood burning technology being offered today. We have also tried to stress that is no less important that we demand that the homeowner (end user) has equally as much knowledge as the manufacturer of these systems. When using the right modern technology, or even that of traditional wood burning, it has a very positive future. Wood burning is a very big, important step in the change towards a long-term and sustainable energy heating system.

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