Biomass combustion

Heating history: no changes in 100 years!

Before the 1900s, the world as a whole used wood (including wood converted to charcoal) for heat in homes and industry. By the 1920s, however, coal and heating oil had largely replaced renewable energy sources in industrialized countries, although wood for home heating and hydroelectric power generation remained in wide use. At the end of the twentieth century, nearly 90 percent of commercial energy supply was from fossil fuels. The world population in the year 2100 will be in excess of 12 billion. If the current trends in technological progress and innovation continue, the demand for energy then will be five times greater than what it is now. If we continue the policy of using coal, oil and gas at the present rate, then by the year 2010 the global temperature will have increased by two degrees Celsius. We do not need reminding of the adverse effects of this: the increased risk of flooding in lowland areas, the processes of desertification, and changing climate all over the world.

The awareness of the need for change in the way heat and power are produced has directed significant technical effort in providing viable alternatives. One of the most promising alternatives is actually going back to a widespread use of wood and other solid biomass as heat and power energy source.

At the beginning of 1990's the technology to generate heat from wood was basically at the same technology level as it was 100 years earlier. At a single house level a wood fired boiler needed manual ignition, manual feeding of fuel, frequent manual cleaning and ash extraction. In addition, the combustion quality depended on the boiler construction and chimney characteristics. The result of such system is an extremely poor efficiency level (only as little as 30-40% of the biomass energy is transferred to water for heating) and on the other hand large emissions of dangerous gasses (CO, NOx, ...) and particles. When we consider the volume of wood logs needed to replace a say 2.000 liter tank of heating oil it is obvious that such systems could not compete with the performance and comfort provided by a gas or heating oil boiler who need a maintenance intervention once a year.

Pellets: the first important step after 100 years

By the year 2000 the situation drastically changed. The first important step is represented by the pellet. Pellets are made of sawdust and have 2 great characteristics - they can be automatically fed within a combustion system and they have high energy density value compared to other wood fuels. The second step is the use of electronics to control the process of biomass burning or combustion. The use of sensors, sophisticated algorithms to control feeding and ventilator motors allows modern biomass fired boilers to achieve efficiency levels of more than 95% and emissions 10 times lower than traditional systems.

Biomass combustion: how does it work?

In order to ensure complete biomass combustion with low emissions and low slagging the quantity and method of supplying the combustion air is of extreme importance. To optimize the combustion it is necessary to divide the combustion chamber into a primary and a secondary combustion zone, where each zone has its own air supply. In the primary zone the primary combustions takes place which consist of two phases, the drying phase, the wood gasification phase (also known as pyrolysis) and the final combustion. During drying the remaining water is released and evaporated from the wood biomass. Then the dry wood biomass will be decomposed in combustible, volatile components and char. The primary combustion requires energy input and takes place with an air ratio below the stoichiometric ratio (air/fuel ratio), i.e. with deficient air supply. During the secondary combustion the flammable gases are combusted in the secondary zone with excess air. Simultaneously the char is combusted in the primary combustion zone. During both oxidations energy is released. In the primary combustion zone primary and secondary combustion take place at the same time, when unburned pellets are fed. For an optimized combustion a proper mixing of the secondary air with the flue gas is required. This can be achieved by accurate dosage of air in the combustion chamber. The longer the flue gas stays in the furnace the more complete the combustion will be. The amount of excess air in the secondary zone is not only of importance for carbon monoxide (CO) and unburned hydrocarbons (OGC).

There is a tradeoff of between this emissions and the emission of nitrogen oxides (NOx). Too little air will result in increased emissions of CO and OGC, but will keep the amount of NOx in the flue gas small. With greater excess air, more NOx will be released from the burner. Measurements have shown that biomass burners often emit 2 to 4 times more NOx compared to oil burners. The biomass boiler and burner without electronically controlled combustion process are developed to be predominantly operated with high excess air. A better control of the air supply by using a variable speed fan controlled by a CO or a Lambda sensor would help to minimize the emissions. Biomass boilers with Lambda control are already state of the art, whereas the most of biomass heating devices often allow only a manual adjustment.

Another two important parameters for low carbon monoxide, unburned hydrocarbons and nitrogen oxides are the residence time of the flue gas and the temperature in the combustion chamber. A high temperature and a long residence time reduce the emissions of CO and OGC to almost zero. The content of nitrogen oxides in the exhaust gas is increasing with increasing temperature unless a sufficient residence time is achieved. Consequent air staging in order to obtain high temperature and long residence times is a practical method to reduce emissions from pellet boilers.

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