Combustion Quality

This article sets out in detail the requirements for a good combustion quality.
These requirements can be “boiled down to” “the 3 T’s” (Temperature, Turbulence
and Time). The temperature should be sufficiently high to enable efficient drying, gasification, and combustion. Air and
combustible gases should be mixed adequately (turbulence), and finally there should be space and time for the gases to burn out before they are cooled too
much by the boiler water.



The flue gases pass from the combustion chamber to the part of the boiler, where the heat is given off to the circulating boiler water. Most often, the boiler is situated above the grate. The flue gas flows inside the tubes that are water cooled on the outside surface.
In small systems, the combustion unit and the boiler may be completely separated, since wood chips are burnt in a separate pre-combustor, from where
the flue gases are passed into the boiler.
In the boiler unit or as a section after this unit, an economiser may be installed that cools the flue gas down to a temperature of approx. 100 °C. The increased cooling improves the efficiency.
The boiler room should be large enough for repair work and for ordinary maintenance work, including boiler purifying, to be carried out in a proper way. The building round the boiler should be designed so as to give room for purifying of the boiler tubes and replacements of tubes.
With respect to the boiler life, it is important that the temperature of the return water to the boiler is sufficiently high. It is recommended to keep a return water temperature of at least 75-80 °C in order to reduce the corrosion of the boiler tubes in particular.
The life of tubes varies a lot at the various wood chip-fired plants. In addition to the operating temperature, the boiler life depends on the
operational patterns, fuel, combustion quality, and choice of material.


Flue Gas Purifying - Fly Ash

The fly ash is the part of the ash that remains in the flue gases on its way through the boiler. Flue gas purifying is
first and foremost a question of reducing the amount of fly ash emitted through the chimney. The emission of other pollutants is discussed later on in this chapter.
The fly ash is transported from the flue gas purifying unit to the remaining part of the ash system by screws. The separation of fly ash from the flue gas
may be accomplished either by means of multicyclone, bag filter, or other flue gas purifying equipment.

The fly ash from the combustion of wood consists primarily of relatively large particles that can be trapped by means of a multicyclone. Most plants are equipped with multicyclones. A well-dimensioned system can purify to a level of approx. 200 mg/m3n /ref. 61/ (1 m3n is a normal cubic metre, i.e., a cubic metre of gas converted to standard conditions 0 °C and 1 bar). Multicyclones that are inexpensive to buy and maintain, are used for precleaning before the flue gas condensation unit.
Bag filters can purify to a level of 10-50 mg/m3n. Normally, bag filters are only capable of withstanding flue gas temperatures of up to approx. 180 °C. In
order to avoid embers and sparks in the bag filters, the flue gas must pass cyclones or a filter chamber situated before the bag filters. Bag filters are automatically deactivated if the max. temperature or the max. value for the oxygen content in the flue gas are exceeded.
Like the bag filter, the electrostatic precipitator (ESP) cleans efficiently, but it is more expensive to install in relatively small wood chip-fired systems. However, operating costs are lower, however, than those of the bag filters. Bag filters, ESPs etc. are not extensively used today at wood chip-fired district heating plants.


Flue Gas Condensation

Flue gas condensation units are now in general use in both new and existing systems.
It is a technique that both purifies the smoke/flue gas for particles to a level almost similar to that of bag filters at the same time of increasing the energy efficiency.
Most of the Danish wood chipfired district heating plants have either been delivered with flue gas condensation or have had the equipment installed
with the boiler system. Like most other fuels, wood contains hydrogen. Together with oxygen from the air, the hydrogen is converted to
water vapour by combustion, and the water vapour forms part of the flue gas together with other products of combustion.
Furthermore, wood chips used at district heating plants typically have a moisture content of 40-55% of the total weight. By the combustion, this water is also converted to water vapour in the flue gas.
The flue gas water vapour content is interesting because it represents unutilised energy that can be released by condensation.
The theoretical amount of energy that can be released by the condensation of water vapour is equal to the heat of evaporation for water plus the
thermal energy from the cooling.
When flue gas is cooled to a temperature below the dew point temperature, the water vapour will start condensing.
The more the flue gas is cooled down, the larger is the amount of water that is condensed, and the amount of heat that is released is increased. The
lowering in temperature from the normal flue gas temperature of the system to dew point temperature automatically increases the heat output. The effect increases, however, when the condensation starts, and the heat of evaporation is released. Figure 22 illustrates in percentages the increased generation of heat that can be achieved by lowering the flue gas temperature. The normal operating situation that forms the basis of the calculations is a flue gas temperature of 130 °C with CO2 being 12%.
The various lines in the figure illustrate various values for the wood chip moisture content in percentage of the total weight.
The curves show the theoretical improvement of the efficiency that can be calculated on the basis of the moisture content and the flue gas temperature.
Experiences acquired from condensation units in operation indicate that an increase in efficiencies can also be achieved in practice. Thus, the annual efficiencies for almost all plants are above 100% (based on the net calorific value of the fuel which does not include the condensation heat).

The return water from the district heating system is used for cooling the flue gas. The water should be as cold as possible. The flue gas cooling unit is
therefore the first unit the water passes when it returns from the district heating system.



Condensate consists of water with a small content of dust particles and organic compounds from incomplete combustion.
There is also a minor content of mineral and heavy metal compounds, and of chlorine and sulphur from the wood.
The pH value of the condensate varies a lot from system to system, and it also varies with the operational pattern.
A typical value lies between pH 6-7, but there have been measured pH values from 2.7 to above 8. The dust particles contained in the condensate affects the
pH value heavily. High pH values are connected with large particle contents - i.e. the fly ash seems to be alkaline/basic, and the majority of it by far is dissolved in the condensate. Indissoluble particles only contribute 10%.
The condensate should be treated before being discharged. The minerals and heavy metals contained in wood, such as cadmium that has been absorbed during the growth in the forest, concentrate in the condensate and may
reach a level exceeding the limit values for discharge. Investigations have shown that the large amount of cadmium contained in the condensate is found in the condensate particles and not in dissolved form in the water.
The particles can be removed from the condensate liquid by filtering, so that the cadmium content is reduced to below the limit values for discharge.
This is the reason why filtration equipment for the separation of condensate particles is being installed in an increasing number of plants right now.
After treatment and neutralisation, the condensate is generally discharged into the municipal sewage system.
When the flue gas leaves the flue gas condenser, it should pass through an efficient mist eliminator for the collection of entrapped droplets, thereby avoiding
mist being carried further into the tube, exhaust fan, and chimney.

The first prerequisite of success with flue gas condensation is a return flow temperature in the district heating system that is so low that the vapour in the flue gas can be condensed. In addition, the fuel should have a high moisture content. Wetter fuel increases the overall efficiency of the plant! This applies only as long as the moisture content is not so high as to result in incomplete combustion.
Forest chips with a moisture content in the range of 40 and 50% are ideal for systems with flue gas condenser.
The installation of flue gas condensers may often make the installation of other equipment for flue gas purifying unnecessary.
If the installation of a bag filter can be avoided, the money thereby saved can often pay the investment in the flue gas condensation unit. Consequently,
the energy saved is almost free.


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