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Biomasses

WHY BIOMASSES FUELS MUST BE DRIED

Biomass used as fuel, normally, has a residual humidity rate ranging from 40 to 60%. The high humidity rate does not allow for direct use on fire chambers, but require a pre-drying process. If we examine the combustion process, it could be divided into three phases, based on the temperature levels achieved into the combustion chamber: Drying – Gasification – Combustion. Drying phase consist on the water evaporation, a part of combustion generated energy is used for that and therefore the temperature is reduced to a lower level and thus slowing down the thermal process. Practically it has been demonstrated that a 60% or higher residual fuel humidity would not support the combustion process and therefore humidity rate is a major important parameter for combustible biomasses exploitation. Additionally, the humidity excess will slow down the combustion process and increase ashes, soot and creosote generation. Creosote rate, combined with steam condensation, especially when heat exchangers are used, will build up tough incrustations which could be removed only by intensive mechanical systems not usable while boilers are in running conditions. Incrustation and scale will build an insulating layer which will drastically reduce the thermal exchange between hot gases and process fluids, tremendously downgrading the boiler thermal performances. A steam excess on flue gases, moreover, will take volume on the combustion chamber disturbing the smooth thermal stream and increasing consequently carbon oxide concentration. To avoid all the mentioned problems it is advisable to reduce the residual humidity to a rate between 15 and 20% on fixed grate hearth so to duplicate the naturally dried wood characteristics. On reciprocating grate hearth it would be possible to use fuels with 25 to 30% residual humidity. We feel important for new combustion chamber dimensions designers to keep in mind that a reduced fuel humidity will result on a reduced fuel quantity inside the hearth and will allow for a better utilization of available space and volumes.

Here some examples :

To generate 400.000 Kcal or 465 KW it could be used:

Dry wood 100 Kg

Wood with 10 % residual humidity 111 Kg

Wood with 20 % residual humidity 125 Kg

Wood with 50 % residual humidity 200 Kg




HOT STREAMS AVAILABLE FOR DRYING PROCESS

Boilers chimney exhausted fumes are normally accounting for a 12 / 15% combustion chamber generated heat. Their temperature may vary from 210 to 230 °C. This heat value is quite enough for biomass drying and reduce them to a residual humidity acceptable ratio. Our drying proposed system take advantage from cogeneration principle by using the exhausted fumes enthalpy as heat source. Another advantage of drier utilization is that fumes temperature and volume are reduced allowing for smaller and simpler fumes cleaning systems.




CONCLUSIONS

Biomass fuel with a reduced residual humidity content advantages will be widely appreciated during boiler’s utilization. As a matter of fact a satisfying thermal performance could be obtained with a lower fuel quantity and a sensible maintenance cost reduction due to extended running time between programmed shut-off operations.

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EU Cookie Directive Module Information