DEPOSITS

 

 

During the combustion of lower quality fuels, a measurable and progressive lowering of efficiency is caused by the decrease in heat transfer rates from the combustion gases to the useful fluid (i.e. steam).  This lowering of efficiency is due to the accumulation of deposits on the metalic surfaces of the heat exchangers.  The structure of these deposits is shown in this picture.

          

Layer 1 is formed by metal oxides, thickness of 0.1 to 2.0 millimeters and has a porous structure which is crisscrossed by very fine capillary chanels. This layer is built as a “protective” covering that aids to the process of corrosion of the metalic surfaces on which the layer is formed.

          The corrosion in this zone is mostly sulfuric.  The sulfuric acid that is formed, practically during combustion, is diffused together with the water vapors through the capillary chanels of Layer 1 and is retained there.  At boilers operating temperatures the molecules of sulfuric acid are stable and the probability of combination with water molecules is very low.

          When the boiler is stopped or during steam/water blowings for cleaning the deposits, as the temperature is lowered, the sulfuric acid is dissolved in the available water, forming a very corrosive solution.

          The resulting salts act as a strong adhesive between the surface of the metal and Layer 1, this is why Layer 1 is almost impossible to be removed with blowings while operating or by mechanical means during shut-downs.

          Layer 2 is formed supported by Layer 1. The superior hydrocarbon molecules, especially the poly-nuclear aromatic and cyclical which require high energies to break the chemical bonds, attach to Layer 1 surface and grow in time by attracting other non-burned particles and inert residual materials from the fuel composition.  As the thickness of Layer 2 deposit grows, the heat transfer efficiency is drastically lowered forcing the user to shut-down the boiler for cleaning.

          During the shut-down/start-up cycle the corrosion phenomenon of Layer 1 is at its highest which results in loss of metalic elements from the heat exchangers.

          Due to these phenomena (deposits accumulation and corrosion) it is necessary a solution to lengthen the uninterrupted operating periods between shut-downs. As the period between shut-downs is lengthened, more and more fuel is used to maintain the required amount of energy transferred with lower and lower heat transfer coefficients. At the same time, the exhaust gases temperature increases. Worse than that, the Layer 2 deposits which are rich in unburned molecules present a very large danger of self-ignition with destructive consequences for the boiler.

          Using ECOBIK® assures the cleaning of old deposits (starting with the base – Layer 1), minimizing the possibility of re-deposits and maintains the combustion system (the boiler’s internal surfaces) functioning normally for as long as it is needed.  It becomes clear at boilers’ shut-downs, that the corrosion phenomena is inhibited.

 

 

SLAGGING

 

 

        In typical combustion applications, especially when solid fuels are used, slagging phenomena appear formed by ash that contains large amounts of unburned substances that clog the combustion gas passages and forces the user to stop the boiler to mechanically clean these deposits. The unburned substances act as a “binder” for the ash, sometimes resulting in hardening of these accumulations. Also, in the low combustion gas velocity zones (at walls), which are important zones for heat transfer, the tubes are completely covered in ash, zones where the heat transfer coefficients are reduced to zero.

        The phenomenon of slagging of the gas passages is evident by decreased pressure in the firebox, and by increased amperage of fans moving the air and combustion gases (forced and induced draft fans).

 

        Using ECOBIK® attains in the first place the quasi-complete and rapid oxidization on unburned particles eliminating the “binder” for the accumulating ash.  At the same time, by more complete oxidation the ash specific mass is lowered and the ash is moved more easily by the combustion gases.

 

NOTE: While the old deposits are removed, when using ECOBIK®, in the case of boilers with heavy deposits, it is possible to clog some parts of the heat exchangers, like the economizers, and especially the air pre-heaters. This phenomenon is only possible at the beginning, after which the process of slagging is arrested.

 

 

BLOWINGS

 

 

        During the operation of industrial combustion installations, the accumulation of deposits on the heat exchanger surfaces is fought with periodic blowings with steam, water, compressed air or other fluids to remove part of these deposits. The blowings are discontinuous, are done at fixed time intervals or when a temporary need for them appears.

        The use of blowings generates multiple negative effects in the operation of combustion installations. Thermal condition (temperature gradient), of the zones where blowings are done, is abruptly lowered, requiring multiple changes in a short time of the combustion installation. For example, in a thermal boiler, the steam temperature drops dramatically after blowing necessitating making injections of steam from other zones to maintain the steam parameters needed at the entry of the steam turbine.

        Blowings, at the same time, are one of the principal causes of heat exchangers’ metal surfaces corrosion and the acceleration of masonry erosion due to large variations in temperature forced into the refractory materials.

 

 

        The use of ECOBIK® allows for complete elimination of blowings, with the eventual exception of regenerating air pre-heaters.

        Direct savings generates by ECOBIK® through blowings elimination reach 1% to 3% of the total operational costs of the combustion installation.