Fouling in a boiler heat transfer surface is different from slagging. Even though both affect heat transfer, fouling happens in the convection heat transfer surface in boiler and slagging happens in the radiant area. Both are results of inorganic material in the fuel. The causes for fouling are mainly due to the low melting point of alkaline constituents of the inorganic material in the fuel. Fouling on the convective heat transfer surface is caused by condensation of the vaporized volatile inorganic elements in fuel during combustion when the gas temperature decreases.
Fouling in boilers is dependent on the type of boiler design and the fuel being fired. Fouling in a chemical recovery boiler, a coal fired boiler, and a biomass fired boiler are different in many ways. In a chemical recovery boiler, fuel being used has a large quantity of alkaline inorganic material. This makes it necessary to operate the boiler in a narrow band and to soot blow the heat transfer surface more frequently when compared to a coal fired boiler. In the case of a biomass fired boiler, even though the inorganic material percentage is very low, a large amount of fouling due to its very low melting temperatures can occur. The presence of alkaline materials like sodium and potassium salts make the coal ash have a very high tendency toward fouling in the convection heat transfer surface in boiler. Primarily the sulphates like calcium sulphate and sodium sulphate in vapor form in flue gases make the first layer of deposit on the heat transfer tubes which gives the binding strength for the deposit.
Fouling in a chemical recovery boiler starts from the bank tubes, followed by the economizer if provided. These fouling deposits increase the pressure drop across the bank tubes and economizer. Soot blowing at regular intervals, depending upon the way in which the deposits are formed, can help to keep the unit on line for more time between water washing cycles or off line cleaning. Factors like fine spray, high black liquor temperature, high black liquor pressure, and low bed operation are some of the major contributing factors for fouling of heat transfer surface in chemical recovery boilers in paper plants.
In the case of coal fired boilers, the ash composition and the combustion regime maintained contribute to fouling of the convection heat transfer surface. The ash composition of coal ash is grouped into three main categories. Namely, these are the low melting, the medium melting, and the high melting constituents. The low melting constituents found in coal normally in small percentage are NaCl, Na2SO4, CaCl2, MgCl2, and Al2(SO4)3 with a melting temperature varying from 700 to 850 degree C. The medium melting constituents are those with a melting temperature of around 900 to 1100 degree C, which include FeS, Na2SiO3, K2SO4, etc. The high melting constituents form the bulk of the ash quantity and are SiO2, Al2O3, CaO, MgO, Fe2O3 etc. The melting temperature of these constituents lies in the range of 1600 to 2800 degree C.
The main factor that leads to fouling in coal fired boilers is the increase in percentage of the low melting constituents. A small percentage increase in NaCl in fuel can substantially increase the fouling tendency in boilers. There are many correlations bought out by many researchers to predict the fouling tendency in boilers, which are little more reliable and universal than the slagging parameters.
The fouling deposits in boiler have two main layers. The first layer gives the bounding strength to the deposit and is a very thin layer. The second layer is the bulk of the friable deposit. This gets formed on the back side of the tube relative to the gas flow direction.
Apart from the chemical composition of ash, the deposition rate is affected by the gas velocity, the type of heat transfer surface arrangement, and the particle size. The lower the velocity, the higher is the tendency for the particles to settle on the tubes. The velocity range will depend on factors like gas temperature, composition, etc. A staggered arrangement of a heat transfer surface has a higher tendency to fouling than an inline arrangement.
Since fuel ash composition is totally an independent factor, the boiler operator or the designers has little say on the change but for a major guideline to be adopted. However, the combustion regime can be maintained by proper tuning of combustion to the optimal condition. Maintaining the required gas velocity and not adopting the staggered arrangement all will depend upon the designer choosing the right design for the fuel under consideration. Operating the soot blowers at the required frequency reduces fouling to a great extent.