Secondary Benefits of Clean Boost™
Although there is not much information available on the combination of combustion catalysts and vanadium fouling, two points can be raised.
Combustion catalysts favour high oxidation-state compounds, such as V2O5, and may have an adverse effect on vanadium fouling. The less highly oxidized V2O3 and V2O4 are high melting and innocuous, while V2O5 is low melting and troublesome. Therefore the addition of combustion improvers to high vanadium fuels, without using magnesium, may exacerbate high temperature fouling.
In the case of fuels with high asphaltene levels the use of a combustion catalyst, such as Clean Boost™, with the magnesium additives is recommended. Unless conventional magnesium additives are supplemented by a combustion catalyst, partially burned asphaltenes are likely to compound high-temperature corrosion and deposit problems.
High-temperature corrosion is caused by deposits of low-melting point (less than 1250°F) ash containing primarily sodium, vanadium and sulphur. These complex alkali sulphates and vanadates cause fouling problems which interfere with heat transfer to the tubes. Additives are considered one of the better techniques for high-temperature corrosion and fouling problems because they can raise the melting point of the ash deposits and prevent the sticking action.
The five elements which have been used in treating high-temperature fouling are magnesium, aluminum, silicon, manganese and calcium.
While magnesium is the primary element used to combat vanadium fouling some reports have suggested that other elements may be beneficial. It now appears that there is no synergistic effect of the other elements on the activity of magnesium. Therefore the preferred additives are compounds of magnesium.
Magnesium reacts with the sodium vanadates to form high melting sodium magnesium vanadates and thus reduces vanadium fouling. This effect is stoichiometric, not catalytic, and at least as much magnesium must be added as there is vanadium in the fuel. Typical additive rate are three parts magnesium for one part vanadium in the fuel. Thus a high vanadium fuel with 100 ppm vanadium would require 300 ppm magnesium added to the boiler to combat high temperature fouling.
Utility companies may use a water-slurry of MgO which is injected through the soot blowers in the convection pass to control high-temperature corrosion. MgO pastes are the least expensive form of the additive and are approximately 30 percent magnesium by weight. Although this approach involves a significant capital investment and high feed system maintenance costs, it allows use of lower cost grades of MgO.
Smaller utilities and boiler operators that do not want to invest in the equipment use an oil soluble form of magnesium. This material is a carbonate/hydroxide magnesium colloid in oil which contains about 14 percent magnesium by weight. These additives dissolve directly into the oil and thus do not require any retrofitting or maintenance cost. The additives cost about the same amount as the MgO powders however they contain roughly half the magnesium and are therefore twice as expensive (to treat a given vanadium concentration).
Cold-end corrosion is caused by the formation of sulphuric acid, H2SO4, which is formed from the reaction of SO3 and H2O. The sulphuric acid attacks the iron used in the boiler construction to form scale deposits of ferrous and ferric sulphate.
The fuel bound sulphur is readily oxidized to SO2. Greater than 99 percent of the sulphur leaves the combustion chamber as SO2. Further oxidation of SO2 to SO3 can occur to a very small extent in the combustor and this small amount of SO3 results in cold-end corrosion. The SO2 has no negative impact upon the combustion systems but does react with water and oxygen in the upper atmosphere to produce acid rain.
Two types of chemical additives can be effective in controlling problems related to sulphuric acid. Combustion catalysts, such as Clean Boost™, can reduce the formation of sulphuric acid while chemicals like the magnesium based additives neutralize the acid after it is formed. The important points regarding the impact of combustion catalysts on SO3 formation are:
SO3 concentration in the flue gas can vary by alternate adsorption and desorption on soot. Therefore reducing soot is important in the elimination of bursts of high SO3 concentrations.
Clean Boost™ allows reduction in excess air which will lower SO3 formation. More efficient combustion and lower excess air levels can reduce the amount of SO3 formed.
Combustion catalysts generally can not control sulphate formation completely and must be supplemented with a neutralizing chemical, usually a magnesium additive.