Performance with Oil Firing
Steam generators have been fired with both distillate fuel oils and residual oils. The design of the boiler does not change much for distillate oil firing compared to gas firing. The fouling factor used is moderately higher, 0.003-0.005 ft2 h °F/Btu, compared to 0.001 ft2 h °F/Btu for gas firing; rotary soot blowers located at either end of the convection section are adequate for cleaning the surfaces for distillate oil firing. With heavy fuel oils, retractable soot blowers are required. Economizers also use rotary blowers in oil-fired applications. Solid fin tubes of a fin density of three or four per inch may be used if distillate fuels are used, but if heavy oil is fired it is preferable to use bare tubes or at best 2-3 fins/in. The emissions of NOx will be higher on the basis of fuel-bound nitrogen, because it can contribute to nearly 50% of the total NOx. Flue gas recirculation has less effect on NOx in oil firing than in gas firing.
With residual fuel oil firing, there are several aspects to be considered.
1. High temperature corrosion due to the formation of salts of sodium and vanadium in the ash has been a serious problem in with heavy oil boilers fired. The furnace exit region is a potentially dirty zone prone to deposition of molten ash on heating surfaces. The use of superheaters in such regions presents serious performance concerns. Retractable steam soot blowers are required, with access lanes for cleaning. Tubes should preferentially be widely spaced at the gas inlet region to avoid bridging of tubes by slag. Vanadium content in fuel oil ash should be restricted to about 100 ppm to minimize corrosion potential.
2. Superheater materials used in heavy oil firing applications should consider the high temperature corrosion problems associated with sodium and vanadium salts. The metallurgy of the tubes should be T22 or even higher if the tube wall temperature exceeds 1000°F. A large corrosion allowance on tube thickness is also preferred. This is yet another reason for preferring a convective superheater design to a radiant superheater.
3. Steam temperatures with oil firing will be lower than on gas firing as discussed above.
4. Furnace heat flux will be higher in oil firing than in gas firing. Therefore one has to check the circulation and the furnace design.
5. One of the problems with firing a fuel containing sulfur is the formation of sulfur dioxide and its conversion to sulfur trioxide in the presence of catalysts such as vanadium, which is present in fuel oil ash. Sulfur trioxide combines with water vapor to form sulfuric acid vapor, which can condense on surfaces whose temperature falls below the acid dew point. Q6.25 illustrates the estimation of dew points of various acid vapors. Sulfuric acid dew points can vary from 200 to 270°F depending on the amount of sulfur in the fuel. If the tube wall temperature of the economizer or air heater falls below the acid dew point, condensation and hence corrosion due to the acid vapor are likely. I have seen a few specifications where a parallel flow arrangement was suggested for the economizer to minimize acid dew point corrosion. Because the feedwater temperature governs the tube wall temperature and not the flue gas temperature, only maintaining a high water temperature avoids this problem, as shown in Q6.25c. One could use steam to preheat the feedwater or use the water from the exit of the economizer to preheat the incoming water in a heat exchanger. Experience and research show that acid corrosion potential is maximum not at the dew point but at slightly lower values, about 15-20°C below the dew point. Hence one may use a feedwater temperature even slightly lower than the dew point of the acid vapor in order to recover more energy from the waste gas stream. In waste heat boiler economizers, other acid vapors such as hydrochloric acid or hydrobromic acid may be present. The dew points of these are much lower than that of sulfuric acid, as discussed in Q6.25, so care must be taken in the design of economizers or air heaters in heat recovery applications.
Table 3.7 Shows the boiler performance with distillate oil firing. The efficiency on LHV basis is nearly the same as for gas firing, but on HHV basis there is a difference. The flue gas analysis with 15% excess air is shown. The flue gases have less water vapor but more carbon dioxide than flue gases from natural gas combustion.