Conversion from gas to oil firing

The first step in such a conversion is to conduct an engineering study directed at determining how the unit can best be altered to meet the requirements for oil firing.

The design of the burners must be reviewed to determine the changes necessary to adapt them to fire oil on a continuing basis. Oil atomizers would have to be added. The existing gas burners would be checked to determine if they would remain undamaged when firing oil or whether burners of a different design would have to be installed.

The circulation of water in the furnace wall tubes would be calculated to determine if the water circulation is adequate for the additional furnace heat absorption from oil firing. In a once-through boiler the higher furnace heat absorption which occurs with oil firing might result in temperatures for the furnace circuits which would be above the design temperature. If this were the case, it would be necessary to reduce furnace heat absorption by some means, such as adding a gas recirculation system.

Undoubtedly, with higher heat absorption in the furnace the heat absorption in the superheater and reheater will be less than with gas firing, and design steam temperatures probably cannot be maintained. To obtain design steam temperatures it will usually be necessary to add surface to the superheater and/or reheater, or to reduce furnace heat absorption by removing some heating surface from the furnace, such as division wall surface, or by adding a gas recirculation system. A check must be made of the metal in the tubes and the tube supports to make sure the material is adequate for the temperatures which will exist with oil firing. Space or other physical limitations may prevent making the changes necessary to meet design conditions and it may be necessary to settle for lower than design superheat and/or reheat steam temperatures.

If the unit has an extended surface economizer (close spaced fins on the tubes) the spaces between the fins can plug with ash and it will usually be necessary to replace the economizer with a bare tube economizer. The new economizer has to be designed to get equivalent heat absorption and this will require more tubes and more space.

If the unit has a regenerative air heater, heating surface at the cold end must be replaced with heating surface with larger gas flow lanes, less subject to plugging with oil ash. For any type air heater, a safe metal temperature from a dewpoint corrosion standpoint must be determined and provision must be made for maintaining this safe metal temperature. This may involve adding a steam coil air heater for the entering air, adding a system for air recirculation from the air heater outlet to the Forced Draft fan inlet, or removing some air heater surface to increase the temperature of the flue gas leaving the air heater.

As mentioned earlier, the ash in the oil can stick to the heat absorbing surfaces in the furnace, superheater, reheater, economizer and air heater and retard the heat transfer to these heating surfaces. The ash also can plug the gas flow lanes in the superheater, reheater, economizer and air heater. To minimize the detrimental effects of this ash, it is necessary to add soot blowers to clean the heating surfaces in furnace, superheater, reheater, economizer and air heater. The study must determine the most practical location for these blowers, but since the heating surface was arranged originally without consideration for soot blowers there can be no assurance that the soot blowers will do as good a job on a converted gas fired unit as they would on a unit designed initially to burn oil.

The soot blowers extend out from the sides of the unit and require supporting steel and platforms for access. These platforms must be designed and the structure checked to be sure the soot blowers can be supported on existing steel.

The air flow requirements for oil firing must be calculated and the draft loss must be determined for the altered boiler. A check must be made to determine whether the existing fans will be capable of supplying sufficient combustion air for these new conditions.

Provisions must be made for water washing the air heater, and in some cases for water washing the superheater, reheater and economizer during periodic outages.

Means must be provided for disposing of the ash that will collect in the furnace and other parts of the unit.

After the study is completed and the required alterations have been determined, drawings must be made, new equipment ordered and manufactured, and field construction has to be done to install the new equipment.

As indicated at the beginning, gas fired units can usually be converted to oil firing as some expense in outage time, money and perhaps some reduction in power generating capacity.

Boiler design for coal firing

Coal contains a considerable amount of ash. The quantity can range from somewhat less than 10% to more than 20% of the fuel fired; so for a utility boiler of reasonable size there are many tons of ash per hour. This ash becomes fluid at temperatures existing in the furnace and the ash in many coals remains liquid, or at least sticky, until the temperature is reduced to 2000F or lower. The ash in some coals has the undesirable ability to sinter into a very strong mass at temperatures in the 1400F to 1700F range, the normal temperature of the flue gas in parts of the superheater and reheater.

Some coals have constituents such as sulfur and sodium, which can cause corrosion of the high temperature parts of the unit, as well as the low temperature portion of the air heater. Because of the additional combustion air requirements for burning bituminous coal, as compared to gas, and because of the greater weight of the fuel fired, the total products of combustion will be about 20% greater than for gas firing. This can be over 40% when firing lignite.

The design of a coal fired unit must take into account all of these characteristics of the fuel. The furnace must be of sufficient size to provide time for complete combustion of the fuel and must have sufficient heating surface to cool the products of combustion to a low enough temperature so the ash particles will not stick to the close spaced superheater and reheater surface.

In a high pressure utility boiler, where approximately 50% of the heat in the fuel must go into superheating and reheating the steam, it is necessary to have the flue gas entering the superheater and reheater at a temperature no lower than about 2400F in order to have enough heat to obtain design superheat and reheat steam temperatures. Thus, superheater and/or reheater heating surface is installed directly in the furnace of a coal fired boiler. This is usually done by installing platens or wing walls of superheater or reheater surface in the upper part of the furnace on very wide centers, 4 feet or more. With surface on such wide centers the ash cannot build up to a point where pluggage of the flue gas lanes can occur.

An alternate method of obtaining sufficient heat for the superheater and reheater but with lower gas temperature is to recirculate flue gas from the economizer outlet to a point near the top of the furnace to reduce the products of combustion leaving the furnace from a temperature of about 2400F to the point where the ash will not be sticky. This reduces the flue gas temperature and cools the ash particles but keeps the heat available for superheating and reheating the steam.

The convection surface (superheater and reheater) must be arranged on wide centers starting at about 24" centers at the furnace outlet and decreasing to 18", 12", 9", etc. centers farther back in

55-305 - 75 pt. 3-4

the unit as the flue gas temperature is reduced and the ash particles have less tendency to stick to the tubes. By comparison, gas fired boilers usually have convection surface on no more than 6" centers. The bank depths must be kept to a minimum so the soot blowers can do an adequate job of cleaning the ash from heating surfaces.

The total area for the flue gas to flow around these heating surfaces must be such that the flue gas velocity is in the range of 65 ft/sec, depending on the character and quantity of the ash, to minimize erosion of the pressure parts by the ash particles carried in the flue gas. This compares with flue gas velocities of over 100 ft/sec on gas fired units.

Hoppers must be provided as the bottom of the furnace and at other strategic points throughout the unit where the tons of ash may collect, and ash removal equipment must be provided for removing ash from these hoppers.

Conclusions

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Based on the design considerations given above, if it should be necessary to fire coal at a station where the boiler was designed for gas or oil firing, even with extensive alterations to the boiler, many of the design requirements for coal firing - - furnace size, convection surface tube spacing, flue gas velocities, etc - cannot be met. It would not be possible to operate the unit with coal firing for any appreciable period at a rating even approaching 50% of design rating without encountering intolerable problems with such things as slagging, fouling and pressure part erosion. Thus, a complete replacement of the boiler with a unit designed for coal firing would be necessary before that station could produce rated output with coal as the base fuel.

These comments will generally apply whether the boiler is a natural circulation drum boiler, an assisted circulation drum boiler or a once-through boiler.

On behalf of the American Paper Institute I am enclosing a statement containing our views and recommendations on S.1777, the "National Petroleum and Natural Gas Conservation and Coal Substitution Act of 1975."

We hope our views will be helpful to you and your Committee in the development of this very important piece of legislation.

EALjr/mm

encl.

Sincerely,

Estocks, Jr.

Edwin A. Locke, Jr.