The determination of the optimum CO₂ for operating number 6 boiler

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Virginia Tech


The operating characteristics of a boiler prove that pressure can usually be maintained by the supply of fuel and air, regardless of the combustion efficiency; however, to achieve the desired efficiency the burning of the fuel must take place with a minimum amount of excess air.

Thus the criterion for boiler operation becomes percent co. in the flue gas. With a CO₂ meter on the control board of the Number Six Boiler it became desirable to acquaint the boiler operators with the conditions in the boiler for a corresponding reading on the CO₂ meter.

The percent CO₂ in the flue gas of Number Six Boiler has four effects on the operating characteristics of the boiler when operating under a steam demand of 50,000 pounds per hour. First, it effects the energy loss due to dry exit gases; second, it effects the energy loss due to unconsumed carbon in the refuse; third, it effects slagging and possible deterioration of the refractory walls in the combustion space, and last, it effects the temperature of the refuse as it is discharged from the grate.

From a survey of the data obtained from the test runs a decrease in the energy loss due to dry flue gas is seen with an increase in the percentage of CO₂ in the exit gases. This indicates the desirability of operating the boiler at as high a CO₂ as possible in regard to losses occurring from dry flue gases. Similarly, the energy loss to unconsumed carbon in the refuse decreased with an increase in CO₂. Though this is contrary to results obtained in most boilers, the low CO₂, indicating a high excess air, resulted in the boiler being cooled below maximum combustion temperature; consequently, a high percentage of carbon in the refuse occurred with the low CO₂ This too indicates the desirability to operate the boiler at a high percent CO₂.

The limiting factors on the maximum percent CO₂ at which the boiler can be operated, with a steam demand of 50,000 pounds per hour, occurred at 12.9 percent CO₂. At this condition the walls began to Slag. This can be harmful to the walls, eventually causing their replacement, an economical loss which would outweigh the slight advantage of increased efficiency when operating at this percent CO₂ over a long period of time. The test runs were not long enough to determine the maximum effect slag would have on the boiler walls; nevertheless, it was noted during the test that slag running off the walls caused the fuel bed to become distorted and that the slagging has necessitated replacing parts of the front wall. Another economical loss would result from the increase temperature of the refuse discharged from the grate when operating at a higher CO₂. Since the refuse is manually removed from the ash pit to the vacuum ash removal system, and since there is a limit to the temperature the operator can withstand and still remove the ashes efficiently, the hotter refuse requires a longer time to be pulled from the ash pit. An additional load of 4,000 pounds of steam per hour, therefore, must be carried over the prolonged period.

The optimum percentage of CO₂ for a single boiler is that which will minimize the total losses due to dry flue gases and unconsumed carbon in the refuse and yet keep the refractory deterioration and temperature of the refuse as low as possible.

The desirable percentage of CO₂ to be maintained in Number Six Boiler for operation at a load of 50,000 pounds of steam per hour is 12 percent. This allows a maximum feasible boiler efficiency of 73 percent and corresponds to an excess air of 39 percent. It is not feasible to operate the boiler above 12 percent CO₂ because of the increase cost of maintenance due to the formation of slag on the walls and the increased time required to remove the refuse.

A CO₂ recorder is a desirable method of obtaining combustion efficiency when the same type of fuel is fired, but it requires periodical checks to maintain it in proper adjustment.