Reducing Fuel Costs Via Improved Boiler Feedwater Quality

A certain amount of water must be removed, either continuously or periodically, from an operating boiler in order to limit the concentration of impurities in the boiler water. Because this water, commonly called blowdown, has been heated, it represents a certain heat value imparted by the fuel, but not available to make steam. It follows, therefore, that if the amount of blowdown can be reduced, a corresponding reduction in fuel costs can be realized for the same steam generation rate. At current fuel costs, even a slight reduction in blowdown can represent a significant cost savings.

DETERMINATION OF CONCENTRATION LIMITS

Blowdown rate is expressed as a percent of total feedwater demand. Thus, a 5% blowdown rate means that 5% of the boiler feedwater is lost through blowdown and the remaining 95% is converted to steam. The blowdown rate (expressed as a decimal) is calculated as the reciprocal of the number of times the feedwater is concentrated in the boiler. For example, if the feedwater can be concentrated twenty times, the blowdown rate required will be 1/20 or 0.05 or 5%. Since the only reason for blowdown is to limit the concentration of undesirable impurities in the boiler water; and because these impurities originate in the feedwater, it follows that if feedwater impurities can be reduced, the feedwater can be concentrated more times. Higher feedwater concentrations, in turn, result in a lower blowdown rate. Thus, if feedwater quality can be improved such that it can be concentrated thirty times rather than twenty, the blowdown rate can be reduced from 1/20 or 5% to 1/30 or      3-1/3%.

Certain feedwater impurities must be limited due to their tendency to form deposits in the boiler. Calcium and magnesium (also known as hardness) fall into this category. Other impurities, if allowed to concentrate too high, can result in boiler water carryover. Alkalinity and total dissolved solids are the primary offenders in this case. Table 1 summarizes generally accepted limits for various feedwater constituents. This table can be used to estimate the maximum allowable feedwater concentrations, based on the present boiler feedwater quality. To do this, analyze the feedwater for all the listed constituents. For greatest accuracy, analyze several samples or obtain a twenty-four hour composite. Next, divide the maximum boiler water value indicated in the table by the feedwater value (in ppm) for each constituent. Take the lowest of these quotients as the actual concentration limit.

The following example illustrates the procedure:

Plant Data: Package water tube boiler operating at 100 psig with no turbine drives

Analysis

ppm in Feedwater

Maximum Allowed

Concentration

Limit

Total alkalinity as CaCO3

70

700

10

Silica as SiO2

4

150

37

Total Dissolved Solids

250

3000

12

The limiting constituent in this case is the total alkalinity, which can be concentrated only ten times. In this case, a blowdown rate of 1/10 or 10% would be required to maintain the boiler water quality within recommended limits.

CONDENSATE RECOVERY

Boiler feedwater consists of returned condensate plus whatever amount of make-up water is required to satisfy the boiler demand. Condensate, unless contaminated, is quite low in dissolved solids. Thus, it adds very little in the way of impurities to the feedwater. Makeup water, on the other hand, usually contributes most of the feedwater impurities. Hence, it is helpful to think of condensate as diluting the makeup water impurities; and the greater amount of condensate returned, the better the feedwater quality.

That said, it is best practice to check the condensate quality to determine the level of impurities. Some condensate can be contaminated with iron or process contaminates, which may adversely affect the feedwater quality.

Of course, there will always be a certain amount of condensate that cannot be recovered for various reasons. Also, some condensate sources may be unsuitable for reuse due to unavoidable contamination. Contaminated condensate should be segregated from other plant condensate as close to the source as possible. All good quality recoverable condensate should be returned to the feedwater system.

CALCULATION OF FUEL SAVINGS

In the example above, let’s assume that the feedwater consists of 60% condensate and 40% makeup. By recovering additional condensate, the feedwater is improved, resulting in a lower blowdown rate. For example, if additional condensate recovery results in a feedwater of 67% condensate rather than 60%, the total alkalinity will be reduced from 70 ppm to 58 ppm. This permits an increase in feedwater concentration from 10 to 12. The blowdown rate can then be reduced from 1/10 or 10% to 1/12 or 8-1/3%. The actual blowdown and feedwater requirement in pounds can be calculated as follows:

Assume a steam production rate of 1,000,000 pounds per day. Then,

F = S/(1 – %BD)

Where:

F = feedwater demand in pounds per day

S = steam generation rate in pounds per day

%BD = percent blowdown expressed as a decimal

Using this formula we can calculate the impact reduced blowdown has on the feedwater demand.

At 10% blowdown, F = 1,111,110 pounds per day

At 8-1/3% blowdown, F = 1,090,870 pounds per day

The difference in feedwater demand represents the actual blowdown reduction.

1,111,110 – 1,090,870 = 20,240 pounds per day

We can now calculate the fuel cost savings by applying the following equation:

$ Savings = (br x H) / (V x %E)

Where:

br = blowdown reduction in pounds per day

H = heat content of blowdown in B.T.U. per pound

C = cost of fuel in $ per unit

V = heating value of fuel in B.T.U. per unit

%E = boiler efficiency expressed as a decimal

Using our former example and burning No. 6 fuel oil with a heating value of 142,440 B.T.U. per gallon at a cost of $3.80 per gallon, we can calculate the following daily savings:

20,240 lbs/day x 309 Btu/lb x $3.80 = $208.56 / day

(142,440 Btu/gal x 0.80)

As shown in this example, by increasing condensate return by a modest 7% of feedwater, a significant savings has been realized. Also, the heat content of the returned condensate would yield additional savings.

These calculations are based on the assumption that blowdown heat recovery is not being utilized and that the hot blowdown is going to drain. A blowdown heat recovery system would, of course, reduce the potential savings.

CONCLUSION

Overall, the quality of the boiler feedwater has a significant impact on boiler efficiency. By improving boiler feedwater quality, such as returning more condensate, boiler blowdown requirements can be reduced, which translates into significant fuel cost savings.

TABLE I

Drum pressure

TDS, ppm

Total Alkalinity

Suspended Solids

Silica

Firetube Boiler

With Turbines

Without Turbines

0 – 300

3500

700

800

100

150

301 – 450

3000

600

400

75

90

451 – 600

2500

500

40

60

601 – 750

2000

400

25

30

TDS = total dissolved solids in ppm

Total alkalinity is expressed as ppm calcium carbonate

TABLE II

Pressure, psig

Heat of Saturated Liquid, Btu/lb.

Pressure, psig

Heat of Saturated Liquid, Btu/lb.

10

208

140

333

15

219

160

344

20

228

180

353

25

236

210

366

30

243

235

376

40

256

260

385

50

267

285

394

60

277

335

410

70

287

385

424

80

294

435

437

90

302

485

450

100

309

585

472

120

322

685

493

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