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The total heat available from the flue gas for steam generation becomes

(32,800 lb.W.G.) x (360 Btu/lb.W.G.) = (11.8 x 106 Btu/h)

The amount of steam that may be generated is determined by a thermodynamic heat balance on the steam circuit.

Enthalpy of steam at 300 psig saturated h3 = 1203 Btu/lb

Enthalpy of saturated liquid at drum pressure of 300 Psig hf = 400 Btu/lb

Enthalpy corresponding to feedwater temperature of 200°F h1= 188 Btu/lb

For this example, assume that boiler blowdown is 10% of steam flow. Therefore, feedwater flow through the economizer to the boiler drum will be 1.10 times the steam outflow from the boiler drum. Let the steam outflow be designated as x. Equating heat absorbed by the waste-heat-steam generator to the heat available from reducing the flue-gas temperature from 1562°F to 285°F yields the following steam flow:

Therefore, steam flow, x = 11,388 lb/hr feedwater flow = 1.10(x)= 1.10(11,388)= 12,527 lb/hr boiler blowdown = 12,527 - 11,388 = 1,139 lb/hr

Determine the equivalent fuel input in conventional fuel-fired boilers corresponding to the waste heat-steam generator capability. This would be defined as follows:

Fuel input to conventional boilers

= (output)/(boiler efficiency)

Therefore,

Fuel input = (11.8 x 106 Btu/h)/(0.85) = 13.88 x 106 Btu/h

This suggests that with the installation of the waste-heat-steam generator utilizing the sensible heat of the reformer furnace flue gas, the equivalent of 13.88 x 106 Btu/hr of fossil-fuel input energy could be saved in the firing of the conventional boilers while still satisfying the overall plant steam demand.

As with other capital projects, the waste-heat-steam generator must compete for capital, and to be viable, it must be profitable. Therefore, the decision to proceed becomes an economical one. For a project to be considered life-cycle cost effective it must have a net-present value greater than or equal to zero, or an internal rate of return greater than the company's hurdle rate. For a thorough coverage of economic analysis, see Chapter 4.

5.3.4 Load Balancing

Energy Conservation Opportunities

There is an inherent variation in the energy conversion efficiencies of boilers and their auxiliaries with the operating load imposed on this equipment. It is desirable, therefore, to operate each piece of equipment at the capacity that corresponds to its highest efficiency.

Process plants generally contain multiple boiler units served by common feedwater and condensate return facilities. The constraints imposed by load variations and the requirement of having excess capacity on line to provide reliability seldom permit operation of each piece of equipment at optimum conditions. The energy conservation opportunities therefore lie in the establishment of an operating regimen which comes closest to attaining this goal for the overall system in light of operational constraints.

How to Test for Energy Conservation Potential

Information needed to determine energy conservation opportunities through load-balancing techniques requires a plant survey to determine (1) total steam demand and duration at various process throughputs (profile of steam load versus runtime), and (2) equipment efficiency characteristics (profile of efficiency versus load).

Steam Demand

Chart recorders are the best source for this information. Individual boiler steam flowmeters can be totalized for plant output. Demands causing peaks and valleys should be identified and their frequency estimated.

Equipment Efficiency Characteristics

The efficiency of each boiler should be documented at a minimum of four load points between half and maximum load. A fairly accurate method of obtaining unit efficiencies is by measuring stack temperature rise and percent O2 (or excess air) in the flue gas or by the input/ output method defined in the ASME power test codes. Unit efficiencies can be determined with the aid of Figure 5.3, 5.4, or 5.5 for the particular fuel fired. For pump(s) and fan(s) efficiencies, the reader should consult manufacturers' performance curves.

An example of the technique for optimizing boiler loading follows.

Example: A plant has a total installed steam-generating capacity of 500,000 lb/hr, and is served by three boilers having a maximum continuous rating of 200,000, 200,000, and 100,000 lb/hr, respectively. Each unit can deliver superheated steam at 620 psig and 700°F with feedwater supplied at 250°F. The fuel fired is natural gas priced into the operation at $3.50/106 Btu. Total plant steam averages 345,000 lb/hr and is relatively constant.

The boilers are normally operated according to the following loading (top of following page).

Analysis. Determine the savings obtainable with optimum steam plant load-balancing conditions.

a) Establish the characteristics of the boiler(s) over the load range suggested through the use of a consultant and translate the results graphically as in Figures 5.11 and 5.12.

b) The plant determines boiler efficiencies for each unit at four load points by measuring unit stack temperature rise and percent O2 in the flue gas. With these parameters known, efficiencies are obtained from Figures 5.3, 5.4, or 5.5. Tabulate the results and graphically plot unit efficiencies and unit heat inputs as a function of steam load. The results of such an analysis are shown in the tabulation and graphically illustrated in Figures 5.11 and 5.12.

(Unit input) = (unit output)/(efficiency)

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