a closed water reuse system for power plant cooling and digester heating

WATER RESOURCES BULLETIN VOL. 13, NO. 6 AMERICAN WATER RESOURCES ASSOCIATION DECEMBER 1977

A CLOSED WATER REUSE SYSTEM FOR POWER PLANT COOLING AND DIGESTER HEATING'

Yishu Chiu and William Guey-Lee'

ABSTRACT: A model consisting of closed water reuse and productive use of various types of wastes for energy generation is presented. The sewage after treatment would be used as the cooling water for power plants, and the condenser discharge therefrom be used as heating water for sludge digesters. The water is then purified for municipal water supply for continuous use. The advantages of this system are that water resources and energy are conserved while various types of wastes including waste heat are controlled.

With a preliminary system analysis, it appears that the design for power plant based on the total heating value of wastes and digester capacity based on sewage sludge generation is feasible in terms of acquisition and full utilization of various types of wastes as generated in a single metropolitan area. The system as shown in this design is in balance among various factors such as the generation rate of municipal refuse, municipal sewage, waste heat in the condenser discharge, and raw sewage sludge. (KEY TERMS: water reuse; waste heat utilization; digester heating; energy generation; pollution control.)

INTRODUCTION

Due t o increase of human activities and the growth of industries, the total amount of water required is increasing steadily. Where power industries are concerned, the amount of water required for cooling has doubled in less than ten years. By the year of 2000, it is estimated that all the surface water in the United States will be withdrawn for cooling purposes if once-through cooling is used. Cooling water effluent usually contains a significant amount of waste heat; if discharged t o environment, it not only causes thermal pollution but all of the recoverable heat energy is lost. With an increasing share of the power generation from nuclear plants and with their lower thermal efficiency, the total amount of waste heat will become much larger. The yearly waste heat generation is ex- pected to be increased to 2x1Ol6 Btu by 1980 (Jaske,etal., 1970).

The cooling water discharge, with a temperature, only 10-20°F higher than the receiv- ing water, is actually not hot enough for use in ordinary heat exchangers. However, it has

'Paper No. 77036 of the Water Resources Bulletin. Discussions are open until August 1, 1978. 'Respectively, Formerly Assistant Professor, Department of Civil, Mechanical, and Environmental

Engineering, George Washington University, Washington, D.C. 20052; Present address, Nihon Suido Consultants Co., Ltd., 10 Nishiokubo 3Chome, Shinjuku-ku, Tokyo, Japan; and Civil Engineer, District of Columbia Department of Housing and Community Development, Washington, D.C. 20005.

1203

1204 Chiu and Guey-Lee

been successfully utilized for aquaculture t o grow fish (Nash 1970) and for irrigation, building heating, etc. An alternative is t o use cooling water discharge as a heating source for a sludge digester t o produce fuel gas (Chiu, 1974). In this regard, a conceptual model consisting of closed water reuse and productive use of waste heat along with other types of wastes for energy generation is shown in Figure 1 . For closed water reuse, treated sewage would be used as cooling water for power plants, and the condenser discharge therefrom for heating water for sludge digesters. The water is then purified for municipal supplies and the sewage generated would be treated and recycled for continuous use. In the digester, sludge separated from sewage effluent would be treated t o generate methane gas, while steam produced by combustion of refuse, along with waste oil and digested sludge, would be used to generate electricity in the power plant. Advantagesof thissystem are that water and energy are conserved while various types of waste, including heat, are controlled. A preliminary analysis of the model, based on a population of one million living in a metropolitan area is presented in this paper.

DESIGN CONSIDERATIONS

The system design will generally concentrate on the areas of power generation and sludge digestion as related t o the reuse of water along with various types of wastes. The sizing of power plant will be based on either total heating value of wastes or total power requirement for one-million population, while the digester capacity will be on the basis of either fresh sludge inflow or the total amount of waste heat available in the cooling water discharge. Formulae required for calculations toward the amount of waste heat carried off by the cooling water discharge as well as its temperature increase are listed in the following.

Total refuse:

Total waste oil:

Total digested sludge solids:

Total cooling water:

Q = q x N

Total power requirement:

P = p x N

A Closed Water Reuse System for Power Plant Cooling and Digester Heating 1205

waste oil I I STEAM I I I I

I

Thermal combustion

refuse solid waste 11111111

I I

3 ' i 5 1 0 1 U I 9 1 .P 1 u 1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

m 3

Sewage treatment

I I

I I FUELGAS

digestion

~f91111911119111999199999 digested sludge r

Figure 1 . A Conceptual Model Consisting of Closed-Loop Water Reuse and Productive Use of Waste Heat Along with Other Types of Wastes.

Total power generation from wastes:

Pw = Wref'ref + WoilHoi1 + WDSSHDSS)~


Total waste heat from power generation:

or

H = ('-e)Pw e

Heat gained by cooling water:

HCW = r x H

Temperature increase of cooling water:

HCW A T = - QC

Where:

N = population

= 1x106

wref = per capita generation rate of refuse

= 6.0 lb/capita/day (assumed)

= per capita generation rate of waste oil

= 0.086 lb/capita/day (assumed) woiI

wDSS = per capita generation rate of digested sewage sludge

= 0.10 Ib/capita/day (assumed)

q = per capita flow of sewage

= 100 gal/capita/day (assumed)

P = per capita power requirement

= 2 1 kwh/capita/day (assumed)

Href = heating value of refuse

= 6,000 Btu/lb (assumed)

Hoil = heating value of waste oil

= 100,000 Btu/gal= 15,000 But/lb (assumed)

A Closed Water Reuse System for Power Plant Cooling and Digestcr Heating 1207

HDSS = heating value of digested sludge

= 5.500 Btu/lb (assumed)

e = efficiency of thermal power plant

= 0.40 (assumed)

r = percentage of waste heat carried off by cooling water

= 85% (assumed)

C = specific heat of water = 1 .OO Btu/lb/'F = 8.33 Btu/gal/'F

Digester Capacity

The key element in the whole system design is the digester. It is designed t o use the waste heat in the condenser cooling water discharge t o heat the incoming sewage sludge as well as other organic wastes and to recover fuel from the digester gas phase. Depending on the types of digestion and the total capacity of digesters, the extent of waste heat utilization and fuel gas generation is different. An ideal digester would carry out the digestion process in a completely mixed, continuous-flow system. To conform to the ideal system, a high-rate digestion process will be considered herein.

Digester capacity is basically a function of sludge flow rate and detention time. Sludge detention time depends on the digestion temperature and can be decreased as temperature is increased. The digestion volume can be computed from:

where:

V = digestion volume

Qs = sludge inflow rate

t = detention time

Digester Heating

The heat requirement for the digester is a function of the inflow sludge temperature, the heat loss incurred by the digester, and the digestion temperature. To heat the digester, the hot condenser discharge will be circulated through pipe coils located inside the tank close to the wall. The area of the heating pipe in contact with sludge inside the digester can be determined by (Babbit and Baumann, 1958):


where :

Ap Q, = sludge inflow rate

C, = specific heat of sludge

A = surface area of digester tank

U1

U2

T = digestion temperature

Ta = ambient air temperature

Ts = inflow sludge temperature

Tw

= contact area of pipe

= overall heat transfer coefficient for digester tank

= overall heat transfer coefficient for pipe coils

= mean hot water temperature

The total length of heating pipe is therefore

L = - AP 3.14d

where:

d = pipe diameter

The value of overall heat transfer coefficient for pipe coils, U 2 , has been reported to vary from 10 to 39 Btu/hr-ft**'F. However, it can be as high as 3 5 0 Btu/hr.ft2='F with new forced convection heat exchanger, or as low as 4.5 Btu/hr.ft'*OF for an old pipe with scaling (Eckenfelder and O'Connor, 1961).

PRELIMINARY SYSTEM DESIGN

For a preliminary system design, analysis is made according to the following three cases. The digester will be designed on the basis of high-rate digestion with detention time of 10-1 5 days and digestion temperature of 90-95'F.

Case 1. Power Plant Based on the Total Heating Value of Wastes and Digester Capacity Based on Sewage Sludge Generation

From Equations ( I ) , (2), (3), and (6), the electric energy generation rate for the wastes is calculated in the following using values previously given to each parameter and the con- stant.

A Closed Water Reuse System for Power Plant Cooling and Digester Heating 1209

Pw = [(6.0~6000 + 0.086~1500 + 0.1~5500) Btu ] (lo6 persons) (0.4) capitaoday

= 1.5x1010 = 180,000kw day

This is the capacity of the power plant that can be accommodated on the basis of wastes as fuel.

With 40% power plant efficiency and 85% of waste heat carried off by the cooling water, the increase in the water temperature is calculated applying Equations (71, (8), and (9) as follows:

0.85( - 0.40 ) ( 1 80.000 kw) 0.40 = 2 . 7 ~ ] 0 - ~ (kw.day) OF = 33oF

( 1 00 MCD) (8.33 a gal. OF

Btu

Assuming a sludge generation rate of 0.07 cu ft/capita/day, then the total generation rate from metropolitan area of one million population is 7x104 cu ft/day. With detention time of IS days. the digester capacity based on Equation ( 1 0) will be:

V = ( 7 ~ t 0 ~ ~ f t ) ( I S d a y s ) = 1 . 0 5 ~ 1 0 6 c u f t day

Considering 3057 allowance. the capacity requirement is increased t o 1 .3x106 cu ft. Heat requirement for digester heating varies according t o the season. The total length

of heating pipe can be determined from Equations ( 1 1) and ( 1 2). On the basis of a summer water temperature of 70°F and a digestion temperature of 90'F. approximately 18,000 ft of heating pipe are required, provided that d = 6 in., U1 = 0.1 5 Btu/hr.ft'*'F, and U 7 = 2 5 Btu/hr*ft2*OF. In winter when the incoming cooling water temperature drops below 70°F. a portion of the cooling water inflow must be diverted in order to attain the required heating water temperature.

Case 2 . Capacity Based on Sewage Sludge Generation

Power Plant Based on Power Requirement for the Population and Digester

From Equation ( 5 ) . the total power requirement for one million population is:

P = (21 kwh ) ( l o 6 persons) = 2.1 x I O7 kwh = 880.000 kw capitamday day

The heat input requirement for the power plant is obviously much greater than can be accommodated by wastes alone. Solid waste will be considered only as a supplement to fossil fuel. Assuming a heating value of 14,000 Btu/lb. approximately 6.000 tons of coal are required t o be mixed with the solid waste each day.


The increase in cooling water temperature is:

O M ( - 0.40 )(880,000 kw) 0.40 = 1 . 3 ~ 1 0 - ~ (m ) OF = llO°F

(100 MGD)(8.33 L) Btu gal*OF

For a digestion temperature of 90°F, the temperature required for heating water will be sufficient throughout the year. The total length of the heating pipe required would be 7,000 ft, which is designed on the basis of possible lowest inflow cooling water temperature of 25'F.

Case 3. Power Plant Based on Power Requirement for the Population and Digester Capacity Based on the Total Available Waste Heat

As in Case 2, the power plant capacity is 880,000 kw, while the digester will have a capacity to handle other organic wastes in addition to sewage sludge. The maximum amount of organic wastes, including sewage sludge that can be handled, is determined by the following heat balance equantion:

where:

Q = heating water flow rate

Q, = sludge flow rate

TwI = inlet temperature of heating water

TWO = outlet temperature of heating water

T = digestion temperature

Ts HL = heat loss

= incoming organic wastes temperature

= 0.3 Q(TW1 - Two) (assumed)

Design on the basis of possible lowest incoming sludge temperature of 25'F, the total capacity of digesters that can be sustained by the total available waste is estimated to be 1x108 cu ft, providing a digestion temperature of 90°F. Capacity is reduced by increasing digestion temperature.

DISCUSSION AND CONCLUSIONS

A theoretical examination of the above three cases based on energy recovery shows that Case 3 is an optimum situation because it utilizes all of the available waste heat in the condenser discharge and provides for the maximum energy recovery from the digesters.

A Closed Water Reuse System for Power Plant Cooling and Digester Heating 121 1

Since sewage sludge represents only about 1% of the total digester capacity, the remain- ing capacity can be used for other organic wastes such as those from animals. Based on the human population equivalent, the per capita generation rate of animal wastes is estimated at 5 cu ft/day, which is approximately 70 times that of sewage sludge. If the system is designed for a metropolitan area, such a great quantity of organic wastes may not exist within the area. Even if there are enough wastes, problems could also arise with the transportation of materials such as animal wastes from many diverse areas t o a cen- tral digester location.

Case 1 is an alternative which appears to b e feasible in terms of acquisition and full utilization of various types of wastes generated in a single metropolitan area. This system is balanced among various factors such as the generation rates of municipal refuse, municipal sewage, waste heat in the condenser discharge, and raw sewage sludge. A "balance sheet" for the Case 1 system is shown in Table 1. To summarize, 100 MGD of treated sewage effluent are used to cool the power plant, which primarily use municipal refuse as fuel. The total wastes have a heating value of 38 billion Btu per day or 1.6 million Btu per hour, which generate 180 thousand kw of electricity and 980 Btu/hr of waste heat; and 85% of the waste heat is carried off by the cooling water, which results in a temperature rise of 23'F. The resulting condenser discharge would be hot enough during warm seasons t o heat the digesters, which would ferment a total daily volume of 70 thousand cu ft of sewage sludge at a temperature between 90 and 95'F t o generate approximately 1 million cu ft of fuel gas with a heating value of 600 million Btu. Di- gesters with a total capacity of 1 million cu ft would be required rfor this system. Total length of pipe coils required for circulation o f heating water is inversely proportional to the pipe diameter. For a 6-inch pipe, assuming a digestion temperature of 90°F and inflow heating water temperature of 93'F, approximately 18,000 f t of pipe are required. If the inlet water temperature is higher than 93'F, then a bypass system which will chan- nel a portion of the heating water around the digester is required. In order t o obtain heating water of higher temperatures during cold seasons (assuming higher than 93'F), a portion of the cooling water would have to be diverted and bypass the condenser. For example, with a cooling water temperature of 6S°F, 20% of the flow needs t o be diverted. If the temperature is decreased to 40°F, about 60% diversion is required. The relationship between the cooling water temperature and the fraction of flow used for cooling is shown in Figure 2 .

Thus the system is becoming operative by regulating the water flowing through the power plant condenser and/or the sludge digester.

An experimental investigation on the use of condenser discharge as a heating source for the sludge digester was conducted by introducing hot water at various temperatures ranging from 130°F to lOOOF into a spiral coil inside a bench scale tank. Depending on the conditions. the temperature of incoming raw sludge can be improved by 30'F.

In order t o maximize the net profit from sale of fuel gas, the system may be optimized with respect to sludge digestion. Parameters involved in this optimization are detention time, digestion temperature, and percentage of methane contained in the digester gas. All three of these parameters are somewhat interrelated. In general, as digestion temperature increases, sludge detention time decreases. However, raising the temperature not only increases the total heat energy required, but also decreases the percentage quantity of methane gas in relation to the total production of the digester gas. Instead of 90°F, which is the digestion temperature arbitrary selected here, an optimum temperature of digestion has t o be established.


TABLE 1. Values of Variables and Outputs for an Area of One Million Population.

Term Value

Refuse generation

Waste oil generation

Digested sludge generation

Total heating value

= Total heat input to the power plant

= (l)x6000 + (2)x15000 + (3)x5500 - -

- -

Waste heat from the power plant

= (4) x 0.60 = - -

Waste heat carried off by cooling water

= (5) x 0.85

Cooling water flow - -

Temperature gained by cooling water

Sludge inflow to the digester

Digestion temperature

Sludge detention time

Digester capacity

Energy output:

a) Power plant = (4) x 0.40

b) Digester

6 6.0 x 10

8.6 x lo4

1.0 lo5

I0

9 3.8 x 10

1.6 x 10

4.7 lo5

2.8 lo5

9.6 x 10’

8.2 x I O U 1.0 lo3

7.0 lo4

4.2 x lo6

2.3 x 10

9.0 x 10

1.5 x 10

1.3 x 10 6

5 1.9 x 10

6.0 x lo8

Ib/day

Ib/day

Ib/day

Btu/day

Btu/hr

kw

kw

Btulday

Btu/day

MGD

gal/hr O F

ft3/day OF

ft 3

days

kw

Btu/day

In conclusion, it is feasible to heat the sludge digester by use of hot condenser discharge. The major problem is sludge caking on the heating coils when hot water temperature exceeds 135’F in summer. This effectively reduces the heat transfer quality of the pipe. Steps could be taken t o reduce the input heating water temperature, such as pond- ing the water and dissipating waste heat into the air.

ACKNOWLEDGMENTS

This study is a part of the project entitled “Environmental Pollution Control System with Energy Recovery and Water Reclamation,” sponsored by the National Science Foundation under the progam of Engineering Research Initiation Grants 1975.

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LITERATURE CITED

Babbit, H. E. and R. Baumann, 1958. Sewerage and Sewage Treatment. John Wiley and Sons, New

Chiu, Y., 1974. Thermal Pollution and Methane Gas. Chemical Engineering Progress, Vol. 70, p. 14. Eckenfelder, W. W., Jr. and D. J. O’Connor, 1961. Biological Waste Treatment. Pergamon Press,

Jaske, R. T., J . F. Fletcher, and K. R, Wise, 1970. Heat Rejection Requirements of the U.S. Chemical

Nash, C. E., 1970. Marine Fish Farming. Marine Pollution Bulletin, Vol. 1, pp. 28-30.

York.

Oxford, England.

Engineering Progress, Vol. 66, pp. 17-22.

a closed water reuse system for power plant cooling and digester heating

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