Cement Plant Waste Heat Recovery

Cement production is one of the most energy-intensive processes in the world. Nearly 50%–60% of the cost of cement is due to energy cost. Energy is consumed in grinding mills, fans, conveyors, and in the process of calcination. Coal used is in the range of 150– 250 kg per ton of cement, and energy used is about 80–125 kWh per ton of cement. A few decades ago, cement plants were venting the waste gases from the preheater and clinker cooler into the atmosphere as they were dusty and were also at low temperatures in the range of 350°C–280°C. Waste heat boilers for such low gas inlet temperatures required large multiple-pressure steam generation to cool the gases to an economical temperature that made the economics unfavorable. However, today, the high cost of energy is forcing all cement plants to recover energy from the preheater and the clinker cooler exhaust gases. The exit gases from rotary kilns, preheater, and calciners are used to heat the incoming feed material, and the gases are cooled to about 300°C–350°C in a four-stage preheater. With more stages, the exhaust temperature can be 200°C–250°C. Part of this gas is used in raw mills and coal mills for drying the coal. The solid clinker coming out of the rotary kiln at 1000°C is cooled to about 120°C using ambient temperature. This generates hot air at about 270°C–310°C. Part of this is used as combustion air in kiln furnaces and rest vented or used in waste heat recovery. Typical flue gas data from a 1500 TPD cement plant is shown later. A scheme of a typical cement plant is shown in Figure 4.32.

A vertical downward gas flow boiler with horizontal tubes is often used for preheater and clinker cooler applications. Vertical tube design with horizontal gas flow natural circulation design is also used. Circulation pumps may not be required if the drum is located properly and adequate streams are used for the evaporator circuit; the tubes may have to be slightly inclined

218 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

TABLE 4.18

Performance with and without Additional Surface

With Additional Surface

General Data Exhaust gas flow

54.00 kg/cm 2 g Exhaust gas temp.

kg/h

Steam pressure

378 ±5°C Exhaust gas pressure

±5°C

Steam temp.

48,806 kg/h Heat loss

1.00 kg/cm 2 a Steam flow

109 ±5°C Firing temperature

Feed water temp.

0 kg/h Burner duty, LHV

0 ±5°C

Process steam

1 % Burner duty, HHV

0.00 MM kcal/h

Blowdown

0.00 % Flue gas flow out

0.00 MM kcal/h

Eco steaming

0 m 2 Process Data

0 kg/h

Steam surface

Surface

Evap. Econ. Gas temp. in, ±5°C

412 286 Gas temp. out, ±5°C

286 206 Gas spht, kcal/kg °C

0.2674 0.3 Duty, MM kcal/h

6321 5183 Gas pr drop, mm wc

Surface area, m 2 187

3.78 3.68 4.66 4.33 40.56 29 Foul factor, gas

0.0002 Steam Side

Steam press., kg/cm 2 g 55 55 55 55 55 55 Steam flow, kg/h

48,806 49,294 Fluid temp. in, °C

265 109 Fluid temp. out, ±5°C

270 265 Pr drop, kg/cm 2 0.3 0.2 0.0 0.0 0 0

Foul factor, fluid

Without Additional Surface

General Data Exhaust gas flow

54.0 kg/cm 2 g Exhaust gas temp.

kg/h

Steam pressure

360 ±5°C Exhaust gas pressure

±5°C

Steam temp

48,446 kg/h Heat loss

1.00 kg/cm 2 a Steam flow

109 ±5°C Firing temperature

Feed water temp.

0.0 kg/h Burner duty

0 oC

Process steam

1.0 % Furnace duty

0.00 MM kcal/h

Blowdown

0.0 % Flue gas flow out

0.00 MM kcal/h

Eco steaming

0 m 2 Process Data

0 kg/h

Steam surface

Surface

Evap. Econ. Gas temp. in, ±5°C

416 290 Gas temp. out, ±5°C

290 213 Gas spht., kcal/kg °C

0.2677 0.2611 Duty, MM kcal/h

5618 4146 Gas press. drop, mm wc

Surface area, m 2 149

3.03 2.96 4.70 4.36 36.22 23.71 Foul factor, gas

0.0002 0.0002 (Continued)

Waste Heat Boilers 219

TABLE 4.18 (Continued)

Performance with and without Additional Surface Steam Side

Evap. Econ. Steam press., kg/cm 2 g 55 55 55 55 55 55

48,446 48,930 Fluid temp. in, C

Steam flow, kg/h

262 109 Fluid temp. out, ±5°C

270 262 Press. drop, kg/cm 2 0.20 0.18 0.00 0.00 0.00 0.19

Foul factor, fluid

Tube Geometry Data with and without Additional Surface

Evap Econ

With Additional Surface Tube OD

38.1 38.1 50.8 50.8 50.8 44.5 Tube ID

30.6 30.6 44.1 44.1 44.1 37.8 Fins/in. or fins/m

0 0 0 78 177 177 Fin height

0 0 0 12.5 12.5 19 Fin thickness

0 0 0 1.9 1.9 1.9 Fin width

0 0 0 0 4.37 4.37 Fin conductivity

0 0 0 35 35 35 Tubes/row

38 38 38 38 38 42 Number of rows deep

5 5 4 2 18 10 Length

8.2 8.2 8.2 8.2 8.2 8.2 Transverse pitch

101 92 Longitudinal pitch

76 76 21 parl = 0, countr = 1

1 1 1 Without Additional Surface

Tube OD 38.1 38.1 50.8 50.8 50.8 44.5 Tube ID

30.6 30.6 44.1 44.1 44.1 37.8 Fins/in. or fins/m

0 0 0 78 177 177 Fin height

0 0 0 12.5 12.5 19 Fin thickness

0 0 0 1.9 1.9 1.9 Fin width

0 0 0 0 4.37 4.37 Fin conductivity

0 0 0 35 35 35 Tubes/row

38 38 38 38 38 42 No deep

4 4 4 2 16 8 Length

8.2 8.2 8.2 8.2 8.2 8.2 Transverse pitch

101 92 Longitudinal pitch

76 76 21 parl = 0, countr = 1

Preheater

Waste heat recovery

water steam

Turbine

Generator

S team G

Waste heat from

the process

Cooling water

(liquid or ﬂue gas)

Condensate pump

Cooling water pump

Cooling tower

Waste heat

Clinker cooler

Typical cement plant heat recovery system.

ee rs

Waste Heat Boilers 221

to the horizontal. Since the gas temperature is quite low to start with, medium-pressure steam in the range of 15–20 barg is generated with steam temperature about 20°C–30°C lower than the gas inlet temperature. It is shown in Chapter 5 on HRSG simulation why with low inlet gas temperature, exit gas temperature from boiler will be high and vice versa. Since a single- pressure unit cannot cool the flue gas to an economical low temperature, low-pressure steam for deaeration is also generated in the clinker cooler boiler. Condensate preheating is also carried out to minimize the steam for deaeration. Many of these streams do not contain acid vapors, and hence, water vapor dew point alone limits the heat recovery.

Since the preheater exhaust gas contains high dust content, rapping mechanisms are used for cleaning the boiler tubes to prevent the dust from settling and fouling the tubes. Fouling factor in the range of 0.002–0.02 m 2 h °C/kcal is used in the design (0.01–0.1 ft 2 h °F/Btu). The nature of fouling is shown in Figure 4.33a. When the cleaning mechanism is used, fouling temporarily reduces and the performance improves for a short duration till the dust settles on the heating surfaces again. The frequency of cleaning is determined by operating experience. A higher gas velocity helps to lower fouling as the dust can get carried away with the flue gas, while too high a gas velocity can also lead to erosion of tubes due to the presence of ash particulates (Table 4.20).

Without cleaning

ouling factor F With cleaning

Steam temperature

300 9500 s 280

Steam ﬂow

260 9000 erature

Exit gas

mp 240 am ﬂow kg/h Te

Eco exit water temperature

Fouling factor m 2 h°C/kcal

FIGURE 4.33

(a) Fouling characteristics in a boiler.- (b) Effect of fouling factor on performance of a waste heat boiler.

222 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

TABLE 4.20

Flue Gas from Preheater and Clinker Cooler

Source

Preheater Flue Gas

Clinker Cooler

Mass flow, kg/h

100,000 Gas temperature, °C

280–370 % volume CO 2 28 0

11 1 N 2 58 78 O 2 3 21

Dust content, mg/N m 3 75–90

Clinker cooler boilers often use finned tubes with fin density varying from 2 to 3 fins/in. as the exhaust gas is mostly air with low dust content. The fouling factor in this boiler often ranges from 0.002 to 0.004 m 2 h °C/kcal (0.01–0.02 ft 2 h °F/Btu). Since fouling factor is a complex factor in the design of any boiler with dusty flue gas, it is often based on experience from the operation of similar units with similar raw materi- als. Presented in Figure 4.33b is the performance of a preheater boiler at various fouling

factors. Gas flow is 145,000 kg/h at 370°C. Gas analysis is % volume CO 2 = 28, H 2 O = 11, N 2 = 58, O 2 = 3. Feed water is at 105°C and steam pressure is 13 kg/cm 2 g. When the boiler is very clean, with a gas-side fouling factor of 0.0002 m 2 h °C/kcal (0.001 ft 2 h °F/Btu) (0.000172 m 2 K/W), it makes 10,187 kg/h steam at 343°C with exit gas temperature of 200°C. As the fouling factor increases to 0.02 m 2 h °C/kcal (0.0172 m 2 K/W), it makes only 7660 kg/h at 321°C with 245°C exit gas temperature. Plant engineers may obtain such performance data from the boiler supplier and monitor the operation to see how the fouling is trending. They may also study the effect of varying the frequency of cleaning using the rapping mechanism. Before ordering the boiler, they may also calculate the actual fouling factor used by the boiler supplier based on the surface area provided and the methods discussed in Appendices A and C.

In order to estimate the actual fouling factor in a fouled boiler, one should have the field data such as the gas flow, temperature at inlet and exit, and the steam-side duty. The U may be computed from the Q, A, and ΔT values. Then the convective heat transfer coefficient in the fol- lowing equation may be obtained (for plain tubes) from which the fouling factor is estimated.

h o In this equation, h i ,h o may be estimated as discussed in Appendices B and C. ff i is known.

U o hd i i

The only unknown is ff o , the outside fouling factor, which may be computed at various loads or periods of time. Typically, a boiler with heavy fouling characteristics such as the preheater flue gas boiler in a cement plant will exhibit a fouling trend as shown in Figure 4.33a. Often, a maximum fouling condition is reached, and all parameters such as steam flow and exit gas temperature remain unchanged. While purchasing boilers, plant engineers should get an idea of their boiler performance as a function of fouling factor as shown and get an idea of all the major operating data under normal and heavily fouled conditions.

The heat balance and steam generation for a typical cement plant generating HP and LP steam are shown in Figure 4.34. The clinker cooler gas boiler consists of the HP superheater, evaporator, and economizer and a condensate heater. The preheater gas boiler consists of HP evaporator and LP superheater and evaporator. The economizer of cooler flue

Waste Heat Boilers 223

% volume H 2 O = 1, N 2 = 78, O 2 = 21

159,000 kg/h 430 % volume CO = 26, H O = 3, N = 67.4, O = 3.6

Clinker cooler gas

253,000 kg/h

358 Preheater waste gas HP superheater

HP evaporator 203 HP evaporator 212

HP economizer 142

LP superheater 209

Condensate heater LP evaporator 181 107

2 26,800 kg/h HP steam at 15 kg/cm 2 a at 400 ° C 3,300 kg/h LP steam at 4.5 kg/cm a at 196°C 13,300 kg/h HP sat, steam

FIGURE 4.34

Gas–steam temperature profiles in a cement plant from cooler gas and preheater gas streams. gas boiler handles the feed water heating of both the boilers and preheats the feed water

from 105°C to 190°C and sends it to the HP drum operating at 15 kg/cm 2 g. The preheater HP evaporator generates about 13,500 kg/h of HP steam and the cooler boiler about the same amount, and the total flow of 26,800 kg/h is superheated in the cooler boiler to 400°C. As mentioned earlier, one has to generate steam at dual pressures to recover additional energy from low gas temperature streams. Hence, in order to improve the overall energy recovery, such combining of water and steam flows is necessary, which is why the Kalina system is more attractive.

Cement Plant Waste Heat Recovery