Process engineering of steam boilers

All the steam boilers in conventional steam power plants are water-tube boilers, which can be divided into natural circulation (NC), forced circulation (FC), once-through (OT) and combined circulation boilers, which are basically a combination of FC and OT boil-ers. (Teir et al. 2002, p.6 & p.21) The NC and FC boilers can only be used in subcritical

2002, p.6 & p.17) One important design variable of steam boilers is the circulation number, which is defined as the ratio of the amount of water evaporating within the steam boiler and the total amount of water-steam mixture circulating in the evaporator.

(Huhtinen et al. 1994, p. 110) In NC boilers, the circulation number is between 5 and 100, in FC boilers it is between 3 and 10, and in OT boilers it is 1. In other words, the feedwater circulates over five times in NC boilers before it is evaporated, whereas all the feedwater is evaporated in OT boilers. Thus, the amount of feedwater inside the boiler is the largest in NC boilers and smallest in OT boilers.

Natural and forced circulation boilers include a steam drum in order to separate the pro-duced steam from water based on their density difference (Figure 25). The density dif-ference between water and steam decreases, as the steam pressure increases and in the critical point (p = 221 bar) the density is same for water and steam. Therefore, NC and FC boilers are not suitable for supercritical units, as the steam drum cannot be operated in supercritical pressures. (Huhtinen et al. 1994, p.106) In addition, steam drum boilers are sensitive to pressure variations, as pressure decrease at the steam drum releases en-ergy from the steam drum and vice versa. However, the steam drum can act as buffer to small load changes due to its capability to store energy on structure of the drum and its water volume. (Teir et al. 2002, p.7) In both boilers, the steam drum is also used to blow out some of the boiler water in order to prevent the formation of boiler scale (Huhtinen et al. 1994, p.103).

Figure 25. Boilers with steam drum a) NC boiler and b) FC boiler. Adapted from Huhtinen et al. 1994, p.105 & p.111.

In natural circulation boilers, the circulation between the steam drum and the evaporator is based only on density difference between the steam and water. Thus, the pressure of the live steam needs to be less than 170 bar. NC boilers are the oldest technique of wa-ter-tube boilers, which configuration is also simpler, and they are more tolerant to feed-water impurities than other feed-water-tube boilers. Therefore, the investment costs and in-ternal consumption of NC boilers are lower, and they are more reliable than other water-tube boilers. On the other hand, NC boilers are slower in start-ups and shutdowns than other water-tube boilers due to large volume of water inside the boiler. Furthermore, the pipes of the NC boilers are larger than in FC boilers. Thus, the NC boilers need to be accurately designed and the heat surfaces should be optimized in order to minimize the required amount of space and steel. (Teir et al. 2002, p.6-8)

In forced circulation boilers, the circulation between the steam drum and the evaporator is based on a circulation pump, which assures the circulation in the evaporator. Thus, the live steam pressure can be greater than 170 bar, but it still needs to be less than 190 bar, as the separation of steam and water in the steam drum is still based on density difference. The increased live steam pressure increases the attainable power output.

However, the forced circulation pump needs to be installed directly under the steam drum in order to prevent cavitation in the pump. In addition, the circulation pump in-creases the internal consumption and the investment costs of the boiler. Furthermore, feedwater quality has to be higher in FC boilers than in NC boilers due to the circulation pump, which is sensitive to boiler scale. Moreover, the recirculation pump needs to be co-operated with the feedwater pump. Thus, the reliability of the FC boilers is lower than NC boilers. (Teir et al. 2002, p.14-15)

Once-through boilers do not include a steam drum. Thus, the amount of evaporated feedwater is directly proportional to the power output of the plant. In addition, the OT

Figure 26. Once-through boilers a) Benson and b) Sulzer boiler. Adapted from Huhtinen et al. 1994, p. 115-116.

In Benson boiler, the end point for evaporation is transient, whereas the end point for evaporation can be fixed in Sulzer boiler due to a water separation flask. Originally, the water separation flask was used to demineralize the boiler water by blowing some of the water out, but the development of water treatment technologies has eliminated this re-quirement. Nowadays, the water separation flask is used in start-up and partial load sit-uations, and it is also installed to Benson boilers to ease the operation of the boiler.

(Huhtinen et al. 1994, p. 114-116) Due to smaller water volume inside the OT boilers, they are quicker in start-ups and shutdowns than steam drum boilers. However, the con-trol of OT boilers needs to be more sophisticated than steam drum boilers, and the feedwater pump has to overcome large pressure losses of the boiler, which increases the internal consumption. (Teir et al. 2002, p.18-20) The overall advantages and disad-vantages of NC, FC and OT boilers are presented in the following Table 1.

Table 1. The advantages and disadvantages of natural circulation, forced circulation and once-through boilers (Teir et al. 2002, p.7-20; Huhtinen et al. 1994, p.112).

Boiler Advantages Disadvantages

NC - Steam drum can act as “buffer” for small load changes

- Large circulation number makes NC slow in start-ups and shutdowns

- Larger pipes increase the requirement for space and steel. Thus, boiler needs to be ac-curately dimensioned

FC - Steam drum can act as “buffer” for small load changes

- Low circulation ratio (3-10)

- Circulation pump assures the circula-tion in the evaporator

- Wider power range than NC boilers - Smaller tube diameters than NC boilers

- Sensitivity to pressure variations

- Location of the circulation pump needs to be directly under the steam drum in order to prevent cavitation in the pump

- Higher internal consumption and investment costs than NC boilers

- Higher feedwater quality than in NC boilers - Control and regulation of the co-operation of feedwater pump and circulation pump - Lower reliability than NC boilers OT - Suitability for supercritical pressures in

larger units

- Evaporation is directly proportional to power load demand

- Quicker in start-ups and shutdowns than NC and FC boilers

- No capacity buffer due to lack of steam drum

- More sophisticated control regime is re-quired due to small water/steam volume - High pressure losses in the boiler which causes high internal consumption of the feedwater pump

As a conclusion of the process engineering of steam boilers, the NC boilers are the most reliable of the water-tube boilers, as its configuration is the simplest. In addition, its internal consumption is the smallest. Furthermore, NC and FC boilers can also store some energy to their steam drums in order to apply it to small load changes, whereas OT boilers cannot store any energy, as the evaporation is directly proportional to the power output of the plant. In addition, control of NC boilers is simpler than in FC and OT boilers. However, NC and FC boilers are slower at start-ups and shut downs and more sensitive to pressure variations than OT boilers.