Where does the Steam come from?
Steam is final product of power plant boiler that converts chemical energy in fuel into heat energy through combustion of fuel in furnace boiler. The heat produced then used for heating up and vaporized feed water in boiler tubes. The heat transfer process in boiler then convert feed water phase into superheated steam with certain pressure and temperature that is required by turbine specification. Well, if we elaborate the system, it can be fully thermodynamically explanation. The simplest way to understand the energy conversion in the system is with Rankine cycle as given below.
Okay, we leave for a moment the thermodynamic explanation in this posting. We will go through coal fired power plant process description. The Process can be clustered into some big system, so we can easily understand the process part by part, that is:
1. Water and Steam System
2. Fuel Feeding System (Solid Fuel and Fuel Oil)
3. Combustion Air System
4. Auxiliary System (Lime Stone Feeding, Sand System, Soot blowing System, etc)
The boiler operation system described below is Circulating Fluidized Bed (CFB) Boiler, where have different boiler part from other type of boiler.
Water and Steam System
The feed water coming into boiler from mill condensate and make up water from demineralization plant, then it is collected in feed water tank. The purpose of water system is to transfer feed water into boiler while it is in operation, also to raise operating temperature of feed water as per required by boiler operation. The oxygen dissolved in feed water must be removed in feed water and de-aerator system because it can be harmful for boiler tube operation.
The feed water is pump from the feed water tank flowing into economizer and heating up to a certain temperature, for example 230 C depend on boiler specific design. In economizer, preliminary heat transfer in boiler is taken place with utilizing flue gas that is flowing out from furnace in counter current way, so it will reduce the flue gas temperature outlet and increase water temperature in economizer. The thermal energy of flue gas is recovered in economizer.
From the Economizer, water is introduced to the steam drum in upper part of boiler. The steam drum purpose is to separate water and steam in the system, and operate a natural circulation of water/steam circulation in boiler. In boilers with natural circulation the circulation of water and steam/water mixture is based on the change of specific density, i.e. the heavier steamless water column in the downcomers presses the lighter steam/water mixture from the steam generating pipes to the drum.
Water from the steam drum is flowing down via down comers through furnace wall and raise up to through loop seal and cyclone wall (might be different in other boiler type) then back to the steam drum as water and steam mixture. From the steam drum the saturated steam flows to the superheaters. The superheated steam temperature is controlled in two-stage steam attemperators by spraying feedwater to steam flow.
Berhubung sudah mulai lelah, ceritanya saya dilanjutkan lain waktu deh.. pembahasan yang di atas pun masih sangat umum, tiap equipment yang tersebut di atas masih punya segudang cerita, baik dari deskripsi prosesnya, aspek termodinamika, mekanika fluida, instrumentasi, control system, dll. Yang pasti bakalan gak habis2 dibahas dalam semalam.
Let’s share the ideas!
Tofan Azhar Hakim