1.1 Boiler Water Treatment
1.1.1 What is a boiler?
A boiler is a closed in which a liquid is heated and changed into vapour. The steam produced is either used to produce electricity in turbines or used in other processes in factories. The fuels used in boilers are mainly coal, heavy oil and gas. The fuel chosen is a factor to be taken into consideration while designing a boiler. For example, coal is burnt on a conveyor grate in the furnace while heavy oil is ejected from a burner.
188.8.131.52 Types of boiler
There are numerous types of boilers and mainly categorised as fire tube boilers and water tube boiler boilers.
- Fire tube boilers: as the name itself suggests, fire tube boilers are boilers where we have hot gasses in the tubes and water all around. Hot gasses from the furnace pass in the tubes and the surrounding water gets heated by means of convection and conduction. Fire tubes are also known as shell boilers. Usually fire tube boilers are low pressure boilers.
- Water tube Boilers: water tube boilers are different from fire tube boilers in the sense that the water in now in the tube and the fire (source of heat) is all around. Water tube boilers consist of a feed drum and a mud drum which are connected via the tubes in which water flows. The water in the tubes get heated from the heat in the furnace and is transformed into steam in the upper drum as they rise up due to being less dense. On the other hand, the water from the feed drum flows down and gets heated. The drum bellow is also known as the mud drum. Types of water tube boilers are as follows: longitudinal drum boiler, cross drum boiler, bent tube. Below is a schematic of a water tube boiler.
1.1.2 Boiler water treatment
The water to be supplied to a boiler that is converted into steam is called feedwater and the sources of water are
1. Condensate water(condensed steam which returned from processes)
2. Make up water( water supply from utilities or any other water source)
In order to understand what a given boiler needs for treatment, a basic study of its feedwater must be done. Three types of impurities exists which can cause a wide range of problems in the boiler. These impurities are
1. Suspended solids,
2. Dissolved solids,
3. Dissolved gases.
Water picks the impurities from the ground in contact and the air it falls in. Geography, amount of rainfall and season change the character of the water. Water from underground will have more minerals(dissolved solids) than water from reservoirs.(surface water). Most problems due to impurities present in the water can be solved chemically, one way or another. It is usually most economical, however, to address the impurities by physical or mechanical means, using chemical treatment only to clean up the remaining traces left after physical treatment.
A maintained boiler is the heart of an efficient plant. Boiler water need to be conditioned before being converted into steam for the following reasons:
1. Optimization of heat transfer and hence have a better energy and fuel consumption
2. Minimisation of chemical cleaning and as a result reducing maintenance cost
3. Corrosion elimination
4. Safe operation of boiler
Pre-treatment methods for boilers
Suspended solids Dissolved solids Dissolved gasses Filtration Ion Exchange(softening) De-aeration Clarification Demineralisation Degasification Reverse osmosis Chemicals
Suspended solids can cause corrosion and deposition hence inhibiting heat transfer and thus reducing the efficiency of the boiler. Filters, filter beds, clarifiers and settling tanks are means to remove any suspended solids in the water.
Dissolved solids are minerals such as calcium, magnesium, iron and many others. Since the main types of boiler scales contain calcium and magnesium, it is very important to remove sine they form scale which inhibits heat transfer. Removing calcium and magnesium from the water removes the majority possibility of scale formation. The makeup water is treated in a softener to remove the calcium and magnesium and hence making the water "soft". Water softening (refer to chapter 1.3) is performed by ion exchange whereby unfavorable calcium and magnesium hardness ion are exchanged for favorable sodium ions by means of ion exchange resin beads. After a certain period, the resin is exhausted and needs regeneration. The unfavorable calcium and magnesium ions are drained in backwash and the regenerant draw is conducted and the resins are again replaced by sodium ions. The softener can again be ready for service.
Similarly a demineralization plant (refer to chapter 1.3) can be used to remove nearly all dissolved solids. Demineralization plants are used to treat makeup up water for high pressure boilers as the silicate ions cause pitting in the turbines. Demineralization plants are more expensive and gives a better dissolved solid control. Demineralization plants use ion exchange to remove the dissolved solids. Unfavorable positive ions are exchanged for hydrogen ions and unfavorable negative ions are exchanged for hydroxide ions. The demineralization plant is regenerated with hydrochloric acid for cation beds and sodium hydroxide for anion beds.
Reverse osmosis is yet another means of removing all the dissolved solids by means of a membrane and not by ion exchange. (Refer to Chapter 1.3)
Dissolved Gases are removed by means of de-aerators which use the physical properties of temperature and pressure to remove any dissolved gas. The two types of de-aerators are atmospheric head and pressurized de-aerators.
Oxygen is a highly reactive gaseous element. Oxygen forms localized corrosion areas referred to as pits. Oxygen pits can rapidly drill through metal surfaces, leading to metal fatigue and failure. Oxygen corrosion is an electrochemical process similar to a simple battery. Iron dissolves at the anode and releases electrons which are subsequently consumed by oxygen at the cathode. This localized mechanism causes pitting.
At the anode:
Fe Fe2+ + 2e-
At the Cathode:
2e- + H20 + O2 2OH-
Since oxygen is a gas, it will flash in the condensate steam and turbine and may cause pitting. This eventually weakens the metal and sends dissolved iron in the boiler which in turn causes overheating. Therefore it becomes very important to remove the dissolved oxygen from the water. This maybe done by physical means by heating the water to reduce its solubility and then de-aerated but even small amounts of dissolved oxygen can cause serious damage to the boiler. Oxygen Scavenger is used to remove the dissolved oxygen.
Also we will also have high risk of corrosion since we have the 3 factors responsible for corrosion to occur which are iron (boiler vessel), water and oxygen.
It is the most effective way to remove oxygen from water.(oxygen removal capacity of 80 %) Oxygen scavengers are strong reducing agent chemicals which remove oxygen from boiler feedwater water. Sodium sulphite is a highly effective oxygen scavenger. It should be noted that sodium sulphite can only be used to certain limits. For example it will decompose at 900 psig. Other oxygen scavengers may also be used such as hydrazine. We have 3 forms of sulphite which are
- Catalysed Sulphite
- Decharacterised sulphite
- Uncatalysed Sulphite
These are used according to the water content, industries requirements (food, textile, thermal power stations).
Dissolved carbon dioxide if cannot be removed or neutralised, the damage must be prevented.
Dissolved carbon dioxide is often present in feedwater in the form of carbonic acid and this causes the pH level to fall. Proper pH control will correct this but carbon dioxide is also released in boilers due to heating of carbonates and bicarbonates. These decompose into caustic soda with the release of carbon dioxide. This may need to be dealt with by use of a condensate corrosion inhibitor, to prevent corrosive attack to the condensate system.
1.1.3 Schematic of Boiler House
As we can see in this schematic of a boiler room with all the arrangement, the de-aerator removes and chemical feed remove dissolved gases and the softener removes any dissolved solid in the feed water.