Industrial Visit

Recently we got a project to visit any industry and observe their Boiler and record the different quantities related to it's working. We decided to visit a cement factory known as Deewan cement. Got the appointment and were ready the day of our visit. The journey was about 1.5 hours and we reached the factory at about 10 am. It was really huge this was the first time we visited a fully functioning factory. Here is a report of what we experienced.

Cement Manufacturing

Cement manufacturing is a complex process that begins with mining and then grinding raw materials that include limestone and clay, to a fine powder, called raw meal, which is then heated to a sintering temperature as high as 1450 °C in a cement kiln. In this process, the chemical bonds of the raw materials are broken down and then they are recombined into new compounds. The result is called clinker, which are rounded nodules between 1mm and 25mm across. The clinker is ground to a fine powder in a cement mill and mixed with gypsum to create cement. The powdered cement is then mixed with water and aggregates to form concrete that is used in construction.

Combustion in the manufacturing of cement is initiated using natural gas which after attaining a certain temperature it is switched to coal. This process takes place at a very high temperature. Previously, the company use to discharge these gases to the atmosphere even though these gases still had a lot of energy left in them. Waste heat from combustion gases can be used for drying of raw materials and fuel. Depending on the humidity of the raw materials and the cooler technology, additional waste heat is available from the kiln gases (preheater exit gas) and cooler exhaust air. This heat can be used for electricity production.
Recently, the company has installed a waste heat recovery system (WHRS) also known as waste heat recovery. In this plant, we recover heat from the discharged gases and use them for a process from which we can generate electricity. WHRS has a lot of advantages such as, it helps in decreasing the greenhouse gases emissions and also decreases the operation cost of the whole powerplant.
 
The Dewan cement plant have a total power consumption of 25 MW of which they produce 5 MW with the help of WHRS & they purchase the remaining 20 MW from the grid station.

Water Treatment

The primary source of water for the company is KDA water plus the company do some valve mixing. As the water enters the boiler, a small amount of dosing chemical is pumped along with the help of dosing pump at the inlet of the boiler to maintain the chemistry of water in the boiler. The pH of the water inside the boiler is maintained on the alkaline side ~10. pH control is related to Acidic Corrosion- Low makeup or feed water pH can cause serious acid attack on metal surfaces in the boiler system. Even if the original makeup or feed water pH is not low, feed water can become acidic from contamination of the system. Therefore to prevent it pH is maintained in Alkaline range. Anti-scaling agents are also used while feeding water to prevent the formation of scales. The TDS in the water is maintained below 500 ppm. The company runs all the necessary tests on water twice a day so that it should not cause any damage to boiler.

Generating Steam in Boilers.

There are two boilers installed in the factory the AQC boiler meaning (Air Quenching Combustion boiler) and the SP Boiler meaning ( Super Preheat Boiler) these boilers work on the principle of Waste Heat Recovery System that is, the waste heat during the heat process of forming clinkers is utilized within the boiler hence the heat which was released into the atmosphere is now used for forming steam within the boiler.
Both the boilers used in the company are water tube boilers. It is a type of boiler in which water circulates in a tube and when the hot flue gases comes in contact with the tubes it changes the water into steam. The water is feeded to the boilers at a temperature of 42ºC with the help of a feed water pump of power 75 kW.
The flue gases comes inside the AQC boiler from the stack at a temperature of 380-400ºC, The flue gases then transfer their heat to the water in the tubes changing it into steam. The steam leaves the boiler at a rated temperature of 360ºC whereas the inlet temperature of flue gases in SP boiler is ~330-350ºC. the steam leaves the boiler at a rated temperature of 315ºC. The AQC boiler has a capacity of 13 ton/hour whereas SP boiler has a capacity of 14 ton/hour so the combined capacities of boilers is 27 ton/hour. The rated working pressure for the boilers is 1.17 MPa.
Steam generation causes the formation of scale, sludge and corrosion in boiler. The formation of scales in the boiler increases creep, solidify during boiling, decreases efficiency and performance so in order to prevent formation of scales we add anti-scaling agents such as Phosphate base. Scale forming substances are Calcium Carbonate, Calcium Sulphate, Iron oxides, Silica etc. Sludge can be removed with the help of blowdown. It is not as harmful as scales but it is still harmful for boiler.

Generating Electricity

The steam from the boiler goes to the turbine where the steam strikes the turbine blades causing them to rotate which produces output in the form of a rotating shaft. The shaft is connected to a generator which produces electricity.
 
The turbine used in dewan cement is a 7-stitch multistage turbine having three LP stages and 4 HP stages. The max pressure that the turbine can handle is 11.7 bar aand the flow rate of the turbine is 26 ton/hr. the rated inlet temperature of turbine is 321ºC. The max temperature that the turbine can handle is 380ºC exceeding it will create an alarming situation. The trip temperature & trip speed for turbine is 400ºC & ~3270-3330 rpm.
Steam after striking the blades loses its energy and goes to the condenser. A vacuum is maintained in the condenser so that the steam can easily flow and more work can be extracted from the steam in turbine. Condenser as the name suggests condenses the steam but there are some dissolved gases in the condensate. To remove them, it is send to the deaerator where the dissolved gases gets remove from the condensate.

Conclusion

The trip was our first experience on hand with a working boiler and production of cement, it was really exciting for us as students of Mechanical Engineering. The staff was really cooperative and we had a nice time inquiring about different specifications of thw working factory and engaging in discussions with the supervisors. It was a good day.