CASE STUDY

Case study for steam utilization optimization
in Cement Industry

Industry

Cement

Background

After completion of curing process, the excess removal of high pressure steam present in autoclave leads to wastage of energy.

Highlights

Venting of live steam present in autoclave after completion of curing process

Energy Conservation Measure
  • Process instrumentation and control simulation
  • Re-sequencing operating trends of autoclaves with respect to heating phase.
Estimated savings $58,500 USD per day
  • Boiler Capacity – 14TPH @ 9 barg
  • Fuel – Furnace oil
  • Payback – 1.5 yrs.
Explanation

The basic raw materials used in the manufacture of AAC are Portland cement, lime, water, sand or fly ash, and aluminum. The sand is ground to a powder-like consistency And mixed with the remaining materials to form slurry. The slurry is then poured into molds. Entrained air bubbles are created by a chemical reaction between the hydration products and the aluminum powder, which causes the material to rise in the mold. After curing for 45 minutes, the product is cut into the Unit sizes by piano wires. The units then are steam cured under pressure in autoclaves for 8 to 12 hours, which transforms the material into a hard.

At present total seven number of Autoclave batches are taken in a day, at the end of every batch excess steam and other gases present in autoclave has to be removed in order to prepare the autoclave for new batch. This is done by vacuum operated by high pressure steam.

The excess removal of high pressure steam present in autoclave after completion of curing process led to wastage of energy. If the other autoclave is in heating phase than this excess steam is taken to other autoclave for heating purpose otherwise this is vented in atmosphere.
During the visit this venting was observed 3 to 4 times in 12 hrs.

Savings estimation from Autoclave
PARAMETERS Value UNIT Remark
Autoclave Length 45 Mts
Autoclave Diameter 3 Mts
Autoclave Inside Area 318 Cu. mts
Estimated Steam Space
in the Autoclave
40 % Assumption
Net Steam Space in
Autoclave
127 Cu. mts
Sp Volume at 12 kg/cm2 0.06 Cu. mts/kg Steam @ 12 kg/cm 2
6.25 Kg/Cu. mts
Steam Quantity in
Venting
795 Kg/hr Steam supply and transfer
valve are in closed
No. of venting batches
in a day
4 nos/day
Steam Loss in Venting
per day
3179 Kg/day
3.2 TPD
Steam to Fuel Ratio 14.6
Fuel Loss in Venting
per day
0.25 TPD
254 Lts. of FO /day

This loss due to Venting of steam was around 3.2 TPD i.e. 254 Lts. of FO per day for 4 nos. of batches in which steam has to be evacuated to atmosphere in vacuum phase. It was not avoidable since, it was process constraints and governed by the sequencing of the autoclave.

However, proper sequencing was adopted at plant autoclaves, but it was noticed that during audit that still 4 to 5 times the exhaust steam still needs to be evacuated from autoclaves to atmosphere, since the other autoclaves was not ready for next phase.

This can be further minimized by proper optimization of the sequencing method of the autoclaves in which exhaust steam from the trailing autoclaves was supplied to another autoclave which should be ready for its next batch.

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