Case Studies in Chemical and Environmental Engineering (Dec 2021)
Power generation in white cement plants from waste heat recovery using steam-organic combined Rankine cycle
Abstract
Cement is one of the highly energy-intensive industries. Producing one ton of cement consumes 3-4-GJ which represent about 40% of the variable costs of cement production. Moreover, fuel cost represents 26–34% of production costs. Increasing energy efficiency is essential in the cement industry, especially in the white cement production. The thermal energy consumption of the white cement production process is twice the average thermal consumption of grey cement manufacture. Improving energy efficiency in the cement industry reduces CO2 emissions in addition to reducing production costs. This study considered the recovery of waste heat in a white cement plant to improve its energy efficiency. The conventional power generation cycles, i.e., steam Rankine cycle, organic Rankine cycle were studied as well as steam-organic combined Rankine cycle. Six different schemes were proposed for the different Rankine cycle systems. A comparative study was made to determine the optimum type of Rankine cycle, scheme, working fluid, and operating conditions based on both thermodynamic and economic evaluation. This research provides a general methodology to evaluate working fluids of different organic Rankine cycle systems to recover high-temperature waste heat. Isopentane and its different isomers -n-pentane, cyclopentane, spiropentane and neopentane-were selected as working fluids. Spiropentane produced the highest net power “4483 kWh” and the highest thermal efficiency “41.69%”. Although spiropentane achieved the best performance, cyclopentane is recommended to be used as a working fluid due to its availability and relatively cheap price. It requires the lowest capital cost among the other working fluids. Moreover, it has achieved an irreversibility rate lower than spiropentane and its thermal efficiency is lower than spiropentane by only 1.6%. Steam-organic combined Rankine cycle is the best choice for recovering high-temperature waste heat. it can generate larger amount of power than traditional steam Rankine cycle. Moreover, it has lower capital cost than the steam Rankine cycle because it has been operating at lower pressures.
