A new DSM simulation model for South African cement plants
Venter, George Stephanus
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Eskom is currently experiencing problems with its electricity supply because of rapidly increasing electricity consumption in Southern Africa. One of the relatively short-term solutions to this problem is demand side management (DSM) through load shifting. DSM load shifting occurs when large electricity-consuming equipment is stopped during the peak hours of each weekday. In the cement industry, the two largest electricity users in a cement manufacturing plant are the raw mill and the finishing mill. When load shifting is applied to cement plants for testing, because of the complex system, production can be lost. A cement plant will not tolerate any loss in production hence the need for a simulation model to simulate the effects of load shifting in a cement plant. In this study, a new simulation model was developed to determine the viability of a DSM project in the raw mill and finishing mill sections of South African cement plants. For a DSM project to be possible there must be no loss in production. In the production process, the silos store the milled products. If the silo runs empty, there is no material for the kiln or packaging plant to process, which will result in a loss in production. The level of the silo is therefore vital in the simulation model. The simulation model consists of two parts. The first part simulates the silo level over a period of one month to determine whether the level remains within the specified limits. The second calculates the optimised baseline versus the historical baseline, load shifting potential and possible annual cost savings. It is critical that the correct inputs to the simulation model are obtained to acquire accurate results. The second part of the simulation can only be applied if the silo level is within specifications. The simulation was applied to the raw milling section of two different cement plants and also to the finishing milling section of two different cement plants. Both raw milling sections showed a potential for DSM intervention. For confidentiality purposes, the cement plants will be referred to as Plant A, B, C, and D. In Plant A, five hours of load shifting, realising a maximum potential of 2.08 MW in morning peaks and 2.05 MW in evening peaks, were possible per weekday. Plant B had a possible 0.79 MW in morning peak hours and 1.96 MW evening peak hours load shifting potential per weekday for two hours each day. An annual cost saving of R 474,000 for Plant A and R 293,000 for Plant B could be realised. There was possible load shifting potential of 3.52 MW in morning peaks and 3.94 MW in evening peaks in the finishing milling section of Plant C. Five hours of load shifting per weekday means an annual cost saving of R 898,000. The silo simulation on the finishing milling section of Plant D showed that the silo level could not remain within the limits when load shifting was applied. Hence there is no scope for a DSM project in the finishing milling section of Plant D. The simulation model developed in this thesis provides an accurate indication of the silo level over a period of one month and projects the possible load shifting and annual cost savings where a cement plant is found to have a viable DSM load shift potential.
- ETD@PUK