In cement plants, getting the best out of the equipment does not necessarily give the desired results. A close study through an optimisation exercise can do the job, says PR Raghavarao.

A case study of optimisation of a grinding unit has been presented in this paper. A grinding unit having vertical roller mill with a grinding capacity of 250 tph PPC and having a fineness of 4,000 cm2/g Blaine was commissioned and completed performance guarantee tests. The specific power consumption was found to be on the higher side. The plant management decided to reduce the specific power consumption. The Project Management Approach (PMA) was adopted in implementing the project, taking up optimisation as a project.

The objective of the project was to reduce the specific power consumption of the total grinding system by 10 per cent in a period of six months, with minimum cost and no capital outflow. There were nine steps involved in the project implementation.

Introduction
This case study is about a clinker grinding unit having a Vertical Roller Mill LM 56.3 +3C supplied by Loesche.

The grinding system consists of

• Feeding arrangement
• Hot air generator
• Vertical roller mill with in-built dynamic separator
• Baghouse for product collection
• Bucket elevator for product transport
• Cement silo
• Dedusting system

The system was designed for grinding 250 tph PPC having 35 per cent fly ash ground to 4,000 cm2/g Blaine. Total specific power consumption for the total system was foreseen to be 32kWh/t.

The unit was commissioned in the year 2010. The supplier carried out a number of modifications in the VRM and achieved the guaranteed performance values. However, the plant management was not satisfied and wanted to reduce the specific power consumption values by 10 per cent in a year.

Optimisation by PMA
PMA is a smart implementation method of executing a project as a team, involving all stakeholders. The whole exercise is taken up after getting approval from top management.

This approach involves nine steps:

• Step 1. Assessment of situation
• Step 2. Stakeholders? analysis
• Step 3. Search for lessons learnt
• Step 4. Definition of service
• Step 5. Milestone schedule
• Step 6. Project organisation
• Step 7. Estimation of project cost
• Step 8. Risk identification
• Step 9. Agreement with client

Role of Process Engineer
Normally in cement plants, the engineer in charge of a shift or daily operations carries out the optimisation exercise. However, due to daily workload coming from operations and administrative jobs, he is not able to devote the necessary time and focus on an optimisation exercise. Therefore, plants are opting to nominate one process engineer from the plant team and provide him with adequate training in the area of process engineering. He is assigned a project as a part of the training with a specific objective and time period. The purpose is to recover the cost of training by way of benefits accrued from the optimisation project. Later he becomes a resource for the plant.

Method of Implementation
Step 1 Assessment of situation
By detailed evaluation, it was decided that the current specific power consumption of the system of 30 kWh/t was high for a vertical roller mill system. Ball mill systems are operating in the region with similar materials and same product specifica?tions at specific power of 35-37 kWh/t. Vertical mills are expected to reduce the power duty by approxi?mately 10 units compared to ball mill systems. The project was taken up to optimise the grinding operation for reducing specific power consumption by 10 per cent from 30 kWh/t to 27 kWh/t in a one-year period.

Step 2 Stakeholder analysis
The mill operator wanted to achieve higher production target in daily working. Maintenance personnel desired to get maximum maintenance time which is achieved by high production rate and thus lower operating hours per day. Quality Control personnel had to ensure that the product quality is maintained even at higher production rate. The electrical engineer wanted to bring down the consumption of electrical energy for the total tonnage produced in a day. The management understood the influence of the optimisation exercise on market demand, and also wanted to develop resources in the plant to sweat the assets optimally.

Step 3 Search for lessons learned
At the time of project, the conditions specified were conservative. There was a change in characteristics of additive material, i.e., fly ash. Market demand was fluctuating. Now the focus was on optimising the operations to reduce specific power and to meet fluctuating market demand.

Step 4 Definition of product or service or benefit

• Specific: Reduce specific power consumption of total grinding system by 10 per cent, i.e., from 30 kWh/t to 27 kWh/t at same product quality.
• Measurable: The benefit was to be assessed by system audit at the start and end of project.
• Achievable: The team believed that the target was achievable, as there were references.
• Relevant: The project for reduction of electrical energy was highly relevant to reduce cost of manufacture.
• Time-bound: A timeframe of six months was agreed for the total project.

Step 6: Project Organisation
Project Client: …

Project Manager: …

Project team:

1. …
2. ….
3. …

Step 7 Estimation of project cost
Cost items were identified as personnel, material and third-party expenses. The main cost was the cost of training of personnel. Replacement material costs and third-party expenses were met from revenue expenses.

Step 8 Risk identification
Major risk factors identified were low availability of mill system due to fluctuating market demand, variation in size and quality of feed materials like clinker and fly ash, and change in role of team members.

Step 9 Agreement with client at start and end of project
The plant head had signed the Memorandum of Understanding at the start of the project. At the conclusion meeting, he endorsed the closure of the project after realising the benefits.

Actual Implementation
The project was implemented by the project team members over the course of six months. A progress report was prepared every month and reported to management and the coach.

Actions taken

• Actions were taken on the findings of the mill assessment.
• High false air was found mainly in the baghouse inlet, mill inlet and outlet. The leakages were rectified over a course of four weeks.
• Feed was not distributing at the centre of the table. Mill feed
• chute was integrated with separator cone. The distribution became even.
• Mill hydraulic pressure was maintained low, at 55 bar. It was increased to 70 bar.
• Separator seal gap was reduced. The residue on 45 microns sieve was also reduced.
• All three water spray lines were repaired to get uniform water on the mill table below three rollers.

Project benefits

• The main target of reduction of specific power consumption was completely achieved; the reduction was 3 kWh/t
• In addition the product quality improved as the residue on 45 microns reduced by 2.5 per cent
• The mill operation was stabilised with low vibration levels.

After six months, the findings were presented by the Project Management team to plant management and coach. The results were accepted and the project was declared a complete success.

The author, PR Raghavarao, holds a B Tech in Chemical Engineering from Banaras Hindu University. He has worked in various organisations like Larsen & Toubro Limited, Prism Cement Limited and Ambuja Cements Limited. He retired as a Senior Vice President from Ambuja Cements Limited. Raghavarao was associated with the cement industry throughout his career, in the fields of process engineering, commissioning, troubleshooting, process audits and plant optimisations. He is based in Mumbai and works as a freelance consultant.