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In late 2007 one such working group was tasked with looking into electricity consumption on the group cement

factories and the means to reduce that consumption in light of escalating electricity costs. This working group was faced with a widely varying situation across the 22 cement plants of the group. Unit electricity consumption ranged from 93 to 150kWh/t of cement produced. All variants of cement manufacturing processes and equipment were deployed on the factories, with some works having a mixture of such technologies. Pure Portland and a wide range of blended cements were produced of all strength classes. Some plants deployed captive power units and some were in the process of installing such power plants. Significant capital projects were underway to extend capacity and modernise a number of the factories.

Against this background it was decided that electricity consumption audits should be carried out on nine of the highest capacity cement facilities. The objectives of these electricity audits being to explain the variation in electricity consumption across the factories and to identify options for reducing electricity consumption and cost. It was also decided that these electricity audits should be carried out by a single, outside consultant in order to provide a consistent approach and a fully independent assessment of electricity consumption on the works.

In March 2008, Whitehopleman were selected as the consultants to carry out these audits and tasked with completing the them on the nine factories and reporting by the middle of August 2008. The remainder of this article will describe Whitehopleman’s methodology to complete these electricity audits and the key findings and recommendations of the audits.

MethodologyWhitehopleman’s methodology to complete these electricity consumption audits was broken down into five steps: (i) an analysis of the current electricity consumption on each of the plants, essentially a current situation analysis, (ii) development of electricity consumption models for each of the factories, (iii) a SWOT (strengths, weaknesses, opportunities, threats) analysis of each of the cement works to identify opportunities and options for electricity consumption savings, (iv) a preliminary options analysis to estimate savings and cost effectiveness of each option, and (v) prioritised recommendations for each of the factories.

The analysis of the current electricity consumption on each of the plants was itself completed in two stages. A questionnaire was completed by each of the factories followed by a visit to each of the plants by the Whitehopleman auditing team. The questionnaire gathered factory performance data over the previous three years and also information on the equipment installed on each factory, its installed electrical capacity and the electricity drawn by the equipment during operation. The visits were of two days duration for a single line factory and an additional day for

each additional line. During those visits discussions were held with the factory management and a tour of the electrical network and process undertaken to understand the electrical and process technology issues faced by each factory.

Central to Whitehopleman’s methodology was the development of an electricity consumption model for each of the works. This modelled the build-up of electricity consumption as intermediate products (crushed stone, raw meal, clinker, etc.) and final products (different cement grades) were manufactured. The models were then used to simulate the effects of changes in the product mix, replacement of equipment with more electrically efficient alternatives and changes in relative overtaking capacity on electricity consumption and cost. This proved to be an essential element of the auditing process demanding that every electricity consumer be documented on each of the plants. The models were especially useful for exploring the impact of changes in the product mix for each of the factories.

The heavy building materials (cement) division of the Irish multinational CRH, operates with a number of working groups targeting operational performance improvements in various aspects of the business. CRH does not operate with a large, central technical centre or staff and therefore the members of these working groups are drawn from the cement companies and factories in the various countries around the world in which CRH operates. This approach ensures that the working groups focus on the issues of immediate concern to the constituent companies and factories and that any findings or recommendations are immediately carried back to and implemented in the cement factories.

by Barry Leonard, Irish Cement Ltd & Dr Michael Clark, Whitehopleman

ICRH electricity audits

All variants of cement process technology were deployed

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These first two steps of the methodology were then followed by the analysis and reporting stages of the auditing process. CRH required that individual factory electricity audit reports be produced and delivered within two weeks of completion of the fieldwork visits to each of the factories. A deliberately mechanistic approach of SWOT and options analysis was employed to ensure consistency of the approach between different plants. All opportunities, however small or unlikely were considered, if only to document and then reject them as options. Management of the individual factories then had the opportunity to comment upon and provide feedback on the reports before their finalisation.

These individual cement factory electricity audits were all completed in a three-month period between mid April and mid-July 2008. That was followed by the preparation of an overall electricity consumption report highlighting the key findings and opportunities across the cement plants of the CRH Group. That overall report with its findings and opportunities was presented to the electricity working group at their periodic meeting in mid-August 2008.

FindingsIt is well documented in the literature that one of Whitehopleman’s regular benchmarking customers, Shree Cement in India, was successful in reducing electricity consumption from 118kWh/t in 1985 to 73kWh/t today. Whitehopleman did not expect savings of this magnitude would be possible on the CRH cement factories audited. However, CRH were insistent that all options to minimise electricity consumption be documented, no matter how improbable were those options.

Shree Cement achieved their success while expanding capacity from 0.6 to 2.6Mta and switching to the production of blended cements with a low clinker content (a Portland Pozzolanic Cement – PPC) with significant additions of fly ash). While expanding capacity they were afforded the opportunity to select and install cement manufacturing equipment with inherently low electricity consumption and made a point of doing so. This success of Shree Cement pointed the way for all cement companies and for the CRH cement factories to reduce electricity consumption.

In CRH another working group was tasked with reducing clinker content of cements produced, demonstrating that electricity consumption, or any other aspect of cement factory performance cannot be divorced from the wider operations. Nevertheless, the narrow focus of the electricity audits proved to be highly effective, allowing opportunities relating to electricity consumption to be identified in isolation. Whether these electricity reduction opportunities made sense in the wider operating context of the individual factories was for the local management to determine.

Deploying more electrically efficient cement manufacturing equipment is the other major way to reduce the electricity usage of a cement factory. As the raw and finish grinding sections of a cement factory consume ~2/3 of the electricity consumed in the manufacture of cement that has to be the first focus for reducing electricity consumption. This involves the replacement of ball mills with more electrically efficient vertical roller mills or using roller presses in conjunction with ball mills. Vertical roller mills for raw grinding were already deployed on six of the CRH factories audited. Vertical roller mills for cement grinding were already being used, or in the process of being installed on four of the facilities. Roll presses and Horomills were deployed on other factories. Nevertheless there remained significant opportunities to reduce electricity consumption by the introduction of roll presses for pregrinding raw materials and clinker ahead of existing ball mills on a number of factories. However, it was recognised that such projects are difficult to justify on electricity cost savings alone, if additional output is not required.

In this regard the differential in electricity tariffs at different times of the day is an important consideration. On some of the factories there was no differential between day and night-time electricity tariffs. In these cases there is no point maintaining more equipment than is necessary and balanced production capacity is the optimum, albeit with some overtaking capacity of the milling systems compared with the kiln to allow for maintenance.

However, on two factories in particular, there were differentials of more than double in the cost of day-time electricity compared with night-time electricity. In that situation it is definitely beneficial to have excess raw and cement grinding capacity to minimise the use of day-time electricity. Having high grinding capacity becomes a more important consideration than reducing the electricity consumption of that grinding capacity. This is the situation where the installation of a roll press ahead of the finish grinding ball mills becomes doubly attractive. Not only will the electricity consumption be reduced by ~30 per cent, but production rate can also be increased by up to 200 per cent, allowing less (or no) expensive day-time power to be utilised.

Moving beyond the grinding systems of the cement factories the next consideration was the process fans used for drawing air into, and exhaust gas through and out of the kilns and mills. Here the major opportunity arises when the flow through a fixed speed fan is controlled by a damper which operates less than 80 per cent open. That is an indication that the fan speed could be reduced resulting in significant reductions in electricity consumption. These opportunities had been largely realised on the CRH factories audited, but there were remaining opportunities.

After grinding systems and process fans the next consideration is the material

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46 ICR JULY 2009 Horomills for cement finish grinding

Visits to each of the cement factories

transport between process sections. As a rule of thumb mechanical transport will consume ~40 per centof the electricity that would be consumed by pneumatic transport. So if a kiln feed airlift can be replaced by an elevator then 60 per cent of the electricity consumed will be saved. The same applies to the replacement of the pneumatic transport of the cement to the silos from the finish mills. Again these opportunities had been realised on some of the CRH factories but other opportunities remained.

Across all the nine factories a total of 54 options for the reduction of electricity consumption were identified. One example highlights the effectiveness of this process. On one factory the audit identified the operation of two induced draft fans with fixed speed motors drawing ~1800kW and with inlet dampers 60 per centopen as a major electricity saving opportunity. The recommendation was

made that these fans be converted to variable speed drives. However this would have required major attendant work on the electrical system of the factory. Local management identified a better solution

being to shorten the blades of the fans, causing them to draw less volume, the inlet dampers therefore needing to be opened and electricity drawn to be reduced by ~370kW. This was on a factory burdened by higher than average electricity costs and the savings from this modification alone were sufficient to more than pay for the electricity audits on all nine CRH factories within one year of operation.

ConclusionsThe CRH electricity working group continues to meet on a regular basis in its ongoing efforts to reduce electricity consumption. The audits of the nine factories have provided a framework for the projects being followed by the working group

and on each of the factories. The process may be repeated again in 2010, dependent on the situations faced by the factories when that time arrives. _________________________________ I

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Process fans drawing air into and exhaust gases through and out of the kilns and mills