Reduce Energy Consumption: Cement Production

How conducting manufacturing energy assessment can help identify a wide range of changes to help reduce consumption. Metering, power quality, load shedding, motor management, power factor, and energy optimization all can help cement operations and other plants, as well.

08/25/2011


Cement producers have faced a significant rise in energy costs with the introduction of dry-process kilns, with a record average consumption of 100-200 kWh per ton of cement, according to the 2009 Cement Plant Operations Handbook. This complex challenge, coupled with rising fuel and energy costs, has prompted cement manufacturers to implement energy management programs to help reduce costs while maintaining competitiveness and increasing profits.

Many cement producers have lowered energy costs up to 20% by adopting a holistic approach to industrial energy management. This strategic process helps customers identify cost-saving measures and evaluate the tools best suited to specific plant needs, including:

  • Power and energy management systems
  • Variable frequency drives
  • Model predictive control systems
  • Energy assessments.

These tools help cement manufacturers:

  • Find and eliminate operating inefficiencies
  • Modify equipment and processes
  • Drive energy efficiency in product design
  • Expand plant operations to include comprehensive energy management programs that will quickly provide measurable results and remarkable cost savings.

This is a time of unprecedented complexity for cement producers. Managing production while balancing supply, pricing, demand, process efficiencies, compliance with regulations, and other demands can be difficult. At the same time, the rising cost of energy, including water, air, gas, electric and steam (WAGES) resources, compounds these challenges.


Divide the plant into energy allocation centers (EACs), as this diagram shows with a cement plant. Courtesy: Rockwell Automation

As Paul Scheihing, technology manager, Industrial Technologies Program, U.S. Department of Energy, explained, “The cost of purchasing the energy needed for production by an industrial facility is viewed as managed input and typically receives significant attention, while the use of that energy once it is inside the factory is often viewed as simply the cost of doing business. While not true in all industrial facilities, experience has shown that unless the facility actively manages energy use and has a documented plan for doing so, these facilities are significantly less energy efficient than they could be. Without performance indicators that relate energy consumption to production output, it is difficult to measure or document improvements in energy intensity.”

For the first time in industrial applications, the automation control, optimization, and information solutions necessary to conquer this energy challenge are in place or readily available to be applied immediately to achieve measurable results.

Energy assessment

Prior to beginning any energy management program, conducting an energy assessment can help companies identify a wide range of changes that they can make to help reduce consumption. These can be simple, such as a walk-through of a building or facility to identify quick-hit opportunities, or much more detailed efforts. Assessments can help establish the scope of an energy savings effort, define key metrics, and put resources in place to take a holistic view of energy for the entire organization.

Recommendations resulting from the assessment may include low-investment or no-investment behavioral modifications, such as shifting maintenance operations to nonpeak times, or may be more involved, such as programming changes to equipment. Evaluation and prioritization of capital improvement opportunities can also be included in the analyses.

Energy monitoring

After an assessment, the first step toward managing energy consumption is to gain awareness of energy usage patterns and trends throughout the facility. Building management personnel can leverage the facility’s metering infrastructure, including power monitoring devices, historical utility bills, and prior energy or process assessments, to collect data about all the energy resources in relation to equipment usage and environmental conditions. This process should include all points where energy is used, from an industrial process to critical building systems.

This data is then logged and time-stamped in an energy historian software program in order to establish trends or discrepancies in energy quality and consumption, and to establish benchmarks for future improvement. With this big-picture view of a facility’s overall WAGES use, building management personnel can then identify and make operational changes to help reduce energy consumption and related costs, such as shedding loads or temporarily lowering power levels when the facility is approaching peak use.

Energy metering

The first step is to meter the main incoming utilities and divide the plant into energy allocation centers (EAC).

For electricity, producers might install main incoming utility meters on:

  • Generators
  • Plant substations
  • Kiln(s)
  • Motors over 200 hp
  • Major electrical consumers per EAC.

For fossil fuels, producers might use the following metering scheme:

  • Main incoming gas meter(s)
  • Kilns
  • Major gas consumers.

Production equipment monitoring also provides knowledge about how specific assets consume energy. Identify useful data collection points across equipment and processes, and program the information system to store and analyze that data.

Load profiling

Load-profiling exercises chart energy consumption patterns by measuring and recording energy usage to identify peak demand periods, correlate consumption with facility activities and production in real time, and forecast energy demand.

Power quality monitoring

Gathering and reviewing power quality information can help identify power system anomalies and calculations like the cost of a power outage.

Power quality monitoring measures, displays, records, trends, and alarms on power quality parameters, such as:

  • Harmonics
  • Voltage excursions
  • Distribution system events.

With a log of historical data, building management staff can identify power quality issues, such as voltage sags or harmonics that can cause damage to equipment inside the plant and cause power factor problems on the energy grid. By knowing these risks, manufacturers can better protect their equipment and avoid incurring penalty fees from utility companies.

Improve efficiency

Next, cement producers can implement control strategies like emergency load-shedding, cost allocation, and demand management to help improve energy efficiency throughout the plant.

Emergency load-shedding

To justify the cost of an emergency load-shedding system, simply identify what a power outage would cost in lost production, using data collected during the assessments conducted earlier. Most cement producers find that an emergency load-shedding solution will pay for itself in one or two outages.

Emergency load-shedding applies to sites with generation or multiple utility sources and helps producers:

  • Protect generators from dangerous, damaging overloads
  • Maintain critical loads during outages
  • Optimize sources and critical loads
  • Eliminate the costs associated with damaged equipment and downtime.

Energy consumption, cost allocation, shadow billing

By reviewing energy usage data collected previously, cement producers can reveal where energy dollars are consumed, and in what proportion. This can help allocate costs by department, process, or facility; verify the accuracy of utility bills through “shadow billing”; and evaluate alternate energy rates and contracts.

Demand management

A demand management system limits energy demand through load shedding and peak shaving strategies. It helps reduce demand charges and manage real-time power purchases or to minimize load during a curtailment period.

For example, a steel mill was using 90,000 MWh of electrical energy per month, at a cost of $2.7 million each year. By replacing the facility’s unreliable demand management system and updating its control algorithms to more efficiently shed loads, improve power factor, and reduce voltage sags, the $300,000 system experienced a complete payback in five months. In addition, the company is enjoying an ongoing savings of $70,000 per month from reduced damage levels.

Motor management

Nearly 70% of all electricity used in industry is consumed by some type of motor-driven system. In a 10-year life cycle, a motor could accumulate energy costs amounting to 100 times its original purchase value.

However, cement producers can significantly reduce motor energy usage by implementing intelligent motor control solutions, such as variable frequency drives (VFDs).

In cement plants, VFDs are used to save energy and control process parameters, and retention times in applications with variable torque characteristics such as gas flow and fluid flow or in constant torque applications such as material handling and grinding equipment. Drives also are used to power roller mills for grinding different blaine of slag for cement, and for starting and running multiple roller mills, ball mills, and overland conveyors.

A China-based cement plant used VFDs to significantly reduce its energy consumption in its dry-process kilns, responsible for production of 1.4 million tons of cement each year. Traditional damper control systems used a fixed amount of energy, so fans at the plant always ran at full capacity even when the facility wasn’t producing product—wasting energy and causing unnecessary wear on the equipment. By using variable frequency drives to automate the speed control of its kiln head exhaust fan, kiln main electric fan, high-temperature fan, coal mill exhaust fan, and kiln tail exhaust fans, the system now only uses the amount of energy necessary to produce the required amount of airflow. As a result, the company reduced specific energy consumption by 10%, generating annual savings of $124,000.

Typical VFD applications in a cement plant include:

  • Induced draft/forced draft fans
  • Kiln drives
  • Mill drives
  • Material handling systems
  • Centrifugal pumps and fans
  • Compressor controls.

Power factor

Power factor (PF) measures how well cement producers use the power they draw from the grid. A PF of one is equivalent to 100% efficiency; in other words, all the power drawn is used. A PF of zero means there is an entirely reactive power flow.

To improve power factor, companies can implement of VFDs, use synchronous motors (such motors inherently have a PF of one), or install power factor correction capacitors. Power factor correction capacitors are used to improve the poor power factor of induction motors.

Optimize efficiency

Once cement producers have addressed opportunities for improved efficiency, the next step is to examine opportunities for optimizing the entire process. Indeed, controlling a major process unit effectively usually means dealing with multivariable systems, but it is extremely unlikely that treating each control loop independently will provide optimal control since in most situations, the control action of one loop affects the other loops. Model predictive control (MPC) technology, a multivariable control algorithm, can provide:

  • An internal dynamic model of the process that is subject to process constraints and based on a fixed frequency cycle
  • A history of past control moves to determine deviations
  • An optimization cost function over the prediction horizon to calculate the optimum control moves and control future behavior.

MPC technology allows the controller to receive information about the current operating condition of the process, then uses a model to predict process response to a sequence of future moves in manipulated inputs over a specified timeline, or “prediction horizon.” Next, an optimal control problem, solved online, determines the best sequence of future moves in multiple manipulated variables to minimize a particular objective function while obeying various process constraints. A fraction of the resulting control trajectory is then applied to the process and new process measurements reflecting modified operating conditions are obtained, allowing comparison of process outputs to desired reference trajectories. With this new information, the system repeats the optimization and control move process.

MPC systems can deliver optimization across key areas of the production process through applications for raw material preparation, including pyro-processing, cement grinding, and material blending. Energy savings can be generated by optimizing the combustion process, controlling temperature profiles, optimizing the heat recuperation process, and others. On average, MPC systems allow cement plants to reduce their energy consumption by 3% to 5%, as well as provide better product quality and capacity improvements.

Energy management profit

Armed with optimized production information, manufacturers can then project, in advance, how much energy will be required for similar loads or batches. Cement producers can then include energy requirements in resource planning and scheduling decisions in the same way they consider the availability of raw materials or other inputs on the bill of materials.

Empirically tying WAGES consumption requirements to the bill of materials allows a plant manager or production scheduling manager to make proactive production decisions and better manage energy investments in a way that will generate a greater return. For example, by knowing that certain cement batches require more natural resources, managers can move those batches outside peak windows.

At this point in the industrial energy management process, energy and its associated greenhouse gas emissions are no longer fixed allocations that are simply part of unavoidable overhead. Manufacturers who add WAGES resources to the bill of materials can actively manage it as an input to achieve higher profitability. In addition, this unit-level energy consumption information becomes valuable input to sustainability scorecards and other reporting mechanisms, allowing companies to better optimize their full supply chain to enhance sustainability and energy programs.

For example, in the cement factory of the future, a manufacturer might wish to enhance operations to support an ideal “sustainability score.” The company might choose a production facility based on the price of slurry, and on the potential carbon or energy footprint of shipping the raw materials to the facility. Additionally, the transportation routes for the outbound product can be optimized to account for weather factors that might impact the energy needed to store the product.

Holistic energy management

Manufacturers who have adopted this holistic approach to energy management have been able to do so by leveraging existing automation and power system investments to make more of their WAGES resources. Using intelligent automation solutions to get the big picture of energy use in a cement plant helps to identify where operational changes can be made to reduce energy consumption and costs.

Key concept: Affinity laws – a little less speed, a lot less energy

VFDs employ affinity laws to reduce energy use by using the minimum amount required by the motor application. For example, in centrifugal applications such as fans and pumps, a reduction in speed translates to a proportional reduction in flow (head pressure varies as the square of speed). A reduction in speed also translates into a reduction in energy (power varies as the cube of speed). Therefore, a flow rate of 50% equates to a power requirement of only 12.5%. In other words, a fan speed of 80% equates to a 50% reduction in energy.

Typical audit scope and timeline

 

 

 

Assessment

 

Audit

 

Approximate time to complete

 

2-3 days

 

2-3 weeks

 

Energy-saving project identification

 

x

 

x

 

Utilities usage review

 

x

 

x

 

Detailed utilities
analysis

 

 

 

x

 

Detailed quotations for sustainability projects

 

 

 

x

 

Return on investment calculation for projects

 

 

 

x

 

Prioritization of projects

 

 

 

x

 

 

Courtesy: Rockwell Automation

- Patrick Murray is manager of industry sales, Rockwell Automation. Edited by Mark T. Hoske, CFE Media, Control Engineering, www.controleng.com.

www.rockwellautomation.com 

www.controleng.com/channels/sustainable-engineering.html 

References

 

Phil Kaufman, White Paper: “Industrial Energy Optimization: Managing Energy Consumption for Higher Profitability,” pub. SUST-WP002B-EN-P, January 2011.

Philip A. Alsop, PhD, Hung Chen, PhD, and Herman Tseng, PE. The Cement Plant Operations Handbook, Fifth Edition, October 2007.

Craig Resnick, “Sustainable Production Imperatives and Opportunities from Rockwell Automation,” ARC Advisory Group, ARC View, May 2009.

George Seggewiss, PEng, Nathan Schacter, PEng, Greg Obermeyer, PEng, and Gary Bankay, “Considerations for Implementing MV Drives in a Cement Plant,” IEEE Conference 2008.

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