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Waste Heat Recovery for Power Generation

By Francisco M. Benavides

Hot process gases that are vented to the atmosphere by a Process Plant represent a potential for the generation of electric power. The installation of a Waste Heat Recovery (WHR) System is a “green” option that must be considered.

Figure 1. Typical WHR System

The Waste Heat Recovery (WHR) System consists of a steam generation unit and a power generation unit. The steam generation unit is a set of boilers placed in the path of the waste gases. The heat in the gases, sometimes supplemented by additional heat in the co- generation process, is used for generating steam. The steam can be used for other process requirements within the industrial plant, or used for driving a turbine that is connected to the generator. The generated power can either be used for running plant equipment or fed back to the power grid.

The recovered waste heat represents “green energy” since it is a direct savings in the use of fossil fuels with the consequent reduction of carbon dioxide emissions. Furthermore, cooling of the process gases is done without wasting scarce water or diluting with ambient air that would increase the energy consumption of the fans.

Several challenges exist, such as sticky and abrasive dust in the gas, or corrosive vapors such as SO2. The boiler must be custom-designed carefully to handle the specific characteristics of the off-gases. In special cases, like when there is a low gas temperature, an Organic Rankine cycle can be chosen instead of the steam cycle. The recovered waste heat represents “green energy” since it is a direct savings in the use of fossil fuels with the consequent reduction of carbon dioxide emissions. Furthermore, cooling of the process gases is done without wasting scarce water or diluting with ambient air that would increase the energy consumption of the fans.

Figure 2. Waste Heat Recovery System

PEC Consulting’s feasibility studies evaluate the characteristics of the process plant’s off-gases and assess the quantity of heat that can be usefully recovered. PEC Consulting will evaluate space limitations and find a solution to place the boilers and power generation system and integrate new equipment with the existing. PEC Consulting’s feasibility studies provide the client with an assessment of the power potential, a financial analysis with capital and operating cost estimates, and a layout to integrate the Waste Heat Recovery System with the existing process plant.

Waste Heat Recovery (WHR) Systems help process industries to become part of the green revolution by conserving natural resources. In addition to providing a reliable electrical supply, the reduction of the carbon footprint helps the environment. A co-generation plant is also a great benefit when the power grid supplying the plant is unreliable or when the plant is subject to interruptible power.

PEC Consulting can help the process industries with the realization of this noble and profitable goal.

About the Author(s)

Francisco M. Benavides, P.E.

Mr. Benavides, Principal Consultant at PEC Consulting Group LLC, has many years of experience in project management, design and construction. He has conducted bankable feasibility studies, economic analysis of mineral transport alternatives, plant valuations for acquisitions and for financing purposes, and due diligence studies. He holds an MBA from Kellogg School of Management, Northwestern University, Evanston, Illinois; a Bachelor of Science and Professional Degree in Civil Engineering from the Missouri University of Science & Technology, Rolla, Missouri; and has completed Graduate Studies in Engineering Management and Environmental Engineering at Missouri University of Science & Technology.

PEC Consulting Group LLC | PENTA Engineering Corporation | St. Louis, Missouri, USA

How can we help you? Get in touch with our team of experts.

May 25, 2013/by Rita Merz

Developing Frac Sand Quarries

By Thomas W. Hedrick

Silica sand is one of the most abundant minerals on the Earth’s crust. However, not any sand will work for gas and oil extraction. The industry is looking for specific gradations of sand between 1.2mm and 0.2mm. This sand should be clean, well rounded and have a Mohs hardness near 7.

Hydraulic fracturing is a key method of extracting unconventional oil and gas resources. As a rule, formations of shale gas resources have lower permeability than conventional gas formations and therefore, depending on the geological characteristics of the formation, specific technologies, such as hydraulic fracturing, are required. Although there are also other methods to extract these resources, such as conventional drilling or horizontal drilling, hydraulic fracturing is one of the key methods making their extraction technically viable. The multi-stage fracturing technique has facilitated shale gas and light tight oil production development in the United States and is believed to do so in the other countries with unconventional hydrocarbon resources. Significance of the extraction of unconventional hydrocarbons lies also in the fact that these resources are less concentrated than of conventional oil and gas resources.

The technique of hydraulic fracturing is used to increase or restore the rate at which fluids, such as natural gas, can be produced from subterranean natural reservoirs. Reservoirs are typically porous sandstones, limestones, or dolomite rocks, but also include “unconventional reservoirs” such as shale rock and coal beds. Hydraulic fracturing enables the production of natural gas and oil from rock formation deep below the earth’s surface (generally, 5,000 – 20,000 feet). At such depth, there may not be sufficient permeability or reservoir pressure to allow natural gas and oil to flow from the rock into the wellbore at economic rates. Thus, creating conductive fracture in the rock is pivotal to extract gas from shale reservoirs because of the extremely low natural permeability of shale. Fractures provide a conductive path connecting a larger volume of the reservoir to the well. So-called “super fracking”, which creates cracks deeper in the rock formation to release more oil and gas, will allow companies to frack more efficiently.

Figure 1. Frac Sand Quarry After Blast

Figure 1 shows a sandstone deposit in mid-Texas, typical of high-quality raw materials. The deposit has approximately 30 meters of overburden. The sandstone is mined, crushed and pre-graded, washed in attrition mills to further liberate the unwanted particle size both hydraulically and with vibrating screens.

The process may yield only 25% usable sand. The wet processing includes the removal of the clays and fine silica. After wet processing, the sand is dried, mechanically screened and separated into specific gradations appropriate for fracing.

Water is needed to liberate the fines. On-site water recovery, water management, clarifier and retention ponds are required. Mine permits, quarry plans, core analysis, logistics for the operations are necessary. Air quality, highway access, and use permits are all part of project development.

The frac sand user, the petroleum well driller, needs about 3,000 tons per well of sand. The driller mixes sand with other ingredients and water and injects the slurry into the well at about 50,000 psi pressure. A continuous high quality source of sand is imperative to obtain the maximum yield from the gas/oil bearing shale.

After the sand has been dried and screened, various 4 gradations of frac sand will be ready for transport to the drill site. The operation runs 24 hours per day, 7 days per week and may provide millions of tons of sand per year to customers. Logistically, this means that hundreds of pneumatic tankers of sand will be loaded, sampled, tested and released to customers each day.

To the quarry, it means that for every 5 million tons of sandstone processed each year, the mine operator must handle 3 million tons of sand tailings with reclaiming and revegetation plans.

PEC Consulting helps owners benefit from thorough planning. Our consultants have years of experience designing and operating quarries and logistics systems. We have qualified miners, engineers, materials handling experts, layout professionals and planners. Have us conceptualize and develop your deposit in this fast-growing field.

About the Author(s)

Thomas W. Hedrick

Mr. Hedrick, P.E. is a Senior Project Consultant at PEC Consulting Group. He specializes in materials handling, has been in manufacturing and engineering for 40 years, holds seven U.S. patents, and has written several dozen technical papers for various magazines and conferences around the world. Mr. Hedrick maintains a passion for excellence in client relations and project execution.

PEC Consulting Group LLC | PENTA Engineering Corporation | St. Louis, Missouri, USA

How can we help you? Get in touch with our team of experts.

May 25, 2013/by Rita Merz

Cement Industry – Plant Process Audits

By Narayana Jayaraman

A Plant Process Audit is a comprehensive evaluation of the overall performance of the plant’s operations. PEC Consulting systematically evaluates the plant’s operations, identifies the areas that are not working efficiently, and presents its findings and solutions to optimize the plant.

1. Steps for Process Audits

1.1. Benchmarking

Modern, dry-process cement plants with efficient configuration of grinding and pyro processing systems typically consume less than 700 kcal/kg-cl thermal energy and 100 kWh/mt of electrical energy. Older plants have inefficient systems, which compounded with operational and maintenance inadequacies, tend to have much higher energy consumption. Based on the plant’s conditions and specific requirements, general benchmarking is done to set targets. Plant audits evaluate the operation of a cement plant against the appropriate benchmark. After a detailed evaluation, recommendations for plant optimization are made in three levels of capital investment:

- Step 1: None or very little capital investment — by making adjustments to the operational protocols and improving maintenance.
- Step 2: Minor capital investments – with payback within 24 months..
- Step 3: Major capital investments – with 3- to 5-year payback.

1.2. Historic Evaluation

The plant operational and stoppage data is collected over the past two or more years. The reasons for the stoppages are analyzed in terms of category (mechanical/electrical/instrumentation/refractory/other), duration, and frequency in order to identify causes in order of severity.

The plant performance is also analyzed by department. Often a department’s best performance does not occur at the same time of the best performance of the plant as a whole. If we choose the best performance times of each department and make them occur at the same time, the plant performance would show a considerably higher level of efficiency. Attempts are made to make them happen at the same time, which is not an unrealistic target as the departments had indeed performed at that level in the past.

Through a systematic approach, all departments are made to perform at the highest possible level thus increasing the plant’s overall productivity.

1.3. Thermal Energy

Thermal energy relates to the Pyroprocessing system. For a 1 million mt/year clinker production, savings of 10 kcal/kg-cl would result in annual savings of approximately $185,000.

(1,000,000 tpy*1,000 kg/y*10 kcal/yr * $120/t-coal)
(6,500kcal/kg-coal/1,000 t coal)

Another significant advantage in most cases is that the reduction in heat consumption can be utilized to increase production.

Potential savings can also be derived from:

- Cooler optimization.
- Arresting in-leakages.
- Optimization of operational strategy.

1.4. Electrical Energy

Large fans and mill drives are major consumers of electrical energy.

- Fans: fan power is linked to specific heat consumption and many operational parameters. Optimization of these parameters will help lowering fan power consumption.
- Mills: in the case of ball mills, optimization of the mill charge and upkeep of the mill internals will minimize power consumption. As for vertical roller mills, the inspection of mill internals and separator and adjustments in the operation will bring about improvements in energy consumption and production increase.

1.5. Chemistry and Operations Strategy

Clinker quality related issues are addressed by evaluating the chemistry and operational parameters.

1.6. Emissions Management

The inadequacy of emission management systems generally found in older plants does not meet current emission regulations. PEC Consulting can analyze emission levels and provide solutions to improve emission management.

The expert staff at PEC Consulting Group has the capability to undertake Plant Process Audits and provide ongoing technical assistance to cement plants to improve operational performance. The scope of work generally includes:

- Plant visit and discussions with the plant’s operating personnel.
- Data collection of historical stoppages and operating parameters.
- Analysis of the data to identify areas for improvement.
- Submission of a report providing observations and recommendations, including economic analysis to establish the cost/benefit ratios.
- Develop an implementation program with the Plant Management.
- Work with Operating personnel through periodic goal-setting and audits until the prescribed performance goals are achieved.

A Plant Audit is the foundation to optimize the plant operations and often presents the lowest cost/benefit investment ratio.

About the Author(s)

Narayana Jayaraman

Mr. Jayaraman is PEC’s expert on cement process systems; he has over 45 years of experience in the cement industry. His expertise includes the process of white cement plants, including upgrading the capacity and resolving process issues at two cement manufacturing facilities in India. He has had in-depth exposure to the technical, economic, and commercial aspects of large cement projects and extensive experience in upgrading and optimizing existing plants. He earned a BS in Mechanical Engineering from Osmania University, Hyderabad, India, and an MS in Mechanical Engineering from the Indian Institute of Technology, Kharagpur, India.

PEC Consulting Group LLC | PENTA Engineering Corporation | St. Louis, Missouri, USA

How can we help you? Get in touch with our team of experts.

May 25, 2013/by Rita Merz

Alternative Fuels

The use of alternative fuels, also known as “co-processing”, is the use of combustible waste materials as a source of energy in pyro-processing of cement manufacturing plants, lime plants, and in the calcining of other minerals.

Some forms of alternative fuels are:

- Used tires.
- Plastic waste (bags, bottles etc.).
- Waste Oils.
- Solvents.
- Saw dust.
- Paint.
- Hazardous Wastes.
- RDF (Refuse Derived Fuels).

Benefits

Conversion to alternative fuels benefits the economy of operation by lowering fuel costs. In some cases, a facility that uses alternative fuels receives economic compensation for waste elimination (e.g. use of Hazardous Wastes). In addition to energy recovery, there are considerable environmental/green benefits, such as the reduction of emissions and greenhouse gases into the atmosphere by not using natural non-renewable raw materials and fuels such as coal. In certain countries, these savings can be utilized for Carbon Trading credits.

However, because of their inherent characteristics, the implementation of alternative fuels present a challenge to use which, if not handled properly, may impact the manufacturing process, product quality, and environmental exposure.

What We Do

At PEC Consulting we provide innovative and thoughtful solutions to help the cement industry and lime industry implement co-processing at their plants and become more cost efficient, which improves their economic competitiveness.

An alternative fuels project will evaluate the characteristics of co-processing and recommend ways to handle and utilize fuels in the pyro-process in the most effective and secure way.

Our scope of work includes:

- Studies for handling and firing Solid Waste Fuels.
- Studies for handling and firing Liquid Waste Fuels.
- Studies for handling and firing Hazardous Waste Fuels.
- Conceptual studies to use whole or shredded tires.
- Studies to use gas or petroleum coke (petcoke) as replacement fuel.
- Alternative Fuels / Co-Processing Studies for Cement Plants.
- Alternative Fuels / Co-Processing Studies for Lime Plants.

How can we help you? Get in touch with our team of experts.

May 24, 2013/by Rita Merz

Waste Heat Recovery Systems

Waste Heat Recovery Systems (WHR) utilize the heat in industrial waste gases for the generation of electrical power. WHR Systems are common in cement and lime plants and other pyroprocess industries where Rotary Kilns are used and the price and reliability of grid power supply make the investment economically feasible.

In developing the concepts for the WHR system, the challenge lies in determining the optimum quantity of waste heat available for power generation and in integrating the WHR system with the cement process. PEC Consulting can assist the client to perform a Feasibility Study to assess the optimum size and configuration of the WHR system and the viability of the project. The study will provide recommendations for an efficient and economical system based on several factors such as the thermal capacity of exhaust gases, size and type of heat exchanger, and desired parameters of the cogeneration system operating conditions.

In some cogeneration systems, the exhaust gas from power generator sets using diesel oil or furnace oil is used as a source of heat for the WHR plant.

Benefits

- Reduction of electrical power demand from the grid and consequently of the plant’s operating costs.
- Reliable electrical power supply from waste gases from the kiln and cooler.
- Reduction of the carbon footprint by conserving the use of fossil fuels and thus helping the environment.

What We Do

- Assess the realistic quantity of heat that can be recovered taking into account variations in the drying requirements of raw materials and fuel throughout the year. This analysis results in optimum sizing of the WHR system.
- Determine the energy recovery process to be utilized: The steam Rankine cycle uses water as the thermodynamic medium and is commonly used for generating power from high temperature sources. Depending on the temperature and other situational factors, some plants use the Organic Rankine cycle for generating power, using an organic compound as the thermodynamic medium.
- Optimize the layout of the boilers and power generation system considering plant footprint space constraints.
- Provide an effective integration of the WHR equipment with the existing cement plant or industrial facility.
- Provide our clients with an assessment of the power potential, a financial analysis with capital and operating cost estimates, and an economic analysis to assess the viability of the project.
- Technical Feasibility Studies of WHR Systems for Cement Plants.
- Technical Feasibility Studies of WHR Systems for Lime Plants.

Learn more about WHR; click on the following publication:

Waste Heat Recovery for Power Generation

To download a list or Energy Efficiency/Process Studies, including WHR projects, please click here.

How can we help you? Get in touch with our team of experts.

May 24, 2013/by Rita Merz

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