Cogeneration and On-site Power Production

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Concrete energy savings - on-site power for the world’s cement industry

Considering the enormous size, the growth and the energy-intensive nature of the cement industry, it’s a little surprising that there isn’t more on-site energy generation used in the sector. Jeff Bell describes the current use of cogeneration and on-site energy in the industry and outlines the potential for growth.

Making cement is one of the world’s most energy-intensive industrial activities. It has been estimated that the process consumes about 3% of total primary energy and results in about 5% of the world’s climate-destabilizing carbon dioxide emissions. Investing in power plants integrated into the cement manufacturing process offers unrealized potential to help reduce the energy intensity of the sector as well as displace emissions from the power sector.

Other benefits resulting from on-site power in the cement industry include improved reliability and increased up-time for plant owners, improved economics for cement makers, improved fuel efficiency, reduced pollution and cheaper power prices for other grid customers. Of special interest are the system-level benefits that cement plant owners are in a position to offer via increased investment in on-site power. The larger societal benefits that are realizable do not typically enter the equation when most plant owners consider the pros and cons of building power generation capacity into the operations of their individual plants.


Inside a cement plant in Hungary. The cement industry has vast potential for adopting on-site power and cogeneration, but this potential has been little exploited to date (HeidelbergCement)
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Considerable on-site power capacity already exists in the world’s cement plants but its use is specific to certain regions of the world and considerable potential for increased use of on-site power in the cement sector remains.

CEMENT MARKETS

Cement is one of the most ubiquitous building materials in the world and is widely used in both developed and developing countries. Cement, mixed with some aggregate such as sand or gravel to form concrete, is an affordable yet resilient building material. In 2004 more than 2 million thousand metric tonnes of cement were manufactured globally and production is expected to increase as demand soars, especially in rapidly developing nations such as China and India. Between 2000 and 2004 the top 20 cement-manufacturing nations witnessed a 24% increase in production volumes. Analysts expect similar growth trends to continue as demand for new highway systems, waterworks and buildings escalate around the world. The growth rates of the cement sector’s top 20 cement manufacturing nations are listed in Table 1.

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THE ENVIRONMENTAL IMPERATIVE

Given the energy-intensive nature of the cement-manufacturing process, it is perhaps not surprising that a twentieth of the world’s man-made CO2 emissions can be traced to a single industry: cement. About half of these emissions are a necessary by-product of the chemical reaction essential in transforming the raw material into the finished product. This means that as long as cement manufacture is based on clinker, the sector will always be an important contributor of GHG. Manufacturers are therefore forced to look elsewhere to reduce their carbon footprint: such as more efficient use of fuel to produce the heat and electricity needed for the process.

The other half of the sector’s emissions results from the combustion of fuels for heat and power. In order to catalyse the chemical reaction, extremely high temperatures must be reached. Therefore, the great majority of the fuel input in the industry is for heat. Power accounts for less than 10% of the sector’s emissions, yet an electricity supply is fundamental to the process. Figure 1 shows the impact of the cement industry on overall global emissions.


Figure 1. Global annual CO2 production by industry (total = 30 giga tonnes CO2). Source: WBCSD
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ON-SITE POWER IN THE CEMENT SECTOR

In some areas of the world on-site power in the cement sector is already quite common; still, significant untapped potential remains. On-site power offers many benefits, both to society in general and, in many cases, for the cement plant managers.

Plant-level benefits

At the plant level, on-site power reduces downtime by guaranteeing against production interruptions from utility failures, saves energy costs by improving energy utilization of expensive fuels, reduces the amount of refractories used (special temperature-resistant materials used to line kilns/furnaces in order to increase their lives), allows higher kiln utilization, and increases quality of the finished product.

Larger societal benefits

Perhaps of greater importance, however - if only because it is natural for cement plant owners to overlook them in internal examinations of the cost/benefits of on-site power - are thesystem-level benefits resulting from increased investment in on-site power. The system-level benefits of on-site power have been documented extensively regularly in COSPP. These include significantly reduced capital expenditure for transmission, distribution and generation infrastructure, decreased vulnerability to fuel price volatility for the larger region and vastly reduced smog and climate change-causing emissions. What may be needed is some way of rewarding cement plant operators for the larger public benefit they provide by investing in highly efficient on-site power.

TYPES OF ON-SITE POWER

On-site power applications can be broken down into three types: bottom-cycle cogeneration, top-cycle cogeneration and on-site power-only applications.

Bottom-cycle cogeneration - waste heat recovery

Abundant waste heat of both high and low grade is a natural by-product of cement manufacture and therefore provides significant opportunity of added value if it is captured. Because of its abundance in this industry, the process of recovering waste heat to generate power, i.e. bottom cycle cogeneration, is the most promising on-site power generation opportunity in the cement sector. All waste heat that is successfully recovered directly displaces energy costs (both fuel to provide heat and electricity from the grid) that would have otherwise been borne by the cement producer. Table 2 summarizes the various sources of heat from a cement plant and the related estimates of theoretically recoverable energy.

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Various approaches can be used for this bottom-cycle approach, including the Steam Rankine Cycle, the Organic Rankine Cycle and the Kalina Cycle. Table 3 summarizes three parameters of the various approaches: heat resource requirements, cost and possible output.

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The Steam Rankine approach is the most common approach and the one used in existing on-site power plants in US cement plants (examples there include the Florida Crushed Stone, in Brooksville, Florida, and CalMat in Colton, California, which has been in operation since 1985). The Organic Rankine Cycle (ORC) (also known as ORMAT energy converter [OEC]), though more expensive, can make use of lower-temperature waste heat. An example of a successful ORC installation is in the HeidelbergCement plant in Lengfurt, Bavaria, Germany. Examples of the successful application of Kalina cycles also exist in industrial settings, for example in the Sumitomo Corporation Kashima Steel works in Japan, but there are not yet any commercial applications in cement plants.

Bottom-cycle plants offer the most environmental benefits because they use waste heat to generate electricity, so no additional fuel is required to generate the power. Power thus generated would displace power from the local grid, so the environmental benefits realized depend on the local fuel mix. In the case of China, for example, generation would likely displace the need for power from central coal plants. Indeed, there is actually an un-intuitive multiplier effect of such fuel displacement. Because a central plant is only 30% efficient, each kilowatt-hour displaced by generating power with top- or bottom-cycle cogeneration in a cement plant actually results in three units of fuel being saved.

Top-cycle cogeneration

Opportunities for standard top-cycle cogeneration are also abundant in the cement industry. Top-cycle applications use waste heat or burn fuel to generate power on-site, and subsequently use the heat exhaust from the generator to meet process needs. Process needs could be either needs of the cement plant, or a neighbouring factory or community (for community heating or cooling). In cases where a neighbour is required to use the waste heat in order for the project to be economic, higher transaction costs will result.

Examples of on-site applications on the cement plant premises could include drying of raw meal (limestone/silicate mix), or drying coal prior to its introduction into the system, both of which are desirable because they can increase the efficiency of the plant. In blended cement plants, the raw material (slag, fly ash, etc.) can also be dried using cogeneration applications. For example, a plant in Rozenburg, the Netherlands, uses gas turbines to generate power while the exhaust is used to dry slag.

An almost inexhaustible array of opportunities exists for cement plants to use cogeneration to supply heat needs to neighbouring plants or communities. Examples include greenhouses, aquaculture, textile plants, food-processing plants, and buildings via district heating networks. Aalborg Portland plant in Denmark, for example, feeds heat into a district heating network supplying approximately 15% of the municipal heat demand of the city of Aalborg.


Aalborg Portland plant in Denmark feeds the heat generated from the cement-making process into the district heating network (Aalborg Portland Group)
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For both buildings and industry, cooling is also possible through the use of absorption chillers. Heat and cooling distance is the main limiting factor as it is both expensive and inefficient to move waste heat long distances. As a result, a very promising approach is to gather industries together in industrial networking parks so that efficiency-increasing measures, such as top-cycle cogeneration, can be realized.

In the case of top-cycle plants the environmental benefits arise from increased efficiency. Even the most efficient central CCGT plants max out at about 50%-60% efficiency if they are not used in cogeneration applications. Where fuel in cement plants is burned to generate power and the waste heat is put to use for fuel drying or raw material preheating applications, the overall efficiency can reach upwards of 90%. This efficiency saving directly translates into environmental benefits in terms of reduced emissions (40% less fuel consumption means 40% lower emissions). Additional environmental benefits are achievable via the fuel-switching effect, for example, if gas-fired CHP replaces a combination of coal-fired boilers and/or coal-fired central plant.

On-site standby/baseload generators

Power-only plants to supply electricity for cement plants can be used for standby/emergency use or for baseload. The important distinction is that no heat is recovered. An example of this approach is seen at Grasim Cement in Chhatisgarh, India. Although from an environmental (and long-term economic efficiency) perspective this approach is sub-optimal compared to the above two options, there can be a strong economic case to invest in such technologies. Periodic power shortages and power quality issues have led some cement plant operators to invest in this technology because of its relative cheap upfront capital costs and its separation from the core processes (i.e. power supply can be independent of operating the kiln).

Such an approach has proven especially popular in areas where grid power has been found insufficiently reliable to meet plant needs (such as in India). For example, the Twiga Cement plant in Tanzania ensures continual operation thanks to a 3 MW on-site power supply. The local electricity supply in Dar-es-Salaam is so unreliable that, without the on-site power, the cement company could expect power outages up to five times a day.


The Twiga Cement plant in Tanzania is able to carry out continual operation thanks to a 3 MW on-site power plant (HeidelbergCement)
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Even though no heat is recovered, there may still be environmental benefits if inefficient and dirty central generation (such as from coal) is displaced by cleaner on-site gas or biomass. In some cases, however, the local environment may suffer as emissions shift from a remote central power plant to the on-site generator or where the central grid is largely hydro-based.

Like bottom- and top-cycle cogeneration, from a system perspective there will be some environmental benefits with power-only applications. The elimination of transmission and distribution losses may allow central thermal plants to reduce output by 5%-10% of the cement plant’s load; an amount equal to average transmission and distribution losses. Clearly, an on-site power plant combined with some use of waste heat is optimal, but even power-only can be a move in the right direction.

THE BASELINE AND THE POTENTIAL

WADE has compiled documentation of over 2900 MW of installed electric generating capacity in cement plants worldwide (see Table 4). Data, although imperfect, suggest that on-site plants are installed at a minority of the world’s cement plants. Increased efforts to gather information of the use of on-site power in the cement industry is required to better estimate to what extent the market has been saturated.

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Based on historic clinker production figures, it is estimated that in the top 20 cement-producing countries alone the overall potential to generate electricity at cement plants is about 57 TWh annually. Total potential is estimated to be about 68.3 TWh/year or 0.41% of total global electricity demand in 2003 (including all sectors: other industries, residential agriculture, etc). If all or even some of the potential were realized, considerable emissions from the central power plants serving these cement plants could be displaced. If it is assumed that all power displaces coal, then about 68.3 Mt CO2 of total global emissions could be displaced every year. Opportunities for either top- or bottom-cycle cogeneration, or both, exist at many of the world’s cement plants.

CONCLUSIONS

In short, while the cement sector is responsible for a disproportionately high percentage of global GHG emissions, there remains significant potential for investment in on-site power technologies that can improve competitiveness, increase reliability and reduce environmental damage. Plant owners that invest in on-site power for their plant have much to gain from increased investment but so too does society as a whole. Incentives must be created to link the larger societal benefits to investments at individual plants. Only with concerted efforts to promote on-site power in cement plants will the greater societal benefits be achievable.

Jeff Bell is Program Director of WADE.
e-mail: jeff.bell@localpower.org

This article is based on a recent WADE report called ‘Concrete Energy Savings: Onsite Power in the Cement Sector’. It is available for free download on WADE’s website: http://www.localpower.org/getreport.php?id=944

BIBLIOGRAPHY

The cement-making process

Cement production involves huge amounts of natural resources, capital, labour and energy. Various types and qualities of end-product exist and a variety of processes are used in the making of cement. The most common manufacturing processes include shaft kiln, wet kiln, dry kiln and precalciner. Figure A shows a simplified diagram of the general manufacturing process.


Figure A. Simplified cement-manufacturing process diagram with main energy flows. Source: WADE, from various sources
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The raw material - limestone, along with much smaller quantities of clay or sand - is quarried, transported to the mill, crushed, blended and milled. The mixture is then fed into preheaters which feed a large rotating kiln.

Within the rotary kiln the mineral mix undergoes chemical reaction under extremely high temperatures (between 1480°C and 1870°C), fuelled by a coal-, oil- or gas-fired burner. Out the other end of the kiln emerges the new material, clinker, which is then cooled, ground and blended with various additives depending on the type of cement being produced. For example, small amounts of gypsum or other additive can be added to the cement prior to sale to control setting time. The finished product is then ready for shipping.

Because the process requires so much heat, opportunities for heat recovery are abundant. With the precalciner process some of the waste heat from the rotary kiln is used to preheat the raw materials on their way to the kiln, but much scope typically exists for improved energy recovery. The fact that a reliable supply of electricity is also required to make cement suggests on-site power applications are a natural opportunity that deserves attention.

There are various reasons waste heat recapture opportunities have not been taken fully advantage of. In some cases barriers are technical, for example when exhaust gases first leave the kiln the gases contain dust particles and contaminants which must be removed to protect turbine blades. New solutions for overcoming technical challenges which prevent economic heat recovery are constantly being sought and much progress has been made in the last 20 years. In other cases capital cost for the necessary equipment can be prohibitive, and in still other cases local regulations can make investing difficult. Another major obstacle is that energy efficiency is not a priority compared to production capacity expansion or cement quality assurance. These non-technical barriers are often the most challenging to overcome.

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