Treating waste productively by the cement industry
As the world population grows and the average income goes up, the amount of associated waste will also increase dramatically. It is estimated that the US (1) generates over one hundred tonnes of waste per person during the person's lifetime; with the average US citizen's lifespan of 77.9 years, that translates to more than 3.5 kilograms per day. While India may be way behind in this respect, urbanization, industrialization and modern lifestyles are contributing to an increase in the amount of waste generated in the country, gradually reducing the gap between the two countries.
Interestingly, waste is increaingly being considered as a potential alternative resource which could be used in industrial processes. In order to reduce consumption of valuable primary resources and to safeguard precious land, landfilling of waste is increasingly deincentivised or viewed as a temporary option or last resort before alternatives are found and implemented.
Global demand for construction materials such as concrete ( for which cement is the glue) is steadily rising due to the expanding population, growing economies and accelerating urbanization. The Indian cement industry is the second largest in the world with a total installed capacity of three hundred and twenty million tonnes per annum (MTPA). The production of cement is basically local. It usually takes place at locations rather close to urban areas, i.e in the proximity of cities where municipal waste is generated.
Co-processing of fuels and raw materials in the cement sector can therefore provide an ideal solution for waste management in urban centres while at the same time benefiting cement companies through the provision of energy and raw materials derived from co-processing.
In response to the escalating rate of waste production on the one hand and the rising resource demands from the cement manufacturing process on the other hand, cement companies around the world have been considering waste as a source of raw material and energy since the late 70's. The sector is now making a considerable and valuable contribution to the waste management system in many countries step by step. When and where waste material is available and cannot technically or economically be avoided, minimized, reused and/or recycled, cement manufacturing provides a more sustainable solution than landfill or dumping due to the full energy and material recovery in the process.
It is estimated that India generates about 6.2 million tonnes of hazardous waste annually, of which 3.09 million tonnes can potentially be recycled, 0.41 tonnes incinerated and 2.73 million tonnes landfilled(2). The Shakti Foundation, an NGO that promotes clean energy, says that 'thermal substitution of just five per cent in the cement industry can reduce India's emissions by about 1.5 million tonnes of CO2 annually.ö Today, the Thermal Substitution Rate (TSR) is below one per cent in the country, compared to about 4.5 per cent worldwide and 13 per cent for CSI members globally (source: CSI Getting the Numbers Right (GNR) database at www.wbcsdcement.org/gnr). Many cement producers are starting to get involved in co-processing activities including some processing of hazardous waste.
Co-processing is a term used to refer to the use of suitable waste materials in manufacturing processes for the purpose of energy and/or resource recovery and resultant reduction in the use of conventional fuels and/or raw materials through substitution. The co-processing of waste is a service that the cement plants can provide to their communities for mutual benefits. For cement plants, the primary benefit is having a reliable local supply of fuel or material that replaces natural resources; while the community and the society at large benefit from a more sustainable local solution that avoids, economically speaking, the large capital expense of incinerators and waste-to-energy plants and ecologically speaking undesirable environmental impacts, increasing land-use and resource inefficiencies associated with landfills.
Governments, municipalities, companies and agricultural organizations that are faced with decisions on how to manage their waste are often guided by the waste hierarchy (see figure 1).
The demand by society for waste prevention and re-use options followed by recycling is growing continuously. The co-processing of waste in a cement kiln is an advanced and innovative recovery process combining material recycling (for the non-combustible part of the waste) and thermal recovery. The mineral portion of the waste is recycled during the process and replaces virgin raw materials. At the same time the energy content of the waste is very efficiently recovered into thermal energy thus saving conventional fuels. Therefore, in the waste hierarchy, co-processing has a position just below recycling, reflecting the added benefits it provides compared to incineration with energy recovery alone.
A well balanced and accepted waste management system should be developed in conjunction with all relevant stakeholders. Co-processing development must be accompanied by a clear, comprehensive and transparent dialogue with neighbours, employees (unions), customers, national regional and local authorities, NGOÆs and waste generators. The dialogue with stakeholders must be based on several key issues relevant to co-processing activities including environmental impact, health and safety, truck-movements, quality management of the alternative resources, potential impact on process and cement quality, reporting and public information.
(i)Alternative raw material use
Selected waste and by-products containing useful minerals such as calcium, silica, alumina, and iron can be used as raw materials in the kiln partially replacing raw materials such as clay, shale, and limestone.
(ii) Alternative fuel
Selected waste and by-products with recoverable calorific value can be used as fuels in a cement kiln, replacing a portion of conventional fuels, like coal or petcoke.
In most cases, a specific waste pre-processing treatment will be carried out in order to economically provide an engineered alternative fuel for the cement manufacturing process which usually includes a homogenization process in order to ensure a uniform source with almost constant thermal properties. After the pre-processing, the alternative fuel produced keeps the status of the input waste and is managed by the waste regulation.
(iii) Alternative raw material and fuel
Because some materials have both useful mineral content and recoverable calorific value, the distinction between alternative fuels and raw materials is not always clear. For example, sewage sludge has a low but significant calorific value and burns to give ash containing minerals useful in the clinker matrix.
For this specific case, these waste types must be treated as a fuel and processed in a high temperature environment where the organic phase is completely destroyed
(iv) Waste or b y-product cement additives
These materials can be used with the clinker to produce different types of cement. They may help to control the setting time of the cement (synthetic gypsum); they may have cement-like properties in their own right (blast furnace slag) or may affect the consistency of the cement mortar. The use of these alternative constituents is extremely important in reducing the environmental impact of cement production. They can reduce the quantity of energyintensive clinker required for each tonne of cement, further reducing CO2 emissions per tonne. Indian cement standards allow up to thirty five per cent clinker substitution by fly ash (compared to fifty five per cent in European Standards) or seventy per cent by blast furnace slag.
(v) Pre-treatment of waste materials for the cement manufacturing process
The key to the safe use of waste derived fuels is understanding the consistency of the fuel utilised in the kiln; pre-processing of the waste is the methodology used to assure both the quality (what is in the waste derived fuel) and the consistency of the fuel.
Feeding points for waste materials to the cement manufacturing process
Given the differences in temperature between different parts of the process, it is important that the waste fuels and raw materials are introduced through an appropriate installation at the correct points in the process to ensure complete combustion or incorporation and to avoid unwanted emissions. For example, raw materials with volatile organic components may be introduced in the cement kiln at the main burner, in mid-kiln, in the riser duct or at the precalciner. They should not be introduced with other raw materials except where tests demonstrate that this will have no effect on gas emissions. Therefore, in most cases, a specific additional installation needs to be built for the input of these materials into the system.
The production of cement requires rigorous control of the chemistry of the main ingredients: CaO, SiO2, Fe2O3, and Al2O3 as well as other minor constituents such as SO4, K2O, Na2O, TiO2 and P2O5. Alternative raw materials and fuels must be used in quantities and proportions with other raw materials in order to achieve the desired balance of material composition in the kiln product clinker. Significant work has been carried out by various groups including industries, universities, research centres and the government to understand the effect alternative fuels and materials have on the final concrete product.
Addressing stakeholders' concerns
Effective measurement, monitoring and transparent and comprehensive reporting of air emissions from the cement manufacturing process contributes to the understanding, documenting and improvement of the industry's environmental performance. This is true in any condition, whether co-processing technology is implemented or not.
More specifically, many stakeholders are concerned or worried about the potential for some specific constituents contained within some waste to influence the cement and concrete quality or being released from the cement product or concrete if used as alternative fuels or as raw material. This concern is heightened as concrete is a major component of residential construction and is often used in pipes. This is not a new question and the domain has been the subject of numerous researched studies for more than twenty years. Aggressive testing carried out by ANSI/NSF Standard 61(3) (a third party certification process for drinking water pipes in USA) has shown that metals in the cement become bound in the concrete calcium-silicate structure and in this form do not leach from the product. Similar results have been reported in many other reports by ATILH(4), CTL(5), Forschungsinstitut der Zementindustrie(6), Cembureau(7), CEN(8), and others(9).
In addition, CSI members follow the guidance on best environmental practices for the prevention or minimization of the formation and subsequent release of unintentional Persistent Organic Pollutants (POPs) from cement kilns co-processing hazardous waste published by the Stockholm Convention Secretariat. See more details on the relevant CSI webpage at www.wbcsdcement.org/emissions.
As recommended in the CSI Guidelines for the Selection and Use of Fuels and Raw Materials in the Cement Manufacturing Process (www.wbcsdcement.org/fuels), there is substantial evidence that cement manufactured from the different types of waste does not change the performance or characteristics of the cement or concrete; high levels of some minor components can affect cement performance and the manufacturer needs to take care that specific thresholds are not exceeded.
3 'A Comparison of Metal and Organic Concentrations in Cement and Clinker Made with Fossil Fuels to Cement and Clinker Made with Waste Derived Fuelsö, Colucci M., Epstein P., Bartley B., NSF International, Ann Arbor, MI, March 1993.
4 'Leaching of Heavy Metals by Mortar Bars in Contact with Drinking and Deionized Water', Germaneau B., Bollotte B., DefossT C., Association Technique de l'Industrie des Liants Hydrauliques (ATILH), March 10, 1993
5 'Leachability of Selected Chemical Elements from Concrete', Kanare H., West P., Construction Technology Laboratories, Inc., March 1993, PCA
6 'Leaching characteristics of cement bound materials containing organic substances and inorganic trace elementsö, Thielen G., Spanka G., Rechenberg W., Forschungsinstitut der Zementindustrie, Dnsseldorf, Germany,
7 'Trace Elements Leaching from Concrete and the Use of Alternative Resources', Cembureau, Feb. 22, 2005
8 CEN, 1999. A study of the characteristic leaching behavior of hardened concrete for use in the natural environment. Report of the Technical Committees CEN/TC51 and CEN/TC 104, European Committee for Standardization, Final Draft, 59p.
9 Interpreting Soil Test Results What do all the numbers mean, Hazelton P., Murphy B., NSW Department of Natural Resources 2007, CSIRO Publishing, Australia.
World Business Council for Sustainable Development
- The World Business Council for Sustainable Development (WBCSD) is a CEO-led organisation of progressive companies that stimulates the global business community to create a sustainable future for business, society and the environment. Together with its members, the council applies its thoughts and effective advocacy to generate constructive solutions and take shared action. Leveraging its strong relationships with stakeholders as the leading advocate for business, the council helps drive debate and policy change in favour of sustainable development solutions.
- The WBCSD provides a forum for its two hundred member companies - who represent all business sectors in all continents with a combined revenue of more than $7 trillion - to share best practices on sustainable development issues and to develop innovative tools that can change the status quo. The Council also benefits from a network of sixty national and regional business councils and partner organizations, a majority of which are based in developing countries.
Cement Sustainability Initiative (CSI)
The Cement Sustainability Initiative (CSI) is a global effort by twenty four leading cement producers with operations in more than a hundred countries. Collectively these companies account for around thirty per cent of the world's cement production and range in size from very large multinationals to smaller local producers. In India, the CSI members account for sixty per cent of production. All CSI members have integrated sustainable development into their business strategies and operations as they seek strong financial performance with an equally strong commitment to social and environmental responsibility. The CSI is an initiative of the World Business Council for Sustainable Development (WBCSD).