Solid Waste & Recycling

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The Cement Kiln Option

Every time a person pitches an empty bread wrapper or other piece of film plastic in the garbage, they commit a valuable energy resource to a fate of slow and uselessness decay in one of our many grow...


Every time a person pitches an empty bread wrapper or other piece of film plastic in the garbage, they commit a valuable energy resource to a fate of slow and uselessness decay in one of our many growing landfills in North America.

It does not have to end this way for these materials.

Many European countries are far ahead of North America in terms of recycling and the efficient utilization of byproduct streams. Specifically, cement kilns have long played a role in capturing the value in these good combustible streams while also reducing the amount of material going to landfill. In Europe, most municipal waste is segregated into recyclable materials and then the combustible fraction suited for energy capture in a cement kiln or purpose-built waste-to-energy plant.

Such energy utilization conserves landfill space, which is at a premium in many European regions; and even helps meet greenhouse gas emissions targets through the replacement of conventional materials with greenhouse-gas neutral materials. Such fuel substitution can also reduce operational costs at cement plants.

These efforts have been reinforced further through legislation referred to as the European Directive. One part of this directive now prohibits the landfilling of materials that contain more than 6 Gigajoules/metric tonne of heat value. In comparison, thermal coal typically ranges in heat value from 22 to 24 Gigajoules/metric tonne.

Cement plants in North America and countries around the world are increasingly participating in such opportunities. For example, millions of scrap tires are used for fuel every year in cement kilns. It’s worth mentioning up front that a huge benefit of cement kilns is that the cost of stand-alone waste-to-energy incinerators is avoided. Policy discussions about waste-to-energy in Canada sometimes overlook the fact that large cement kilns are already up and running, generating an important product, and already consume fuel (e.g., coal) while meeting strict emissions standards.

The process

To better understand how materials going to landfill can be redirected for use by the cement industry, it’s necessary to understand the cement manufacturing process.

First, raw materials that contain lime, silica, alumina and iron are combined in exact proportions to produce a raw mix. The mix is finely ground in a roller or ball mill and fed into a kiln where it’s heated to a temperature of over 1400C (2500F). The raw materials reform into a “gravel like” material called clinker. After the clinker has cooled, it’s finely ground with a small percentage of gypsum to produce the fine powder commonly referred to as “Portland cement.” The majority of the Portland cement is then shipped to plants where it is mixed with water, sand, rock and other additives to produce the common building material called “concrete.”

Opportunities for the beneficial utilization of byproducts exist in two parts of the cement manufacturing process: in the replacement of the conventional raw materials that make up the raw mix, and in the replacement of conventional fuels with other combustible materials.

Replacing raw materials and fuels

The raw materials mix is comprised of materials containing lime, silica, alumina and iron. The conventional sources of these chemical elements are mined materials such as limestone, sand, clay, shale and iron ore. However, byproducts containing these elements also can supply the needed chemical components. The cement industry has long played a role in the reduction of materials going to landfill through the use of byproducts that include (but are not limited to): spent foundry sands; smelter slags; spent abrasives sands or slags used for metal cleaning; metal impacted soils; alumina bearing sludges; or spent catalysts used in industries such as oil and gas processing.

The amount of these materials that a cement plant can use is limited by either the volume of material available for use or by the chemical composition of the byproduct. Any chemical limitations can be assessed and determined by cement plant personnel on a case-by-case basis.

The heating and subsequent burning of the raw mix in the cement kiln is a highly energy intensive process that consumes large quantities of combustible materials. The conventional fuel of choice for the cement industry is coal; however, cement kilns are fairly flexible and can utilize many different types of material for fuel.

Cement kilns are well suited to safely use a variety of fuel sources with their relatively long burning-zone residences times of up to three seconds, high burning-zone temperatures (+1400C) and the alkaline environment that naturally exists in a kiln. Furthermore, cement kilns are already equipped with elaborate process-monitoring equipment and dust collection systems so that alternative fuels can be readily introduced to a kiln with only the addition of appropriate alternative fuel-handling and feed systems.

The aforementioned attributes allow cement kilns to safely use both hazardous and non-hazardous materials for fuel. Typical alternative fuels include: the combustible fraction of municipal solid waste; wood; plastics (PVC and other chlorinated materials should be limited); waste oils and solvents; whole and chipped tires; textiles, carpets and foam; and meat and bone meal from the rendering process. Other cellulose products ranging from coated paper to the cut-out pieces from the production of diapers are also used.

The challenge in using these alternative fuel materials is often in the segregation of usable materials from a mixed stream. For example, streams such as municipal solid waste have a significant combustible fraction; however, this fraction needs to be separated from the non-combustible components. This can be accomplished through a number of different proven processes, the most common utilizing a series of screens and air separation systems.

After materials are isolated, there also may be a need to reduce the size for proper combustion in the cement kiln. This size reduction depends on the specific material and also where it will be introduced on the kiln line (as newer vintage kilns have multiple points where fuel is introduced). The size reduction can be done using readily available shredding and grinding systems that are available from a number of manufacturers.

Permitting can also be a significant hurdle for a cement kiln to overcome. Significant amounts of time and effort are needed to educate the public, regulators and other officials of the benefits in the utilization of these alternative materials.

Regardless of these hurdles in using these alternative streams, the benefits in the utilization of these byproduct materials are clear.

Win-win situation

The use of alternative raw materials and fuels by the cement industry represents a win-win situation for both the environment and the companies utilizing these materials. The use of byproducts replaces conventional materials that would otherwise have to be mined or quarried. Diversion of materials from landfill eliminates any long-term liability from materials decomposing in a landfill and potentially leaching into the environment.

Many of these byproduct streams are considered greenhouse gas neutral when used in this manner and therefore can play a role in helping companies and countries meet their Kyoto Protocol commitments.

Many opportunities in the beneficial utilization area lie ahead for the cement industry. Lafarge is one such company that is aggressively moving ahead throughout North America with a number of innovative fuel initiatives modeled after successes achieved in Europe.

For example, one Lafarge plant in Austria obtains in excess of 40 per cent of its fuel requirements from non-hazardous alternative fuel sources which include tires, meat and bone meal and a solid fuel stream composed of various municipal, commercial and industrial byproducts. Several of Lafarge’s kiln’s in Ohio and Kansa

s currently get even higher fuel replacement rates through the use of hazardous wastes as well as non-hazardous materials.

Lafarge personnel believe that these efforts speak to the belief that the beneficial utilization of materials in an economically, socially and environmentally acceptable manner is a preferred option to disposal into the environment. This practice also is aligned with sustainability of our natural resources.

Randy Gue is one of five Lafarge Directors of Resource Recovery working throughout North America. He is based in Vancouver, British Columbia and is responsible for the Western Region of North America. Contact Randy at Randy.Gue@lafarge-na.com or at www.lafargenorthamerica.com

Lafarge’s Bath Plant Proposal

Lafarge’s Bath, Ontario plant (located about 30 minutes west of Kingston) has worked with the public and regulators for almost two years to gain approval for the use of some select alternative fuels. Specifically, Lafarge’s Bath plant personnel are requesting approval from Ontario’s Ministry of the Environment to use up to 100 tonnes per day of the following alternative fuel materials: 1. Scrap tires; 2. Plastics; 3. Meat & bone meal; 4. Solid shredded materials including a small amount of fuel pellets.

The plant has traditionally used coal and petroleum coke as its primary fuel source (like many cement kilns around the world). The goal of the project is to partially replace the conventional fossil fuel while diverting combustible materials from landfills or similar out-of-province disposal uses. It’s anticipated that the use of some of these alternative fuel materials will be considered greenhouse-gas neutral, which will assist Canada meet its Kyoto commitments.

Considering scrap tires, the residents of Ontario generate over 10 million passenger tire equivalents (PTE) per year. Much of this volume is already shipped out of the province for use as a fuel. The Bath plant could consume as many as three million of these tires annually, thus keeping the benefits of energy recovery at home.

Another stream of alternate fuel that the plant would like to utilize is material generated by institutional, commercial, and industrial (IC&I) businesses, much of which are destined for landfill. Source segregation, sorting and size reduction will yield a high quality fuel stream for use at the cement plant.

Fuel pellets will be produced from curbside household waste in a separate process being proposed by a third party. The household material will be sorted to remove the recyclables and hazardous components. The remaining volume will be further processed and then finely shredded and mixed with other non-recyclable combustible materials to form a pellet for fuel use both in Ontario and in export markets.

In fact, the use of alternative fuels in the cement industry has a fairly long history in Ontario. St. Lawrence Cement (located in Mississauga) currently has a Certificate of Approval (C of A) to combust and recover the energy in some used liquids. St. Lawrence has been using these streams since 1971. Essroc in Picton has a C of A to use scrap tires but has not used any tires to date.

Moving forward, in order to use alternative fuels in Ontario, cement plants must now meet a set of emission standards known as Guideline A-7. These standards are among the strictest in the world. Based on past experience, Lafarge personnel are confident that the Bath plant operation can meet these standards. The Bath plant will be the first plant required to meet the latest revision of these new emission standards in the province.

In November 2005, Environment Minister Broten determined that the Bath alternate fuel project should be reviewed under the Environmental Protection Act (as opposed to the Environmental Assessment Act). As a result of public and ministry comments, Lafarge made minor changes to the proposal that were posted on the Environmental Review website for public comment.

The project has now been posted on the Environmental Review site a total of three times. Over 3000 people have been personally briefed on the merits of the project, over 600 pages of scientific reports have been submitted, and 11 government agencies have commented on the project during the past two years of public consultation. Lafarge staff anticipate a decision by the ministry this summer. (Editor’s note: The ministry’s decision will be reported in this magazine and also on our website atwww.solidwastemag.com)


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1 Comment » for The Cement Kiln Option
  1. saeed says:

    Hi,I am cement engineer and I need information about cement klin
    tanks

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