Case study: Resource recovery

• By Gerald F. Connell
• Jun 01, 2000

Adding a ton of slag during the kiln stage of the cement making process eliminates almost a ton of carbon dioxide

"Byproduct synergy" — just another catchy yet essentially meaningless buzzword? Not according to one cement and structural steel company. Dallas-based Texas Industries Inc. (TXI) has taken the lofty ideals of industrial ecology and applied them to something as everyday as cement. The CemStar^SM process significantly decreases both carbon dioxide (CO₂) emissions and fuel consumption, while increasing cement production by an average of 10 percent — all through the use of a byproduct of steel manufacturing previously used as road fill — slag.

A new take on an old idea

Slag — the scum that collects on top of molten steel — has been used in cement making since 1774. Previously, however, it was assumed it had to be ground into a fine powder and injected into the flame end of the kiln, an energy-intensive and expensive process. The result was that few cement makers bothered to use slag.

In 1994, two TXI employees hit upon a new idea. Rom Young, a cement chemist at the company's Midlothian, Texas, cement manufacturing operation, and Libor Rostik, vice president of engineering at TXI's adjacent steel-producing subsidiary, Chaparral Steel, realized the potential of what lay just next door for each of them.

The new process calls for injecting 2-inch chunks of electric arc furnace slag into the cement kiln at the start of the process, eliminating the grinding step. The slag, which contains alumina and iron compounds, is mixed with the raw feed that is made up of limestone, sand and clay. Due to the slag's chemistry, which is similar to finished clinker (the main ingredient in Portland cement), and its low melting point, it readily mixes and bonds with the other ingredients in the kiln.

Saving energy

Slag and clinker essentially come from the same raw materials, but slag has already been subjected to calcination, the heat process that clinker receives in the kiln. Calcination removes carbon dioxide and forms clinker's principal compound, dicalcium silicate, the building block for Portland cement. This saves energy in the cement process, since this step doesn't have to be repeated.

Slag's low moisture content also translates into substantial moisture reduction and fuel savings when the process is used in wet process operations, as at TXI's Midlothian plant. The result is that one ton of slag yields a ton of incremental clinker and eliminates almost a ton of CO₂ emissions.

The process also reduces the need for raw materials such as clay or shale. These are significant sources of hydrocarbon and sulfur emissions, as well as being expensive raw materials. The slag supplements production by contributing additional constituents to the mix essential in the production of cement such as silica, aluminum, iron and calcium.

According to TXI, Portland cement made using the new process is indistinguishable from that made with standard raw materials. The process allows plants to expand production without the heavy capital expenditures required for traditional capacity enhancement projects. At TXI's Midlothian plant, production has increased 10 percent on average with no plant expansion, virtually no additional energy requirements and a reduction in emissions.

Lowering emissions

The process reduces hazardous air emissions of CO₂, nitrogen oxides (NO_x) and sulfur dioxide. The slag has already been processed at a high temperature in steel production, with many chemical reactions having taken place before it is added to the cement process. This contributes to a lower required processing temperature and associated lower fuel consumption.

A reduction of approximately 0.75 tons of CO₂ emissions can be achieved for every ton of steel slag processed. Additionally, reductions in NO_x emissions also take place. A reduction of approximately 0.06 tons of NO_x emissions occurs for every ton of slag processed at the plant.

In 1999, TXI calculated that its Midlothian plant had reduced emissions of CO₂ and NO_x by 241,707 tons and 18,499 tons respectively since 1994 through use of the CemStar process.

In addition to the company's own facilities, eight cement plants throughout the nation belonging to competitors have licensed the process (see Playing matchmaker below.) In total, plants using the process have reduced CO₂ emissions by 500,000 tons a year, based upon emissions totals reported to state and federal environmental agencies. According to TXI, the process has the potential to reduce annual CO₂ emissions in the U.S. cement industry by 9 million tons. Worldwide CO₂ reduction potential is 90 million tons. In addition, companies may one day receive carbon-trading credits for their byproduct synergy efforts.

Fuel reduction

Use of the process also results in lower fuel usage per ton of cement produced. Tests at the Midlothian plant showed a best-case fuel reduction of 0.69 million British thermal units (MMBtu) for each ton of clinker produced during the test. Average cases yielded a lower reduction of approximately 0.545 MMBtu per ton of clinker produced. According to TXI, use of the process at the plant has resulted in a reduction of carbon-based fuel use of more than 2,725,000 MMBtu since the beginning of the program in 1994. This equates to approximately 115,460 tons of coal. Use of the process at the projected annual rates of 100,000 tons a year will result in an annual reduced fuel demand of between approximately 54,500 and 69,000 MMBtu, generating considerable cost savings.

Good timing

The innovation could not have come at a better time. U.S. cement production falls far short of demand. According to the Portland Cement Association, an annual production shortfall of approximately 20 million tons is filled by imported cement and clinker from Europe and Asia. Transportation and handling increase the cost of imported cement significantly. According to TXI, the process provides an alternative to assist domestic cement companies in lowering the U.S. cement trade deficit at the lowest alternative cost.

The process has gained recognition. In 1999, it won two major awards from the U.S. Environmental Protection Agency (EPA) — a Climate Wise Special Recognition Award and the Climate Protection Award. EPA recognized significant benefits of the process, primarily a reduction in emissions. However, for industry the concomitant cost savings are just as important.

Reaping the profits

TXI's environmental innovation has translated into increased profitability for TXI. Steel slag that once sold for $3 to $8 a ton is now converted into Portland cement that sells for an average of $70 a ton. TXI also collects royalties from competitors who have licensed the process. The company now derives substantial profit from Young and Rostik's brainstorm.

The CemStar process illustrates an important point: The principles of sustainable development make good business sense. To take advantage of byproduct synergy, businesses must be willing to widen their focus and seek creative alternatives. Byproduct synergy requires a collaborative approach that breaks down the traditional barriers between industry sectors, individual companies and countries. Joining forces to consider cross-industry synergies benefits the parties involved financially while reducing pollution and promoting sustainable development.

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Playing matchmaker

The Business Council for Sustainable Development for the Gulf of Mexico (BCSD-GM), of which TXI is a member, acts as a matchmaker between companies interested in participating in a byproduct synergy project. The Gulf Council (www.bcsdgm.org) is a member of a worldwide network of Business Councils for Sustainable Development led by the World Business Council for Sustainable Development (www.wbcsd.ch), based in Geneva.

One Gulf Council project involves 21 major companies in the Mexico seaport of Tampico. The participants, who include chemical and petrochemical companies as well as the local Coca-Cola bottling company and an electricity producer, met several times over the course of a year. They collected data on each company's materials and energy flows, eventually identifying 29 potential synergies with immediate commercial possibilities.

Participants decided initially to pursue 13 demonstration projects. One company's 51,000 tons of unusable butadiene (a hydrocarbon used in making synthetic rubber) became a source of cheaper combustion gas for another industry. An industrial-gas company plans to manufacture CO₂ using waste CO₂ generated by several nearby businesses. Another company converted its polyvinylchloride residuals into shoe soles.

Other synergies involve sets of companies that generate the same byproduct. While individually the companies can't afford to recover and resell the wastes, together they can. In one case, six companies that together produce 134 tons per year of polyethylene/polypropylene wastes will sell the waste to a seventh company that plans to build plastic platforms for ship-loading operations.

Paradoxically, these companies have gained an edge over their competitors by adopting a cooperative rather than confrontational approach through their willingness to experiment with innovative methods of recycling resources.

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This article appeared in Environmental Protection magazine, June 2000, Vol. 11, No. 6, p. 77.

This article originally appeared in the 06/01/2000 issue of Environmental Protection.