Tires fuel oil field cement manufacturing

Bill Caveny
Daniel Ashford
Halliburton Energy Services Inc.
Duncan, Okla.

Jose G. Garcia
Capitol Cement
San Antonio

Rex Hammack
Halliburton Energy Services Inc.
Alice, Tex.

• An hydraulic lift tilts a trailer, causing tires to fall into a hopper (Fig. 2 [14,480 bytes]).
• A conveyer system transports tires to the kiln (Fig. 3 [23,125 bytes]).
• CEMENT PHOTOMICROGRAPHS: Angular alites and rounded belites are common in samples of Capitol's conventional cement (left [21,154 bytes]).
• A sample of Capitol's tire-burn cement reveals the characteristic good signs of angular alites and rounded belites. The white C4AF matrix is another indication that the tire-burn cement does not differ from conventional cement (right, Fig. 4 [22,192 bytes]).

In a new process, waste automobile tires added to the fuel mix of gas, coal, and coke help fire kilns to produce API-quality oil field cement.

Capitol Cement uses this process in its cement-manufacturing plant in San Antonio, in which it also produces construction cement.

The tires provide a lower-cost fuel and boost the temperature at a critical stage in the kiln "burn" process. Also, steel-belted tires add iron content to the mix.

According to lab results, tire-burned cement slurries will perform the same as conventionally burned cement slurries (Table 1 [30,004 bytes]). Actual field applications have proven that cement produced by burning tires performs no different than conventionally produced slurries.

Tires have previously been included in the manufacturing process of construction cement, but attempts to use them to produce oil field cement have generally failed because these processes did not produce an oil field cement with the required specifications.

Capitol developed an automated system of incorporating tires into the burn process while working closely on cement-quality issues with the Halliburton Energy Services Inc. Duncan, Okla. technology center. Fig. 1 [106,616 bytes] illustrates the manufacturing process.

Capitol's plant uses both dry and wet processes, with separate kilns running both processes at the same time. Cement clinker is partially fired by waste tires in both kiln processes.

The tires represent 12% of the fuel consumed by the plant, a number that is expected to increase. Capitol burns about 200 tires/hr, or about 1.6 million tires/year.

Material source

A limestone quarry, part of the upper Cretaceous Austin group, lies a few yards away from Capitol's San Antonio plant. The quarry provides an ideal source rock for cement.

At Capitol's quarry, the Austin lies on the upthrown side of a fault and is adjacent to the normally overlying Anacacho (Pecan group) limestone of the Taylor group. Thus, the same quarry has the primary calcareous resource rock (CaCO₃), the Austin, as well as the Anacacho marls that are a source of Si₂, Al₂O₃, and Fe₂O₃.

Oil field cement

Portland cements must meet certain chemical and physical standards, depending on their application. In the U.S., API provides specifications for oil well cements. The basic cement raw materials are limestone (calcium carbonate), silica, and clay or shale. Iron and alumina are frequently added if the clay or shale contain insufficient amounts.

In the cement-manufacturing process, the raw materials are first blended, either wet or dry. Then, the materials are fed into a rotary kiln that fuses the limestone slurry at temperatures from 2,600 to 3,000° F. into a material called cement clinker.

Upon cooling, the clinker is pulverized, with about 5% gypsum, into a powder known as Portland cement. The cement may be stored in bulk or in bags.

Cement clinker is composed of four major crystalline phases, as follows:

• 3CaO*SiO₂, tricalcium silicate (C₃S)
• 2CaO*SiO₂, dicalcium silicate (C₂S)
• 3CaO*Al₂O₃, tricalcium aluminate (C₃A)
• 4CaO*Al₂O₃*Fe₂O₃, tetracalcium aluminoferrite (C₄AF).

For oil well cementing, cement properties must be consistent and dependable enough to meet, at times, extreme conditions such as:

• Temperature ranges from permafrost, below freezing, to more than 600° F. in geothermal wells
• Pressures as great as 20,000 psi
• Salt water cross flows that tend to dislodge the slurry during pumping and after it is placed
• Density effects caused by cement columns as high as 10,000 ft
• Compressive strength of at least 1,000 psi after curing for 24 hr
• Flexibility to withstand possible shifts in the earth or other shearing forces.

Tire-burn system

The tire-burning systems was successful because of the automation and knowing exactly when and how to drop tires into the kiln.

In its process, Capitol follows a fully automated tire-feed process, burns whole tires, automatically measures the amount of tire material consumed, inserts tires into the kiln when temperature is high, and monitors cement by microscopy.

In the past, others had tried manually inserting the tires, burning whole tires, and chipping tires into small chunks. These attempts failed.

Manual tire insertion introduced an unpredictable human factor. It left too much of the process to chance by losing the benefit of weight metering and other quality-control measures.

Burning whole tires at the beginning of the kiln burn process can cause "cool" burning, which adversely affects critical properties of oil field cement.

Chipping tires added cost to the process and led to the accumulation of chipped rubber in the kiln.

Capitol's process is about 90% automated. From the time the tires are dumped from the cargo truck into the hopper (Figs. 2 and 3), the process runs automatically.

Along the route to the kiln, the tires move along conveyers and pass sensors that determine if the tires are too large or too small to be used. The sensor data also allow control of the fuel proportion furnished by the waste tires.

Tires not meeting specifications are recovered by the supplier for other disposal methods. Capitol employees do not routinely handle the tires at any point in the process.

Each time the kiln revolves, two tires are fed into it when kiln temperature is above 1,900° F. Therefore, the tires burn "hot" and do not produce soot. From the point of insertion onward, tires make a significant contribution to kiln heat, and are superior to coal in BTU output by weight.

In Capitol's wet manufacturing process, the raw materials are mixed in water and fed into the kiln as a slurry. Heated chains hang into the kiln to rapidly raise the temperature of the wet mixture. Tires are inserted on the hot side of the chain section.

In conventionally fired operations, kiln temperature just after the chain section is 1,200° F. The post-chain temperature averages 1,600° F. when tires are inserted on the hot side of the chain section. This temperature boost reduces energy requirements for the cement in the burn zone, and reduces kiln damage and consequently maintenance.

Microscopic evaluation

Microscopy is one key to Capitol's successful manufacturing process. Halliburton and Capitol have long advocated that cement quality control include microscopy to ensure that a highly consistent, good-quality oil well cement is produced.

Both use microscopy techniques to examine clinker samples and cement. A large amount of data has been accumulated from both chemical and microscopic analysis.

Microscopy helps ensure proper crystal formation, size, and homogeneity to ensure a clean burn.

Other cement manufacturers have tried but failed to produce a quality, tire-burn, Class-H oil well cement. The cements failed to meet API specifications and exhibited properties of premature gelation and poor response to retarders and other chemical additives.

Two procedures that could have contributed to poor cement quality are:

1. Chipping the tires into small pieces for burning
2. Adding the rubber early in the kiln process when temperature was low.

Before full-scale production was started both Halliburton and Capitol, evaluated a small batch of the new tire-burn cement. This experimental batch was found to be nearly identical to conventionally produced Class-H cement.

The tire-burn Class-H cement was evaluated and compared to regular Class-H cement with a microscopic examination, a complete chemical analysis using quantitative X-ray diffraction, a routine clinker and oxide analysis, a particle-size distribution, and a series of physical tests.

Polished sections of the samples were prepared by routine methods and etched with a CDTA (cyclohexaneamine-tetraacidic acid) solution. Microscopic examination showed almost identical characteristics between the regular and tire-burn Class-H samples.

The alite (C₃S) and belite (C₂S) crystals were similar in size, color, and general appearance (Fig. 4). The matrix, C₄AF-white phase, was also similar, with a fairly clean, white appearance in all samples.

The whiteness of the matrix in the photomicrographs indicated that a clean burn was achieved in both the traditional and tire-burn fueling methods. These photomicrographs were so similar that one could believe they all came from the same cement sample.

Chemical analysis of the oxides present in regular and tire-burn samples also showed consistency between the two samples.

Cement performance

For a recent job, laboratory tests were conducted in which various properties of a tire-burn cement slurry were compared to a slurry made from conventionally burned cement.

The lab tests were designed to simulate the pressure and temperature conditions for a particular well. In percentage by weight of cement, both cement slurries contained 53% water, 35% silica flour, 37% NaCl, 14% high-density additive, 0.5% friction reducer, 1.3% fluid-loss additives, and 0.1% retarder.

The variances between the two slurries were within normal ranges for any two cement batches. Viscometer data collected for the two slurries were within the range expected from any two separate cement batches. Rheological tests also indicated that tire-burned cement is an alternative to conventionally burned cement.

In practice, cements created by burning tires can be managed and used in the same ways as cements created with conventional fuels. Tire-burn cements present no unusual difficulties in bulk handling and no abnormalities with clumping or cohesion. These cements do not produce excessive dust or any other adverse features that would affect cement performance or usage.

Tire-burn cements do not appear to have any unusual sensitivities to different waters. Examination of the performance of Capitol's tire-burn cements shows them to be indistinguishable from conventionally burned cements.

Environmental impact

Capitol's tire-burning process meets state and federal air-cleanliness guidelines. The company uses electrostatic precipitators to control dust output from its wet process and a bag-house filtering system to collect and control dust produced by the dry process.

Early in the process of planning to include waste tires as fuel, Capitol realized that the San Antonio government and many private persons would be alarmed at burning waste tires within city limits. To overcome these fears, Capitol invited city and school officials, the media, and representatives of civic groups to visit the plant, hear a briefing on its plans, and discussed its proposal to safeguard air quality.

Since the operation began in 1996, Capitol has hosted numerous plant tours to show the tire-burning process. The company has produced and distributed a 5-min video that explains tire-burn cement manufacturing and cites the many environmental benefits derived from the process.

The Authors