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Carbon monoxide recovery and utilization in special applications.

The CO-rich gas produced at the QIT-Fer et Titane Inc. smelter in Sorel, PQ, has been recovered, pressurized, dried and cleaned for use as the principal fuel source by its subsidiary plant, Les Poudres Metalliques du Quebec in the production of metal powders. The project, funded in part by Enerdemo, was completed in 1986 and since then has replaced up to $1-million per annum of purchased natural gas.

This was not a simple routine alternate fuel application like dual fuel burners on a boiler system. Many new features had to be developed which were not current practice for QIT, accustomed to handling large volumes of low pressure gas for miscellaneous heating applications throughout the smelter complex.

The PMQ application required the development of burners and control equipment to handle a gas with very different characteristics from natural gas such as calorific heating value, combustion air requirements, toxicity, flame temperature, dirt and water content and irregular constancy of supply. Dual burners were designed for the Drever furnaces featuring automatic switch-over from one fuel to the other in the event of CO supply curtailment without upsetting the critical temperature profile in the iron annealing operation.

Finally the project entailed the design of a fully integrated system to ensure the safe handling and combustion of a pressurized toxic fuel.

Carbon Monoxide Recovery System

The QIT-Fer et Titane Smelter located in Sorel, PQ operates nine smelting furnaces with a total combined power input of 435 megawatts.

Ilmenite ore containing iron and titanium oxides is reduced with coal in the furnaces providing two main products - Sorelslag an enriched TiO2 bearing raw material used extensively by the paint pigment industry and Sorel-metal a high purity pig iron used in the foundry industry. A major by-product of the reduction process is the off-gas, rich in carbon monoxide and hydrogen representing a valuable source of fuel gas produced at a rate of 42,500 scfm at design capacity. This gas, with a total fuel value of 800 million BTU per hour is used for steam generation, kiln firing, plant heating and other process requirements. In spite of the extensive use of furnace off-gas in the smelter, significant quantities of gas are periodically flared to the atmosphere as excess waste gas.

An adjacent plant Les Poudres Metalliques du Quebec Ltee., owned and operated by QIT-Fer et Titane, used up to 770 MSCFD purchased natural gas as fuel for heating and process requirements in the production of specialized iron powder products. In 1983 development work was undertaken to develop fuel transmission and combustion systems that would allow PMQ to take advantage of this proximity to a large source of surplus gas.

The Starting Point

The furnace gas collection system, in use for a number of years, to supply the process gas requirements in the smelter is shown in Figure 1 (not shown). The gas, containing a high quantity of fume and particulate material, is extracted from the furnaces via disintegrator scrubbing and moisture elimination units. The supply pressure at the exhauster discharge is normally in the range of 1-2 pounds per square inch, a pressure suitable only for very short transmission distances using large diameter pipelines. After this primary cleaning the smelter gas is, compared to natural gas, still loaded with particulate material and is saturated with water vapour. The equipment selection and configuration developed for the new supply system is shown in Figure 2 (not shown).

Gas Compression Drying and Cleaning

The NASH compressors installed in the plant are ideally suited for the application. Nominal discharge pressures of 25 lbs/in 2 allowed transmission of the fuel to PMQ using standard size 12" diameter steel pipe. These compressors use a continuous flow of water as the sealing medium between the impellers and compressor housing which has several advantages.

To dry the gas, a refrigeration process was selected as the best available technology to cool the gas from 100 degrees F discharge temperature after compression to an exit gas temperature of 40 degrees F.

As a final step before transmission to PMQ the gas, containing particulates in the size range of 1-10 microns, was filtered in a pulse jet bag filter.

High Pressure Storage

To minimize interruptions in the supply of fuel to PMQ a high pressure storage facility, providing 50 minutes retention time, was installed to ensure backup during short outages of the smelter gas collection system. Statistical analysis of operating records for the smelter gas system determined the appropriate storage capacity for the standby supply, taking into account the capital cost of the high pressure system and the savings in purchased natural gas that it would provide. The compressors selected were of the reciprocating diaphragm type manufactured by Pressure Products Industries Ltd. The discharge pressure obtainable with this equipment provides gas storage at 150 pounds per square inch in the high pressure system. To ensure that submicron particulate material did not result in long term damage to the sensitive metal diaphragm material an in line cartridge filter was installed upstream of the compressor inlet.

Safety Hazards and Design Considerations

The safety hazards associated with handling carbon monoxide gas at high pressures were recognized at the beginning of project development and given high priority during all phases of engineering and design. The toxicity of carbon monoxide is a concentration-time dependent function as shown in Figure 3 (not shown). In the compressor station facility detailed design review identified the potential leaks, resulting from equipment failure. Continuous systems for monitoring of carbon monoxide were employed to provide early warning alarms should the concentration exceed 50 parts per million in any area of the plant.

At PMQ flame detection and safety lockout systems were designed to meet or exceed Canadian codes applicable to industrial combustion equipment.

Combustion Characteristics of the Co-Product Fuel

The design of combustion systems at the PMQ plant provide for dual fuel firing of natural gas and co-product gas using the same burner for either fuel, with automatic switchover from one fuel to the other. The composition of the coproduct fuel, shown below has combustion characteristics that differ significantly from natural gas.

CO- 83-87% N2, CO2 1-2%

H2- 10-15% H2S 130-150 PPM

(by volume)

The co-product gas has a fuel value of approximately 320 BTU per cubic foot resulting in a significantly different fuel to air ratio as compared to natural gas. To achieve the equivalent heat input therefore requires a volumetric input for three times that of natural gas. As shown below, both fuels, mixed in stoichiometric proportion with air result in a nearly identical fuel value per cubic foot at the combustion point.

Placement Instruction:

Fuel Air Ratio diagram

Even though the heating value of the fuel air mixture for natural gas and carbon monoxide is the same the combustion characteristics are quite different. Figure 4 (not shown) shows that higher flame temperature, much wider range of combustibility and greater flame speeds are inherent characteristics that result with the carbon monoxide-hydrogen fuel mixture. The combination of high flame temperature and speed requires careful selection and sizing of combustion flow control equipment.

PMQ CO Fuel Distribution

As shown in Figure 5 (not included), there were five major areas which required conversion. The most critical application was on the Drever annealing furnaces due to the large number of burners involved (in excess of 50) and the need to avoid any temperature disturbance when switching from one fuel to another. Considerable effort was expended with a test unit before the proper burner configuration was developed. It was found that due to the higher flame speed and wide flammability range of the CO gas, the normal practice of pre-mixing with primary air could not be tolerated. Consequently, as shown in Figure 6 (not shown), design changes had to be made to the North American Combustion Ltd. burners. The control system was more complex than usual as indicated in Figure 7 (not shown) in order to control firing conditions with two different fuels.

The other users with single nozzle mix burners were generally simpler to convert but the product dryer was of the inspirating type and resulted in an unacceptably noisy operation. As a short-term solution two burners, one for each fuel were used; ultimately a forced air nozzle mix configuration will be installed.

Conclusions

Successful completion of this project is evidenced by the 90% reduction in the use of purchased natural gas, achieved within a few months of completing the installation. This is not to say that problems were not encountered and indeed much was learned during the commissioning stage of this unique project. In the compressor stations the NASH compressors functioned very well and have proven to be reliable machines. The gas drying and polishing systems have not reached anticipated performance levels, with entrained moisture carryover and migration of very fine particulate matter being the major problem areas. Due to higher flame temperature the co-product fuel was found to be much more efficient than natural gas. This resulted in lower required firing rates than anticipated.

For fuels with wide flammability limits and high flame speed, burners of the nozzle mix type would seem to be the most suitable. The custom-designed dual fuel burners and associated control equipment on the Drever furnaces have performed well handling two entirely different gases. The automatic switch-over system has operated safely and satisfactorily without adverse effect on the critical temperature profile.
COPYRIGHT 1989 Chemical Institute of Canada
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:McMaster, S.; O'Rourke, D.; Siscoe, R.
Publication:Canadian Chemical News
Date:Nov 1, 1989
Words:1556
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