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Technical Sheet of THERMIE Project Rational use of energy in Industry and Energy Industry (RUE-5)
INDUSTRIAL SECTOR: CHEMICALS


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General Information

Project number : IN 0031/94/NL
Title of project A TECHNOLOGICAL BREAKTHROUGH IN RADIANT EFFICIENCY - MAJOR FUEL SAVING ON A STEAM REFORMING FURNACE
Contractors
  • HYDRO AGRI SLUISKIL, a chemical industry based in Sluiskil, Holland.
Project location The project was implemented at the Hydro Agri ammonia plant Unit C, located at Sluiskil.
Pay-back time of the project The expected pay-back time is under one year.
Manufacturer
  • NUOVO PIGNONE
  • FOSTER WHEELER
Contact Details
Mr. Wim VERSTEELE
Hydro Agri Sluiskil
Postbus 2
4540 AA Sluiskil (NL)
Phone: +31 115 474 385
Fax: +31 115 474 223

Aim of the Project

This project is part of a retrofit of a 20 years old ammonia plant. Its aim is to improve the efficiency of a primary reformer furnace/gas turbine combination by extensive preheating of the mixed feed going to the furnace and by the installation of a highly efficient gas turbine of which the operating conditions can be adapted to suit the oxygen requirements of the furnace.

The extensive preheating of the mixed feed reduces considerably the heat duty which has to be transferred in the radiant section.

The installation of a highly efficient two shaft gas turbine improves the furnace efficiency by reducing the exhaust flow through the fire box down to the minimum amount required to achieve complete combustion of the furnace fuel.

The above combined effects, together with the high efficiency of the gas turbine will yield important energy savings and reduce consumption of natural gas.

Project description

The steam reforming of natural gas to produce syngas consists of two integrated parts: the primary and secondary reformer.

The primary reformer is a furnace type reactor, in which catalyst tubes are exposed to radiant heat from the burner flames.

The secondary reformer is an adiabatic reactor where air is added to the gas. The nitrogen is needed for the ammonia synthesis, while the oxygen is burnt releasing more heat to complete the reforming reaction.

Before introduction to the secondary reformer, the air is compressed by a compressor, driven by a gas turbine. The gas turbine exhaust gases are used as preheated combustion air for the primary reformer furnace.

As the total fuel gas consumption is an important factor in the ammonia production cost, efforts were directed to savings in fuel gas consumption.

Steam reforming of hydrocarbon feeds is the principal process for the production of hydrogen gas and hydrogen containing gas mixtures. The desulphurized feedstock is diluted with large quantities of steam, then preheated to elevated temperatures and finally contacted with nickel catalysts.

Following reactions are involved in the generation of hydrogen:

  • CH4 + H2O -----> CO + 3 H2 Reforming reaction
  • CO + H2O -----> CO2 + H2 Shift reaction

Since several decades, this process takes place inside catalyst filled tubes, which are suspended in the radiant section of a furnace.

Unfortunately, only 30 - 35 % of the heat released by the burners is effectively absorbed by the reformer tubes. The balance of the heat carried by the flue gases that leave the radiant section is recovered in various convection coils. Once the temperature is reduced sufficiently, the flue gases are discharged into the atmosphere by induced draft fans.

The heat requirements of the radiant section determine the total fuel gas consumption of the furnace , as this heat is needed at that highest temperature level.

To reduce the fuel gas consumption, the duty of the radiant section was decreased by extended preheating of the hydrocarbon/steam mixture before it reaches the catalyst tubes. This was done by installing a new highly alloyed mixed feed preheat coil in the convection section of the furnace. In this way, radiant heat is substituted by heat available in the convection section at a sufficiently high temperature level.

A second key factor in achieving the fuel gas savings was the installation of a second generation gas turbine. The amount of oxygen available in the exhaust gases from this machine matches closely the oxygen requirements of the primary reformer furnace.

In this way, the flow through the radiant box of the furnace is minimised allowing to reach the high temperatures required in the fire box with a lower fuel gas consumption.

Due to the low oxygen surplus, the furnace burners have required modifications to assure an adequate distribution of gas turbine exhaust over the burners. This is important to obtain complete combustion of the furnace fuel and to assure a uniform heat release in the fire box. This last issue is critical for the lifetime of the catalyst tubes, as local overheating of the tubes might cause premature tube failure.

Low oxygen surplus gives the benefit of reduced NOx emissions.

Fuel gas consumption of the gas turbine is decreased as well because of the high efficiency of this new machine.

Because of the reduced fired duty and because a larger share of the total heat is absorbed in the reforming process (Mixed feed preheat coil + radiant tubes) a lower heat load is available for the rest of the convection section.

For this reason, all convection coils in the heat recovery section were rechecked for the revamped process requirements and, where necessary, additional surface was added.

The rearrangement of the convection coils which is an integral part of the proposed furnace revamp, also serves another objective: the heat conservation is further optimised by reducing the stack losses.

The furnace was coupled to a gas turbine, which generates 7MW mechanical power and was delivering its exhaust gases as combustion air to the main and auxiliary burners. Gas turbine exhaust was at 425 oC and contains 17% of oxygen. Part of the gas turbine exhaust was sent to the radiant section, the balance was dumped in the convection section to recover its residual heat.

In the previous situation, the gas turbine power was not sufficient to drive the process air compressor. Therefore, a helper steam turbine was necessary to supply extra power to match the power absorbed by the process air compressor.

After revamp, the power of the new gas turbine is sufficient to drive the process air compressor, so the helper steam turbine was eliminated. The gas turbine exhaust gases are at around 520 oC and all exhaust is sent through the radiant section to provide sufficient oxygen for combustion in the furnace.

Project Results

The revamp project has achieved the following results:
  • Reduced specific energy consumption = Feed + Fuel from 8.6 to 7.43 Gcal/T. The net energy consumption was 7.3 Gcal/T
  • Compliance with the most stringent emission regulations. NOx emissions < 200n mg/Nm³
    • Reduction in CO2 emission by 18% - noise level < 82 dBa
    • Elimination of CO2 solution containing heavy metals as corrosion inhibitor.
  • Increased capacity: The guaranteed capacity is 1100 T/d - New equipment designed for 1200T/d. The capacity is limited to 97.2 % of design due to lower efficiency of process air compressor with respect to what was predicted at design stage.
  • Improved availability
  • State of the art efficiency of the new turbine: 32% versus 19% of the former machine
  • Good match of exhaust with O2 demand of primary reformer furnace.

Financing

ECU

%

European Community

700,000

12,.5

Contractors

5,008,504

87.75

Total investment

5,708,504

100

Environmental impact

  • Reduction in CO2 emission by 18%
  • NOx emission complying with BEES regulations valid from 1998
  • Elimination of CO2 solution containing heavy metals as corrosion inhibitor
  • · No surplus of CO2 condensation
  • New equipment: Noise < 82 dBA

Potential for further installations and dissemination of results

The project in general demonstrates the high potential for energy savings with integrated energy technologies and process innovations in the chemical industry. In process industries, substantial innovations to be demonstrated often are not limited to a specific new equipment but a combination of newest equipment developments an process changes; the specific know-how and innovation is in the systems engineering and application development.

The main supplier in this project, the UK engineering company Foster Wheeler has combined innovative heat exchanger technology, latest gas engine development and reforming process know-how in a process revamp with very positive results. Payback time under one year is of extraordinary economical viability for the kind of investment encountered with this project.

Project results can directly be disseminated to all similar ammonium plants which have a substantial market potential in Europe and especially outside Europe. Furthermore, this successful example should be used to stimulate further integrated energy technologies and process innovations in the chemical industry.

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Last Updated: July 20, 1998