The energy crunch and high energy prices facing the South African industry has led smelter engineering firm Metix, a subsidiary of German group SMS Siemag, to offer the furnaces industry energy-recovery systems.

Metix has developed power-saving solutions that enable cogeneration and that are suitable for the local market.

Cogeneration is characterised by the simultaneous production of electricity and heat from a single fuel source, such as natural gas, biogas, biomass, coal, oil or waste heat.

“Combined heat and power (CHP) is an integrated energy system, which can be modified, depending on the requirements of the end-user,” says Metix deputy sales director Klaus Schmale.

Owing to their higher efficiency, CHP systems use less fuel to produce a given energy output.

Within the ferroalloy production sector, different types of submerged arc furnaces (SAFs) are applicable: open, semiclosed and closed. The last two are suited for energy recovery.

Within the semiclosed type furnace, carbon monoxide (CO) and hydrogen (H₂) are generated but directly burned completely owing to sufficient false air, which enters the furnace through the doors and presribed openings.

Therefore, huge amounts of fully combusted offgas with a high amount of sensible heat is generated – on average it reaches temperatures of about 650 ºC.

The offgas of closed type furnaces is even hotter (up to 1 800 ºC) and contains large amounts of chemical energy in terms of CO and H₂. The low calorific-value (LCV) is usually one-third of the LCV of natural gases.

Metix plans to generate power with a process gas-fired boiler combined with a turbine and a generator unit.

“The boilers burn LCV process gas in a combustion chamber. Our standardised boiler is a two-pass boiler. The first pass is the combustion chamber and the heat exchangers (superheater, evaporators and economisers) are located in the second pass.

“The thumb rule for efficiency is 33% and the superheated steam runs the turbine island to produce steam. Boilers using up to 360 t/h of steam at 540 ºC can typically produce 20 MW to 160 MW of electrical energy,” states Schmale.

Metix says Turkey-based ferrochrome producer Eti Krom has ordered this type of energy- recovery solution from Metix for use in two semiclosed ferrochrome SAFs. Each furnace is equipped with one waste-heat boiler.

The system is expected to start up early in 2013 and will produce steam, which will be used in a steam turbine with an attached power- generation unit to produce 5 MW of electrical energy.

With this system, Eti Krom expects an amortisation period of less than three and a half years.

Offgases
Beside the closed type SAF, the 650 ºC offgases of semiclosed type SAFs can be used for energy recovery.

“Up to 40% of the energy spent during smelting in a furnace leaves with the offgas. These offgases are always hot and are mostly bursting with energy; Metix offers energy-recovery systems for steelmaking, as well as for ferroalloy production, which will save up to 55% of the waste energy,” says Schmale.

The waste energy of an operating plant, which is not normally used but heats up the environment instead, can save other energy sources and, in turn, reduce energy costs.

Using the waste heat of process gas also reduces carbon dioxide emissions, which further reduces the amortisation period for Eti Krom.

Long-Term Plans
Metix plans to produce its own electricity from waste energy.

“A directly controllable and constant price gives you an advantage, compared with using electricity from the grid and from a power supplier that is increasing its prices,” says Schmale.

As a service supplier, Metix will also provide training for internal and external plants using its CHP technology.

The company also plans to approach State-owned power utility Eskom to present the features of the technology and to discuss how it can help overcome the shortfall in electrical energy.

Metix hopes that Eskom will support the introduction of the energy-recovery system as part of its range of energy-saving initiatives.

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