Transforming waste energy into sustainable processes

Around half of the primary energy consumed in industrial processes is currently wasted. Even steel produced by recycling scrap metal in electric-arc furnaces, for instance, requires approximately 370 kWh per metric tonne. In this type of furnace an electric arc is struck between several electrodes. The resulting heat causes the steel to melt before emitting a mixture of gases at up to 1700°C - a huge waste that might otherwise be used in the process or to generate electric power.

Predictions suggest that around 50% of costs from operating a mine, for example, will come from energy use, making it even more of an incentive for industries to reduce energy consumption as well as manage its waste more cleverly.

When considering energy efficiency, one often thinks of energy-saving technologies for lighting, heating and air conditioning, for example. But energy efficiency is so much more than this - it’s about reducing consumption while at the same time making the most of waste energy by using it in other parts of the process (eg, to preheat combustion air or serve another thermal load) or by condensing it to generate electricity.

Research has shown that up to 20% of the energy needed for melting scrap metal could be recovered from waste heat. That would reduce CO₂ emissions per metric tonne of steel by about 40 kg. Current systems emit about 270 kg of CO₂, 220 kg of which results from power generation. This means that CO₂ emissions from a typical 120-tonne furnace could be reduced by more than 30,000 tonnes annually.

While it makes sense to recover waste heat, most industrial waste heat is still not captured for re-use and unfortunately, around half of the primary energy consumed in industrial processes and energy generation currently goes to waste. Part of the problem is that there are still hardly any economically practical and technically mature processes available for waste heat below 300°C.

But in a thermal management study, Siemens has identified a process called Organic Rankine Cycle (ORC) technology - particularly suitable for recovering waste heat from furnaces in the glass industry, diesel or gasoline engines, gas flaring at refineries as well as gas turbines in compressor stations. The non-polluting organic medium used in this process ensures optimal efficiency for low waste-heat temperatures and low power. In a trial at Moscow State University, the ORC technology produced about 800,000 kWh of extra electricity per year with an efficiency of about 20% and a payback within three years.

In addition to producing electricity, waste heat can also be used to clean and demineralise water through an evaporation and condensation process. This not only generates new freshwater for re-use but at the same time reduces the amount of wastewater that might otherwise go to sewage. Paper mills, soft drink bottling plants and other industries where wastewater cannot be readily disposed via a sewage treatment plant could potentially benefit from this type of technology.

*Ed Sugay is Head of Energy Efficiency, Siemens Ltd. Before heading Siemens’ Energy Efficiency portfolio in Australia, Ed was Siemens’ Global Director of Energy & Environmental Solutions for Asia Pacific. Ed brings 20 years’ experience in energy generation and energy efficiency, as well as an MBA from the University of California.