There’s been strides in innovation at Energy-from-Waste (EfW) facilities and, in particular, thermal treatment plants in recent years. Improving waste recycling rates in line with the principles of the circular economy and driving up air quality are top priorities for many countries around the world. The design engineers at Hitachi Zosen Inova (HZI) have taken on this challenge head-on.
Amal Jugdeo, Senior Engineer at HZI Australia says HZI continues to search for smarter ways to combine new and complementary technologies.
“We are improving existing technologies to meet and exceed the challenges of tighter emissions requirements, maximising waste recycling and reducing carbon emissions,” Amal says.
One such example is the upgrade of the KVA Horgen plant in Switzerland. Amal says that as one of the first EfW plants in the world with dry bottom ash discharge technology, KVA Horgen is a pioneer when it comes to future-oriented material recycling.
“Most plants feed the slag (incinerator bottom ash) into a water-filled wet extractor for cooling. However, this changes chemical compositions, particularly in the fine fraction of materials, making it difficult to extract valuable non-ferrous metals such as aluminium, zinc, copper, and gold contained in the bottom ash,” Amal says.
“By contrast, the new dry discharge system used in Horgen maximises the recovery of materials sent for recycling with recovery efficiencies more than doubling for the non-ferrous metal fine fraction. These materials are recovered and fed back into the economic cycle as raw materials, thus forming part of the concept of ‘urban mining’, which increases the re-use and recycling of valuable resources.
“As a result, there is also less bottom ash to dispose of in landfill – up to a 20 per cent mass reduction –compared to existing processes. KVA Horgen is one of the first plants in the world to use this environmentally friendly approach to dry bottom ash discharge.”
The retrofitting process in Horgen also included the reduction of combustion lines. Before the upgrade there were two combustion lines. Now, one line is generating the same power export in the form of electricity and heat for district heating from 60 per cent of the waste previously required.
Amal says the plant upgrade included improvements and automation of the combustion system. Because of the lower excess air in the combustion process, the amount of flue gas is reduced and heat is recovered more efficiently. This also means lower NOx production and utility consumption.
Constant analysis of the flue gas, fire position on the moving grate via video analysis, bed height measurements, 3D laser scans at the feed hopper and the complete automation of the feed waste crane allow for steadier combustion and steam production.
The moving grate consists of four optimised and individually driven zones that facilitate the different phases of the combustion process (i.e., drying, ignition, gasification and combustion of volatiles – where the excess oxygen is essentially zero and char burn-out).
Amal says that these combustion system enhancements have also been applied by HZI to the 330,000 tonnes per year EfW plant under construction in East Rockingham, Western Australia, which is scheduled for start-up in 2023.
Increasing efficiency
Another example of pushing the technology boundaries can be found at the world’s largest EfW plant under construction in Dubai. Once completed in 2024, it will process up to 45 per cent of Dubai’s current municipal waste, converting 1.9 million tonnes of residual waste a year into electricity for about 120,000 households.
“With a net energy efficiency of more than 30 per cent, Dubai’s future waste incineration plant will rank among the very best in the world,” says Amal.
The location in the Dubai desert brings with it a number of challenges such as ambient temperatures that allow limited air-cooling of the steam. The plant design included provision for a larger air-cooled condenser (ACC).
Another potential issue is desert sand clogging up the surface of the ACC and preventing air flow. To counteract this, the ACC has to be cleaned more often and thoroughly than it does in other plants in cooler climates. Amal says that despite this unusual environment for an EfW plant, the Dubai project will achieve world-leading energy efficiency.
EfW plants require substantial capital outlay. Any improvement in the capital cost per tonne of waste processed has a material impact on the business case for such a facility. The Dubai project and HZI’s Newhurst Energy Recovery Facility in the UK (with the latter set to enter commercial operations in 2023) will be the largest single-line boiler designs in the world (124.6 MWth and 126.3MWth respectively).
HZI engineers conducted fluid dynamic modelling prior to commercialising these ultra large boilers and they continue to push the boundaries by designing even larger single-line facilities. Amal says that such designs offer up to a 20 per cent discount on an equivalent two-line design and therefore result in a discount on the gate fee for such facilities.
Carbon capture
Australia has committed itself to reducing greenhouse gas (GHG) emissions by 43 per cent below 2005 levels by 2030.
“The waste management sector in Australia contributes just under three per cent to Australia’s overall GHG emissions,” Amal says. “EfW technology already presents a quick win by reducing net GHG emissions compared to residual waste disposal via conventional landfills.
“Although landfill gas capture reduces GHG emissions compared to landfill sites without gas capture, studies highlight that the net impact of fugitive methane emissions as ‘carbon dioxide equivalent’ from such landfills far exceed the GHG emissions that residual waste treatment in an EfW would bring.”
To improve the EfW footprint even further, HZI has partnered with technology providers to capture the carbon dioxide (CO2) contained in EfW flue gases. One such process involves the reaction of the CO2 with calcium to form mineral calcium carbonate – a stable, non-toxic, valuable component that has many uses, including as a raw material in the construction industry, fertiliser, as well as a mineral filler in the paper, paint, and plastics industries.
The capture rates can enable EfW plants to become carbon sinks. The system is also geographically independent and does not require storing carbon dioxide in a gaseous state underground, nor additional pipelines for transportation.
“Such technologies will enable us to reduce CO2 emissions from EfW plants to zero or even negative, further embedding our technology within the circular economy,” Amal says.
“The technology exists to produce hydrogen from the electricity produced at the facility. By combining this with recovered CO2 in a processed called methanation, methane is produced that could be injected into the domestic gas network.”
Dr Marc Stammbach, HZI’s Managing Director for the Australian HZI subsidiary, says that with hundreds of EfW plants as reference throughout the world, EfW technology has proven itself to be an environmentally safe and reliable solution for the disposal of residual waste.
“As demand grows to improve overall material recovery and re-use, together with a drive to increase energy efficiency and reduce greenhouse gas emissions in a cost-effective package, HZI has stepped up to the challenge and is ready to offer the next generation of EfW plants,” says Marc.
For more information, visit: www.hz-inova.com