Emerging thermal technologies and their role in energy production

Each year, millions of tonnes of waste materials are sent to landfills across Australia. While Australians are very good at recycling, there will always be waste that is not physically able to be recycled, or where it is not financially viable. Much of that waste, however, has a significant energy content, and a large portion of that energy is from renewable sources.

There are a number of options for recovering energy from waste - landfills with gas collection, incineration and anaerobic digestion are common forms, but emerging technologies based on pyrolysis and gasification reactions are also of particular interest. These processes have actually been around for a very long time; pyrolysis is what we use to turn wood into smokeless charcoal and gasification of coal was used to make ‘town gas’ in the 19th century for street lighting in London and other major cities. Plants were phased out from the 1950s with the discovery of, and access to, massive reserves of natural gas, but there has been a recent resurgence as an alternative to convert waste material into energy.

How do they work?

We know that when enough heat is applied to a carbon-based fuel like waste in the presence of oxygen, it burns and is converted to carbon dioxide. Lots of heat is released and all that is left of the waste is some unburnable ash. What’s really happening, however, is that the carbon is first vapourised into a gas and those vapours burn as a visible flame.

That initial vapourisation stage is actually pyrolysis and if no oxygen is available, the vapours can be captured and used. Some vapours remain as gas while others can be condensed into liquid oil. The remaining solid, char, still contains lots of carbon and all three products - solid, liquid and gas - have a number of potential uses. Pyrolysis requires a constant heat source but the trade-off for this energy input is that all three products contain significant energy content.

Gasification fits in somewhere between pyrolysis and combustion, where there is oxygen available but it is limited and carefully controlled. The partial combustion reactions that occur heat the process and result in a synthesised gas comprising mostly carbon monoxide and hydrogen.

Feedstocks

Essentially almost any carbon-based waste or biomass stream, including a number of materials with little other value, can go into these systems - from household or municipal solid waste and residual plastics to garden organics, paper and cardboard and waste wood.

Some systems are designed to specifically receive certain feedstocks but others can take a range of materials. In most cases, particularly when looking at mixed waste streams or wet materials, pre-processing and drying of the feed will be necessary.

What are the benefits?

Unlike large-scale incineration plants, most pyrolysis and gasification plants are designed, and financially viable, for small to medium scales - anywhere from 5000 to 100,000 tonnes per annum. These smaller plants can be positioned closer to population centres, reducing waste transport costs and allowing other industries to use excess heat and power from the plant.

Considering most pyrolysis systems can fit into a standard industrial shed, they should also have a smoother ride through the approvals process and much less local opposition.

The plants can also be more efficient in recovering energy than incineration, which requires up to twice as much air input than is needed to actually burn the waste. All that unnecessary cold air, 79% of which is nitrogen and of no use, sucks energy from the process. In pyrolysis and gasification, air input can be more carefully controlled and there is much less combustion air.

The real advantage, however, is the potential to use the synthesised gas (syngas) in other power production systems such as efficient gas engine generators which could also provide heat and cooling in a cogeneration or trigeneration system.

The challenges

Well-publicised failures, such as the SWERF project in Wollongong and the Thermoselect plant in Germany, have resulted in the waste industry now being very nervous about dabbling in new technologies and being the first to take a risk.

The cost to develop and test a new process like this can be massive and a lot of the companies developing these systems are small independent companies with limited resources who are very protective of their ideas. This means there isn’t any sharing or learning from mistakes across the industry.

Potential applications in Australia

These systems are designed for small- to medium-sized plants, which will suit smaller Australian cities and regional centres. They are more affordable and a palatable investment for councils that could potentially procure one on their own.

Biochar is a real attraction in Australia. We have a carbon pricing scheme that recognises biochar as a permanent carbon sink that can generate carbon credits. There is also an abundance of arid and degraded land that would benefit from biochar application.

Pacific Pyrolysis is a small Australian company that has developed a slow pyrolysis system, mainly focusing on making quality biochar from garden and wood waste for sale as a soil amendment. They have been very active in biochar research in Australia and are in the process of commercialising their technology. The first commercial demonstration plant looks likely to be in Melbourne, having recently been given $4.5 million in state government funding. It will process 30,000 tpa of garden and wood waste, producing 5000 tpa of biochar and at least 1 MW of power. They have also been working with Ballina Shire Council on a similar project which has recently been offered a small amount of federal government funding.

As well as Ballina Shire Council, the City of Sydney has expressed interest in such technologies to produce gas from waste and biomass to fuel its planned trigeneration network. The ACT has also given serious consideration to these options to manage urban biomass and waste wood.

What happens next?

At the moment, landfill is still the main choice when it comes to managing residual waste in Australia, but that is set to change with rising costs and landfill levies in many states. Financially there is no reason why these projects could not be viable now in some cases, with good revenues available for both inputs (gate fees) and outputs of the process (energy sales, products, carbon credits).

There is still some process risk associated with the new technologies that are not commercially proven, so early adopters who are willing to take the risk need to be supported. Governments have recognised this and there are some grants being offered.

Most importantly though, technology developers need to make sure their process works before they launch it commercially. Any problems with the first commercial plant will be well publicised and disastrous for the technology and industry as a whole, but success stories will provide the opportunity to educate the public and turn around negative attitudes of the past. Nevertheless, these technologies could provide a great alternative for managing waste materials in Australia and generating renewable energy in the not-too-distant future.

*Dominic Schliebs is a senior consultant in Hyder’s Waste and Resource Management team. He has a particular interest in waste-to-energy technologies after spending time in the UK working thermal treatment projects and developing new pyrolysis technologies.