Thermal treatment of waste gets heated
Director of science at The Natural Step (TNS), Dr Mark Everard identifies the sustainability challenges facing advanced thermal treatment technologies.
International sustainable development charity, The Natural Step (TNS), believes that these technologies should be assessed objectively on the basis of sustainability, and has embarked upon a study of thermal treatment technologies. The study assesses their current sustainability, develops a vision of a sustainable future, and identifies the sustainability challenges facing thermal treatment technologies.
The term ‘thermal treatment’ is used to describe a range of technologies using heat to degrade the constitution of solid matter. These include basic ‘mass burn’ as well as variants upon incineration, as well as so-called ‘advanced thermal conversion’ (ATC) technologies such as pyrolysis and gasification.
One of the distinguishing features of ATC is that syngas, the gaseous products generated from heating of waste in the absence of oxygen, can be extracted from the process for cleaning prior to further use. This capacity to extract and utilise syngas opens up three potential uses: combustion of cleaned syngas for energy production with relatively clean emissions; utilisation as an energy carrier; and regeneration of monomers from organic waste, leading to production of polymer. Residual solid ‘char’ from the pyrolysis process may also be used for energy and as an aggregate or other construction material, though it may contain residual pollutants that compromise reuse. Thermal treatment therefore touches upon four policy areas: waste, energy generation, alternative vehicle fuels and resource regeneration – all of which are currently the focus of short-term pressures.
Short or long game?
There are dangers that immediate policy pressures may drive short-term decision-making to meet current regulatory requirements, without adequate foresight about how proposed solutions contribute towards a genuinely sustainable future. Clear thinking about the use of all waste treatment technologies is required to ensure that: (1) their contribution towards the long-term term achievement of sustainability is understood by all; (2) benefits to all policy areas are maximised; and (3) existing practices such as linear and wasteful resource use are not perpetuated, and particularly where energy recovery may create an incentive for continued waste production.
The draft TNS 2020 Vision report highlights that all thermal treatment processes, and their ‘benchmark’ alternatives, are currently far from sustainable. This is hardly surprising in a largely unsustainable society, though some emerging technologies are helping address some of the wider challenges raised by traditional technologies.
Firstly, it makes no sense to view treatment technologies outside of the wider question of society’s pattern of resource consumption, which today gives rise to a huge volume of waste through linear consumption (mine-use-dispose). The context within which waste technologies are proposed and used has a huge – perhaps overwhelming – bearing upon their sustainability implications. Our consumption patterns themselves need to change, and the replacement of one mass disposal method with another will not lead to a sustainable solution.
Something approaching 80 per cent of the mass of waste streams today represents embodied energy, so one of the strongest arguments proposed in favour of thermal treatment technologies is energy recovery from low-grade material. This form of value recovery would be lost if waste were landfilled, and it can also potentially reduce dependence upon fossil carbon resources. Pyrolysis and gasification technologies also offer the potential for recovery of syngas for onward conversion to chemical feedstock, which may make a positive contribution towards more cyclic use of resources (with approximately 50 per cent loss through the total process from plastic waste to new polymer) and reduction in use of fossil fuels.
This flexibility of operation provided by some technologies – notably pyrolysis – means that kilns can be operated to match energy demand curves (maximising premium energy charges), or even some resource regeneration, but also may be tuned to declining waste stream in future. They may therefore be adapted to changing requirements in the future which, from a sustainability perspective, creates the flexibility for their migration towards energy production from biomass as progress is made towards sustainable resource flows.
Significant ethical and human issues may arise from the potential for adverse health effects of pollutants, but also from disturbance and nuisance from plant operation. These will inevitably affect the acceptability and siting of disposal technologies.
Theory and reality
The issue is not merely one of whether various technological solutions have a theoretical technical place in delivering sustainability. The way they will be used in the ‘real world’, particularly in the light of the short-term regulatory and economic pressures bearing down upon decision-makers, is equally important. For example, tenders for waste contracts that commit municipalities to provide developers with stable or increasing quantities of waste, of a predictable quality, obviously mitigate against progress towards waste minimisation and recycling, regardless of the technologies that are proposed. Equally, generation of energy from waste has benefits but also risks, as already discussed. Waste technologies have therefore to be planned carefully, with an eye to the longer-term achievement of sustainability.
The long march
Achievement of a ‘cyclic resource use’ and ‘zero waste’ economy is the only realistic long-term sustainability goal. None of the technologies assessed in this report represent a ‘magic bullet’ that can make today’s unsustainable resource and waste practices sustainable. Rethinking of resource flows is required, with the now-outmoded concept of ‘disposal’ replaced by one of resource and value recovery. This means delivery of maximum social utility per unit physical resource, and the maximum possible recovery of value of those resources at end-of-life of products.
Whether any of the thermal treatment technologies are considered to be fully sustainable or not is, for many, not the most pressing question given the growing waste problems we face today. The trick will be to implement solutions that offer clear ‘next steps’ towards the longer-term goal of sustainability. Technologies that can operate at smaller scales and under variable loads appear to represent amongst the best options available today, and particularly where operation can be adjusted to accommodate changes in waste volumes and composition resulting from waste reduction, reuse and recycling. Some technologies – gasification and pyrolysis amongst them – offer flexible solutions that could adapt to changing needs as we make progress towards sustainability. The end-goal of sustainability has to be factored into every aspect of planning and contract-writing. In the absence of such strategic thinking, no technology on earth can help us!