by Paul Dumble6 minute read
What do you think the municipal solid waste of the future is going to be like? Recycling rates may have seen a downward blip last quarter, but if we’re to meet our targets you can rest assured they’ll resume their upward trend. That means less paper, card and plastic in the residual mix, and more organic material being separately collected for composting and anaerobic digestion. This entirely foreseeable trend has really profound implications for the waste to energy industry that we seem not to be taking account of – but which I believe will be soluble with the right technology.
Bad for the waste line
The UK’s waste to energy (WtE) sector is, ironically enough, itself pretty wasteful. While many thermal treatment plants on the continent bolster their efficiency by making productive use of the heat they produce, the UK has tended to build electricity only plants, as district heating schemes and opportunities for industrial steam usage are rare. Average net efficiencies for plants solely producing electricity average about 20%, and plants of less than 100kt capacity do rather worse. Efficiency values for combined heat and power (CHP) vary greatly depending on the balance between the two outputs, but values in excess of 50% are claimed.
Our lacklustre performance, however, seems set to get worse. Currently, refuse derived fuel (RDF) from municipal solid waste (MSW) in the UK has an average calorific value (CV) of around 14 MJ/Kg. Much of this comes from the organics, paper and plastics that we’re increasingly looking to separate out. If recycling and composting levels continue to rise over the next 20-25 years to reach the EU target of 70%, the average calorific content of MSW-based RDF will almost halve, to something closer to 8 MJ/Kg. Such rates aren’t implausible: Scotland’s national targets are a recycling and composting rate of 60% by 2020, increasing to 70% by 2025.
The decline in CV clearly presents a problem: WtE plants set a minimum CV of 9 MJ/Kg for their feedstock. Over the next 10 to 20 years we will therefore see the efficiency of our incinerators drop even further if important feedstock streams increasingly produce below-specification RDF.
Municipal incinerators will, no doubt, try to address this by securing alternative waste streams, such as commercial waste. This may have a higher calorific value but increased recycling is likely to have an impact on its potential as fuel as well. The logical thing for us to do is to actively improve the quality of the fuel, by removing the inert and other non-organic fractions from the RDF.
Helpfully for those thinking about what makes good RDF, WRAP has recently developed a grading system for waste derived fuels, which ranks them on a 1–5 scale based primarily on biomass content but also looking at issues such bulk density, moisture and heavy metals content. If widely adopted, this should help buyers and sellers improve shared understanding of how good the fuel they trade actually is!
There is of course a bit of a quandary for anyone concerned about the quality of RDF. Given that recycling and composting rates are currently still below 50%, we must still be burning materials that we should be looking to push up the waste hierarchy. Incineration infrastructure appears both to be a waste from an environmental and economic outlook and in terms of investment. We should be actively working to make its feedstock disappear over time.
On the other hand, a future in which we reach such high levels of recycling that incinerating the residue is not worthwhile still seems a little way away, especially to policy-makers focused on current conditions. Since we are still landfilling, there is a case for more incineration capacity. But there is a real risk that it won’t be able to be run efficiently through its lifetime if we’re to meet our recycling goals.
I do see a way to escape the horns of this dilemma. The heat energy wastefully vented by many UK WtE facilities is an opportunity: together with a small fraction of the energy generated, it could be put to use, not just to provide power and heat for the day to day running, administration and distribution functions of waste treatment facilities, but in the treatment of mixed MSW residues to improve the quality and value of the recyclable materials and fuel produced.
The environmental benefits of such an approach can be identified through modelling based on embedded energy calculations (such as the University of Bath’s Inventory of Carbon and Energy) although care needs to be taken to understand how the calculations are made). It could be possible to more than double the carbon savings we are making through waste treatment, by capturing the heat energy generated during incineration and using it to enable additional pre-treatment. .
Let’s focus on a specific example, MBT. It is unlikely that we will see all organic material removed from MSW. Biological processing of residual MSW is fraught with problems, and processes are often inhibited by contaminants, or composition variations such as high moisture content. The quality of recyclables recovered by MBT plants is variable, and the saleability of the resulting recyclate is reliant on markets in the Far East, currently far less willing to take in low grade material.
The “compost like output” from MBT is typically of too poor quality for uses other than as landfill cover or capping – in my view basically land filling. Anaerobic digestion’s performance with mixed residual MSW is highly variable, though some process improvements have been developed, such as the use of enzymic hydrolysis as a pre-treatment step. In short, all is not well with the treatment of the organic fraction of residual waste, and the market is ripe for additional pre-treatment to separate out more recyclables and boost the CV of any resulting RDF.
Pre-treatment could also reduce the perverse incentive to maintain a supply of MSW that is rich in organic waste, plastics and fibres, removing a barrier to increased segregation and separate collection of recyclables.
I’m currently working to develop technically efficient and sustainable solutions that I believe could lead to mass burn incinerators being phased out over the next 20 years. Improving the residual waste treatment process will produce far less surplus fuel, but what remains will be of a quality that allows it to be used to generate cleaner power in smaller, more efficient Combined Heat and Power plants.
For those building old-fashioned incinerators, the long-term future is far from secure, and I would counsel against assuming that RDF will continue to look the same as it does now. The current policy position risks leaving us with a legacy of outmoded incinerators unsuited to dealing with gradually degrading RDF. A little bit of coordinated strategic thinking in the UK could put us far ahead of the game, enabling is to create a model of sustainable best practice that the rest of Europe could learn from – and the last time we were that far ahead was the Industrial Revolution.
Anyone who would like to share and help me develop this progressive vision is welcome to contact me through LinkedIn.