by Paul Dumble
6 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!
Fuelish choices
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.
Increasingly baleful fuel? Photo via Wikimedia Commons
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. .
Bio-logic
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.
Residual revolution
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.
Paul
Thanks for your response – i think we’re going to have to agree to disagree on some of these issues!
I’d be interested to hear more about your proposed technology, so happy to continue discussion offline sometime in the future.
Adam
Adam – Happy to develop this discussion further off line. Please get my email address from Peter. Regards Paul
Adam
1. The CV for untreated residual MSW drops by about 2MJ/Kg which would put the lines in my chart shown in the article between 12MJ/Kg and 6Mj/Kg with a 2MJ/Kg variation between each of devolved countries.
2. The business model you refer to is based on what I refer to as the cheap and cheerful with lax regulation (from a policy and enforcement point of view). Your argument in my humble view is based on an unsustainable premise. I started my career as an industrial process chemist and base my arguments on thermodynamics and many years of process/product development experience.
3. The test of a good model is whether is it fits over a period of time. Your model may be crystal ball gazing – my model is working very well thank you (the model was compiled originally in 2007-2008).
4. I am working on such concept. My vision is to take projects such as the recent Gloucestershire PFI which will cost tax payers about £130 for every tonne of waste treated over the next 25 years to a situation that will make this cost look like a payday loan.
Paul / Josh
Thanks for your comments. A few points to make in response:
1.The starting point for RDF CV is probably more like 12 GJ/tonne currently. This is being driven by the export market, which wants relatively low-grade feedstock to maintain high throughput (and thus gate fees). Whilst there are examples of higher CV SRF around, these are pretty much limited to supply to cement kilns;
2. Yes, MBT biodrying does increase the CV of organic wastes, but frankly, there is very little biodrying capacity either operating or under construction currently (less than 1 million tpa) and this doesn’t look like increasing in the near future. I therefore don’t feel that this will have a major influence on the market, as it simply doesn’t make sense to spend heavily on such pre-treatment when most end markets want a less processed fuel (in the UK look at massive plant at SSE Ferrybridge, INEO Chlor Runcorn and Air Products Teesside);
3. My first comment above relating to my view that CV will increase over time is based on mine and colleaugues at Eunomia’s own detailed modelling of future waste composition (i’d be interested to know which researchers have done yours, Paul?). However, we all know that modelling of this nature is somewhat crystal ball gazing, as essentially it depends upon both consumer habits and what new recycling services get rolled out. Most importantly in this context, however, the future CV will depend on future recovery of plastics and food wastes. As stated above, our view is that food waste collection roll-out will outstrip that of new services for plastics and thus reduce the concentration of food in the residual stream when compared with plastics, hence the CV rises.
4. I wouldn’t be averse to seeing a greater move towards ‘higher value’, more processed fuels, but to move the market in that direction, developers and financiers need to see plant on the ground are proven in processing them profitably.
Interesting article, Paul – but I’m not sure I agree with your premise. My recent work on feedstock analysis suggests that we’re actually likely to see an increase in CV in coming years. Food waste, as you rightly say, is likely to be segregated to an increasing extent – but its high moisture content makes it a fairly low CV fuel. Removing it from the residual waste stream will therefore tend to increase the CV of the remainder, if we assume this will continue to contain a range of difficult to recycle plastics.
Adam: Presumably the organic fraction of RDF will have a higher CV than for regular unprocessed organic waste because of the drying process removing some of the water.
Josh, Well spotted – MBT used to produce RDF is a biodrying process and is primarily designed to remove moisture and this will result in rise of the CV. These factors have been used in my model to produce the above chart.
Adam. The chart I used to illustrate the drop in CV’s is based on the work of several researchers who have looked at the impact of recycling and composting rates on waste composition so the impact of different levels of recycling of food wastes, plastics, paper, wood etc has been taken into account. I can vary the recovery rates for each of the organic constituents. So on this occasion I disagree with your comment. Paul