First-generation treatment options for mixed residual household waste – such as incineration with energy recovery and the big, slow mechanical biological treatment (MBT) plants that use a combination of mechanical separation and conventional anaerobic digestion (AD) or composting – are now being overtaken by a second generation of advanced processing techniques.
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While these processes certainly present opportunities for investors seeking to build new sophisticated plants that can extract value from our black bin bag waste, the ‘route to recovery’ can be fraught with complications for the unwary and ill-advised.
During the past decade or so, many people have got their fingers burned through poor decisions, such as building plants of inappropriate scale or using technology that is untried for the type of waste to be processed.
Very often, the technology used is much more sensitive to feed composition than the investors and project promoters expect. As a result, feed preparation systems installed before the advanced waste treatment process are not sophisticated enough and fail to standardise the feed adequately for steady process operation.
Axion’s engineering team has gained expertise in this area in the past decade by evaluating a variety of conversion technologies for residual waste projects, both for our own plants and for external clients. We have also gained experience from research conducted for organisations including WRAP, Zero Waste Scotland and other private clients on converting mixed waste into gas, oil and other products via different techniques.
This also includes expert witness work for companies, helping to sort out legal disputes between clients, financiers and contractors where plants have gone wrong, particularly the first-generation MBT plants.
Last December, we held our first-ever London breakfast event for investors, held in collaboration with Energos and Fiberight. This communicated the lessons we have all learned about when some of these new technologies are appropriate, practical and financeable, which ones might be viable in the future – and those that need more careful thought before selection.
Balancing what is achievable technically with what can be financed securely is vital to the success of big projects of this nature. We chose Energos and Fiberight to join us because their examples of advanced conversion processes strike a sensible balance between advanced resource recovery and use of equipment at a scale and of a type where the technical risk is manageable for financiers.
The main advanced waste conversion technologies currently being promoted regularly to investors in the UK are pyrolysis, gasification and intensified MBT. Within each of these groupings there are many process options available or under development. So which is best?
- Pyrolysis works by heating the waste without oxygen to between 250 and 800˚C. Catalysts are sometimes added to reduce the temperature and improve liquid yield. Long plastic and paper molecules break up into smaller pieces to produce a mixture of oil, gas and char. Heat to drive the process must be provided from outside the waste-containing vessel, so pyrolysis works best at small scale: 250-1,000kg/hr.
Pyrolysis can produce a high yield of oil, up to 70%. As pyrolysis oils are complex mixtures of many different molecules, they are usually suitable only for combustion to make heat. The process works well on materials with high plastic, but low oxygen, content. Oxygen in wood, paper and oxygenated plastics such as PET reacts with the feed material to produce water, reducing yield and degrading the oil quality.
Really high-quality feeds of plastic containing polyethylene and polypropylene allow pyrolysis to generate oils good enough to use as road transport fuel. Sita’s Cynar plant at Avonmouth does this, using mixed plastic film as the feed material.
- In contrast, gasification processes crack long molecules in the feed to much smaller pieces, reducing the material to a mixture of tiny hydrogen and carbon monoxide molecules, called syngas, which is similar to the coal gas that we used in the UK before ‘natural gas’.
The main difference between pyrolysis and gasification is that the former operates without oxygen while gasification uses the oxygen contained within the feed material, plus additional air or pure oxygen injected directly to the gasifier to partially combust the material.
This generates the heat energy and the right chemical conditions to convert all the feed to syngas. Gasification processes can tolerate highly oxygenated feed materials such as wood and paper as well as a higher level of feed contamination than pyrolysis.
Syngas made by gasification can be burned directly to make heat for power generation, as in the process offered by Energos. Processes of this type give cleaner and more complete conversion of the feed than conventional incineration with energy recovery.
Alternatively, syngas can be injected directly to the gas grid for distribution to domestic and industrial users. For this, significant further processing is usually required. Generally it is only possible to do this when syngas has been made in a process that uses pure oxygen rather than air because the 80% nitrogen associated with oxygen in air dilutes the syngas excessively. But using pure oxygen for gasification obviously increases both the cost and hazard of the process.
The alternative to direct gas injection to the grid is to upgrade the syngas to hydrocarbon liquid for use as transport fuel or chemical feedstock. Two techniques are currently used:
1) Fischer Tropsch synthesis – this converts syngas to very clean oils and waxes at high temperature and pressure using sophisticated catalysts. The process was pioneered in Germany during the world wars and refined further at Sasol in South Africa during the apartheid years. Several companies are promoting projects for the UK based on this technology.
2) Fermentation - this uses micro-organisms to convert the carbon monoxide and some of the hydrogen in the syngas to liquid ethanol. Ineos Bio is currently trialling this process on a large scale in the US.
Both techniques make good quality liquid but they have disadvantages, the biggest being that you lose a lot of yield. For 100 tonnes of waste input, you might get only 15 tonnes of oil output plus a lot of heat, which can potentially be used for power generation. Both also work much better when syngas is made with pure oxygen rather than air.
Gasification projects which aim to make liquids really only work commercially at huge scale (40-100 tonnes/hour) and, even then, there is a significant element of technical risk. The capital cost of these large and sophisticated plants runs to hundreds of millions of pounds.
- Intensified MBT processes for mixed waste of the type developed by Fiberight take a different approach and do not rely on thermal degradation of waste.
Paper-making technology separates the different waste components using a wet method. Metals and plastics are separated, leaving a wet ‘soup’ of fats, sugars and other organic matter along with a cellulose fibre feedstock derived from the paper and wood in the residual waste feed.
Bugs easily digest this organic mass that can be fermented using standard intensified high-speed anaerobic digestion (AD), producing a high-quality gas for burning to generate green power. Batch times are 20 times faster than conventional AD and, as a result, the equipment can be much smaller.
Extracted cellulose fibre is converted by special enzymes to make sugar. This can be used as a building block chemical in various industrial chemicals such as adhesives, which is probably the best thing to do in Europe.
Alternatively, the sugar can be fermented with yeast, similar to conventional brewing technology, to make ethanol for use in road transport fuels. Fiberight operates a plant in the US and is developing a project for the UK.
Exciting opportunities await those who want to invest in these second-generation waste treatment technologies. They are definitely the future. But it is essential to obtain good advice from knowledgeable experts who really understand the competing technologies, not just the one that the particular technology promoter is suggesting.
The optimum solution depends on the scale of the waste stream you have access to and its composition. Ultimately, you need to ensure the technology is the most appropriate one for the composition and quantity of the waste you have available. Thanks to positive feedback from investors, we are already planning our next evaluation event.
Roger Morton is director of resource recovery specialist Axion Consulting