Professor Quicker, we regularly hear it promised that gasification processes could replace classic waste incineration and become the next big thing in thermal waste processing. What is your assessment?
First of all, we have to clarify what we mean by gasification. A lot of what comes under this term is actually two-stage incineration. One example from the UK relates to processes that were developed when plants capable of producing calorific gas were being promoted there. This was achieved by first of all incinerating the waste with a small amount of oxygen, which always produces combustible gas. If you were to take a measurement, you could say: I am producing gas by incinerating waste, which I can theoretically use for energy. In reality, though, in such plants air is added again in a second stage that follows on immediately and the combustion process continues. It is therefore two-stage incineration and not gasification.
What about Japan? The country is considered a stronghold of gasification.
Processes are very often used there that are similar, but the two stages take place spatially separated. This development came about when regulations were passed in Japan that required the ash produced by waste incineration to be melted. In these spatially separated two-stage processes, the waste is firstly degassed and then the gas that is produced is burned in a high-temperature furnace together with the coke, i.e. the carbon left over from the degassing process. The idea is that the ash melts during the process. This reduces the volume of the residual material. The slag that remains after the ash has melted is completely inert, burnt out, and free of pollutants. If it is allowed to drip into water, it vitrifies. But the problem with the process is that you have to add additional fossil fuel, e.g. natural gas or coal, to make it work. Moreover, the process is much more expensive than conventional combustion.
But there are other gasification processes that actually produce a gas that is available for further thermal utilisation.
There are fluidised bed gasification plants where the gas produced during waste treatment goes directly into a downstream process, for example into the calciner of a cement rotary kiln. Here too, though, the gas is then burnt so no gas is recovered that can be used for higher-value subsequent utilisation. A gasification technology that aims at motor use of the produced gas is known as the Thermoselect process. Its use has failed in Europe but some plants are still operating in Japan. They gasify the waste to produce a gas that can be used in higher-value processes. However, the performance of these plants is, to put it mildly, very mediocre. One problem is that the very high temperatures required for this process are difficult to achieve with waste on its own. Therefore, natural gas has to be constantly added – a not inconsiderable amount in fact: 40 cubic metres per tonne. And 400 cubic metres of oxygen per tonne are also required. And ultimately, the gas is very often not clean enough for motor use and, despite the effort, it is burnt again in a combustion chamber to run a steam process.
Are you saying that in reality there is not a single functioning plant where waste can be gasified?
I actually don’t know of any case where residual waste is gasified under conditions that come even halfway to making economic sense and where there is an additional benefit compared to conventional waste incineration. In Japan, there is a plant where plastic is gasified and where hydrogen is produced at the end of the process. But I see that as more of a showcase example. For this process to work, it needs such good-quality feedstock that the question immediately arises whether it would not make more sense actually to recycle this material. Moreover, a huge amount of work still has to be done on the feedstock before gasification. The plastic has to be cleaned, shredded, and extruded.
Read more about gasification in Japan here!
And is there nothing else?
Lots of processes are being tested in which plastic is pyrolysed, resulting in oils that can be used as a feedstock for the chemical industry. But the quality of such pyrolysis oils is so low that they can actually only be used in small quantities as an additive to crude oil in the refinery. Or their processing is extremely complex. This may make sense for certain special fractions that are clearly separated, such as mattress foams or thermal insulation boards, and for which mechanical recycling is not possible. I would advise anyone who thinks that in pyrolysis they have found a solution to household plastic waste to have a good look in a recycling bag or dustbin and then consider what they would need to do to separate out single-variety plastics from the rest of the contents.
With the right amount of effort, it’s possible.
With the right amount of effort, anything is possible. You could even make diamonds out of residual waste. Technically, it is possible. You just have to separate out all the minerals so you are left with just the carbon, which you pyrolyse into pure carbon, and then you pressurise it at high temperature for several months to make diamonds. That’s possible. However, I would not like to see the ecological rucksack that you have created in the process. It’s a similar thing with gasification. Here, too, the more complex the process, the worse its balance, both environmentally and economically.
So there is no alternative to waste incineration?
Everyone always asks for an alternative! Yes, there is, but it’s upstream of the bin, not downstream of it. When designing products, much more consideration should be given to their ability to be separated into individual materials at the end of their life cycle, because this will then enable these materials to be recycled. At the moment, however, things are unfortunately going in exactly the opposite direction. Everything is being glued, fused, welded, or cast. And more and more products have inbuilt lifestyle electronics that you can’t remove and they end up in the residual waste. Not every coat has to be able to make a cup of coffee. No, it’s not the fault of waste incineration that so much unnecessary waste is produced. It disposes of it and does so in the best possible technical way. But there is still an urgent need for us to burn fewer valuable resources. The way to do this, however, is through waste avoidance and recycling. It is no use hoping that one day a process will be invented with which – if you’ll pardon the expression – you can turn shit into gold. Even in the Middle Ages, they knew that didn’t work.
Peter Quicker is a professor at RWTH Aachen University and one of the world’s best-known specialists in the fields of waste to energy, alternative waste treatment technologies, and material recovery.
