American chemists discuss a scheme hydrogen production from waste plastic.

[NNadir]

At present, hydrogen is commercially produced mostly by catalytic steam reforming of hydrocarbons: natural gas and naphtha. However, processes using other raw materials to produce hydrogen, especially wastes and byproducts, are also attractive because of promising economic and environmental benefits. In this research we explored possibilities for producing hydrogen from post-consumer waste plastics. Plastics, especially polyolefins, have significant potential but are not yet used as a resource for hydrogen production. Plastics account for 8-9% of today’s waste stream, or about 15 million tons annually,1 that are mostly disposed of in landfills. Potentially, these waste plastics could be used to generate six million tons of hydrogen per year. Though recycling of plastics has a positive environmental impact, in most cases it is not yet economically attractive. So far, industry has focused efforts in plastics recycling on the recovery and reuse of polymers by mechanical processing. However, mechanical technologies require relatively clean feedstocks that are expensive to collect and separate. For this reason, commercial recycling has not had a significant impact with the collection rate of less than 5% of total annual resin sales.

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Here are some caveats before anyone gets too excited: First of all 15 million tons of any form of energy is relatively trivial. Secondly, this is a pyrolytic scheme which requires heat, but heat is available from many sources - including the greenhouse gas minimized nuclear energy. In any case the heat requirement limits the energy efficiency of the process. Third, for all of it environmental problems, and even though the EPA now includes burning plastic as a "renewable fuel," plastics are mostly oil and natural gas based. It is possible to make them synthetically from other sources, but this the current state of affairs is that they are fossil fuel derivatives. As such, they actually represent, in landfills, sequestered CO₂.

However the research does point up a way that as part of a carbon cycle, plastics could be more versatile. The side product carbon dioxide could in theory be recovered and hydrogenated to make clean motor fuels like DME.

The abstract of the article is available here: http://pubs.acs.org/cgi-bin/abstract.cgi/enfuem/asap/abs/ef050354h.html

[skids]

1. What benefits does this offer...

...above the recycled plastic diesel fuel technology? Does it use different types of plastic?

[Oerdin]

2. The molecular depolymerization processes

Could do either one though it would be much more energy intensive to create hydrogen compared to hydrocarbons. Basically you're just using heat, pressure, and time to break long chains of hydrocarbons down to shorter chains of hydrocarbons so if you cook it until every chain is broken then you are left with a good deal of hydrogen gas. That would require a lot of energy and likely we would never recover enough energy to justify the amount we recover in hydrogen. Hell molecular depolymerization doesn't even do that for hydrocarbons which require much less energy inputs to achieve.

Luckily, we don't measure things by the amount of energy needed to create them and instead measure them by the cost it takes to produce them and it doesn't get much cheaper then trash. Of course there is costs associated with building the facility and maintaining it and since it operates at a lose there is a cost associated with that financial lose. So does that mean this technology is a born loser? Not necissarially since there is also costs associated with the disposal of rubbish. Currently slaughter companies use molecular depolymerization to get rid of animal remains which aren't marketable since it is cheaper to run this process, make oil & water (along with a bit of carbon black), and then sell the stuff at a slight lose then to pay to landfill it in some areas. Municipalities are looking in to doing this with plastics since they'd lose less money (and it is always a money loser) then they would if they had to build new landfills to replace maxed out landfills. Also if clean water standards increase, not going to happen with the current administration but possible with future admins, then the disposal of sewage via molecular depolimerization becomes attractive for several large cities.

The current molecular depolymerization plants all rely upon burning the recovered natural gas though if you're heating things until it breaks down to hydrogen then obviously you will recover much less natural gas and no oil. That means in addition to requiring more energy to operate you would also recover less energy to subsidize the whole process. All in all it is a money loser.

[Dead_Parrot]

3. Herein lie one of the reasons...

.. for my guarded support of nuclear power: a lot of the fossil replacements - depolymerisation, H by electrolysis, processing algae/whatever for biofuels need lots of heat and/or electricity to make them work. Without that, we're screwed.

We're screwed already, but that a different thread. :)

[NNadir]

4. The economics of the process are indeed subject to process heat costs.

However there are different ways of defining "economics."

Traditional economics only includes internal costs. This might be fine in a world inhabited by one billion people but it is not fine in a world inhabited by 5 billion people or 6 billion people or even, really, 3 billion people.

In order to maximize the already small probability of human survival it is therefore necessary to include and charge external costs, specifically, the cost to the environment.

According to this link, a mean price of 19 euros per ton of carbon dioxide have been assessed: http://europa.eu.int/comm/research/headlines/news/article_05_10_21_en.html

I do not know that the external cost of landfilling plastic has been systematically analyzed, but as a child of New York's Long Island, where landfills are located over sandy glacial moraines, where the effluent of these landfills filters directly into the water supply, I well know that landfills are not free when considered on an external cost basis.

I believe that carbon dioxide can be hydrogenated - if - convenient sources of high concentrations of carbon dioxide can be found. One may pick and choose the hydrogenation products, depending on need. One approach of course is the brute force approach of isolating carbon dioxide from air. This too, is energy intensive and thus the external cost is dependent on the external cost of energy. Another consists of the thermal decomposition of waste into carbon monoxide and carbon dioxide in the presence of water, possibly waste water.

One thing is certain: Plastic recycling as it now exists is only working marginally. It can and should be accomplished in a much more efficient and comprehensive way, maybe through the conversion to simple and versatile intermediates, in particular syn gas. Since syn gas is generic and can be used in an almost infinite variety of ways with quick responses to market conditions - it may prove to be the most economic way of processing mixed organic wastes including plastics. I expect that the external cost of plastic recycling as it is now practiced is somewhat higher than we realize because of the need for separation and attendant trucking and processing. A thermal treatment other than traditional garbage burning, it seems to me, is a worthy enterprise. I say this in full knowledge of failed depolymerization units like the Changing World Technology. That is why I am interested and excited by research such as this. I read it with the same enthusiasm as other people read about Brad, Jen, and Angelina.

I note that carbon dioxide obtained from the air - if possible - and hydrogenated into plastic precursors does represent fixed and sequestered carbon dioxide.

Process heat in theory is not really difficult to obtain and combustion - the traditional way - is but one way to obtain it.

In general I agree that economics is an extremely important factor in all environmental energy decisions. If I did not agree I'd be running around making grand claims for solar PV energy, but I'm not. However, even acknowledging that, I am not really a free marketeer. I believe it is the role of government to regulate the common space - including the atmosphere - and this is only possible through the imposition of an external cost fee: Something some people would call taxes. If carbon taxes were paid, recycling plastics might become economically viable: I don't know but I do call for the analysis of the problem in a systematic way.

Personally I have no problem whatsoever with taxes. I believe that taxes are what you pay for living in a civilized - and hopefully sustainable - world. I would like taxes applied fairly, and the revenues used efficiently, but I am glad that I get to pay taxes.