Among waste management and recycling technologies, pyrolysis remains little known to the general public. I first encountered it in the late 1980s when I served on a team that created a database for the American Public Works Association.

My part was to index articles from their library’s collection of periodicals. I got a great general overview of public works. In the area of waste management, I encountered many articles about landfills and incineration. A few described pyrolysis, and at first, I had no idea what it meant. Nowadays, most people don’t know any more about it than I did 30+ years ago.

It has important advantages over more common waste management technologies. Landfills merely bury wastes, where they create leachate that can pollute groundwater. Incineration reduces wastes to ash and contributes to air pollution in the process. Pyrolysis turns wastes to useful products that command good prices on the market, with few if any emissions.

What is pyrolysis?

The word pyrolysis comes from two Greek words: pyro (fire) and lysis (separating). It means heating organic material in an atmosphere without oxygen. In those conditions, large molecules break down into smaller ones. That is, the components of the original molecules separate from each other in extreme heat.

What starts out as plastic, biomass, old tires, or other feedstock becomes a solid residue called biochar, a liquid called pyrolysis oil, and various gases including carbon dioxide, carbon monoxide, hydrogen, and methane. The chemical reaction can take place in rotary kilns, rotary hearth furnaces, or fluidized bed furnaces. These need not be large.

Various catalysts improve the efficiency of the process. They can reduce the necessary temperatures and help control the properties of the pyrolysis oil. Many catalysts also increase the cost of pyrolysis. Natural zeolite catalysts have proven much less expensive.

Without oxygen, nothing can burn. Burning anything emits large quantities of greenhouse gases. Since no burning takes place, pyrolysis has no emissions. The gas residues remain in the chamber until they are removed for use.

Moisture content of the feedstock ought to be about 10%. Therefore, it is not a suitable process for dealing with sludge, manure, or meat processing wastes unless they are first dried.

Pyrolysis also demands a small particle size. Some kind of size reduction process must reduce particles to no more than 2 mm.

Pyrolysis products

There are three types of pyrolysis, depending on the final temperature and how quickly it is reached.

• Slow pyrolysis takes place at temperatures of about 450º C at a slow heating rate. It takes several hours to complete and produces mostly biochar.
• Fast pyrolysis takes place at somewhat higher temperatures with a rapid heating rate. The feedstock spends less than one second as a vapor. Rapid cooling after that brief time produces the pyrolysis oil.
• Ultra-fast pyrolysis takes place at temperatures above 800º C and produces mostly gas.

Pyrolysis oil has a fuel value of about 50-70% of fuels from crude oil. As it is, it’s useful as boiler fuel. It is possible to upgrade it to make biodiesel or syngas. It is also useful as feedstock to make a wide variety of specialty organic chemicals, including new plastic.

Biochar makes an excellent soil amender. Highly absorbent, it helps the soil retain water, nutrients, and farm chemicals. Therefore, it reduces runoff and water pollution. It also sequesters carbon.

The gases plus a portion of the solid or liquid can provide all the energy needed to operate the equipment. If used that way, the process becomes a closed loop. The output provides the energy to run it. Refinement of syngas includes removing particulate matter, hydrocarbons, and other impurities.

Pyrolysis feedstocks

Pyrolysis reduces any kind of organic waste. It is especially useful for municipal solid waste, biomass, plastic, and tires. Pyrolysis of municipal waste first requires separation of glass, metal, and inert materials, leaving organic feedstock.

Biomass can include scrap wood and agricultural wastes. Biomass itself is very awkward to transport. After pyrolysis, it becomes much denser and more cost effective to transport.

Pyrolyzing plastic and tires requires fuller explanation.

Plastic pyrolysis

While global production of plastic increases, recycling rates remain stuck at about 15%. Pyrolysis may help keep more of it out of oceans and help solve what to do with plastic recovered from oceans.

Polyvinyl chloride is currently the one plastic that pyrolysis can’t deal with.  It has a wide range of applications but presents special challenges for waste management. The chlorine renders most techniques of plastic recycling useless.Pyrolizing polyvinyl chloride requires dichlorination. Otherwise it produces toxic hydrogen chloride, which contaminates the output.

Current technology can either dechlorinate before thermal degradation or scrub the chorine from pyrolysis residue. This extra step makes it too expensive for commercial pyrolysis and still leaves too much chlorine.

Ongoing research seeks a catalyst that can capture the chlorine in a single-step pyrolysis process.

Tire pyrolysis

Waste from manufacturing tires amounts to about 50 thousand tons of mostly body ply sent to landfills every year. That’s in addition to worn out tires removed from vehicles.

Scrap tires present several hazards. They become breeding grounds for rodents, snakes, and insects. Piles of them are unstable. Falling or rolling tires can injure or kill people. If they catch fire, they can burn for months. And if illegally dumped, rubber causes water pollution. Incineration of tires requires careful emission controls.

Fortunately, piles of scrap tires have become much less common than they used to be. Now, special machines cut the tires into pieces. Various mechanical or chemical processes reduce the rubber to granules or powder. Somewhere in the process, it is necessary to remove the steel, which is itself a valuable resource.

The crumb rubber can become components of products such as synthetic turf, highway additives, or playground floors.

Pyrolizing tires, as of anything else, produces pyrolysis oil, non-condensable gases, and biochar. In addition, it leaves the steel behind, which comprises about a third of the weight of a tire. All of these products have greater value than crumb rubber.

Where can pyrolysis be useful?

Agilyx and other companies operate large-scale pyrolysis facilities. They exist to turn plastic waste into new plastic. The technology works on a smaller scale, however.

The US Department of Agriculture envisions small-scale pyrolyzers on farms, although the page I used for this post doesn’t explain why pyrolyzing them would be better than leaving crop residues in the field. It appears that retrieving biochar for a soil amender and selling pyrolysis oil to refiners can make economic sense. But plowing crop residues under likewise sequesters carbon dioxide.

Several years ago, Montgomery, Alabama announced a recycling program that wouldn’t require residents to separate recyclables from other trash. It was a resounding failure. More recently, it has modified the concept and tried again.

One trouble with such mixed waste processing is that it renders otherwise valuable paper and cardboard unfit for further use. Yet recycling rates remain stubbornly low.

What if pyrolyzers became common at landfills? Local recycling programs would divert the most valuable materials from the waste stream and run them through a clean materials recovery facility (MRF). A dirty MRF at the landfill would remove recyclable metal and glass and send them to the clean MRF, which would process and sell it.

Remaining organic material could be set aside either for composting or pyrolysis, either of which would produce other saleable commodities. Only a small fraction of the waste stream would have to go to the landfill. And with the organic material removed, the landfill would no longer have to deal with vermin, leachate, or methane buildup.

Some environmentalists seem to think that pyrolysis is nothing more than a synonym for burning and stand in ignorant opposition to it. This technology ought to be better known, better understood, and more widely practiced.

Sources:

Biomass pyrolysis / Salman Zafar, AltEnergyMag. February 1, 2009
How are automobile tires recycled? / Utires. February 5, 2017
Making inroads with tire pyrolysis / Maura Keller, American Recycler. July 2017
Pyrolysis of municipal wastes / Salman Zafar, BioEnergy Consult. July 29, 2020
What is pyrolysis? / USDA Agricultural Research Service. April 14, 2017