Pyrolysis is not a new technology per se. However, new applications of the technology are gaining interest from the scientific community in light of the emerging global energy and global warming crises, complex waste problems and agricultural soil quality issues. Pyrolysis can be used as an "appropriate technology," by extracting more value and energy from materials in the waste stream like agricultural stover and post-consumer food wastes, bio sludge, forestry byproducts, some kinds of plastics and tires.

Now there is more focused research and money available from government stakeholders, more analytic tools and complementary technologies available to bring to market some new applications of pyrolysis.

This website aims to compile and summarize some of this more recent research.

More Information About Sustainable Pyrolysis Applications

Recovery of Fuels and Chemicals Through Catalytic Pyrolysis of Plastic Wastes

Life Cycle Assessment of Biochar Systems: Estimating the Energetic, Economic, and Climate Change Potential

Rubber Tire Pyrolysis - Production of Highly Enhanced Marketable Products

Pyrolysis of the Tetra Pak

Low Oxygenate Bio-Oil Through Two Stage Hydro Pyrolysis

Pyrolysis of Algal Biomass in a Solar Furnace Reactor

Climatelab: Biochar

Biochar for the home gardener


 

 

  • What is it?
    ...
  • How does it
    work?
  • What are some
    applications?
  • Novel
    research
  • Benefits of the
    technology
  • Alternatives
    ...

What is it?

Torrefaction, pyrolysis and gasification are all irreversible endothermic, thermolytic reactions involving organic materials that take place in low or no oxygen environments at high temperatures and under high pressure. The process simultaneously changes the phase and chemical composition of the reactants; the products are almost always smaller in molecular weight. When the reactants are from just biomass, the reaction products are "synthetic gas" and tarry "bio-oil," and a form of solid stable carbon called "biochar" (usually about a 1:1 ratio of elemental carbon and hydrogen) that is agriculturally useful as a soil amendment that retains water and nutrients.

Torrefaction (from the French word for "roasting") is a lower-heat form of pyrolysis, which typically occurs at temperatures of 200 to 320 ºC. There are plenty of applications of torrefaction in food processing and production. For the purposes of this research website, it is interesting that torrefaction produces higher proportions of the stable carbonaceous “biochar” (like charcoal) from biomass feedstocks than the other two processes. There is research being conducted presently at the Schatz Energy Research Center at Humboldt State University on this very topic. Biochar can be used as a soil amendment, for cooking or in co-generation as a cleaner-burning substitute for coal in power plants (when pelleted).

Anhydrous pyrolysis (in the absence of oxygen and water) is generally conducted at temperatures between about 400-550ºC, when the desired products are some ratio of the syngas, bio-oil and biochar. Compared to torrefaction, higher proportions of energy-dense organic gases, liquids and oils are produced than the biochar. A survey of the research abstracts on the internet seems to indicate that there is particular interest in the optimization of pyrolytic processes for the production of replacement fuels from biomass wastes, since there is an enormous amount of chemical energy available from them.

Other research has focused upon the application of pyrolysis to the recovery of energy and chemical manipulation of complex waste items like tires, "tetra pak" packaging, food wastes, and mixed scrap plastics remaining from junking cars. Some of these applications will be explored in this website.

It should be noted that the pyrolysis process at varying temperatures is used to create a wide swath of products in diverse industries, from fuels like methanol, chemical feedstocks for pharmaceuticals, activated carbon (which has important adsorbant properties), PVC, and carbon fiber, to coke (used in metallurgy) to name a few. Also, so-called stream cracking, or hydrous pyrolysis, is used in the petroleum industry to refine oil into other lighter hydrocarbons using superheated water under pressure.

Gasification typically requires temperatures above 700ºC, and the products are even larger proportions of gas, liquid and oil organic fuels, in less time. Still higher temperatures can produce coke (1000-2000 ºC) and carbon fiber (1500-3000 ºC).

How does it work?

Pyrolysis processes can vary wildy depending upon the input constituents and the desired output, whether or not there are catalysts, which byproduct pollutants need to be controlled for and so forth. The diagram below summarizes what a pyrolysis process would look like if the feedstock were mostly mixed plastic wastes from scrapping cars, if the desired products were to be optimized for "liquid organics," and if catalysts were used. Liquid organics are unrefined combustible fuels with high boiling points, high energy density and high heating values that could be used as primary chemical feedstock for industrial applications, and as a substitute for gasoline (also known as syngas).

The pyrolytic process below is a particularly complex example, and does differ stepwise from a process whose feedstock is say purely lignocellulosic from forestry byproducts, or pure corn crop stover. It is a very intriguing idea to be able to extract energy and enriched chemical value from a mixture of polypropylene, acrylic, nylon, PVC, ABS, polyurethane foam, tar, wood and glass-reinforced polyester as this example shows. This example is still relevant to show essentially how the pyrolysis process can be applied, as well as the refining methods used to segregate useful hydrocarbons by their thermal properties.
The methods that are fairly commonly used across the range of pyrolysis applications are as follows:

  1. A pre-heater and drier removes as much moisture as is practical to achieve, to migate the production of less useful or valuable oxygenates during pyrolysis, and other potentially toxic compounds.
  2. A hopper shreds the material feed to a desired size to maximize surface to volume ratio and speed up the reaction times.
  3. Solids are fed into a highly pressurized anaerobic reactor, heated to typically somewhere between 150°C and 400°C for torrefaction and 400°C and 550°C for pyrolysis, depending upon the desired chemical output. Pressure inside the reactor and chambers can vary greatly as well.
    1. Chemical bonds of the molecules in the solids are broken and new ones are formed (usually of lower molecular weight). Solids, liquids (in vapor form) and gases are formed.
    2. Residence time of the input also varies greatly. Typically more time produces more char whereas "fast pyrolysis" produces more liquids/oils.
    3. The type of reactor used can vary depending on the desired reaction products and scale. Most commonly the fluidized bed reactor is used since they can be employed at industrial scales. Different carrier gases like N2, CH4, CO, CO2, or H2 are used to inject the input material at high velocity into a bed of super-heated sand. Like a popcorn air-popper works, a critical point is reached where the force of the particles and the aerodynamic drag (against the particles and sand as they fall back down) is greater than the combined weight of the sand and particles, and everything in the reaction chamber starts to behave as an expanding fluid conforming to the shape of the reaction chamber. The different residence times and temperatures produce varying proportions of liquids with different chemical constituents and heating values, and so do the different carrier gases [reference]. One benefit of the fluidized bed reactor design is that the biochar residue build-up on the chamber walls also gets removed by the collisions of sand.
      However, there are several more types of reactors that may be used in pyrolysis:
      1. Double Rotary Kiln (used in the schematic below)
      2. Circulating Beds
      3. Transported Beds
      4. Cyclonic Reactors
      5. Entrained Flow
      6. Ablative Reactors (hard to scale up)
      7. Microwave Reactors
  4. Regardless of the type of reactor, as the reaction effluent leaves the reactor, its constituents are separated by properties like mass, density, and phases within the temperatures and pressures of the environments inside and out of the reactor.
  5. The lightest fraction of gases and water vapor are among the first to be separated out into another chamber. These will be cooled to STP (sometimes their heat is exchanged and reused in the system). Of this fraction, the heavier are separated as they cool slightly, and the rest of the gas moves to another chamber. Here much of the water vapor is removed, and the lightest of the gases are condensed and treated in a scrubber, then collected. These gases include ethylene, propene, butene (olefins), methane, ethane, CO2 and methyl chloride.
  6. A common device is a cyclone that spins the constituents leaving the reactor. In the diagram below there are two. Solids hit the sides and fall out due to their mass and density, while hot gases and vapors will rise and their heat can be exchanged and used to pre-heat and dry the new material being fed into the system. Also the gases and their heat can be reinjected into the reactor as carrier gases. Most systems have more than one cyclone. The hot gases rising out of the other cyclone will be condensed, while the solids drop out.
  7. Then a series of boiling, fractionation and condensation steps separate the constituents by their boiling point, density and molecular weight properties.
  8. The oxygenates dissolved in water are boiled to be separated (mostly mild organic acids and glycols).
  9. The liquid organic fraction of hydrocarbons is heavier my molecular weight, so these separate from the aqueous fraction easily.
  10. Again the liquid organics are boiled and the "high boilers" (the really dense, heavy hydrocarbons) are separated, while the other aromatics and branched aliphatics are repeatedly segregated by boiling point (the former below, and the latter above 190°C). This is essentially how a refinery works.

It may seem miraculous that such a system could be economical, considering the enormous amount of energy required to boil water, let alone compounds that boil at near twice the temperature of water, especially at higher than atmospheric pressure. Most of the gases and liquid organic products from these reactions can be combusted to provide the heat for pyrolysis and boiling, and with careful consideration of design and materials, heat loss can be minimized for efficiency.

Source: http://onlinelibrary.wiley.com/doi/10.1002/ep.670170114/pdf

Pyrolysis Diagram

What are some applications?

Pyrolysis can be applied at large and small scales for both pollution control and energy efficiency improvements, most beneficially if its feedstocks are biomass waste materials, though as it was just shown in the previous tab, one of the applications of pyrolysis is to transform a mixed source of plastics from junked cars into concentrated, value-added chemicals and synthetic fuels. Likewise with used tires, pyrolysis can be applied to produce activated carbon, synthetic oil feedstocks comparable to Number 1 Fuel oil, and synthetic gas. It is an exciting proposition that such materials can be diverted from an eternal resting place in a landfill, where they will resist degradation for thousands of years, and instead new uses can be found for their embodied energy.

Good examples of biomass material candidates for pyrolysis are agricultural solid wastes, industrial food processing wastes, animal feedlot wastes, municipal sewage and solid wastes, forest product processing wastes, and urban forestry and landscaping wastes. Now, most of these sources carry their own sustainability challenges, for instance due to their inherent role in the ecosystem or their material composition. But a well-managed and considerate use of these materials can provide more benefit than other disposal methods. By contrast, a common method of dealing with excess agricultural biomass (especially in the developing world) is to burn it in the open environment, which emits large quantities of carbon dioxide and black carbon, which is also a worrisome greenhouse gas that is more potent than CO2 and yet also easier to control. Furthermore, animal feedlot wastes are biological disease vectors and sources of water pollution, and huge sources of methane pollution (a potent greenhouse gas).

Of all of these potential feedstocks, probably the least complex step-wise pyrolysis application is to "stover" (agricultural biomass waste). Studies show that it can be an economical and efficient way to derive net energy and heating value from stover and also to sequester carbon into a stable form ("biochar") that can be amended to agricultural soil. Much of the agricultural sector in this country is focused on monoculture crops, which has its own problems, but the consistency of the crop waste (stover) and the existing infrastructure on large farms can make the large-scale adoption of a biomass to energy program a relatively easy transition. Further, there is a massive and ongoing need for soil amendments to prevent erosion and retain moisture and nutrients on farms. Biochar is stable and persitant (lasting hundreds or thousands of years), and it has preferable physical qualities, like the slow release of micronutrients like selenium and other fertilizers if applied. So biochar could reduce the need for fertilizer (which is very energy and therefore carbon-expensive). Biochar is also good for the soil ecology since it provides good habitat for microorganisms and fungal hyphae.

Now this particular application of the pyrolysis process can be done on site, since a mobile self-powered pyrolysis machine has been developed by Agri-Therm, a spin-off company from the University of Western Ontario. A market-ready solution that is available for the very decentralized agricultural sector and appeals to the self-interest of farmers is a very thoughtful idea.

Image source: http://www.universityaffairs.ca/crop-residue-waste-not-want-not.aspx

Agri-Therm

Novel research

One of the challenges to using syngas and bio-oil products produced from pyrolysis of biomass feedstocks is that they often have impurities or a significant amount of suspended water, which complicates their refinement, lowers their heating value, and makes them less stable. Also, the presence of aqueous organics causes etherification, esterification and polymerization reactions, resulting in both higher viscosity and higher water content [reference]. There is some research looking into refining the process to reduce the oxygenates in the syngas during pyrolysis. One research proposal found aims to refine a two-stage hydro pyrolysis process for this purpose [reference].

Since pyrolysis requires heat, why not use solar thermal energy? One team of researchers, through the P3 contest, proposed pyrolysis of algal biomass in a solar furnace reactor [reference]. This biomass is what is left after the production of biofuel, and if algal biofuel reaches an economy of scale, this will be a big challenge to make use of. Since this kind of biomass would be so moist, pyrolysis would not be the best option unless there were an economical way to preheat and then pyrolyze. This is an ingenious way to use a solar furnace (see diagram below).

Reference: http://visual.merriam-webster.com/images/energy/solar-energy/solar-furnace.jpg

Solar Furnace

Benefits of the technology

Perhaps the lynchpin for widespread application of biomass pyrolysis is the buy-in from the agricultural sector. To balnce the sometimes competing needs for energy content and heating value, economic interests, carbon sequestration and land use and improvement, research was conducted for a life cycle assessment of biochar systems. It was found that "late stover" (drier agricultural waste) was the most beneficial among yard waste, switchgrass and "early stover," for carbon sequestration; its GHG balance was net negative, -864 kg CO2 emissions per ton dry biomass. It also yielded high energy/heating values compared to the other samples [reference]. This is good news. The net balance of the products from pyrolysis of stover is energy-positive and carbon-negative, even as some of the energy from the process is used to run the reaction, and this is still true for early stover, but with less energy benefit. There is market value in agricultural waste for use as fuel, and also as a soil amendment as mentioned previously. More information about the soil ecology benefits of biochar can be found here.

The life cycle study also accounts for transportation and embodied energy averted due to a decreased need for fertilizers due to the physical and chemical properties of the biochar soil amendment. Critically, it evaluates the net emissions if the biochar is combusted as a fuel. If it replaces coal or natural gas, then in terms of GHG emissions, biochar is a better option (and also in terms of other emissions from coal).

For market mechanisms to work on a climate exchange, so that farmers would be willing to spend the time and money for pyrolysis machinery,and alter their production inputs and logistics, the study suggests that the price of carbon sequestration be set at least to $40/ton. The study seems to advocate against the production of crops solely for pyrolysis, given the negative impacts of land changes.

The author of the life cycle study also takes a global view of CO2 emissions, in light of the IPCC recommendations for 50% emission reductions from 2007 levels by 2050. The author concludes that pyrolysis of just 50% of the currently wasted biomass in the US agriculture sectors could achieve a 4% reduction in global emissions, and moreso if forestry residues were used, more still if the US were to adopt agressive policies to use replacement renewable biofuels (which would of course include this process).

Logistics, economics and carbon considerations of transportation make large-scale adoption particularly challenging. Though interestingly, the emissions are least affected by transportation distance of biochar; rather, net energy and price are more impacted. This is why the Agri-Therm portable pyrolysis machine built specifically for stover-type biomass is so intriguing. The biomass can be converted on site, without transportation energy and labor costs involved, if the producer is willing to invest in that capital equipment.

Source: http://pubs.acs.org/doi/pdfplus/10.1021/es071361i

Energy Balance

Alternatives

If the concerns of, say, a waste managment company are to maximize the volume of wastes coming into a landfill, minimize the volume that is actually landfilled, and lower capital costs as much as possible, they might be inclined to use an incinerator to double as a cogeneration plant. Incinerators combust material in open air, which can lead to toxic reaction products like PCBs (polychlorinated biphenols), and PCDFs (polychlorinated dibenzofurans), which are known carcinogens. This is an atrocious practice that spews out airborne toxic wastes and produces endless tons of toxic fly ash, which sometimes is approved for use as daily cover. Scrubbers only go so far toward mitigating such impacts. It is hard to believe that these plants are still legal to build, but it is true.

Pyrolysis presents a cleaner alternative to incineration, although it is a much more complex one. The economics of resource recovery are surveyed in the paper, "Recovery of Fuels and Chemicals Through Catalytic Pyrolysis of Plastic Wastes." Given that one of the goals of a waste management company is to extend the landfill life by reducing the volume landfilled, while also increasing the volume it manages, it may in fact be in its economic interest to run or collaborate with a pyrolysis recovery operation at its landfill facility. What better place to find chemical feedstocks for virtually any purpose?