Biomass pyrolysis is the thermal decomposition of biomass occurring in the absence of oxygen. It is the fundamental chemical reaction that is the precursor of both the combustion and gasification processes and occurs naturally in the first two seconds. The products of biomass pyrolysis include biochar, bio-oil and gases including methane, hydrogen, carbon monoxide, and carbon dioxide.
The biomass pyrolysis process consists of both simultaneous and successive reactions when organic material is heated in a non-reactive atmosphere. Thermal decomposition of organic components in biomass starts at 350 °C–550 °C and goes up to 700 °C–800 °C in the absence of air/oxygen. The long chains of carbon, hydrogen and oxygen compounds in biomass break down into smaller molecules in the form of gases, condensable vapours (tars and oils) and solid charcoal under pyrolysis conditions. Rate and extent of decomposition of each of these components depends on the process parameters of the reactor temperature, biomass heating rate, pressure, reactor configuration, feedstock etc
Depending on the thermal environment and the final temperature, pyrolysis will yield mainly biochar at low temperatures, less than 450 0C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 800 0C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is bio-oil.
Slow and Fast Pyrolysis
Pyrolysis processes can be categorized as slow or fast. Slow pyrolysis takes several hours to complete and results in biochar as the main product. On the other hand, fast pyrolysis yields 60% bio-oil and takes seconds for complete pyrolysis. In addition, it gives 20% biochar and 20% syngas. Fast pyrolysis is currently the most widely used pyrolysis system.
The essential features of a fast pyrolysis process are:
- Very high heating and heat transfer rates, which require a finely ground feed.
- Carefully controlled reaction temperature of around 500oC in the vapour phase
- Residence time of pyrolysis vapours in the reactor less than 1 sec
- Quenching (rapid cooling) of the pyrolysis vapours to give the bio-oil product.
Advantages of Biomass Pyrolysis
Pyrolysis can be performed at relatively small scale and at remote locations which enhance energy density of the biomass resource and reduce transport and handling costs. Heat transfer is a critical area in pyrolysis as the pyrolysis process is endothermic and sufficient heat transfer surface has to be provided to meet process heat needs. Biomass pyrolysis offers a flexible and attractive way of converting organic matter into energy products which can be successfully used for the production of heat, power and chemicals.
A wide range of biomass feedstock can be used in pyrolysis processes. The pyrolysis process is very dependent on the moisture content of the feedstock, which should be around 10%. At higher moisture contents, high levels of water are produced and at lower levels there is a risk that the process only produces dust instead of oil. High-moisture waste streams, such as sludge and meat processing wastes, require drying before subjecting to pyrolysis.
Furthermore, the bio-char produced can be used on the farm as an excellent soil amender as it is highly absorbent and therefore increases the soil’s ability to retain water, nutrients and agricultural chemicals, preventing water contamination and soil erosion. Soil application of bio-char may enhance both soil quality and be an effective means of sequestering large amounts of carbon, thereby helping to mitigate global climate change through carbon sequestration. Use of bio-char as a soil amendment will offset many of the problems associated with removing crop residues from the land.
Biomass pyrolysis has been garnering much attention due to its high efficiency and good environmental performance characteristics. It also provides an opportunity for the processing of agricultural residues, wood wastes and municipal solid waste into clean energy. In addition, biochar sequestration could make a big difference in the fossil fuel emissions worldwide and act as a major player in the global carbon market with its robust, clean and simple production technology.
Not all charcoal is biochar. True biochar is the result of heating biomass in an emission free pyrolysis reactor devoid of oxygen. Biochar has been shown to be a very effective soil amendment in numerous studies in South America and Japan. It is becoming popularized enough in the US that Biochar Xtra is now even being sold on Ebay. Others are using the bio-oils derived from biochar production to replace fossil fuels. Some folks are alarmed at the possibility of vast tracts of land being denuded to produce biochar. This is not a valid concern because, due to its very low density of from 20 to 35 pounds per cubic foot, the transport of biochar over long distances is not economically feasible.
I’ve been researching this for the book I’m working on, “Pumping the Brakes on Climate Change.”
Last year’s wildfire season should have been a wake-up call. Climate change is killing our forests, and drying them to tinder. Much of Oregon’s coast range (I was just there, for some video of overgrown forests) and a whole lot of the rest of the West is so thick with underbrush you can’t move through it (neither can the deer and elk); it is literally impenetrable jungle. We need to clean that up–run tree farms like farms and pull the weeds–before future megafires destroy more towns and homes and lives.
Our age-old way of clearing brush is slash and burn. That wastes all of the vast energy in that woody biomass; and open burning is inefficient, creates a lot of nasty air pollution–which kills 177,000 Americans, maybe 7 million people around the world, each year–and dumps all of that sequestered carbon back into the atmosphere, where it speeds climate change, forests drying, future wildfires…. Feedback loops grow exponentially. This is how we turn the world to desert.
Or we can harvest that energy and a lot of useful chemicals, and make char, cleanly, without all the air pollution, with pyrolysis plants. Some of the dozens of different pyrolysis devices already being manufactured around the world are small enough to drag out to the woods behind a pickup truck; others fit/are mounted in a shipping container or three, or they can be built big enough to turn the solids from a sewage treatment plant or waste disposal facility into energy and char. Pharmaceuticals in sewage are messing with creatures throughout the ocean food chain, where they concentrate and come back to us. Pyrolyzing the solids destroys those chemicals, and should decompose most other molecular nasties. If we can remove the heavy metals we might use that char for fertilizer; otherwise pumping it deep underground sequesters those toxins, too, and char is a lot easier to handle than CO2.
We’d be replacing fossil fuels with carbon that came out of the atmosphere in the first place; much less harmful cycling it in and out than adding fossil carbon. You get to sequester 20 to 50 percent of the carbon in the wood as char with every cycle, and you are improving the soil so trees and crops grow faster and sequester more carbon. Preventing inevitable future wildfires becoming megafires is almost a side benefit.
Use a big truck to pull right-size pyrolysis units out to a logging landing, clean up all the slash, and only pull the useful, concentrated oils (a stew of useful chemicals, or a replacement for fossil fuel oil), wood vinegar, and as much char for urban gardens as we can’t make from urban landscaping wastes, back to a refinery. Pull a trailer full of crushed silicate rock back out to the woods/farm, add minerals and biofertilizers, and blow it onto the soil (Enhanced Weathering), along with the char. Silicates (basalt, serpentine, olivine) combine with atmospheric CO2 as they decompose, sequestering it forever, The right silicates (already-crushed mine tailings with the right mineral profile?) could hugely improve the soil, and any that wash into the oceans will help decarbonize and de-acidify the waters. Including larger particles would improve the structure of the poor clay soils of so much of the mountain West, while continuing to pull carbon out of the atmosphere for decades—centuries?
Put right-size pyrolysis plants on articulated, off-road trucks like the Mercedes-Benz Unimog, put big, soft off-road tires on the trucks, and take the pyrolysis plants right to the slash/brush/trash/crop residues you want to clean up.
We need to clean up many thousands of square miles of overgrown forest, and we should do it again, in rotation, every decade or three. There are a lot of jobs in this. It will be hard work, mostly in incredibly rugged terrain; it has to pay well enough to attract workers.
Prevent future megafires;
Prevent megatons of killing air pollution;
Slow climate change by not uselessly dumping a lot of carbon back into the atmosphere, and by replacing fossil fuels;
Help reverse climate change via sequestering carbon and enhanced weathering;
Carbon-neutral fuel oil; replacements for a number of petrochemicals; wood vinegar can replace some herbicides;
Improve soils to grow more timber and crops for the too many people of the future;
Cleanly clean up all kinds of wastes, some pretty nasty; and
Lotsa Jobs.
If we can’t find enough value in all of that to pay people to do it, add in carbon credit monies.
And if we’re to get people to do this, government needs to lend, not grant, startups the money, maybe provide guidance and training….
John O.