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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. Depending on the thermal environment and the final temperature, pyrolysis will yield mainly biochar at low temperatures, less than 450 degrees C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 800 degrees C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is bio-oil.
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. Pyrolysis offers a flexible and attractive way of converting solid biomass into an easily stored and transported liquid, which can be successfully used for the production of heat, power and chemicals.
Figure 1: Process conditions for pyrolysis of biomass
Feedstock for Pyrolysis
A wide range of biomass feedstocks can be used in pyrolysis processes. The pyrolysis process is very dependent on the moisture content of the feedstock, which should be around 10 percent. 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.
The efficiency and nature of the pyrolysis process is dependent on the particle size of feedstocks. Most of the pyrolysis technologies can only process small particles to a maximum of 2 mm keeping in view the need for rapid heat transfer through the particle. The demand for small particle size means that the feedstock has to be size-reduced before being used for pyrolysis.
Figure 2: A glance at feedstock availability and energy products from biomass pyrolysis
Types of Pyrolysis
Pyrolysis processes can be categorized as slow pyrolysis or fast pyrolysis. Fast pyrolysis is currently the most widely used pyrolysis system. Slow pyrolysis takes several hours to complete and results in biochar as the main product. On the other hand, fast pyrolysis yields 60 percent bio-oil and takes seconds for complete pyrolysis. In addition, it gives 20 percent biochar and 20 percent syngas. Fast pyrolysis processes include open-core fixed bed pyrolysis, ablative fast pyrolysis, cyclonic fast pyrolysis, and rotating core fast pyrolysis systems. 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 one second.
Quenching (rapid cooling) of the pyrolysis vapours to give the bio-oil product.
Uses of Bio-Oil
Bio-oil is a dark brown liquid and has a similar composition to biomass. It has a much higher density than woody materials which reduces storage and transport costs. Bio-oil is not suitable for direct use in standard internal combustion engines. Alternatively, the oil can be upgraded to either a special engine fuel or through gasification processes to a syngas and then bio-diesel. Bio-oil is particularly attractive for co-firing because it can be more readily handled and burned than solid fuel and is cheaper to transport and store. Co-firing of bio-oil has been demonstrated in 350 MW gas fired power station in Holland, when one percent of the boiler output was successfully replaced. It is in such applications that bio-oil can offer major advantages over solid biomass and gasification due to the ease of handling, storage and combustion in an existing power station when special start-up procedures are not necessary. In addition, bio-oil is also a vital source for a wide range of organic compounds and speciality chemicals.
Importance of Biochar
The growing concerns about climate change have brought biochar into limelight. Combustion and decomposition of woody biomass and agricultural residues results in the emission of a large amount of carbon dioxide. Biochar can store this CO2 in the soil leading to reduction in GHGs emission and enhancement of soil fertility. In addition to its potential for carbon sequestration, biochar has several other advantages.
Biochar can increase the available nutrients for plant growth, water retention and reduce the amount of fertilizer by preventing the leaching of nutrients out of the soil.
Biochar reduces methane and nitrous oxide emissions from soil, thus further reducing GHG emissions.
Biochar can be utilized in many applications as a replacement for other biomass energy systems.
Biochar can be used as a soil amendment to increase plant growth yield.
Conclusions
Biomass pyrolysis has been attracting 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.
For information on purchasing reprints of this article, contact Tim Tobeck ttobeck@energycentral.com. Copyright 2010 CyberTech, Inc.
Thanks for a good article on one of my favorite topics.
You focus on production of bio-oil, which is one of the paths for utilization of biomass. But it's by no means the only one, nor is it necessarily the best. We don't have enough experience to judge. Alternatives for initial processing at local sites are pelletization -- a purely mechanical process -- and torrefaction.
Torrefaction is a very mild pyrolysis process. Making it taxes the available energy in the biomass by ~ 10%, but results in a hydrophobic product that can be ground and stored for long periods without degrading. (Roasted coffee beans are an example of torrefied biomass.)
One drawback to bio-oil that you neglect to mention is that it doesn't store well. It's a witches brew of unstable decomposition products. Over time -- a matter of days, as I understand it -- molecular fragments will begin to link up and form tars. It can be chemically stabilized, but that adds cost. So it generally needs to be burned as fuel or refined into other products rather quickly after being produced.
I'm personally biased toward a lower temperature pyrolysis that leaves a higher percentage of char and produces more gases and less smoke and condensible liquids. The char from low temperature pyrolysis has better characteristics as a soil ammendment. It retains a skeletal cell structure that seems to function as an analog of a coral reef for soil microorganisms. The downside is that the gaseous fuel that it produces has lower energy content than the bio-oil derived from fast pyrolysis.
Thomas Saidak 3.10.09
Either thermal or catalytic depolymerization has proven at two different plants the ability to turn waste into oil. I would argue that since oil is harder to get or has higher costs in terms of independence, trade deficits and incumbent military and diplomatic postures that anything that can displace drilled oil should be done. There are a host of reasonable alternatives for electricity, but liquid fuels will have the most problems in scaling up.
Ron Wagner 3.10.09
Great article, and comment. I am also a fan of biomass and bio char. I just read that Canada is losing millions of acres of Boreal forest to the pine beetle. A great chance to use a lot of biomass and then reforest with other species. Biochar could also be used to fertilize the area. I am in favor of the part that is suitable for that use. Thousands of jobs could be created in harvesting, processing and reforestation.
Ron Wagner
Paresh Trivedi 3.11.09
I like Ron Wagner's idea ...adding to that I like to see no forest fire in USA and we use that fuel before it becomes fire incident ....Use dangerous forest as bio mass input energy.... Also Forest Fire Fighter should start work before fire season and use potential fire hazard forest as energy generation project and minimize prescribed burning. Let us give them job before fire season start and transform them in Clean Energy soldiers. With regards,
Paresh Trivedi ( I just got Forest Fire fighting training) NJ pareshtri@gmail.com
Malcolm Rawlingson 3.12.09
One of the reasons for these infestations is that we are so good at controlling forest fires. Fires have for many years been a natural way of cleansing the land. Ask the aboriginal people. Most forest fires are caused by lightning strikes. Unfortunately we have constructed millions of homes in what used to be forests so now we are obliged to control forest fires or lose all the property.
What sort of fuel had you in mind for transporting the hundreds of millions of tons of biomass from the northern forests. Sounds like you would need to burn a lot of diesel fuel to do what you are proposing. It sounds much like the energy efficiency of growing corn to make ethanol. The oil producers are just laughing at us all the way to the bank. We think we are saving oil and in fact we are burning more of it than every before.
We could use horses and elephants to haul the logs I suppose - that way they could fertilize the land to grow still more trees to turn into biomass. But elephants pass gas a lot so that would be bad for the environment....aha but we could harness the gas to make electricity......???
M
Fred Linn 4.2.09
I don't see how this is any different from Fischer-Tropsch process first developed in1924. F-T uses varying temperatures, syngas mixtures, pressure and catalysts to produce a wide area of hydrocarbon products from methane, to simple alcohols like methanol and ethanol to long chain hydrocarbons like diesel fuel and lubricants.
F-T process was used widely in German to produce fuels from both coal and waste wood during WW2 after the loss of North Africa and the Allied bombing of Ploesti virtually shut off petroleum supplies. Synthetic and biofuels powered everything from Panzer tanks, submairines, V1 and V2 rockets, and even the Me 262 Swallow-the world's first operational jet fighter.
This was over 60 years ago. It has been used in South Africa to produce fuels particularly diesel fuels since 1980.
There is nothing new, or untried about Fischer-Tropsh process---it has been used on commercial scale at least twice I know of. Range Fuels is set to open a 100 million gal/yr plant in Soperton GA to produce ethanol from logging and millwork wood waste, early next year. Abengoa the Spanish national energy company produces 500 million liters per year using this process and is currently building 6 plants that will be the largest in the world in China to produce ethanol from bamboo. The Chinesse have already put cars into production that run on hydrous ethanol(straight from the still, no mixing)---and have scheduled a production quota of 2.8 million vehicles for this year, up from 1.7 million last year.
Fischer-Tropsch processi is not rocket science. In fact, without Fischer-Tropsch, there would BE no rocket science. The birth of rocket science was in Germany with the development of the V1, V2, and V10 rockets----all powered with methanol made by F-T process.
Fred Linn 4.4.09
From Malcolm R-----"What sort of fuel had you in mind for transporting the hundreds of millions of tons of biomass from the northern forests. Sounds like you would need to burn a lot of diesel fuel to do what you are proposing. It sounds much like the energy efficiency of growing corn to make ethanol. The oil producers are just laughing at us all the way to the bank. We think we are saving oil and in fact we are burning more of it than every before. "-------- Fischer-Tropsch process can produce cetanes(diesel fuel) as well as ethanol from wood. If transporting raw materials is too expensive, then construct mobile F-T units, and take them to the raw materials. Use the fuels you are producing to power the power the synthgas and conversion units---there is nothing new or unproven about diesel powered generators. Leave the ash and char final product in the forest to return to the soil, just like nature has been doing all along. The only difference is that we would be catching the energy produced in forest fires and converting it into a usable form that we use in the vehicles we currently have.
Biodiesel can be used in any diesel powered vehicle with no conversion at all. Just fill the tank.
E85(85% ethanol) can be used in any Flex Fuel vehicle. It costs nothing or only minimal to buy a flex fuel vehicle that can run on either E85 or gasoline, just fill the tank with whichever is available in any combination. Flex Fuel vehicles are currently in production and have been for many years. There are over 8 million Flex Fuel vehicles on the road now in the US.
