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Pyrolysis OIl and Terra Preta
Submitted by Dan Sweeney on Thu, 2008-02-21 12:12.
Pyrolysis oil may be fairly said to be the poor relation of biofuels. There is no well established market for it, and, until recently, there has been little advocacy on its behalf. That may be about to change.
Pyrolysis oil has traditionally been produced by heating biomass to temperatures of several hundred degrees Celsius in the absence of oxygen, a process which vaporizes the biomass and produces a complex chemical soup with a heating value approximately equal to that of bunker oil, the bottom of the barrel residue gotten from oil refineries.
More recently the flash pyrolysis production process has come to predominate. Here the biomass is heated to a considerably higher temperature for a very brief duration which results in higher production efficiencies and better yields. The Canadian firm Dynamotive is the chief proponent of this process today, although it did not invent the process which has been used for coal pyrolysis for decades.
Pyrolysis oil in its native form has a lot going against it, which explains its prior unpopularity. It is nasty, corrosive stuff and is unstable to boot. It is also fairly viscous. While it has been used experimentally to fuel engines, it is far from established in this application, and most of what has been produced is designated as boiler or furnace fuel. It is also used by the food industry as an ingredient of "liquid smoke". There are, however, processes for converting it into close analogs to refined petroleum products, and, according to UOP which has patented one such process, the economics are good.
Most pyrolysis oil, also known as biocrude, has been produced from wood or paper waste, and the relatively few economic analyses of the production by this means have been favorable. One doesn't require the high temperatures of gasification, and it's all done in a single reaction chamber which doesn't use catalysts. Post processing and refining of course adds to the cost, but it still looks good compared to crop based biofuels such as grain ethanol and biodiesel from soybeans simply because the feedstock is so cheap—at least for now.
The drawbacks are that the refining techniques are proprietary and still experimental and that the process produces a great deal of waste char. Furthermore, the emphasis on wood waste as a feedstock has tended to situate the industry, such as it is, within a few locales, such as Scandinavia and Canada, with extensive timber industries.
Now biocrude may be about to emerge from its relative obscurity, and the change is due largely to the terra preta phenomenon which is currently engaging the environmental community.
Black Earth, Brown Earth
Terra Preta, if it weren't very well documented in the scientific literature, would scarcely be credible. It's like something out of Chariots of the Gods. But it does exist and it has recently begun to arouse intense interest among those concerned with reducing the presence of carbon dioxide in the atmosphere.
Terra Preta, Portuguese for black earth, is a type of soil encountered in large concentrations in the Amazon and in much smaller deposits in Africa. The seventeenth century Dutch are also believed to have produced it, and it may have accounted for the unbelievable agricultural yields they were getting during the Age of Rembrandt when Holland was enjoying its own abortive industrial revolution.
Terra preta is soil, which almost certainly of human origin, has a very high concentration of elemental carbon, sometimes as much as a third by weight. Usually the carbon laden soil is mixed with pottery shards and nitrogen-rich wastes such fish heads, termite nests, manure, etc. The carbon itself aids in the absorption of nitrogenous nutrients by plants, serves to fix nutrients in the soil, permiting yields approaching those on good land saturated with artificial ammonia-based fertilizers. Furthermore, the land can be worked for extensive periods without further fertilization or crop rotation.
The existence of terra preta has been known since the earliest days of Spanish and Portuguese exploration, and naturalists have studied it since the late nineteenth century. Its extent in the Amazon Valley is a matter of dispute, but most agronomists who have studied it believe that a sizable fraction of one percent of the land falls into this category.
Terra preta contains char that was almost certainly derived from the incomplete combustion of crop residues, ground cover, or felled trees, and possibly from charcoal production for use in bronze smelting which was known in the western coastal region of South American. Once the char was produced, it was laboriously worked into the soil to the depth of several feet. Similar use of char was made among primitive agricultural communities throughout the world, but only in the Amazon were vast tracts of extremely carbon rich soils created.
According to the first European explorers of the region in the early sixteenth century, the banks of the Amazon and its major tributaries were lined with settlements containing thousands of inhabitants, and in some cases the black soil itself would extend in mile wide strips over a distance of thirty miles. Sometimes smaller islands of black soil would be linked by stretches of ground with elevated but somewhat reduced carbon content known terra muleta, brown earth. This was usually devoid of additional fertilizers and was not actively cultivated.
European diseases largely destroyed the cultures that created terra preta, and it is uncertain if any Amazonian Indian tribe is still producing it today. The sixteenth century settlements lost approximately 90% of their inhabitants, and labor intensive terra preta kitchen middens could no longer be maintained. The survivors purchased steel axes from the Europeans and began practicing an environmentally destructive form of slash burn agriculture that is still seen today.
In the past terra preta was widely used by horticulturalists and farmers of specialty crops, and trainloads of the soil were taken away from the forest and into the settled south. Today terra preta is protected in Brazil and is regarded as a national treasure and its removal is prohibited.
So what has terra preta to do with global warming on the one hand and biocrude on the other?
Terra preta soils form remarkably stable carbon sinks so any carbon disposed of in this manner isn't going back into the atmosphere any time soon. Moreover, terra preta appears to be environmentally benign and conducive to sustainable agricultural practices. Furthermore, because terra preta is so rich, the plant yields per acre are higher than is the case for normal soils, and thus more CO2 is captured by the growing plants than would otherwise be the case. In fact, advocates of terra preta claim that it is carbon negative rather than carbon neutral.
The normal process by which bio-char is produced as the first step in the genesis of terra preta involves pyrolysis, and two principal products are derived from the process, the char itself and biocrude. If the biomass is taken from a terra preta cultivation area, then the whole process can be nearly zero emissions with essentially no CO2 being released into the atmosphere, and unlike most forms of organic farming this will produce yields approaching those of conventional mechanized farms relying on heavy infusions of fossil fuel derived fertilizers. In effect what one has is a closed carbon loop.
So is this the ideal solution to our energy and environmental problems? Not necessarily. As with any biofuel one faces limits in the amount of readily available biomass. Terra preta is fertile soil but it does not fundamentally alter the carrying capacity of the land. There is also the problem of a knowledge base. The last vestiges of terra preta cultivation survive in West Africa, and they are strongly rooted in the local ecology and are extremely labor intensive. Placing terra preta within the context of modern mechanized mass production agriculture is a challenge that has yet to be met, though experiments are taking place.
Notwithstanding these difficulties, a surge of entrepreneurial activity is taking place in this area with companies such as Eprida, Bicarbo, Bioenergy LLC Bioware, Bioaware, Cleanfuels, Carbon Diversion Technologies, Renewable Oil International, LLC and Terra Humana Clean Technology Ltd. providing intellectual property and in some cases hardware to would be biocrude/biochar producers. Dynamotive and Ensyn, the two giants of biocrude are also active. Interest in the phenomenon is international.
Where this is going is difficult to say at this time. Biocrude is not apt to be a major industry for years, and terra preta will be subject to much experimentation before it becomes commonly accepted even among organic farmers let alone big agribusiness. Still, the logic of the approach is compelling. You really are getting two birds with one stone.