What is Slow Pyrolysis?

Slow pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. The feed material is dried and fed into a stirred, heated kiln. As the material passes through the kiln, a combustible synthesis gas (syngas) is evolved and is continuously removed from the kiln. Approximately 35% by weight of the dry feed material is converted to a high-carbon char material that is collected on the discharge of the kiln.

How Slow Pyrolysis works

Slow pyrolysis technology which consumes biomass waste streams while creating continuous syngas and carbon-rich end products. The syngas is composed of combustible gases including hydrogen, carbon monoxide, methane, and lower molecular weight hydrocarbons, as well as nitrogen and carbon dioxide. This gas is cleaned by a series of unit operations before being recycled back to the plant or exported. A portion of the gas generated is combusted and used as a heat source on the pyrolysis kiln itself. An additional portion of the gas is combusted and used to dry the incoming feed material for pyrolysis. The excess syngas gas represents the net energy output and can be utilized as a fuel for an engine, an industrial boiler, or as a feedstock for down stream processes which refine the syngas into a liquid fuel.

Rather than use underground heat, geothermal heat pumps attached to buildings capitalize on the steady temperature of the ground or deep water wells. In effect, they treat the Earth like a giant energy savings bank, depositing or withdrawing heat depending on the time of year.

“Impossible!” you say. “Even wind and solar have carbon emissions from their manufacturing, and biofuels are carbon neutral at best. How can a fuel be carbon negative?” But listen to people working on gasification and terra preta, and you’ll have something new to think about.

We’ve mentioned terra preta before: it’s a human-made soil or fertilizer. “Three times richer in nitrogen and phosphorous, and twenty times the carbon of normal soils, terra preta is the legacy of ancient Amazonians who predate Western civilization.” Although we don’t know how it was made back then, we do know how to make it now: burn biomass (preferably agricultural waste) in a special way that pyrolisizes it, breaking down long hydrocarbon chains like cellulose into shorter, simpler molecules. These simpler molecules are more easily broken down by microbes and plants as food, and bond more easily with key nutrients like nitrogen and phosphorus. This is what makes terra preta such good fertilizer. Because terra preta locks so much carbon in the soil, it’s also a form of carbon sequestration that doesn’t involve bizarre heroics like pumping CO2 down old mine shafts. What’s more, it may reduce other greenhouse gases as well as water pollution: according to Biopact, a network that promotes biofuels and biomass energy,

Char-amended soils have shown 50 – 80 percent reductions in nitrous oxide emissions and reduced runoff of phosphorus into surface waters and leaching of nitrogen into groundwater. As a soil amendment, biochar significantly increases the efficiency of and reduces the need for traditional chemical fertilizers, while greatly enhancing crop yields. Experiments have shown yields for some crops can be doubled and even tripled.

As it happens, the process of burning/pyrolisizing agricultural char is also a way to produce energy. MIT Professor Amy Smith, a recipient of the prestigious MacArthur “genius award,” gave a TED Conference talk in 2006 on using agricultural char as fuel in developing countries. It works because the chemical reactions that break down the long hydrocarbon chains also give off hydrogen gas, methane, and various other burnable fuel gases. (As well as tars and non-useful gases like CO2.) This is gasification. The fuel gas can be burned for heat, or if it’s pretty clean (that is, if the tar levels are low), it can be used to power an engine.

I was first introduced to gasification by Jim Mason at Foo Camp, and helped a bit with his Burning Man project The Mechabolic. It’s an art project designed to use gasification for motive power, electrical power, and gas-powered lights and heat, all at the same time. The Mechabolic is intended to run off of coffee grounds, or whatever ground-up dried-out biomass can be fed to it, with its own “mouth” parts to chop up and pulverize incoming material well enough to be fed into its gasifier tank. Jim points out that gasification is not new — in fact, according to Professor Tom Jeffries at the University of Wisconsin, “Over a million wood gasifiers were used to power cars and trucks during World War II,” when Europeans often lacked access to oil.

There are many kinds of gasifiers, each with advantages and disadvantages for different conditions and input materials. Babcock & Wilson Vølund have a helpful diagram (see below) with explanatory text that shows one kind in detail. The locations and sizes of the different zones will be different for other designs, but the sorts of chemistry described occur in all gasifiers.

Not all gasification is green. The coal industry routinely uses gasification all around the world to create syngas (synthetic gas) as a petroleum substitute of chemical feedstock. However, gasification plus terra preta has potential to be revolutionary.
I can’t promise that using gasification for energy and using the resulting char as terra preta fertilizer will be a carbon negative fuel, because I haven’t seen a credible lifecycle analysis of it. (If anyone has, please post it to the comments.) But it’s quite plausible. Consider that it takes a certain amount of CO2 to grow a crop, such as corn. You harvest the crop and sell the food part, which leaves you with all the agricultural waste. Instead of burning it in the open air, or landfilling it (which is what’s done today — basically topsoil mining), you gasify it. You then burn the fuel gas you get from gasification, putting some fraction of that CO2 into the air; the agri-char (terra preta) that you’re left with contains the rest of the embodied CO2 which the crops sucked up while growing. There’s more carbon here than there was in the fuel gas. You spread the terra preta on the fields as fertilizer to grow more crops, and repeat the cycle — and with each repeat, you pull more carbon back into the soil than you burn, resulting in a carbon negative fuel as well as crops fertilized with fewer petrochemicals. It’s a double win.
Energy Policy and Farm Policy

Gasification and terra preta as a means of sequestering carbon is far cheaper than injecting CO2 into mine shafts, but it’s still not cheap. Biopact calculated that “under a basic scenario sequestering biochar from biofuels produced by pyrolysis would be competitive when carbon prices reach US$37 (carbon currently fetches €21.55 on the European market, that is $30.5, and prices are expected to increase strongly in the near future).”

However, “[T]he great advantage of biochar is the fact that the technique can be applied world-wide on agricultual soils, and even by rural communities in the developing world because it is relatively low tech.” In fact, the guts of Jim Mason’s Mechabolic was mostly built with scrap steel tanks and whatever miscellaneous piping was handy, with nothing but a couple welders and some power tools — nothing a well-equipped farm mechanic wouldn’t have.

One of the 2007 Ashden Awards went to a company in India making gasification / char systems in Kerala:

BIOTECH has succeeded in tackling the problem of the dumping of food waste in the streets of Kerala through the installation of biogas plants that use the food waste to produce gas for cooking and, in some cases, electricity for lighting; the residue serves as a fertiliser. To date BIOTECH has built and installed an impressive 12,000 domestic plants (160 of which also use human waste from latrines to avoid contamination of ground water), 220 institutional plants and 17 municipal plants that use waste from markets to power generators. The disposal of food waste and the production of clean energy are not the only benefits of BIOTECH’s scheme. The plants also replace the equivalent of about 3.7 tonnes/day of LPG and diesel which in turn results in the saving of about 3,700 tonnes/year of CO2, with further savings from the reduction in methane production as a result of the uncontrolled decomposition of waste, and from the transport of LPG.

While still under the radar of most policymakers, gasification and terra preta are starting to appear on the scene. In the US this year, Senator Ken Salazar (D-CO) is promoting legislation that would give subsidies of up to $10,000 for farmers who set up gasifiers and use the terra preta on their fields, and $100 million in related research grants. Biopact has the full text of the bill online, and Biochar International has a summary of the bill.

Image: The Mechabolic at Burning Man. Credit: Michael P Byrne