Climate Change Connections: Pyrolysis For Char Part I

BioCycle February 2009, Vol. 50, No. 2, p. 44

Sally Brown

This month, instead of simply ranting about one topic or another, I thought it might be a nice change of pace to actually provide some information. After you read the column and digest the information, you can decide for yourself if and how you would like to rant about it. What prompted this mature decision were about 20 references to biochar within a one-week period. If you recall, biochar is the carbon soil amendment that is all the rage in soil and organics circles. This is because biochar is seen as the source of fertility for the ancient Terra preta soils in Brazil. Arguments go, “If it was good enough for the Amazon, it is going to be great for us.” Production of char is being looked at as an alternative to landfilling, composting and a range of other organics management options.

This is a topic that gets me ranting very easily. In fact, I have already devoted a column to ranting about char (see “Use What Works? How Novel!” March 2008). During the one-week period that prompted this second column, I was at a Washington Organics Recycling Council meeting. When you say the WORC meeting in Washington, people say, “Oh the compost meeting.” In this particular case it should have been referred to as the char meeting, as three speakers in a row got up and talked about biochar. I got back from the meeting, checked my email and found several messages from the Seattle biochar worship association (that may not be the exact name, but you get the point).

So, now it is time for you make your own judgment about char – after this column and the next one. This month I’ll provide information on pyrolysis, the process that produces char. I’ll do this with some comparisons to anaerobic digestion. Next month I’ll sound more like a soil chemist (what I was actually trained to know in grad school) and talk about char for soil, with compost or materials from anaerobic digesters used as a basis for comparison.

SIMPLE DEFINITION
Pyrolysis is the combustion process used to produce char. Pyrolysis is one type of thermolysis, meaning a way to chemically alter things using heat. It is a term used for combustion with limited oxygen, generally under elevated temperature and pressure. Pyrolysis is a process that we’ve all seen. When you burn wood and stare at the pretty fire, you are actually looking, in part, at a pyrolysis reaction. In a normal fire, like a wood stove, the carbon compounds in the wood are gradually turned into gas as the wood heats up. This gas consists mostly of shorter chain (simple) carbon compounds like phenols, aromatics, methane and carbon dioxide, whereas the wood consists of more complex, longer chain compounds like cellulose and lignin.

In a normal fire, the pyrolysis gas starts to burn the minute it gets enough oxygen to fully combust (i.e., to oxidize to CO2 and H2O). In a fireplace with a log, that happens just a few millimeters past the log, where you see the flame. When you look at the fire, you aren’t seeing the wood burn, you are seeing the pyrolysis gasses released from the wood burn. In a pyrolysis reactor, sufficient heat is present to alter the carbon compounds in the feedstock, however there isn’t enough oxygen to allow actual combustion with a flame.

Pyrolysis facilities don’t have to be high-tech. In our house we have a wood stove. To get it going, we keep the vents wide open. This lets plenty of oxygen in and the pyrolysis is followed rapidly by combustion. We get a pretty fire. At night before bed, we load up the stove and shut all of the vents to reduce the oxygen supply and slow the combustion process. This takes it to pyrolysis alone. Some mornings we come downstairs and find wood with some blackened edges. The fire wasn’t hot enough when we took away the oxygen and it just went out. However, many mornings, we come downstairs and find charcoal from incomplete combustion. This is the primary product you get if you have pyrolysis for extended times at low heat. This is char.

So the process of combustion is basically: 1) Fuel heats up; 2) Pyrolysis starts to happen, forming volatile gases; 3) Gas hits oxygen and combusts. If you control the reaction to eliminate step three, you have pyrolysis.

PYROLYSIS PRODUCTS
The pyrolysis process creates three products: gas, a liquid and char. The gas and liquid are each comprised of a variety of different carbon compounds. Both have use as fuel. The gas is called syngas and is similar in many ways to natural gas. If the gas is allowed to cool before it contacts oxygen it will produce the liquid or tar. This liquid is often called “biocrude” and can be used like unrefined oil. In fact, pyrolysis is a process used in crude oil processing. You also get the char, which is basically concentrated carbon. The char can also be used as a fuel (think of mesquite grilled steaks or salmon). It can also be used as a soil amendment.

The amount of gas, liquid or char produced depends on a number of factors including feedstocks, time and temperature. Typical reaction temperatures for pyrolysis range from 200° to 600°C. At higher temperatures, often called flash pyrolysis, higher concentrations of gas are produced. At low temperatures and long periods of time, production of char is highest. Pyrolysis under these conditions can also be called carbonization and is the commonly used process to make charcoal. It doesn’t require fancy highly engineered systems. Burning waste under a soil cover is likely what produced the Terra preta soils in Brazil and would be considered a carbonization process.

In any version of this process, a certain amount of the energy contained in the feedstocks is required to generate the heat required to maintain this process. If you are running a crude facility with the goal of producing charcoal, you won’t need to completely dry the feedstocks before you start. Material just has to be dry enough to burn on its own and suck all of the oxygen out of the system with sufficient heat remaining to continue to transform. This would be a solids content of 40 to 60 percent, I’d guess. For more highly controlled facilities where you want more than just char, drying feedstocks is required, as the water vapor in the gas will be a contaminant that will lessen the value of the syngas and biocrude.

A review paper by S. Yaman (2003) gives a very long list of different feedstocks that have been processed with pyrolysis. Reaction products vary with the conditions and you can alter conditions to maximize production of one of the three products. With wood, the volatile content is about 70 to 90 percent of the total dry weight. After complete pyrolysis, the remaining char would be about 10 to 30 percent of the original dry solids. Renewable Energy Resources (Twidell & Weir, 2006, 2nd ed.) puts maximum char yield at 25 to 35 percent of dry biomass input.

In addition to chemically altering the carbon, pyrolysis changes the chemical forms of other ions in the feedstocks. At the temperatures used in pyrolysis, all of the nitrogen in the feedstocks will volatilize and be lost to the system. Other nutrients will likely form oxides like calcium oxide or potassium oxide. Just as wood ash makes a good fertilizer for certain ions, there is a potential that char would also provide plant nutrients – except for nitrogen that is.

COMPARISON WITH ANAEROBIC DIGESTION
So that is pyrolysis. How is it different from anaerobic digestion (AD), which also produces chemical changes in the carbon in the feedstocks? With AD, these changes are brought about by microbes rather than by heat. AD is a feeding frenzy for methanogenic bacteria. These creatures can eat carbon, just like we do, but they can do it without oxygen.

Electrons released from the carbon they eat get stuffed onto other carbon compounds and produce methane. So if a microbe wanted to eat a good pound of carbohydrates, a portion of that pound would be converted into microbial biomass (think of a microbe with a big gut). Another portion would evolve into CO2 and methane (CH4), with a little bit left over as an indigestible carbon compound.

This process requires heat, as the microbes are hungrier and more active in a mesophillic environment (about 30°C). It also requires moisture, with wet (traditional) anaerobic digesters operating at about 2 to 7 percent solids and dry digesters operating at a solids content greater than 15 percent. Not all types of carbon compounds can decompose under anaerobic conditions. Lignin, for example, doesn’t change a whole bunch.

AD also takes some time. Retention time in digesters for food waste generally averages 10 to 15 days. In wastewater treatment, longer retention times are common, sometimes in excess of 30 days. Another thing about AD is that the nutrients don’t go anywhere. As it is a microbial transformation, rather than a purely abiotic one, the only nitrogen that is lost to gas is the stuff that was initially present as nitrate to begin with. Most of the nitrogen in these systems is present as organic nitrogen and is conserved in the digestion process. So is the phosphorus and the micronutrients.

So much for Part I. Stay tuned for Part II.

Sally Brown – Research Associate Professor at the University of Washington in Seattle – is a member of BioCycle’s Editorial Board, and authors this regular column on the connections of composting, organics recycling and renewable energy to climate change. E-mail Dr. Brown at slb@u.washington.edu.

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