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Introduction

Biochar is similar to other charred organic matter, like charcoal. It is developed principally for the remediation of soils, whether in an agricultural setting or for more general rehabilitation of the soil. It can be worked into the soil to increase yields, and its high stability means it can store carbon in the soil for long periods of time. Different production methods lead to different byproducts which can be used as biofuels.

History

Biochar is commonly linked to the Terra Preta, or Dark Earths, discovered by Europeans when they arrived in the Amazonian basin in the 1500s. In tropical soils, which are usually nutrient-poor, there were significant swaths that were much darker and more productive than other, lighter soils. It was believed that Amazonian peoples added amendments to the soil in such a way as to greatly enhance its productivity, amendments which were stable for thousands of years. While no way exists to exactly determine the method or rationale behind Terra Preta, some method of charcoal production (via pyrolysis) of organic matter seems to be the linchpin, and improving productivity of the soil the ultimate aim.

Since this discovery, biochar has found its way to various corners of the globe. Since the 1800s there has been a growing appreciation for the positive benefits of charcoal soil amendments. Biochar is a recently developed term, indicating pyrolyzed organic matter developed for the sake of introduction into agricultural fields and improvement of depleted soils. Recently, significant energy is being directed towards characterizing biochar, as a variety of feedstocks can be pyrolyzed under a wide array of conditions to achieve a range of purposes. Biochar is not only useful for improving soil productivity, but can also sequester carbon, utilize waste, and produce energy during the pyrolysis process. All these aspects encourage research efforts in both developed and developing countries. The leading biochar producing countries and their output are summarized below (Methods for producing biochar and advanced biofuels in Washington State): Brazil – 9.9 million tons/yr Thailand – 3.9 million tons/yr Ethiopia – 3.2 million tons/yr Tanzania – 2.5 million tons/yr India – 1.7 million tons/yr Democratic Republic of the Congo – 1.7 million tons/yr

Design

Scientific/Engineering Theory

Various aspects of biochar lend it the favorable qualities it possesses. These include chemical properties, nutrient properties, and microbial interactions.

Chemical Properties

The process of pyrolysis transforms a compound into another by heat alone (Pyrolysis of carbon compounds, p 9). Pyrolysis is used to transform a given biomass feedstock (crop waste, dung, wood, etc) into biochar. An array of factors affects this progression, including rate of heating, temperature, heating time, and size of particles (Lehmann, p 74).

In any organic substance, varying amounts of hydrogen, oxygen and carbon will be found, among other elements. During the process of pyrolysis, elements are released in different proportions. The proportions of different elements are indicative of the stability of the substance. The ratios of hydrogen to carbon (H/C) and oxygen to carbon (O/C), especially, are used to measure the stability of the substance. The decrease in H/C and O/C ratios corresponds to a process known as aromatization, or the formation of aromatic rings. An aromatic compound is significantly more stable. To quantify this, unburned fuel has a rough H/C ratio of 1.5, whereas black carbon is considered to have a ratio of H/C less than or equal to 0.2 (Lehmann, 54). Processed biochar can have a range of H/C values, but those pyrolyzed about 400 C can have H/C ratios below 0.5 (Lehmann, 54). Again, the choice of feedstock can have substantial influence on these ratios. Increasing thermal alteration, due to higher temperatures of pyrolysis, lead to the aromatization of cellulosic compounds (Baldock, Smernick 1099). This is demonstrated in the following table by decreasing H/C and O/C compounds (Table 3, Baldock): Aromatic compounds are much more stable than aliphatic compounds, which is why biochar is considered to last for so long in soils. Some biochars are thought to last for thousands of years, e.g., the Terra Preta in the Amazon. Accumulation of organic matter is due in large part to the stability of such soil amendments, leading to higher soil fertility ((Trompowsky, Benites et al. 2005)).

Nutrient Capacities

A critical aspect of biochar is the effect it has on soil nutrients. Biochar supplies nutrients directly to the soil when it is added. These are also highly variable and dependent on the feedstock. For example, higher concentrations of phosphorus are found in feed stocks of animal origin. Total nitrogen is also greater for biochars produced from just plants (Lehmann, p 68). However, the nutrients directly supplied by biochar are generally seen as minor in comparison to other benefits (Lehmann, 71). It has been demonstrated that biochar increases cation exchange capacity in soils (Liang, Lehmann et al. 2006)). This, in turn, results in a higher retention of nutrients by the biochar, which is made available to plants, increasing yield (Major, Rondon et al. 2010). In addition, biochar increases the pH of soils, which has interaction effects with nutrient availability (Major, Rondon et al. 2010). Notably, some yields were reported as declining to the increase in pH brought on by biochar application (Lehmann, 74). Careful attention must be paid to the circumstances of use.

Microbial interactions

Biochar has many effects on microbial populations. It can provide habitat and protection from predators (Lehmann, p 86). In the Terra Preta soils of the Amazon, compared to surrounding soils, increased microbial biomass is present, along with a lower respiration rate, which indicates higher efficiency (Liang, 2008). Corresponding to this is a lower ratio of CO2 to microbial biomass C, which is considered to be responsible for the longevity of the Terra Preta (Lehmann, 92). Soil aeration and water holding capacity is influenced by biochar additions, which leads to decreased anaerobic pore space. This in turn limits the possible activity for microbes to participate in the denitrification cycle, and results in decreased N2O emissions (biochar induced microbial stuff, 315). Biological nitrogen fixation has also been shown to improve drastically with additions of biochar (Bio N2 fixation). Numerous other interactions go on with a variety of bacteria and fungi. Biochars should be designed for the intended microbial community as closely as possible.


Production

(detailed schematic, how can somebody make one?) When considering production of biochar, it is essential to know beforehand what is desired. Different products are generated at different operating points. Lower temperature pyrolysis (<400 C) produces higher proportions of solids. Fast pyrolysis, occurring between 400 and 600 C, gives primarily a liquid product, and gasification, occurring above 600 C, gives gaseous products (Materials, chemicals, and energy from forest biomass). Biofuels and synthetic gases may be produced from these latter set of parameters, demonstrating that production of biochar is actually an energy-positive process CITE. For those interested in producing as much biochar as possible, lower temperatures would be preferred.

Key factors

The type of biochar produced also varies widely with temperature. Below is a plot that summarizes the competing considerations that factor into biochar production.

Optimum table.jpg

Several things are important to notice: 1. The rapid and extreme increase in surface area past a certain threshold, approximately 450 C. 2. The gradual increase (cation exchange capacity and pH) or decline (carbon recovery) of other factors across the range of temperatures. 3. The optimum presented here occurs around 450-550 C, to both capture the increase in surface area as well as limit the decline in carbon recovery.

Again, choice of factors depends on the ultimate intended purpose of biochar. Greater carbon sequestration occurs with higher carbon recovery, but the biochar will be less stable, due to a limited transition from aliphatic to aromatic carbon. Other benefits will also be limited, such as water-holding capacity and the liming effect for which biochar is often noted. In the conversion to aromatic carbon, mentioned above as increasing with temperature, nutrient availability in the biochar itself also decreases. This occurs alongside increasing cation exchange capacity, which, as was seen earlier, may be responsible for greater availability of nutrients from the soil. Other factors besides temperature also play a critical role. These include heating rate, heating time and particle size.

Reactor Design Considerations

A wide array of reactors are available to process biochar, and can be classified by an array of properties (Methods for producing biochar and advanced biofuels in Washington State)

Heat Transfer Rate Slow pyrolysis occurs at a rate of 5-7 C/min, and produces less liquid and more char. Fast pyrolysis incorporates heating rates above 300 C/min and generates primarily bio-oil. The following table summarizes production of different phases under different conditions.

Mode Conditions Liquid Solid Gas
Fast Moderate temperature, around 500 C, short hot vapor residence time ~ 1 second 75% 2% 13%
Intermediate Moderate temperature, around 500 C, moderate hot vapor residence time ~ 10-20 seconds 0% 20% 30%
Slow Low temperature, around 400 C, very long solids residence time 30% 35% 35%
Gasification High temperature, around 800 C, long vapour residence time 5% 10% 85%

Bridgwater 2007

Mode of Operation Batch, semi-batch and continuous methods can be utilized. Batch methods are primarily for generating biochar, so byproducts like bio-oil and gases are not utilized and often vented, leading to significant pollution. Since it must go through a repetitive cycle of warming up and cooling down, much energy is expended in this method. Semi-batch setups transfer heat among batch reactors, leading to lower energy expenditures. Liquid may be recovered, but biochar is the primary desired product. Continuous reactors are more efficient than either batch or semi-batch, but have their own limitations. Technical expertise, flowrate of feedstock, and capital investment are all significant.

Heating Methods Heating may occur via partial combustion, carbonization via inert gases, or indirect heating. Partial combustion is generally the mode utilized for small-scale reactors. A portion of the raw material is combusted, which generates the energy needed for the process to continue. Inert gases can be heated up outside the reactor with another fuel source, and brought into contact with the feedstock. This leads to carbonization into biochar, and produces high yields. The reactor can also be heated from the outside, with the feedstock kept in an anoxic environment. With the beginning of pyrolysis, gases produced can generate energy to continue the process. Byproducts can be recovered more easily, and yields are high.

Numerous other factors come into play in reactor design, including:

Construction Materials Portability Reactor Position Raw Materials Mechanisms of Loading and Unloading Size of Kiln Ignition of Feedstock Process Control Pressure Pretreatment These are further discussed in Methods for producing biochar and advanced biofuels in Washington State. However, for application in developing countries, it may be best to briefly describe a few particular designs.

Kammen and Lew, 2005 Table, Lehmann, 132

Production Options

Pit and Mound Kilns (picture) Pit and mound kilns are the simplest types of kilns. In a pit kiln, a small fire is started inside a pit and additional wood is added. Leaves and branches are piled on top to make a sort of shelf for dirt to be added on top. It must be continually managed to allow the proper inflow of air. A mound kiln follows a similar type of operation, only pyrolysis takes place above ground, and air inlets provide more regular means of regulating air flow. Both of these kilns have limited yield of biochar and contribute greatly to air pollution. (Lehmann, 128-9)

Brick Kiln (picture) A brick kiln can be utilized to make biochar as well. This is a simple batch reactor made out of brick, with air filtration.

Metal Kiln (picture) An example of a metal kiln is that produced by Tropical Products Institute (TPI). A cylindrical body is covered by a conical section. This cover has steam release ports, and the body has air inlets around the bottom. This design allows air to be much more easily controlled than the mud or brick kilns, and tends to produce more biochar of greater quality. Unfortunately, this design still produces significant amounts of air pollution.

Concrete Kiln (picture) This is a rectangular structure made of concrete with steel doors. It is also known as the Missouri kiln. It can produce about 3 times the amount of biochar as a brick kiln, but at a better quality. While technically complex, this kiln has several advantages. Thermocouples allow temperature monitoring, which facilitates air flow control. Chimneys can be augmented with a flue and afterburner to limit atmospheric emissions.

Many other, more complex kiln designs exist, but these are likely outside the scope of resources for smallholder farmers who would benefit most from biochar applications.

Impacts

(what good does this do? what bad can happen?) Biochar, as mentioned above, has some important features that have led to its popularity. These include:

Sequestration of carbon Utilization of organic waste Energy generation Soil improvement

This article will focus primarily on this last aspect, but brief attention will be paid to the former three topics as well.

Sequestration of Carbon

During the pyrolysis process, carbon structures are converted from aliphatic to aromatic form. This aromaticity leads to high stability in soil. Thus, if organic material, such as crop waste, dung, or wood is pyrolyzed and worked into the soil, the carbon present in these materials will stay in the soil for a significant period, perhaps on the order of thousands of years. It is projected that substantial amounts of carbon can be sequestered in soil (CITE).

Evaluation

Due to the wide variety of conditions that lead to biochar production, numerous effects, both positive and negative, have been reported.

(how do you analyze how well this works? how well has this worked in real settings?)

Waste Removal

Energy generation

Soil improvement

Dissemination

(who is promoting this technology?, how are they effectively disseminating it?, where is the technology being used today?)

Re-design

(what would you recommend be changed? what would you anticipate happening if so?)



  Baldock, J. A. and R. J. Smernik (2002). "<Chemical Composition and bioavailability of thermally altered Pinus resinosa (Red pine) wood.pdf>."

Liang, B., J. Lehmann, et al. (2006). "Black Carbon Increases Cation Exchange Capacity in Soils." Soil Science Society of America Journal 70(5): 1719.

Major, J., M. Rondon, et al. (2010). "Maize yield and nutrition during 4 years after biochar application to a Colombian savanna oxisol." Plant and Soil 333(1-2): 117-128.

Trompowsky, P. M., V. d. M. Benites, et al. (2005). "Characterization of humic like substances obtained by chemical oxidation of eucalyptus charcoal." Organic Geochemistry 36(11): 1480-1489.

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