“Low-temperature slow pyrolysis offers an energetically efficient strategy for bioenergy production.”
Dr. John Gaunt
Director GY Associates & Cornell University
The process of making biochar is called “pyrolysis.” Pyrolysis offers an efficient and sustainable means of generating energy from biomass while simultaneously sequestering carbon from the atmosphere in the form of biochar.
Care must be taken, however, to ensure that the harvesting, transport, pretreatment, production, post-treatment and application processes are accounted for and do not emit more carbon than is captured by transforming the biomass into biochar. This will require innovative, low-carbon strategies for pyrolysis energy generation.
Biochar has been labeled a “carbon negative” energy source because it has this possibility of sequestering more carbon than is produced. The chief benefit of biochar over many other bioenergy technologies is the wide variety of feedstocks that it can be made from. Further, it can also produce a variety of energy outputs from these feedstocks including syngas, electricity, bio-oil, hydrogen, nitrogenous fertilizer, heat, and, of course, biochar. This flexibility in both input and output provides biochar production with a strong advantage over many other bioenergy systems.
Another flexibility that biochar has is that it can be produced at a wide-range of scales from a simple cookstove to mobile farm pyrolyzers and all the way up to large-scale “biorefineries.” Due to transport costs and carbon emissions there is likely to be, in my opinion, inherent limits to geographical scale which will favor smaller-scale and decentralized approaches.
While many systems are currently being researched, piloted, and deployed, there remains significant work to be done on creating economically viable biochar production systems. This market void is likely to be quickly filled by a host of competing technologies with unique production scales and strategies. It will probably take some time during which these technologies will have to run the gambit of real-world application before the best technologies can be identified and the most appropriate means for biochar production determined for various operators and bioregions.
There are a wide variety of systems to make biochar and somewhat complex technical language, but really biochar production is a relatively simple at its core. Essentially, in all pyrolytic processes biomass feedstock is heated between 250-800 degree C in an oxygen free environment. The biomass is essentially “cooked” until various lignin and cellulose products breakdown to produce a hydrogen rich fuel stream that can either be combusted or condensed for energy generation. The high-carbon product that remains is biochar which has essentially been mined of hydrogen.
The initial phase of the pyrolysis process is typically”endothermic” meaning it requires more energy than it producess. In the end, however, pyrolysis is “exothermic” meaning that it produces more heat than is required to originate the 10% of the final energy produced making it a rather efficient process particularly in light that its capturing valuable carbon at the same time.
Under the molecular stress of the heat the biomass is turned into charcoal in a process known as “thermal decomposition” as a steady, energetic stream of carbon monoxide (CO), methane (CH4) and hydrogen (H) gases (known as “syngas“) are driven off of the biomass. All that is left is the carbon exoskeleton latticework of the charred material we call “biochar;” containing approximately 50% of the orginal carbon but only 30% of the residual energy.
The various systems that use the thermal decomposition process include 1) torrefication, 2)carbonization, 3) slow pyrolysis, 4) intermediate pyrolysis, 5) fast pyrolysis, 6) microwave pyrolysis, 7) gasification, 8 ) hydrothermal carbonization and 9) steam gasification. Sure, there’s a lot–and they differ quite significantly from each other in terms of heat required, time it takes to cook the biochar, etc.–but in the end they are all simply driving off gases and turning the material into charcoal.
With many of these systems there is a wide variety of scales of operation ranging from tiny pyrolysis and gasification stoves that could be small enough just to cook a nice garlic and root vegetable soup with locally grown produce all the way up to large scale biorefinery plants capable of delivering heat and power to entire houses, neighborhoods, communities, and countries in a grid of redundat, highly resileint distributed combined heat and power (CHP) plants that could replace coal and oil demand in a carbon negative energy production process
Generally, an equivalent of 10 to 20 percent of energy generated from the process is required to run the process, but this varies depending on the type of process used. Generally, modern pyrolyzers use the energy produced by the provide the requisite energy required to drive the process. Because there is more energy left at the end of the day than was used, the farmer or community power plant is a net energy producer as the entire process (including harvesting and shipping the biomass) has a strong postivie net energy ration, particularly when compared to other bioenergy technologies such as corn ethanol. Indeed, pyrolysis is some 2-20 times more efficient than corn ethanol production and requires no agricultural land or fertilizers.
This leaves 80 to 90% of the original energy to be captured, thus pyrolysis provides an efficient means of converting biomass to energy (Gaunt, 2008). Each of the various methods for producing biochar have specific costs and benefits which needed to be weighed against one another and assessed in terms of the end user’s desired outputs. Slow pyrolysis, for instance, is the most effective means of producing biochar with typical biochar yields of 35 to 50% of dried biomass weight. Fast pyrolysis is the most effective method for producing biofuels with typical biochar yields of 10 to 30%. And gasification is the most effective means of producing syngas and therefore is typically used to generate heat and power with only ash as a byproduct because it is causing a complete combustion and is therefore not typically associated with biochar production (though it can be used make it by simply dampening the incoming oxygen flow and perhaps modifying several other components such as ).
There are several significant challenges that biomass energy will encounter. One is that we don’t have systems ready for collecting, drying, and efficiently combusting biomass materials. (Building an effective rail transport systems would be a good first step to accomplishing this challenge.) Two is that biomass is not nearly as energy-dense as fossil fuels and is bulkier and more difficult to handle. Three, there are few examples of working exclusively biomass fired combined heat and power systems operating at a significant scale anywhere in the world that I am aware of. A sign of hope however, comes from the recent story that an existing coal power plant is being transformed into one that can combust biomass instead and researchers are investigating feasibility of biomass CHP plants.
PYROLYSIS ENERGY GENERATION
The amount of energy that can be generated from pyrolysis is substantial by simply tapping into waste and debris materials such as saw dust, sawmill scraps, urban waste debris, livestock manure, sewage, food waste, storm debris, agro-feedstocks such as corn stalks, rice hulls, and wheat straw, and in order to reduce the potential of dangerous forest fires by pyrolyzing forest biomass and trees attacked by invasive species.
The potential amount of feedstock that could be used to generate much greater amounts of energy could be accomplished by planting purpose grown bioenergy crops (BEC), but this is a controversial approach as some believe it may quickly lead to competition with agricultural lands, forest clearing to plant energy crops, or to large-scale monoculture plantations that would greatly reduce biodiversity, exploit indigenous communities and subsistence farmers and small-scale cottage industries. While these are all valid concerns, growing energy crops for biochar need not have any of these effects because it can be grown on marginal or highly degraded lands where very little is growing and where applying biochar could greatly enhance the soil quality of the area. They also do not require plantation agriculture, though we might consider blending biochar production with eco-agroforestry and food forest design systems that promote local economic opportunity for small scale agriculturalists and subsistence farmers.
The total amount of available biomass produced annually is some 54.6 gigatons of carbon per year (same as a petragram or 1 billion metric tons of C). If we were to estimate a certain percent of it was available as a “waste” stream or byproduct such as sawmill scraps, nut shells, or corn stalks we would be able to approximately calculate how much energy could be produced for pyrolysis at a given efficiency. Figuring out how much biomass is available is also important for determining the extent of CO2 we might be able to sequester from the atmosphere using pyrolysis.
Gaunt, J.L., Lehmann, J. (2008) “Energy Balance and Emissions Associated with Biochar Sequestration and Pyrolysis Bioenergy Production,” Env Sci and Tech, 4152.
GLOBAL BIOENERGY POTENTIAL
The continued availability, environmental impacts and rising costs of fossil fuels has necessitated a transition to renewable and alternative energy sources. The most significant threats of continued fossil fuel dependence are i) emissions of CO₂ leading to dangerous perturbations to the climate system, and ii) the possible near-term peaking of world oil supplies which could lead to social, political and economic difficulty.
Bioenergy production has been proposed as an alternative source of energy to offset fossil fuels along with development of wind, solar, hydro, and nuclear energy sources. Sims et al (2007) have estimated current global potential biomass supply of 46 EJ yr⁻¹, while a review by Berndes et al (2003) found total potential biomass supply by 2050 to range from 100 EJ yr⁻¹ to 400 EJ yr⁻¹ according to various estimates. By contrast, total global energy production in 2005 was 489 EJ (EIA, 2007). As such, it is unlikely that bioenergy sources alone will be able to meet future global demand or offset current fossil fuel use, but is capable of providing a non-trivial portion of future demand.
Several platforms for transferring biomass into energy exist. These include i) direct combustion (e.g. co-firing biomass in coal plants), ii) pyrolysis/gasification, iii) bio-oil production (e.g. palm, algae, jatropha, and soy) and iv) microbial fermentation (e.g. corn, sugar cane, and cellulosic ethanol). Each of these platforms must be critically examined based on a full accounting of lifecycle costs and benefits.
Bioenergy systems have come under intense scrutiny as converting land and food crops to energy production has been heavily criticized. Among the concerns are conversion of “food-to-fuel” leading to increases in grain prices and food insecurity in developing countries; higher rates of deforestation from the conversion of primary forests into monoculture energy plantations; and the removal of crop residues from fields for cellulosic ethanol production which can be highly deleterious since these residues are essential for preserving soil quality through nutrient cycling and erosion control (Wilhelm et al, 2004).
Pyrolysis has three advantages which make it less problematic than other proposed bioenergy systems. First, unlike first generation biofuels such as corn ethanol pyrolysis has a relatively high net energy ratio (energy returned on energy invested—EROEI) suggesting it is energetically efficient. Second, pyrolysis capable of utilizing a wide variety of feedstock materials including waste streams and perennial bioenergy crops grown on marginal lands thereby diminishing competition with land for food crops or need for monoculture plantations. Third, nutrients and C can be cycled back into the ecosystems through application of biochar.
During pyrolysis, solid carbonaceous materials are heated under oxygen (O₂)-limited conditions to between 300 and 700°C (Brown, 2003). The various components of the organic feedstock material including hemicellulose, cellulose and lignin progressively undergo thermal decomposition breaking down into a stream of synthetic producer gases. The resultant “syngas” is comprised of various amounts of hydrogen (H), carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄), and lesser quantities of ethane, propane, and other compounds. These gases can be directly combusted in order to produce electricity or cleaned and condensed into a liquid fuel such as H (Day et al, 2005). Another product of pyrolysis, bio-oil, can be gasified or refined for energy use.
->Sims, R.E.H., et al. (2007). “Energy supply. In Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change” [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
A perceived advantage of pyrolysis is the wide variety of feedstock materials capable of being utilized (Yaman, 2004) including waste materials (such as sawdust or forest waste materials); agriculture crop residues (such as corn stover, wheat straw, rice hulls, or nut shells); and dedicated “low-input high-diversity” (LIHD) bioenergy crops (such as miscanthus, switchgrass, hybrid poplar, karnal grass, or short-rotation willow coppice) which are capable of sequestering C through deposition of root biomass within the soil layer (Tilman et al, 2006).
Biochar can be produced at many scales including cookstoves, distributed combined heat and power (CHP) units, and large-scale biorefineries. Flexibility in feedstock and scale of operation are seen to provide for a wide degree of technological appropriateness and can be selected and optimized based on regional biomass availability and infrastructural development (Lehmann and Joseph, 2009).
Gaunt and Lehmann (2008) have performed lifecycle analyses which suggest a net energy ratio for slow pyrolysis of 2-7 when optimized for biochar production and 3-9 when optimized for energy production. Slow pyrolysis is typically used to maximize for syngas or biochar production. Typical conversion rates for biomass-to-biochar using slow pyrolysis systems are 25-50%. Fast pyrolyzers are capable of creating a larger quantity of “bio-oil” which can be used to offset petroleum. This oil is typically of significantly lower energy density (~25-50%) than that of petroleum diesel. It usually contains relatively high water content which makes it highly corrosive, thus bio-oil of significantly lesser quality than petroleum. Typically bio-oil is gasified, refined into higher quality fuel, or turned into chemical feedstock material. Conversion rates for biomass-to-biochar using fast pyrolysis systems are between 10-35%. Waste heat from biochar production can be utilized for cooking, space heating, industrial processes, or cooling with absorptive refrigeration technologies.
Biochar can be combusted rather than incorporated into soils in order to increase net energy output; however, this would greatly reduce net avoided C emissions (Gaunt and Lehmann, 2008). Other methods of biochar production beyond pyrolysis include gasification, hydrothermal conversion, torrefaction and carbonization. These methods typically have lower biomass-to-biochar conversion rates than pyrolysis.
Quantifying the net energy of the process through a life cycle analysis (LCA) is necessary to determine the energy efficiency of the process. The overall efficiency and economic feasibility are highly constrained by the transport of biomass. For instance, if one fleet of trucks are used for collection of biomass and another for distribution of biochar this would imply that for every one mile in distance from the pyrolysis plant, four miles of transportation costs must be traveled. The above consideration implies that distributed, on-farm, mobile and village-scale approaches to biochar production will have comparative advantages over large-scale biorefineries despite apparent gains by scales-of-economy. A network of distributed combined heat and power (CHP) pyrolysis units may promising strategy for offsetting coal generation with carbon negative electricity from biomass (Laird, 2008).
BIOMASS PRODUCTION & HARVESTING
Slow pyrolysis is the most efficient method of turning biomass into biochar and is therefore commonly cited in the literature as being one of the most promising technologies to produce biochar (Lehmann, 2006).
Slow pyrolyis requires low-to-medium temperatures between 350 and 700 °C at relatively long residence times typically taking hours or days (depending on kiln size) and generates three yields: between 35 and 50% biochar from the original weight of the biomass; water; and a syngas. The properties of the resulting biochar and syngas are heavily determined by feedstock material, temperature, and residence times.
Based on an analysis of two diagnostic samples of snygas from a slow pyrolysis demonstration unit, the syngas stream had an energy content of 8-10 megajoules/kilogram (MJ/kg) with mainly consisting of 10-25% hydrogen (H2), 15-25% carbon monoxide (CO), 8-15% methane (CH4), and smaller amounts of ethane, propane, ethyl alcohol, and acetyle alcohol.
BEST Energies, headquartered out of Madison, Wisconsin, has already developed on demonstration 300 kilograms per hour scale pyrolysis plant in Somersby, New South Wales and is planning on unveiling a fully commercial 2-ton per hour scale slow pyrolysis plant capable of generating fairly large quantities of electricity by 2011.
While biochar production is generally maximized with slow pyrolysis, fast pyrolysis offers the benefit of bio-oil production in addition to biochar. Fast pyrolysis, sometimes called “flash carbonization,” occurs in a matter of 0.5 to 2 seconds with modest temperature requirements of approximately 400-600 degrees C. The process yields approximately 60-70% biofuels, 10-20% biochar, and 10-25% producer gases (Brown, 2008).
Fast pyrolysis takes advantage of a swift biomass-to-biochar reaction which has been used primarily used to convert biomass into bio-oils and secondarily to produce biochar. But fast pyrolysis typically also comes with a cost. Before feedstock material is fed into the reaction chamber, it must be quite small (less than 3 milimeters) and dry (less than 10% moisture content) which typically means there is some energy penalty to be paid in terms of preprocessing as well as active drying of the biomass.
Critical to fast pyrolysis and liquid fuel production from pyrolysis is the refinement and upgrading of bio-oil. Presently, bio-oil is not an altogether high quality product: it contains high levels of both water and oxygen which make it difficult to store due to corrosion issues; it contains roughly 50% the energy content of diesel fuel; it is acidic and has a relatively low viscosity; and it has a tendency to settle, separate and polymerize. Thus, refinement is necessary to turn the turn the low-grade bio-oil into a product that can be shipped, stored, and used more easily. After refinement the bio-oil is most suitable for stationary applications such as heating oil, as opposed to a transport fuel which requires higher grade and more consistent product. Advancements are being made and new pyrolysis systems developed for creating higher grade fuels that could be able to replace transport motor fuels.
Dynamotive LLC, based out of Vancouver, Canada, is one of the leading companies investing in fast pyrolysis technologies. In September 2008, the Iowa State Capital building ran a test trial of the Dynamotive’s BioOil made from hardwood sawdust to replace #2 diesel used for heating oil. Dynamotive reports achieving a 20% biochar production rate from their process.
BIOCHAR AND GASIFICATION
Gasification is a tried and trued method of energy production which has been in commercial development for over 40 years. In the simplest terms we could think of gasification as “pyrolysis plus more oxygen.” The main objective of gasification is energy in the form of syngas. Because the gasification is optimized for total energy output the biomass is typically reduced to ash instead of biochar as additional oxygen to the burn cycle allows for more complete combustion. Gasification consists of converting high-carbon materials into a steady flow of energy-rich and combustible hydrogen and carbon monoxide “syngases” in a high temperature reaction within an oxygen controlled environment that typically generates less biochar production than pyrolysis.
Gasifiers have been modified to produce biochar by modulating the amount of airflow so that less air is released into the combustion zone therefore increasing the amount of charcoal exiting the system. An example of this recently took place on Joshua Frye’s farm in which a gasifier built by Coaltec Energy made to run on poultry litter was retrofitted to optimize for biochar production with assistance from the International Biochar Initiative’s Johannes Lehmann and Stephen Joseph.
Steam gasification is a modification of slow pyrolysis developed by EPRIDA, based out of Florida, in collaboration with the University of Georgia’s Biorefining and Carbon Cycling Program. The process allows for the optimization of syngas production by adding steam into the reaction chamber during pyrolization which liberates greater quanitities of syngas, mostly in the form of hydrogen. Typical syngas yields are 50% hydrogen (H), 30% carbon dioxide (CO2), 15% nitrogen (N), 5% methane, smaller amounts of lower weight hydrocarbons, and carbon monoxide (Day et al., 2005).
EPRIDA has proposed using the hydrogen coming off the steam gasification reaction to fix triple-bonded atmospheric nitrogen into ammonia which can then be added to the biochar, a product which could provide a slow release carbon and nitrogen rich soil fertilizer that they have trademarked and are marketing by the name of “ECOSS.”
->Day, D., Evans, R.J., Lee, J.W. and Reicosky, D. (2005) “Economical CO2, SOx , and NOx capture from fossil-fuel utilization with combined renewable hydrogen production and large-scale carbon sequestration,” Energy 30, 2558–2579.
Hydrothermal carbonization (HTC) has been proposed as an alternative process for creating recalicitrant charcoal-like material which requires significantly lower temperatures and can accomodate feedstock with relatively high levels of moisture levels therefore nixing the need to dry feedstock material prior to pyrolization. Titirici et al. (2007) have demonstrated hydrothermal carbonization at less than 200 degrees C in a slightly acidic enviornment for 4-24 hours.
Intermediate pyrolysis is a modified pyrolysis technique somewhat in between fast and slow pyrolysis in which low to moderate temperatures are required for a shorter duration of time than for slow pyrolysis. The products of intermediate pyrolysis is 50% bio-oil (50% of which is water), 25% biochar, and 25% syngas (powerpoint by Dr. Tony Bridgwater of Aston University).
Intermediate pyrolysis is a new concept and therefore is not in widespread use. Further, there is relatively little literature on it, and I am not aware of any private companies actively working on this type of system. The technology is presently being researched and developed by Aston University’s Bio-energy Research Group (BERG) in Birmingham, UK.
Microwave pyrolysis is proposed means of creating biochar and syngas. The technology is still quite nascent with only a handful of demonstration facilities, from what I have seen. I do not know about microwave pyrolysis, but some general principles of heating with microwave radiation may be helpful. Microwave ovens cook things using “dialectric heating” whereby polar molecules such as water are caused to vibrate and rotate as they attempt to align themselves to the shifting incoming microwave radiation frequencies. The typically household microwave oven is 64% efficient at turning incoming electricity into microwave radiation, the rest of it is lost as heat mostly in the magnetron, the device which transforms high voltage electricity into microwave radiation.
The efficiency with which the targeted material actually absorbs the microwave radiation depends significantly on the moisture content of the material: If there is too few polar molecules (typical moisture from water) little to no heating will occur; whereas if there is a significant amount of excess polar molecules, an excessive amount of time and energy is required to heat the material. Efficiency is also signficantly determined by the extent to which microwave radiation can pass through the material. If the material too large or impenetrable by radiation differential heating is likely to occur.
Important research questions, therefore, for microwave pyrolysis are figuring out the ideal moisture content and size of various feedstock materials in order to maximize heating efficiency. Despite the inherent initial energy costs of converting electricity into microwave radiation, microwave heating benefits from the fact that it is very effecient at only heating the material which is targeted, microwave chamber walls and exterior surfaces are not directly heated (though they can absorb blackbody heat being reradiated from the heated material).
Carbonscape, based out of Blehnheim, New Zealand, has recently announced plans to build a commercial scale pyrolysis plant. Dr. Christopher Turney, professor of Geography at the University of Exter and founder of the company, built a 5 meter long prototype microwave pyrolysis unit which puportedly successfully demonstrated the concept at very competitive price of $65 per metric ton of carbon dioxide. Dr. Tim Flannery, 2007 Australian of the Year and author of the book on climate change “The Weathermakers,” has recently joined the board of the company.
Biochar typically contain somewhere between 20-35 megajoules per kilogram of energy so why not simply burn it to replace fossil fuels? This is an important question to address because it significantly affects the amount of carbon which can be sequestered from the process. Gaunt and Lehmann (2008) have found that, “the avoided emissions are between 2 and 5 times greater when biochar is applied to agricultural land (2–19 Mg CO2 ha-1 y-1) than used solely for fossil energy offsets.” Thus, if we were solelyconcerned about energy production we probably wouldn’t even bother with pyrolsyis, we would simply gasify the biomass, extract as much energy as possible and simply be left with with ash.
The benefits of pyrolysis, however, are that we can still extract a considerable amount of energy (approximately 70%) while retain a large amount of carbon (approximately 50%), thus the process delivers considerable energy for the amount of carbon that is retained (Sohi et al., 2009). Further, the retained carbon in the form of biochar has many potential benefits (both with and without direct financial incentives) such as improved crop production, decrease of greenhouse gas emissions from soils, increased fertilizer efficiency, improved water quality, sequestration and storage of atmospheric carbon, and so forth. It is expected that the various benefits of applying biochar to soils will likely outweigh the benefits of burning it to replace fossil fuels.
COMBINED HEAT AND POWER
Unless all coal power plants are outfitted with carbon capture and storage (CCS) technologies by 2030, it is unlikely that we will be able to reduce atmospheric carbon dioxide to safe levels of 350 parts per million (Hansen et al., 2008). At present, serious questions remain about the feasibility of CCS technologies due to the high price tag (costing 21-91% more for new plants), serious efficiency losses (25-40% energy lost to actively sequestering carbon), and the difficulty to retrofit existing plants. Despite more than three decades of research and hundreds of billions of dollars of research money, there remain but a handful of plants using CCS technologies causing many to wonder about the feasibility mitigating CO2 emissions from coal in the short-term.
Development of distributed regional-scale combined heat and power (CHP or “cogeneration”) by pyrolyzing biomass could be an extremely efficient means of replacing carbon intense energy with carbon negative energy would go a long ways toward achieving climate sustainability. CHP is extremely efficient because waste heat from the process is used to heat nearby homes and buildings thereby increasing the overall efficiency of the process.
“Trigeneration”—also referred to as Combined Cooling Heat and Power (CCHP)—is the simultaneous production of power, heat, and cooling through “absorption chiller” technology. Absorption chillers work much like “compressor chillers” using an evaporated refrigerant to capture heat from inside the cooled space and moving the heat outside of the cooled space. The difference is that instead of using an electrical compressor to change the refrigerant back to a liquid, the absorption chiller uses heat to drive this process, typically uses ammonia as opposed to hydrofluorocarbons, and has no moving parts. While trigeneration is good, we may be able to do even better.
COMBINED COOLING, HEAT, POWER, BIO-OIL, AND BIOCHAR
If we were to use pyrolysis to power a decentralized, regionally distributed power system we could set up a highly resilient, “smart grid” that could be configured to utilize local biomass resources; that could sequester carbon dioxide from the atmosphere; that could generate clean and renewable power; that could provide heat to nearby homes and buildings; that could be used synthesize bio-oils to offset oil imports; that could produce biochar for improving agricultural soils; and that could provide refrigeration capabilities. Thus our pyrolysis system could be a Combined Cooling, Heat, Power, Biofuel, Biochar (CCHPBB) system.
The benefits that efficient sustainable biochar-based energy could have for both the least developed world where people are in desperate need for basic infrastructure to improve daily life could be immense in terms of sustainable development. For the developed world, where incredible gains in efficiency can be made by replacing coal with pyrolyzed biomass, there are incredible opportunities for reducing CO2 emissions.
Community Power Corporation, based out of Littleton, Colorado, offers several different models of CHP systems ranging in scale from 5 kW systems all the way up to 100 kW systems. From what I have gleaned from their website, their systems are essentially using gasification technologies which completely combust the biomass feedstock reducing entirely to ash. If these type of systems can be slightly modified in order to produce biochar through pyrolysis, systems such as these could be used to generate carbon negative CCHPBB.
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RESIDENTIAL HOME HEATING WITH PYROLYSIS
Pyrolysis units for residential buildings may be an appropriate technology where biomass is readily available and economically priced vis-a-vis fossil fuel based fuels such as natural gas, heating oil, or electric heat. For example, heating oil in the United States almost hit a “trigger point” at which many families weren’t able to heat their homes due to exponential increases in oil prices over the past two decades.
Thankfully, the threat of home heating crises has decreased as oil prices have declined. However, this should give little indication that conditions are likely to keep energy prices cheap for a long duration of time. The likely near-to-medium term peaking in world oil prices coinciding with other major threats such as climate change and economic instability within the next 5 to 10 years hence is likely to again drive overall energy prices up.
Home heat and power generation (possibly even with a small separately insulated refrigerator/freezer using an ammonia absorption chiller) may be a really stellar choice for many areas with an acre or more and who have ready supply to inexpensive biomass. These two factors are probably important to insure proper air quality while maintaining sustainability of the biomass availability. It may also work in a more city like-structure as long as every possible off-gas is captured (for which there may be certain interesting possibilities) and it is deemed safe with highest standards of quality. However, the neighborhood grid systems are likely to be the more effecient choice as oppossed to each family installing a separate system–the houses could simply be wired into existing boiler systems and heat the building with water heated from a more efficient neighborhood pyrolysis unit which deliveres hot water via super insulated tubes.
First let us deal with the negatives: There are several considerations which make biomass-based residential heating systems slightly challenging. First, biomass is bulkier and less energy-dense than fossil fuels, which means you need more of it to heat a given space. (Fortunately, it’s often half the price generally in many places.) Second, most people living in temperate climates enjoy heating with little or no labor or even though on their part–the entire heating system is hooked up to a natural gas line, propane or oil tank and regulated by a thermostat. Wood, chip, and pellet systems generally take slightly more work than this. Third, we are presented with the difficulty managing biochar coming off of the system. So there is difficulty in wood systems, but pyrolysis offers the benefits of being both an affordable carbon negative heat and electricity source, and a means of sequestering carbon while likely greatly enhancing degraded soils.
Outdoor Wood Boilers (OWBs) have come under heavy criticism in the United States and several states have banned their use due to their poor emissions controls. Pyrolysis offers a much cleaner burning process that could be integrated into the overall outdoor wood burning boiler/furnace system. Pelletized biomass (i.e. wood pellets) fed into an automated hopper may be an option for offering greater levels of convenience for homeowners. BioHeat USA (formerly Tarm), offers residential wood downdraft gasification units for the residential homeowner which may offer a model for producing similar units capable of also producing biocahr.
BIOCHAR STORAGE AND TRANSPORT
One should take care storing, transporting, or combusting readily flammable materials and biochar/charcoal is no exception. Recent experiences of one gentleman was the smoking of stored biochar. Fortunately the issue was caught immediately. Apparently storing the charcoal in a plastic 55-gallon storage container when a static start causing the material to smoke. The situation was unlikely to be aided by the probable presence of volatile organic compounds (VOCs) that would be readily evaporated from the biochar which is found to be often found to have a fairly significant VOC content, especially for biochars made from fast pyrolysis.
BIOCHAR AND FUEL CELL TECHNOLOGIES
Carnot’s Theorem states that an engine that relies upon the differential heating of hot and cold heating reservoirs is inherently limited in terms of its total energetic efficiency. Carnot’s limit is the case even if a theoretically ideal engine were developed which had no friction between components. This is obviously not the case. The average automobile, for instance, is 20-25% efficiency and the average power station operates at roughly 35% efficiency with the most advanced power stations approaching 45% efficiency, whereas the Carnot efficiency for the Otto cycle is approximately 60% under typical conditions.
The Carnot efficiency discount, however, can be avoided if the chemical energy source is not burned to run a heat engine, but rather directly transformed into mechanical energy as is done in a fuel cell. A fuel cell is an electrochemical device which converts fuel into electricity by forcing the fuel’s electrons to be split off from protons and forced through a circuit. The result can be a more efficient transfer of fuel into electrical energy of around 83% under typical conditions, however, it is not always appropriate to simply compare maximum thermodynamic efficiencies as real world efficiencies can be quite different with significant decreases in fuel cell efficiencies.
Suffice to say that fuel cells perform best for stationary electrical power generation purposes and when fed with a pure stream of hydrogen gas. The advancements described above in increasing the output of the syngas through “steam pyrolysis” developed by EPRIDA with 50% hydrogen may offer a the energy for a fuel cell energy generation plant. Significant challenges, however, remain with manufacturing complexity and price of fuel cells.
BIOCHAR AND STIRLING ENGINES
Stirling engines are external combustion (or heat) devices that transform differences in temperature into mechanical work and, if hooked up to a generator, electrical energy. Use of waste heat, quiet operation, durability and relatively high efficiency have made Stirling engines an appealing technology for small-scale CHP with pyrolysis based systems. The difficulty with Stirling engine systems seems to be sophisticated engineering requirements and fairly large size devised in order to generate substantial energy both of which combine to create a high initial cost for such a system.
The WhisperGen Company , based out of New Zealand and Spain, is in the intial phases of commercializing a CHP Sterling engine system that uses fossil fuel based energy inputs. The feasibility of using a similar device that instead utilizes waste heat coming off of a carbon negative pyrolysis product may offer promising avenues for development.
COMPARING FAST PYROLYSIS SYSTEMS
The various methods for fast pyrolysis include: 1) Ablative pyrolyzer, 2) Vacuum pyrolysis, 3) Augur reactor, 4) Bubbling fluidized beds, 5) Circulating fluidized beds, and 6) Rotating cone pyrolyzer. halocline reactors?
HIGHWAY BIO-ENERGY PRODUCTION
Fast growing tree species such as poplar and willow could be grown for biochar production. For a thought experiment, imagine if the US government decided that it would plant out the medians and birms of the highway system with hybrid poplar. According to the Federal Highway Administration, there are some 161,000 miles (more than 612,000,000 feet) of federal highway in the US (putting asid state and local roadway systems altogether.) If we can estimate that there is, on average, 15 feet of median and berm width that is mowed, sprayed, or otherwise managed that would give us a total acreage of some 15,312,000,000 square feet or approximately 350,000 acres. Being conservative we might say that perhaps only half of this land is suitable for growing and harvesting energy crops, thus reducing the total acreage down t0 175,000 acres.
According to the US Bioenergy Feedstock Information Network, hybrid poplars, “when grown under short rotation silviculture, can produce between 4 and 10 dry tons of wood per acre per year (8-22 dry metric tonnes per hectare per year), and can achieve a hieght of 60 feet (20 meters) in as little as 6 years.” Taking the median number of 7 dry tons of wood per acre per year with our total acreage of 175,000 acres, we find that we could be harvesting some 1,230,000 tons of poplar wood.
The high heating value (HHV), which is the optimal estimate of the energy content of the wood (basically, assuming that it is completely dry), of poplar is between 20 and 25 kJ/g. Accounting for moisture content we could be conservative and use 18 kj/g as a more realistic figure. Converting and multiplying we find that our annual poplar harvest would give us some 22,000,000 Gigajoules of energy or 6.2 billion kilowatt-hours (kWh) of energy. This would be enough energy to offset a 250 megawatt power plant assuming a 30% energy efficiency and to sequester some 1.5 billion pounds (680 million kilograms) of carbon in the form of biochar (according to Johannes Lehmann’s estimate that 30.6 kg of carbon can be sequestered for each GJ of energy produced.)