This study examines the chemical and physical properties of the outputs of two different thermochemical conversion technologies used to process mill and forest residues, with an emphasis on potential uses and markets for those outputs. The two systems were chosen because they are commercially available, can process a wide range of woody biomass feedstocks of varying quality, and are small enough to be used in distributed applications by small and medium scale forest industry facilities. The two systems also represent different ends of the conversion spectrum, with the BSI pyrolysis system designed to maximize biochar fixed carbon and sorption for an exothermic reaction between 350 and 750 °C, and the TEA system gasifier designed to produce high-energy gas at temperatures greater than 1000 °C. The differences between the systems are reflected in differences between production outputs and their potential uses and markets.
4.1. Producer Gas
The energy content of gas produced from forest biomass by the TEA system gasifier is somewhat higher than values reported in studies of pilot scale and laboratory systems. For example, using a Biomax-25 gasifier (Community Power Corporation, Littleton, CO, USA), Elder and Groom [26
] produced gas from pine and mixed hardwood chips with energy content around 6 MJ m−3
. Son et al.
] reported syngas energy content of 4.6 MJ m−3
for wood chips processed in an experimental downdraft gasifier. At 12.4 MJ m−3
, the TEA system gas produced from mill residue in this study has relatively high energy content. The energy content of BSI pyrolysis system gas is below 3.0 MJ m−3
, which is relatively low compared to gasification systems, but similar to gas produced by pyrolysis systems operating in this temperature range [28
]. It would be expected that mill residue feedstocks would result in gas with higher energy content than forest residue feedstocks because of the lower ash content and higher energy density of the raw materials, and this appears to be the case for the TEA system. However, we do not know if the seemingly contrary results of for the BSI system are within the range of variability of the system or represent a statistically significant difference between the feedstocks.
High concentrations of CO2 and N2 are responsible for the lower energy content of the BSI gas. However, it is also worth noting that the BSI system is exothermic and does not require any gas inputs for heating once the primary reactor is fired. By contrast, the TEA reactor is endothermic and is estimated to consume about 1500 MJ h−1 in process heat, which was provided by propane in this study. Using woody biomass feedstocks, process energy needs could be met with about 15% of the gas production of the TEA system. Though the prototype is not presently configured to use producer gas as fuel for its six tangential burners, TEA is currently modifying the system to integrate this option as part of ongoing research and development.
There are some operational considerations related to air-fuel ratios and lower flame temperature, but in general retrofitting gas burners to accommodate producer gas from biomass conversion is relatively straight forward. This means that, with relatively little cost, the gas produced by these systems can be combusted to produce process heat for mill operations such as feedstock and lumber drying, heat treating, and facility heating. For example, many conversion systems require feedstock with low moisture content, but feedstock moisture content less than 10% is below what could be expected from field-dried forest residues in most parts of the country. Both systems can be configured to use waste heat and/or combustion of a portion of the gas stream to dry feedstock. BSI currently offers this option for new pyrolysis systems. Mills that dry solid wood products to meet product specifications or export requirements also have a significant need for heat.
Producer gas can also be used as fuel for electricity generation using an internal combustion or turbine engine, but this application is highly dependent on gas quality. Specifically, particulate matter and tars significantly increase engine wear and must be removed though gas cleaning. Gas quality for use in turbine engines must be especially high, with narrow parameters for particulate matter (<30 ppm), particle size (<5 µm), and alkali metals <0.2 ppm [28
]. Reductions in engine power (i.e.
, derating) are also a challenge when substituting biomass producer gas for natural gas or liquid fossil fuel because of its lower energy density. Though we did not evaluate these gas properties in this study, the need for significant producer gas post-processing for power generation is likely. However, if power is generated, these systems provide another potential market product—electricity to the grid. If power generation meets facility requirements, excess power may be sold or credited against future power use, depending on grid infrastructure and utility sector regulation.
Biomass is the only renewable energy source that can be used as raw material in the production of liquid hydrocarbon fuels and chemicals. Thermochemical conversion systems are associated with two general types of liquid outputs: bio-oil and liquids manufactured from producer gas. Neither of the two systems in this study produces liquid output, but some pyrolysis systems do produce bio-oil, which could be shipped in its raw form to a refining facility [7
]. Industrial systems for the production of methanol (methyl alcohol) from biomass producer gas with high concentrations of CO and H2
, synthesis gas) are well established [18
]. Furthermore, the development of methods for the commercial production of liquid fuels and chemicals using Fischer-Tropsch (FT) synthesis of biomass producer gas is progressing [19
] and even commercialized in some cases [29
]. However, a variety of challenges related to economies of scale in refining operations and the technical demands of liquid fuel production make it unlikely that small-scale catalytic production of liquid fuels and chemicals will be integrated directly into these systems in the near future. For both methanol production and FT synthesis, the major barriers to integration at distributed scales are both technical and financial [19
Biochars produced by the TEA and BSI systems can be used as a solid fuel. Recent research on the use of torrefied wood and biochar as fuel has focused on utility applications, especially co-firing with coal [30
]. Fossil coal energy content is generally higher than that of biochar and ranges from 28 to 40 MJ kg−1
, depending on coal quality. In contrast, the calorific value of fuels produced by pyrolysis of wood biomass have been reported as 20.7 MJ kg−1
for torrefied wood [16
], between 22.8 and 31.8 MJ kg−1
for slow pyrolysis biochar (i.e.
, charcoal, [32
]), and 20 to 26 MJ kg−1
for biochars made from various woody materials [33
]. With energy content above 30 MJ kg−1
, the biochars evaluated in this study are higher in energy than these fuels, but lower than medium and high quality coal. This means that biochar, like biomass, is generally a poor substitute for coal in terms of energy content. However, there may be other reasons that utility companies and other coal users may want to substitute biochar for coal. Among them, co-firing biochar with coal may reduce fuel costs, reduce some types of emissions (e.g., sulfur oxide and nitrogen oxide), diversify fuel sources, and offset fossil fuels with renewable fuels [35
]. In some states, cofiring may also meet requirements for renewable portfolio standards. Whether or not cofiring reduces greenhouse gas emissions depends on the source of the feedstock as well as the carbon accounting methods used, but it is clear that cofiring substitutes biogenic emissions for fossil fuel emissions. In addition, forests supplying biomass for cofiring recover emissions over time through regrowth of harvested stands, as long as forests are not converted to other land uses.
In general, these benefits are similar to those of cofiring biomass directly without conversion. Some types of coal-fired boilers, including stoker boilers and pulverized coal boilers, can substitute raw biomass for coal, often up to 20% by mass, without significant detrimental effects on system performance [36
], but there may be some advantages to using pyrolysis products in these systems rather than biomass. The energy density of pyrolysis products is higher than that of biomass, which is typically around 16 MJ kg−1
for biomass used in cofiring [37
]. In addition to improving transportation efficiency and boiler performance through higher energy density, biochar and torrefied wood have better handling and storage properties than biomass [15
]. Furthermore, boiler systems that cannot cofire biomass directly, such as integrated gasification combined cycle systems, may be capable of substituting biochar directly for coal, depending on biochar properties. Based on the particle size distributions and other properties of the biochars produced in this study, we believe that they could be substituted for coal in most gasification applications.
The use of biochar as a soil amendment is the subject of intensifying scientific inquiry from researchers in agriculture, forestry, mining, and other fields. Biochar additions have received the most attention from efforts to increase carbon sequestration while reducing atmospheric carbon dioxide concentrations [38
]. Increases in carbon sequestration can improve overall soil quality because of the role that carbon plays in chemical, biological, and physical soil processes [39
]. Biochar has a higher surface area and greater porosity than native soil organic matter, which also helps improve soil aggregation. Application of biochar from forest biomass to forest sites can improve the nutrient and water holding capacity of the soil by altering soil texture, aggregation, and organic matter content [40
]. Biochar can also decrease nutrient leaching and increase nutrient availability by altering soil cation exchange capacity and soil pH [42
]. Understanding the interactions of biochar application and soil texture, organic matter, and pH will be the key to determining both long-term impacts and potential market opportunities.
It is difficult to evaluate the use of our biochars for soil applications because most chemical data available for biochar soil amendments are based on agricultural crop feedstocks (e.g., peanut hulls, pecan shells, apricot stones) with little information on woody feedstock biochar. As a benchmark, biochar produced from hardwood forest residues by fast pyrolysis and used in forest soil studies contained 62% C and 18% N, with a pH of 6.8 and a bulk density of 0.25 Mg m−3
]. In comparison to the biochars produced in this study, these chars have higher C, higher N, lower pH, and higher bulk density (Table 3
The greatest impact of biochar additions to forest soil may be the liming effect that occurs as a result of increased pH. Biochar pH ranged from 8.7 to 10.2 for this study (Table 3
). Forest soils generally have a pH range from 4.5 to 6.0. In this case, the liming effect may not be ideal for all forest soil types and plant communities. Many forest plants, fungi, and bacteria thrive at lower soil pH [44
]; therefore altering forest soil pH through the addition of biochar may result in unfavorable shifts in above- and belowground flora. However, there are currently not any guidelines on the amounts of biochar that can be added before a resultant pH shift occurs, as this will likely be soil-specific. Low application rates (e.g., 1 to 2 Mg ha−1
biochar), which mimic the amount of biomass removed during harvest operations, may have little impact on soil pH, but would alter water holding and nutrient cycling conditions enough to improve forest growth. On degraded forest lands (e.g., log landings, skid trails) biochar may reduce soil bulk density and increase plant available nutrients sufficiently to rehabilitate the soil and ensure native vegetation regrowth [45
]. Though field research is progressing rapidly, markets for biochar as a soil amendment are still emerging.
In contrast, markets for AC are well developed and diverse. The chemical and physical properties of carbon that result in improved nutrient and water holding capacity in some soils are also desirable properties for industrial sorbents. High surface area and high porosity are ideal for adsorbing contaminants from both liquids and gases. Physical and chemical activation methods can significantly improve these properties in biochars, potentially adding value by meeting commercial specifications for AC used in filtering applications. Table 5
summarizes published results of BET surface areas and other properties of AC produced from fossil coal and biomass feedstocks. Pollard et al.
] reported that most commercial ACs have a surface area between 400 and 1600 m2
Published BET surface areas of AC produced from fossil coal and biomass feedstocks.
Published BET surface areas of AC produced from fossil coal and biomass feedstocks.
|Feedstock||BET surface (m2 g−1)||Pyrolysis temp. (°C)||Activat. type||Activat. temp. (°C)||Pore volume (cm3 g−1)||Iodine # (mg g−1)||Source|
|Wood||1780||440||H3PO4 + Steam||440||0.130||a|||
In this study we show that the biochar produced by the BSI and TEA systems is suitable for physical activation using standard physical activation methods. The TEA AC is within the commercial range, and biochar from mill residue produces AC with a higher BET surface area than biochar from forest residues. This result is consistent with studies relating high ash content to lower activation levels [51
]. Though we did not activate BSI biochars using the industrial rotary calciner, in laboratory activation BSI biochars were activated to a higher level than TEA biochars, as evaluated by iodine adsorption (Figure 5
). This result is expected based on the temperature of pyrolysis [52
]. Both biochar products could be used as a precursor for the production of AC. Though it is difficult to compete with the economic efficiency and consistency of fossil coal as an AC precursor, biochar is from renewable resources, which may be an advantage in some markets for differentiated carbon products. Further, because woody biomass is available as a byproduct of forest management and timber production in many parts of the country and pyrolysis or gasification technology performs well at distributed scales, AC can be produced from biochar in a more distributed supply chain than characterizes AC from fossil coal. Distributed production might provide advantages in transportation efficiency to some individual AC users and local markets.
Activated carbon is only one potential use and market for biochar. The range of potential market-based uses of biochar, from fuel to soil amendment to AC, can enhance product and market diversification available to producers. This offers the possibility of increasing the portfolio of value-added products that can be produced from what are now waste byproducts of solid wood products manufacturing and forest management.