Proceedings of the IASTED International Conference Environmental Management and Engineering (EME 2009) July 6 - 8, 2009 Banff, Canada

FROM BIOMASS TO BIOCARBON – TRENDS AND TRADEOFFS WHEN CO- FIRING

Hugh McLaughlin, PhD, PE Director of Biocarbon Research, Alterna Energy Inc. #102 3645 – 18th Avenue, Prince George, BC, Canada V2N 1A8 www.alternaenergy.ca, [email protected]

ABSTRACT the utility location, and adapting the operations of the The challenges associated with co-firing a biofuel in an utility to accommodate the characteristics of the new fuel existing combustion train can be conceptually divided (Baxter, 2005). One popular strategy is to co-fire a into assembling a biofuel creation capability, transporting portion of the alternative biofuel with the historic fossil the biofuel to the utility location, and adapting the fuel. In general, the relative portion of biofuel that can be operations of the utility to accommodate the successfully co-fired with the historic fuel is dependent on characteristics of the new fuel. This analysis will develop the magnitude of the differences in physical and and compare the properties of three biofuels capable of combustion properties of the two fuels (Demirbas, 2003; being co-fired: dry wood pellets, torrefied wood pellets Kiel, 2008). and biocarbon pellets (carbonized biomass pellets). The The current programs to utilize renewable fuel physical properties and processing mass balances of these sources are considering a broad range of biomass sources renewable resources will be investigated. Additional that share a common property of all being characterizations will be provided that influence the “lignocellulosic” in composition, but vary widely in most transportation and utilization of the fuel by the utility in other properties, such as plant origin, bulk density, coal and lignite combustion processes. Utilizing recent moisture and ash properties. While ongoing research data from the literature on the production and includes all types of biomass, the vast majority of the transportation costs of biofuels produced in British historic work has focused on the starting biomass source Columbia and consumed by utilities in Sweden, the of wood residues and the thermal conversion to torrefied market value and associated maximum production cost, wood and onto biocarbon (also known as biochar and including profit, is discussed. It is concluded that if the charcoal). For the purposes of utilization as fuel, the overall biofuel supply chain includes significant thermal conversion processes stop at biocarbon (charcoal). transportation costs, relative to the cost of the raw Further processing at higher temperatures yield “bio- biomass and operating costs of the biofuel conversion coke” for metal refining. Additional specialized processes process, then higher energy density products, such as such as formal “activation” can yield activated carbons, biocarbon pellets, potentially represent the most cost- comparable with traditional coconut shell activated effective biofuel and present the most compatible biofuel carbons. for co-firing with coal. This analysis will compare two well-documented options for solid biomass-based fuels, dried wood pellets KEY WORDS and torrefied wood pellets, with a less well studied option, Sustainable Development, Co-firing Biomass, carbonized wood pellets (i.e. Biocarbon). Biocarbon fuels Biocarbon/Biochar Properties, Biofuel Supply Chain are processed at higher “carbonizing” temperatures to Economics produce a higher energy-density fuel per unit weight at lower overall mass yield. As will be discussed, the interaction of producing higher quality biofuels with 1. Introduction lower shipping weight influences the overall economics of the supply chain from biomass “on the stump” to The combination of heightened corporate environmental electricity flowing into the grid. stewardship, public and regulatory pressure to reduce GHG emissions, and supply chain concerns for traditional utility fossil fuels such as coal and lignite have created 2. Materials and Methods strong interest and motivation to utilize renewable biomass sources in traditional utility generating capacity. The underlying data utilized in the discussion that follows The challenges associated with utilizing a different fuel in have been culled from the existing literature and an existing combustion train occur at every step of the combined to isolate the phenomena of interest. As such, process, but can be conceptually divided into assembling the reader is recommended to review the original works to a biofuel creation capability, transporting the biofuel to evaluate the credibility of the raw data. References,

650-811 43 including web links if available, are provided for all 2.2 Predicting the energy content of biomass to literature resources utilized. biocarbon fuels – derived fuel metrics

2.1 Biomass to Biocarbon Properties from the Solid biomass-based fuels are only recently being literature considered for commercial-scale utility fuels, except for a handful of dedicated applications in the pulp and paper Anyone familiar with biomass literature is compassionate industries, such as bark boilers, etc. Of paramount interest with the challenges of defining “typical” properties of to any utility application is the heating value of the fuel, thermally modified biomass. Further complicating the since the application fundamentally generates heat and characterization of carbonized biomass, or biocarbon, is converts that thermal energy into electricity. the variability created by the interactions between starting Heating value metrics come in two versions, gross or biomass and the reaction conditions. A recent review of higher heating value (GHV or HHV), and net or lower charcoal technology (Antal and Grønli, 2003) will provide heating value (NHV or LHV). Net Heating Value, NHV, the motivated reader with a sound overview and a taste of is a far more representative measure of how much useful the complexity of the well-documented interactions when energy the electricity-generating utility can extract from lignocellulosic biomass is thermally modified. the fuel and will be used for this analysis. The difference While there is no such thing as universally between the two measures involves the fate of the water representative biocarbon, the Antal review does contain a formed during combustion (either condensing to a liquid set of data contained in a doctoral dissertation that and relinquishing the heat of condensation or persisting as provides a reasonably coherent set of data to serve as the a vapour and leaving with said energy), and depends on basis for the discussion that follows (Schenkel, 1999). the fuel, with high quality coals and pure carbon Before dissecting the actual data, the reader is (graphite) having little or no difference and methane cautioned that biomass is intrinsically variable and one having a NHV 10% less than the GHV. Since biomass- can get into more trouble than it is worth attempting to based fuels contain significant amounts of hydrogen, one over-quantify the underlying trends. The phenomena finds significant differences between GHV and NHV (see depicted in the graphs of Schenkel’s work have been discussion by Spill-Sorb, 2008). tabulated to allow more facile manipulation as a The literature options for predicting the energy spreadsheet (please email [email protected] density of biofuels were reviewed. The well-established for a reprint, copy of the Excel spreadsheet, and the and historically utilized Dulong’s Formula was utilized, esoteric references cited herein). Since the original raw with the method described by Spill-Sorb to convert GHV data was not available and the original figures show to NHV at 15.6C (60F – which is the historic reference significant scatter in the data, some smoothing of the data temperature for heating value data). The calculated values was applied to emphasize the underlying trends, as shown are shown in Table 1. Table 1 also provides the energy in Figure 1. requirements to dry incoming biomass from some initial moisture content to a reference point of zero residual moisture, which is a state passed through on the way to higher treatment temperatures associated with torrefaction and carbonization.

100% Carbon Hydrogen

90% Oxygen Fuel Yield

80% Fixed Carbon % Fixed Carbon Yield

70%

60%

50%

40%

Mass fraction (%) 30%

20%

10%

0% 200 250 300 350 400 450 500 Biomass Processing Temperature Celsius

Figure 1. Trends on biocarbon properties with increasing carbonization temperatures

44 Table 1 Calculated energy content of biofuels and derived fuel metrics Moisture Fuel Yield Dulong's NHV Energy Yield % of dry wood Energy Density Content & wt fuel per GJ/te fuel GJ/te feedstock energy delivered dry wood=100% Temp C wt dry wood dry wood = 100% 50% 200.00% 6.08 12.17 83.98% 41.99% 40% 166.67% 7.76 12.94 89.32% 53.59% 30% 142.86% 9.44 13.49 93.13% 65.19% 20% 125.00% 11.13 13.91 95.99% 76.80% 10% 111.11% 12.81 14.23 98.22% 88.40% 0%, 200C 100.00% 14.49 14.49 100.00% 100.00% 250C 93.50% 15.14 14.16 97.72% 104.52% 300C 80.00% 16.83 13.47 92.95% 116.19% 350C 58.00% 20.86 12.10 83.51% 143.97% 400C 40.00% 24.10 9.64 66.56% 166.39% 450C 30.00% 26.57 7.97 55.03% 183.43% 500C 27.00% 27.49 7.42 51.23% 189.74%

200% Energy Yield (dry wood = 100%) Energy Density (dry wood = 100%) 180% Fuel Yield (wt fuel per wt dry wood) 160%

140%

120%

100%

80%

60%

40%

20%

0% 50% 40% 30% 20% 10% 200 250 300 350 400 450 500 Moisture Content 0% Highest Treatment Temperature

Figure 2. Derived Fuel Metrics of Biofuels (dry wood = 100%)

In Table 1, a number of metrics have been calculated The 250C and 300C rows represent the range of to allow the variations of biofuel properties to be metrics for torrefied wood and the 350C to 450C rows compared as they relate to actual utilization of the represent the range of biocarbon products. The 500C row biofuels in the utility industries. The rows above “0%, is provided to complete the trends shown in Figure 1, but 200C” represent residual moisture levels in the incoming this temperature is generally too high for production of biomass, ranging from 50 weight percent, representing biocarbons due to yield and energy losses associated with freshly cut wood, to bone-dry wood, shown as “0%, higher processing temperatures. 200C”. Commercial wood pellets typically contain less In Table 1, the “Fuel Yield – wt fuel per wt dry than 5% residual moisture, unless improperly shipped or wood” column has been carried directly from the trends in stored such that they exhibit increased moisture content Figure 1, based on the data available in (1). The after manufacture due to the hydrophilic nature of the “Dulong’s NHV – GJ/te fuel” column is the lower heating product. value of the biofuel, as calculated in the manner discussed previously. The “Energy Yield – GJ/te feedstock” column

45 is the product of the Fuel Yield percentage and the NHV temperatures and due to the loss of energy from the measure. The “% of dry wood energy delivered” column biomass in the form of volatiles, the Energy Yield is the percent of the energy in the benchmark bone-dry decreases, as shown in Figure 2. The Energy Yield biomass, as represented by the 200C row, that is represents the product of the two first two trends, with the contained in incoming biomass with residual moisture and decreasing Fuel Yield and the increasing Energy Density the corresponding fraction retained in the solid biofuel at counterbalancing each other to produce the trend depicted higher processing temperatures. The “Energy Density” is in Figure 2. the relative energy per unit weight of the biofuel as The overriding question is whether the thermal compared to the reference energy density of dry wood = processing of the wood to produce a torrefied biofuel or 100%. This can be calculated as either the ratio of the higher carbonization temperatures to produce biocarbon NHV values or the ratio of the “% of dry wood energy fuels represents an improvement or deterioration in the delivered” divided by the “Fuel Yield” and represent the value of the fuel. Clearly, if the value of the biofuel is same measure of how energy dense the refined biofuel is measured strictly by the content of energy contained, and compared to bone-dry wood. does not incorporate additional costs such as The trends of the derived fuel metrics are shown in transportation to the utility or co-firing properties, then Figure 2. bone-dry wood, with an Energy Yield of 100%, is the preferred product. Unfortunately, substituting one source of heat for another is not that straightforward in co-firing 3. Results and Discussion applications, as discussed at length in several recent publications (Demirbas, 2003; Maciejewska , et al., As can be seen in Figure 2, there is a consistent trend that 2006). as the biomass is dried to lower moisture levels and One critical consideration left out of the Energy subsequently processed at higher temperatures, the Fuel Balance analysis provided in Figure 2 is the energy Yield, represented by the weight of fuel per tonne of requirement required to transform the starting biomass, initial bone-dry wood, decreases and the thermal content which would be expected to contain significant moisture, of the biofuel, as measured by Energy Density, increases. into the finished biofuel suitable for co-firing. Logically, drying biomass, assuming an external source of Figure 3 provides one such set of data, taken from a energy, increases the energy content of the biomass by recent presentation at Ontario Power Generation removing the evaporative burden of the moisture content (MacLean, 2009). of the wetter biomass. As such, the Energy Yield increases as wood is dried. Subsequently, at higher

Figure 3. Summary of Production Energy Requirements for Wood Pellet Manufacture

46 As can be seen in Figure 3, the production of a Recalling that production of one tonne of ODT wood finished wood pellet from the “Biofibre supply” is energy pellets utilized 1.681 tons of incoming 30% moisture intensive. If the split of incoming biomass is 85% to biomass, with a Net Heating Value of 9.44 MJ/tonne, the pellets and 15% to fuel the drying requirement, then incoming biomass contained contains a total of 15.87 MJ. 1/0.85 = 1.176 tons of dry incoming biomass is required After conversion to wood pellets and providing the fuel to to yield 1 dry ton for pelletizing. generate the electrical demand, the finished wood pellets Any incoming moisture content adds to the weight of yields the net energy of 12.35 MJ, for an overall energy the incoming biomass. For example, 1 ODT incoming at conversion efficiency of 77.8% for the finished wood 30% moisture would weigh 1/0.7 = 1.429 tons of pre- pellet with 5% residual moisture. Of the 22.2% energy drying biomass. Thus, 1.176 * 1.429 = 1.681 tons of consumed during converting the incoming biomass into incoming 30% moisture biomass is required to yield one wood pellets, 8.2% was associated with the electrical ton of dried wood for pelletizing if the energy requirement discussed above and 14.0% was associated requirement for drying is derived from the incoming with the thermal requirement for drying the incoming biomass, which is highly likely. biomass. In addition to the material loss as fuel, the wood The approach used above requires knowledge of the pellet process consumes 144 kwh/ODT of pellets. If this electrical requirement for biomass conversion, which electricity is provided by consuming a portion of the cannot be predicted from the overall energy balance of the wood pellet product supplied to the utility generating pyrolysis process. If one ignores the electrical electricity (perhaps by co-firing), then the actual yield of requirements, then one has the upper bound of the net thermal energy from the wood pellets needs to be efficiency of a pyrolysis process for converting incoming adjusted for the electrical demand for wood pellet “wet” biomass into the spectrum of higher energy density production. Therefore, 144 kwh = 0.5184 MJ of biomass-based fuels. This approach is applied to a electricity, which can be assumed to be generated by a number of relevant industrial applications in Figure 4. modern power plant at a thermal efficiency of 40% As shown in Figure 4, and premised on the (electricity output/heat input). Thus, 0.5184 MJ/0.4 = 1.30 assumption that all electrical costs are ignored, the 300C MJ per ODT of wood pellets is the thermal duty on the torrefaction conversion process is iso-energetic with 30% wood pellets required to generate the electricity to moisture content in the incoming biomass. Lower manufacture the wood pellets. torrefaction temperatures or higher incoming moisture Assuming the wood pellet product is 5% moisture, levels (or inclusion of the electrical conversion the Net Heating Value, by interpolating the values in requirements, as shown above) will require supplemental Table 1, is 13.65 GJ/tonne of wood pellet fuel. energy be supplied, often by consuming a portion of the Subtracting the electricity requirement discussed above, incoming biomass. the net energy content of wood pellets is 12.35 MJ/tonne (assuming ODT = 5% moisture in Figure 3).

18.00

Drying = removing 1 ton of water out per ton of dry wood - 2.32 GJ

16.00

14.00

torrefaction at 300C - iso-energetic at 30% moisture

12.00

carbonization at 350C - near iso-energetic with 50% moisture

10.00

carbonization at 400C - 2.53 GJ excess per tonne dry wood Energy for Drying Energy denstiy (GJ/metric tonne) (GJ/metric denstiy Energy 8.00

6.00 50% 40% 30% 20% 10% 200 250 300 350 400 450

moisture content of wood 0% Highest Treatment Temp C

Figure 4. Energy Balance - supplying the energy for Drying from the Pyrolysis Process

47 As can be seen, thermal conversion at higher pellets in British Columbia and selling them into the temp eratures than torrefaction yields additional energy utility market in Sweden, highlights the impact of from the biomass as part of the pyrolysis process to meet transportation costs on the overall value proposition. The the drying requirement of the incoming biomass, even at data contained in Peng are summarized in Table 2. 50% moisture content shown in Figure 4. The reader is As shown in Table 2, the increased production costs cautioned that electrical requirements of the higher of t he torrefied TOP pellets is easily justified by the temperature conversion processes have also been ignored. higher market price commanded by the more refined As such, the ultimate energy efficiency of a pyrolysis biofuel. The cause of the higher value is likely dictated by process will be impacted by the incoming biomass the market forces driving the supply and demand for the handling and size reduction requirements and the exiting torrefied wood pellets in 2005 in Sweden. biocarbon densification, forming and packaging A recent report (Maciejewska , et al., 2006 – Table operations. 18, page 91) provided the data contained in Table 3, Because of the significant weight reductions which summarizes the properties of the native biomass associated with drying biomass, biofuel conversion and the pelletized forms. The reader is cautioned that the processing is typically located close to the source of the energy densities reported in Maciejewska , et al., 2006 starting raw material biomass. The point of biofuel seem systematically higher than the values calculated in consumption, especially in co-firing applications, can be Table 1 of this document for comparable biofuels significantly removed from the location where the produced at similar highest treatment temperatures. finished biofuel is processed. As was seen with the Unfortunately, Maciejewska , et al., 2006 does not inclusion of electrical requirements in the thermal energy provide any background on the energy densities reported balance of pyrolysis, the inclusion of the transportation and it is recommended that the reader treat the energy costs into the overall biofuel economic model can density metrics as relative within a given analysis and not dramatically influence the optimisation of biofuel absolute in magnitude between documents. properties. A recent presentation (Peng, 2007), containing 2005 cost s of producing wood pellets and torrefied (TOP) wood Table 2 Summary of 2005 Wood Pellet and TOP Pellet costs – from Peng (2005) units BC wood pellet BC TOP pellet Feedstock Wood residues Wood residues Production cost $/tonne 79.09 101.76 Transportation cost $/tonne 116.45 116.45 (to energy plant in Sweden) Delivered cost of biofuel $/tonne 195.54 218.21

Market price of biofuel $/tonne 189.00 248.81 (Sweden 2005) Net profit per unit Biofuel $/tonne -6.54 30.60

Table 3 Properties of wood, wood pellets and TOP Pell ets – from Maciejewska , et al., 2006 Properties Unit Wood Wood pellets TOP pellets Moisture content %wt 35 7-10% 1-5% NHV dry MJ/kg 17.7 17.7 20.4-22.7 NHV as received MJ/kg 10.5 15.6-6.2 19.9-21.6 Mass density (bulk) Kg/m3 550 500-650 750-850 Energy denstiy (bulk) GJ/m3 5.8 7.8-10.5 14.9-18.4 Pellets strength - Good Very good Dust formation Moderate Limited Limited Hygroscopic nature Water uptake Swelling/water Poor swelling uptake (hydrophobic) Biological degradation Possible Possible Impossible Handling properties Normal Good Good

48 If one compares the properties of the two pellet generated during the biomass conversion processing products in Table 3, Wood Pellets and TOP pellets, the allows one to estimate the viability of operating a most significant difference appears to be the LHV as pyrolysis process without requiring supplemental energy received. The Wood pellets average 15.9 MJ/kg (=GJ/te) inputs to dry the incoming biomass. For those cases where and the TOP pellets average 20.75 MJ/kg. As such, the the incoming biomass contains appreciable moisture, energy density of the TOP pellet is 30.5% higher than the conversion to oven-dry wood products and torrefied wood Wood pellet, which parallels the 31.6% higher market products are predicted to require utilization of some of the price of the TOP pellet, as shown in Table 2. incoming biomass as an energy source for the front-end If one assumes that the higher energy density of TOP drying operations. pellets drives the higher market value over the dry wood The current market dynamics for biomass-based fuels pellets, then it seems that biocarbon pellets could produced in British Columbia and consumed by utilities command a similar premium over TOP pellets. If the 30% in Sweden were reviewed for wood pellets and torrefied energy content of TOP pellets over wood pellets translates (TOP) pellets. The properties for “biocarbon pellets”, into a similar market value escalation, then it seems produced at highest treatment temperatures above those reasonable that the 30 to 66% energy density associated with torrefaction and featuring significantly improvement of the biocarbon products with a range of higher energy density than the TOP pellet, were presented. energy density of 20.86 to 26.57 GJ/te, over TOP pellets It is concluded that if the overall biofuel supply chain with a range of energy density of 15.14 to 20.86 GJ/te, as includes significant transportation costs, relative to the shown in Table 1 and Figure 2, might result in a cost of the raw biomass and operating costs of the biofuel significant value-added market positon. conversion process, then higher energy density products, Based on the data of Table 1, a 50% premium over such as biocarbon pellets, merit consideration. TOP pellet prices would imply that 400C biocarbon pellet would command a market price of $375/tonne. Since the transportation costs from British Columbia to consuming References utility in Sweden would remain the same on a per ton basis, the extrapolation implies that the 400C biocarbon [1] Baxter, L. (2005). Biomass-coal co-combustion: pellet should command a predicted production cost, opportunity for affordable renewable energy. Fuel, 84, including profit, of over $250 per tonne. This is 1295-1302. approximately twice the production cost ceiling predicted [2] Antal, M.J.Jr. and M. Grønli (2003). The Art, for torrefied wood pellets. Science, and Technology of Charcoal Production. Ind. A final consideration meriting discussion, but difficult to Eng. Chem. Res., 42, 1619-1640. address in a concise analysis, is the cost impact of co- [3] Demirbas, A. (2003). Sustainable cofiring of firing a fuel that differs from the combustion biomass with coal. Energy Conversion and Management, characteristics of the incumbent fossil fuel, especially in 44, 1465–1479. combustion kinetics and the behavior of the ash [4] Hamelinck C.N., R.A.A. Suurs, A.P.C. Faaij constituents. The reader is referred to the references for an (2005). International bioenergy transport costs and energy introduction to these issues (Kiel, 2003; Maciejewska , et balance. Biomass and Bioenergy, 29, 114–134 al., 2006). In general, the more similar the properties of [5] Kiel, J. (2008). IEA Bioenergy Task 32 workshop - the biofuel is to the original fossil fuel, the larger the co- Increasing co-firing percentages in existing coal-fired firing operational envelope and the more manageable the power plants, Geertruidenberg, the Netherlands, 21 compatibility issues. October 2008. Available at http://www.ieabcc.nl/ [6] Maciejewska, A., H. Veringa, J. Sanders, S.D. Peteves (2006). Co-firing of Biomass with Coal: 4. Conclusion Constraints and Role of Biomass Pre-treatment. EUR 22461 EN, ISBN 92-79-02989-4, ISSN 1018-5593, Assembling a predictive model for the economics of the Office for Official Publications of the European supply chain for biomass “on the stump” to electricity Communities Available at flowing into the grid is a formidable task, as demonstrated http://www.techtp.com/Cofiring/Cofiring biomass with by Hamelinck (2005). Once the transportation sequence is Coal.pdf assembled, it becomes apparent that trade-offs are [7] MacLean, H. et al. (2009), Life Cycle Assessment possible that strongly influence the overall cost of of Wood Pellet Use, presented at the OPG Biomass delivering biomass-fueled electricity via existing utility- Workshop January 20, 2009. Available online at based capacity. A dynamic interaction occurs whenever http://www.opg.com/power/fossil/biomass.asp under modification of the biomass prior to transportation affects “Presentations”. the efficiency of the transportation expense, in terms of [8] Peng, J. (2007). A Study of Torrefaction for the net energy moved per unit of transportation cost, and the Production of High Quality Wood Pellets. Available at options for co-firing at the biofuel-consuming utility. http://www.techtp.com/Torrefaction for High Quality Matching the properties of the starting biomass raw Wood Pellets.pdf material with the available excess thermal energy

49 [9] Schenkel, Y. Modelisation des Flux Massiques et Energetiques dans la Carbonisation du Bois en Four Cornue. Ph.D., Dissertation, Universite´ des Sciences Agronomiques de Gembloux, Gembloux, Belgium, 1999; also available on page 1621 of (1) Ind. Eng. Chem. Res., 42 (2003). [10] Spill-Sorb (2008). 26-30. Available at http://spillsorb.com/ss6b.pdf.

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