Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Determining the Severity of Torrefaction for Multiple Biomass Types Using Carbon Content William A. Campbell* and Richard W. Evitts
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Department of Chemical and Biological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, Saskatchewan S7N5A9, Canada ABSTRACT: This research concerns the investigation of an alternative measurement to directly indicate the severity of torrefaction. Composition and process data from both batch and continuous torrefaction experiments using willow, wheat straw, and cattail biomass were combined and analyzed. The mass yield, which is an indication of torrefaction severity was correlated to the net change in residual carbon concentration (ΔC), and second the total change in mass (ΔMt) was correlated to net change in mass of carbon per 100 g of feedstock (ΔMc). Analysis of the experimental data show a polynomial relationship between the dry mass yield (Ym) and the change residual carbon concentration (ΔC). This relationship is Ym = 5.05ΔC2 − 3.96ΔC + 0.98 (R2 = 0.89). The uncertainty in this correlation is ±7.3% (w/w). The relation between total change in mass (ΔMt) and change in carbon mass (ΔMc) meanwhile was found to fit a linear model by ΔMc = 0.36ΔMt + 1.04 with a coefficient of determination of 0.96. Each of these models was then validated by introducing experimental data from numerous published materials focused on biomass torrefaction. That data included bench and pilot scale experiments that examined a wide range of biomass including soft and hardwoods, grasses, and agricultural residues. Analysis of the combined data set confirmed a second order polynomial model is predictive of the mass yield based on the change in carbon concentration where both parameters are expressed on a dry, ash-free basis. This model, Ym = 4.29ΔC2 − 3.66ΔC + 0.98 (R2 = 0.935), is predictive for torrefaction experiments with initial masses of 500 g and greater and for mass yields as low as 60%. The uncertainty in this second correlation is ±4.6% (w/w). Since this expression relies only on the concentration of carbon in each of the feed and output product streams, this model could be used to predict mass yield of continuous torrefaction in real-time if the carbon content of each stream were sampled periodically and if the raw biomass ash content were sampled intermittently. The relation between change in total mass and change in mass of carbon meanwhile was confirmed with the inclusion of all of the literature review data; the linear regression model was found to be ΔMc = 0.37ΔMt + 1.26 which had an R2 of 0.93. In practical terms, this expression indicates that for the average of these experiments, the first 3.4% of mass loss in torrefaction occurs without loss of significant carbon, and below 97% mass yield, carbon will consistently represent close to 37% of total mass loss, a figure which appears to hold to as low as 40% mass yield (w/w). solid matter is thus lowered,12 which also reduces its hydrogen bonding capacity and related hygroscopicity. The directly observable effects include a significant change in the solid feedstock color; from light to dark brown and then black, an overall more homogeneous appearance and character, an increase in brittleness, and increased resistance to microbial decay.13−15 The extent of these changes relates directly to the process temperature and residence time; for a given torrefaction process and biomass, increasing either of these parameters will increase the severity of torrefaction observed in the char product.16 The uses of torrefaction char are considered to be 2-fold; it can be used to partially or fully replace coal in power generation applications, requiring minimal infrastructure and process changes while reducing net CO2 emissions.15,17,18 Torrefaction char has also been shown to be superior to raw biomass as feedstock for gasification, producing fewer tars and overall higher quality syngas.10,19
1. INTRODUCTION The term “torrefaction” refers to the heating of biomass in the absence of oxygen, causing a fraction of the solid matter to volatilize,1−3 and is associated with producing a solid char product that has at least 70% of the starting dry mass.2−5 Torrefaction is sometimes also referred to as mild pyrolysis; pyrolysis is the general term for heating biomass to remove volatile matter and encompasses a wide range of processes. These can range from torrefaction where a high yield of char is the intended product to extreme pyrolysis which produces a low yield of concentrated solid carbon product and a high yield of liquid bio-oil.4 Torrefaction processes typically operate in a temperature range of 200−300 °C,3,5−7 while the torrefaction process time can range from seconds to hours, depending on biomass particle size, density, and heating rate.3,8,9 Torrefaction changes the composition of solid biomass in specific ways; of the three main plant polymers hemicellulose reacts to the thermal energy levels of torrefaction the most thoroughly, undergoing depolymerization, devolatilization, and carbonization reactions to a much greater extent than cellulose or lignin.3 The result is that the hemicellulose content is reduced significantly and converted to a separate stream of volatile matter rich in oxygen and hydrogen.3,10,11 The hydrogen and oxygen content of the © XXXX American Chemical Society
Received: April 26, 2018 Revised: August 6, 2018 Published: August 21, 2018 A
DOI: 10.1021/acs.energyfuels.8b01485 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels A van Krevelen diagram is used to compare the composition of solid fuel types according to their molar ratios of hydrogen, hydrogen and oxygen3,9 and was originally developed for comparing types of coal based on composition of these elements. Referring to the van Krevelen diagram of Figure 1,
An excellent example of how torrefaction extents relate to the change in carbon concentration is the research paper by Lestander et al.23 That study explored extremes of torrefaction and pyrolysis conditions for two biomass types (reed canary grass and Norway spruce) from above 95% to below 20% mass yield (w/w), where the residual carbon concentration increased from 50% to above 90%. Lestander et al.23 concluded that a second order polynomial model fit the relation between pyrolysis mass yield and residual carbon concentration for the experimental results analyzed. The rationale for a second order polynomial model relating mass yield to carbon concentration as illustrated by Lestander et al.23 is as follows: while part of the carbon will be released as volatile matter from heating, the “fixed” carbon content cannot be volatilized and thus represents the lowest possible or boundary mass yield (daf basis). Although some of the total carbon is lost as part of the gas/condensable organic matter during torrefaction, this fixed carbon content maintains an increasing carbon concentration as other mass is lost through volatilization. Further, the carbon concentration will approach 100% as the mass yield reaches its lower limit/boundary defined by the fixed carbon content, which is on average between 17.5 and 18.1% for woody and grass-like lignocellulosic biomass.24 The study by Lestander et al.23 benefited from two biomass types with very similar raw ash-free carbon content, allowing a direct comparison of total carbon content that increased consistently through a wide range of pyrolysis conditions. There are many published examples of torrefaction experiments where carbon concentration clearly increases as mass yield decreases,11,12,25−29 but this relationship has not generally been defined mathematically, compared between a wide range of experiments, or proposed specifically as an indicator for severity of torrefaction. This research takes the work of Lestander et al.23 and others to the next logical step: combining in-house data with other sets of torrefaction experimental results, normalization of the carbon data, and determination to what extent the change in carbon correlates to torrefaction severity, as defined by the change in total mass. The novelty of the work presented here is in how the results of many torrefaction experiments are distilled to a few normalized parameters that allow direct comparison of those experiments. First, the measured change in carbon concentration between raw biomass and torrefaction chars is compared to the process mass yield, a proxy indicator for severity of torrefaction. Second, using the same data, the mass change of carbon is compared to the total mass change for each experiment. Torrefaction experiments were conducted using both a batch and continuous-flow apparatus’, with willow, wheat straw, and cattail feedstocks. The mass yield and ash and carbon content measurements from these experiments were used to develop an initial estimate for the carbon concentration and carbon mass change models. These models were subsequently refined and validated by introducing more than 60 data points from other published works on lab and pilot scale torrefaction experiments, with various wood and grass biomass types.
Figure 1. Van Krevelen diagram; relative ratios of H:C and O:C for different fuel types. Effect of torrefaction line represents a typical data set, where the origin indicates the raw biomass composition and position on the line is relative to severity of torrefaction. (Diagram and torrefaction data adapted from van der Stelt et al.9)
the arrow representing typical torrefaction process data from van der Stelt et al.9 can be understood chemically as parallel to the geological process by which biomass is converted to coal. The position of a particular fuel on this diagram will not only help classify the fuel but also allow an estimation of the heating value (which generally increases from right to left) and amount of volatile matter (which decreases from right to left), each of which can be extremely helpful in designing equipment for converting that fuel to heat and/or electricity. The final resting point of the torrefaction char atomic ratio on this torrefaction “line” relative to the origin indicates the severity of torrefaction applied but also will relate to the change in heating value (increase) and volatile matter (decrease). Improving the torrefaction process control through direct measurement of the severity of torrefaction is the subject of this paper. Quantification of the mass of the remaining solid matter (torrefaction dry mass yield) is normally how the extent or severity of torrefaction are reported at the lab and pilot scale, which relates well with other key criteria such as increase in heating value, energy yield, and friability.20 On an industrial scale, accurate and continuous measurement of the mass yield would be complex and costly, requiring measurement of both the mass flow in and out of the torrefaction process; belt scales, nuclear mass flow meters, loss-in-weight feeders, and Coriolis instruments would be the typical measurement systems.21 This work examines how the change in carbon content may be used as an alternative measure for evaluating torrefaction severity. The char carbon concentration has been shown to increase with the severity of torrefaction, as can also be inferred from the torrefaction pathway illustrated in Figure 1. Carbon can also be measured continuously and relatively rapidly using an automatically fed analytical instrument; an instrument that can automatically weigh, combust, and measure the relative production of CO2 using NIR sensing can report the carbon content at a frequency of only several minutes.22
2. EXPERIMENTAL SECTION This section describes the apparatus and method for conducting continuous torrefaction experiments, which yielded the majority of the data that was the subject of this study. The experimental data examined included the results of 22 batch torrefaction experiments using wheat straw that had been previously published by Campbell et B
DOI: 10.1021/acs.energyfuels.8b01485 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels al.30 as well as continuous torrefaction of coppiced willow using a modified apparatus by Woytiuk et al.10 The details of those apparatus’ and method may be found in the referenced publications. 2.1. Continuous Torrefaction Pilot Plant. The experimental apparatus used to generate the majority of data for this analysis is a continuous torrefaction pilot plant using a novel reactor concept. This plant, based on horizontal moving bed contactors, was built between 2013 and 2015 and is referred to as the continuous torrefaction unit or “CTU”. Figure 2 illustrates the key aspects of this plant in a
Table 1. CTU Torrefaction Parameters temp, °C
time, min
replicates
SRC willow (2015−2017) 220 16.0 229 10.3 229 21.7 235 20.0 250 8.0 250 16.0 250 24.0 255 10.0 265 16.0 271 10.3 271 21.7 280 16.0
1 1 1 1 1 6 1 1 5 1 1 1
cattail (2017) 250 6.8
1
temp, °C
time, min
replicates
SRC willow (2014)10a 240
