Impact of Co-firing Coal and Biomass on Mixed Char Reactivity under

Feb 2, 2016 - This work focuses on the gasification behaviors of the chars formed during devolatilization when co-firing coal/biomass blends...
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Impact of Co-firing Coal and Biomass on Mixed Char Reactivity under Gasification Conditions Matthew B. Tilghman and Reginald E. Mitchell* Mechanical Engineering Department, Thermosciences Group, High Temperature Gasdynamics Laboratory, Stanford University, Stanford, California 94305, United States ABSTRACT: This work focuses on the gasification behaviors of the chars formed during devolatilization when co-firing coal/ biomass blends. In this experimental and modeling investigation, Wyodak coal, a sub-bituminous coal from the Powder River Basin, was the coal employed and corn stover was the biomass. Torrefied corn stover was also used in selected tests. Chars were produced by injecting pulverized fuel particles into a laboratory-scale entrained flow reactor, in which a reducing environment was established at ∼1550 K (1277 °C) and 1 atm. The injected fuel was either pure coal, pure biomass, or a premixed blend of the two. Char particles were extracted from the reactor just subsequent to devolatilization and then analyzed for specific surface areas and reactivity to CO2, H2O, and O2. The results indicated that the specific surface areas of the mixed chars were lower than the specific surface areas of the chars produced from the pure fuels. The mixed char reactivities were also outside the bounds of the reactivities of the chars produced from the pure fuels, being lower in CO2-containing environments and higher in environments containing H2O. The cause of this impact of the fraction of biomass in the coal/biomass blend on mixed char specific surface area and reactivity is a consequence of devolatilizing biomass particles altering the environment in which devolatilizing coal particles are exposed. Mixed char specific surface area and reactivity models were developed that accurately characterize the observations. The models employ a parameter N, which represents the number of coal particles that are affected by a single biomass particle during devolatilization for the fuel blend that yields the lowest mixed char specific surface area. A value of 3 was determined for the Wyodak coal/corn stover blend, and a value of 0.2 was determined for the Wyodak coal/ torrefied corn stover blend, meaning that it takes five torrefied corn stover particles to affect a single coal particle, reflecting the reduction in the volatile matter of the torrefied material.

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INTRODUCTION Coal is the dominant resource used for electric power generation worldwide. In 2010, coal-fired power plants produced nearly 20 trillion kWh of electricity, globally, providing electricity for about 81% of the world’s population.1 The use of coal is projected to increase well into the 21st century, especially in developing countries, due in large part to its abundance and low price. Some of these developing countries have already begun to feel the ill effects of using coal, such as poor air quality. Nevertheless, many of the developing countries are unwilling to slow development of coalfired power plants, as lifting a large number of people out of poverty depends upon it. Although coal use is on the decline in the United States, in 2040, coal and biomass are still expected to account for over 20% of the total energy consumption of the United States.2 In the face of anthropogenic climate change, the high carbon content of coal means that it is paramount that it be used as cleanly and efficiently as possible. Therefore, the need for a quick, economically viable method for improving the cleanliness of coal usage is necessary. Co-firing coal and biomass is an easy-to-implement, inexpensive means of improving the cleanliness of electric power generation when coal is used. There are multiple ways in which co-firing coal with biomass improves the cleanliness of coal utilization. Co-firing can reduce fossil CO2 emissions. If grown, harvested, and processed responsibly, biomass is a carbon-neutral fuel (or carbonnegative, when exhaust CO2 is sequestered). Therefore, offsetting some of the coal with biomass will reduce the © 2016 American Chemical Society

amount of fossil CO2 released per unit of heat released. Cofiring can also reduce the emission of sulfur oxides (SOx), pollutants that contribute to acid rain as well as a precursor to particulate matter. Biomass ash typically has a higher content of alkaline minerals than coal ash, which is effective at adsorbing SO2 molecules.3−5 The biomass ash can act as an in situ scrubber, sequestering the sulfur along with the ash and reducing the total SOx emission of the system. Finally, co-firing has the potential to reduce NOx emissions. The reasons for this are 2-fold. Biomass has a lower nitrogen content than coal and, therefore, offsets NOx in much the same way that it can offset fossil CO2 emissions, by simply displacing those of coal. In addition, biomass can also go beyond mere displacement, because a higher percentage of nitrogen in biomass forms amine radicals compared to coal, and these amine radicals have the potential to reduce NOx to N2.4,5 Economic analyses indicate that, in certain situations, cofiring coal and biomass can be economically feasible,6 which significantly reduces the barriers to market entry often associated with cleaner technology. However, this conclusion depends upon many variables, such as the energy density of the biomass chosen, transportation costs, and more. Another factor Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: October 20, 2015 Revised: January 22, 2016 Published: February 2, 2016 1708

DOI: 10.1021/acs.energyfuels.5b02480 Energy Fuels 2016, 30, 1708−1719

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Energy & Fuels on which the economic viability of co-firing coal and biomass depends is the ability to co-fire into existing burners or gasifiers, without the need for significant retrofitting. Concerns toward this end include how the biomass will fare with the coal feed system as well as whether the high ash content of many types of biomass will lead to prohibitive slagging and how this could be ameliorated.7,8 Co-firing coal and biomass has the potential to reduce both the cost and carbon footprint of coal gasification, thereby speeding the adoption of more advanced utilization schemes, such as integrated gasification combined cycle (IGCC). However, in addition to potential benefits, there are also potential drawbacks, such as rendering the ash of the facility unsuitable for resale,9 exacerbating fouling,7,10 or causing feeding difficulties. Because of this, the biomass levels in cofired systems are not likely to exceed about 20% of the total mass loading to maintain low levels of biomass ash in the system. This paper focuses on yet another potential complication of co-firing: the impact on char reactivity. While the heat content per unit mass of the coal/biomass blend will be an average of the two pure fuels, changes to both the heat release rate and burnout time depend upon the kinetics of the fuel blend, which may not change linearly. Being able to anticipate or avoid reactivity changes as a result of co-firing is important because reactors are designed for specific residence times and to handle specific heat release rates. Recently, there has been an increase in research of these potential nonlinear changes. Many researchers believe they detect synergies during co-gasification, where the char reactivity increases more than what a linear combination of the two fuels would suggest.11 Some even find the co-gasified char to have a higher reactivity than the pure biomass char.12 Others observe the co-fired char to have a decreased reactivity during gasification.13 There is little consensus as to the direction of, the cause of, or even the existence of these nonlinear changes. However, it is agreed that these changes are of crucial importance, because many attribute the observed changes in the syngas yield and content when blending coal and biomass, in part, to these changes in reactivity.14 Knowing how these parameters will change is crucial to an efficient, low-cost implementation of retrofit technology for co-firing coal and biomass. In the work presented, the gasification reactivities of mixed chars produced from co-firing coal/biomass mixtures are compared to the gasification reactivities of the chars produced from the pure coal and biomass components. The work is focused on providing the answer to the following key question: Can linear mixing rules be used to predict the conversion behaviors of the chars produced when co-firing coal/biomass blends employing only conversion data for chars produced from the pure coal and biomass fuels?

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Table 1. Proximate Analyses of Wyodak Coal and Corn Stover (As Received) property

Wyodak coal

corn stover

fixed carbon (wt %) volatile matter (wt %) moisture (wt %) ash (wt %)

35.06 33.06 26.30 5.58

20.0 68.9 ∼6.0 5.1

Table 2. Ultimate Analyses of Wyodak Coal and Corn Stover (Dry Basis) property

Wyodak coal

corn stover

carbon (wt %) hydrogen (wt %) oxygen (wt %) (by difference) nitrogen (wt %) total sulfur (wt %) ash (wt %)

69.8 5.65 15.6 0.94 0.43 7.57

43.9 5.62 44.7 0.36 0.069 5.43

The raw materials were ground, sieved, and classified to obtain particles in the 75−106 μm size range for testing. With particles this small, the data obtained in the reactivity tests were not influenced by the transport of reactive gases to the outer surfaces of the particles or through the pores of the particles. Besides using such small particles, the particles were scattered over the balance pan of the thermogravimetric analyser (TGA) used in the tests, rendering a thin bed through which gases must diffuse. Repeated tests with varying amounts of char indicated that, in tests performed with less than about 20 mg of char placed in the pan, normalized mass loss rates were nearly identical for all sample sizes, indicating that mass transport effects were insignificant. The data reflected only the impact of chemical kinetic effects. The size distributions measured using a Coulter Multisizer are shown in Figure 1. Note that the mean size of the Wyodak coal particles is within the expected range (the 75−106 μm size range, as sieved) but that the mean size of the corn stover particles is not. This observed behavior for the biomass particles is a consequence of the large number of cylindrically shaped, fibrous-like particles, as observed in scanning electron microscopy (SEM) images. The Wyodak coal particle size distribution is described by a Gaussian distribution function, having a mean size of 87 μm. The size distribution of the corn stover particles was observed to be non-Gaussian and was best described by a Weibull distribution function [f(x,A,B) = B/A(x/A)B − 1 B

e−(x/A) ], with a mean size of ∼58 μm. The apparent densities of the raw fuels were measured employing a tap density technique, a technique that involves placing a measured amount of fuel particles in a vertically oriented graduated cylinder and noting the total volume occupied after repeatedly tapping the outside of the cylinder until the particles are well packed. This yields the bulk density (ρb) of the packed bed of particles. Employing a void volume fraction (θv) in the range of 0.3−0.33 (an appropriate range of values for small particles in a large container15), the apparent density of the particles (ρp) in the bed was calculated from the measured bulk density [ρp = ρb/(1 − θv)]. The technique has been used in previous work.16 An apparent density of 0.95 g/cm3 was determined for the raw Wyodak coal particles, and an apparent density of 0.51 g/cm3 was determined for the raw corn stover particles. Fuel blends were prepared by mixing together two fuel types on a raw mass basis. The three primary blend ratios tested were 90:10% (coal:biomass, raw mass basis), 70:30%, and 50:50%. Additional blend ratios between 85 and 100% coal were analyzed to a lesser degree upon discovery that the specific surface area of the resulting char is most sensitive in this regime. Linear mixing rules were used to determine the apparent density of the blends from the apparent densities of the raw materials. Char particles were created by injecting the pure fuel or the fuel blend into a laboratory-scale laminar flow reactor and extracting the

EXPERIMENTAL SECTION

The coal chosen for study is Wyodak coal, a sub-bituminous, lowsulfur coal from the Powder River Basin region of Wyoming. The biomass chosen is corn stover, an agricultural waste product (stalks, husks, leaves, and cob) that is abundant in regions where corn production is high. Torrefied corn stover was also employed in the study. Proximate and ultimate analyses are shown in Tables 1 and 2, respectively, for the raw materials. Note that, with the higher fixed carbon content of the coal and the higher volatile matter content of the corn stover, the volatile matter content of the corn stover is greater than the volatile matter plus the moisture content of the coal. 1709

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Figure 1. Raw Wyodak coal and corn stover particle size distributions.

Table 3. Reactor Inlet Flow Rates and Equilibrium Composition of the Reactor Gases burner input N2 = 39.9 L/min

O2 = 6.0 L/min CH4 = 2.2 L/min equilibrium post-flame mole fractions

H2 = 7.0 L/min

XN2 = 7.46 × 10−1

XH2O = 1.63 × 10−1

XH2 = 5.04 × 10−2

XCO = 2.07 × 10−2

XCO2 = 2.04 × 10−2

XN = 2.38 × 10−5

XOH = 3.62 × 10−7

XO2 = 2.03 × 10−9

XO = 1.39 × 10−9

XCH4 = 1.19 × 10−11

particles just subsequent to devolatilization, at a residence time of nominally 30 ms at the flow rates of gaseous CH4, H2, O2, and N2 employed. The laminar flow reactor used to obtain fuel chars was designed to have low fuel loadings, so that the particles are not affected by particle−particle interactions and experience the same ambient. As is shown in this work, however, the volatile gases do have the ability to alter nearby temperatures, and if devolatilization times vary considerably within the fuel, this zone can extend to other particles. Fuel loadings of this reactor were kept fairly constant at approximately 20 g of coal/m3 of gas. It was hard to guarantee perfect consistency, because the fuel blends sometimes fell out of the feeder in clumps, as a result of the agglomeration of the biomass particles. Repeated tests at slightly different loadings yielded similar reactivity and morphology results, suggesting that the range of fuel loadings experienced in this reactor are insignificant in this study. The laminar flow facility and experimental procedure have been described in previous work.16 Shown in Table 3 are the flow rates of the gases fired to the burner of the laminar flow reactor and the calculated equilibrium mole fractions of the species in the burned gas (the composition of the gas in which the fuel particles devolatilize), assuming a constant pressure/constant enthalpy combustion event. The gas conditions established in the flow reactor were designed to be reducing conditions: essentially no O2 or O exists in the gaseous reactor environment, and their mole fractions are in the parts per billion range. Measured gas temperatures along the centerline of the reactor where particles were injected ranged from 1560 K (1287 °C) near the start of corn stover devolatilization to 1545 K (1272 °C) near the end of Wyodak coal devolatilization. Temperatures were lower than the calculated adiabatic flame temperature (1666 K or 1393 °C), owing to the small flow of room-temperature nitrogen needed to advect the particles into the flow along the reactor centerline. Selected amounts of coal and biomass particles and selected blends of the two were injected at the base of the reactor, along its centerline. The particles passed through the flame established on the burner of the flow reactor and flowed upward in the reactor, heating and devolatilizing in the process. Particle heating rates were greater than 104 K/s, similar to the heating rates that devolatilizing coal particles experience in real commercial, entrained flow gasifiers. Particles were extracted from the reactor using a water-cooled helium-quench solids sampling probe. The probe was positioned at a height of 1.5 in. above

the burner surface, a height at which devolatilization of the coal and biomass particles were complete. Once the particles entered the solids sampling probe, they were promptly quenched with helium, to prevent further reaction. The char particles were collected on glass-fiber filter paper, dried, weighed, and then stored in an oxygen-free environment until used in experiments. The entire amount of feed material placed in the feeder (typically 1−2 g of coal, biomass, or blend of the two) was fed to the flow reactor. The amount of char collected in comparison to the amount of coal or biomass fed yielded the mass loss upon devolatilization. Results indicated that, during devolatilization, the Wyodak coal loss 57% of its mass and the corn stover loss 86% of its mass. These values are consistent with the loss of volatile matter and moisture, as determined from the proximate analysis of each fuel. Increasing volatile yields with the devolatilization temperature is evident with the corn stover. Various properties of the resulting chars were measured, including their apparent densities and particle size distributions in the same manner as with the raw materials. Specific surface areas of the chars were characterized by Brunauer−Emmett−Teller (BET) tests, performed in a pressurized TGA at 10 atm, employing CO2 as the adsorption gas. Char reactivities were determined from mass loss measurements taken in the TGA under a variety of temperatures (from 700 to 1000 °C) and gas compositions (mixtures that contain H2O, H2, CO2, CO, and N2) to assess how co-firing impacts char particle conversion behavior in gasifying environments. In all TGA tests, sample sizes were 20 mg or less, sufficiently small to render mass transport effects insignificant. The measured variations in the normalized mass loss rates were due solely to the effects of chemical reactions. Data obtained with the chars produced from the pure Wyodak coal and pure corn stover have been used to determine kinetic parameters that describe rate coefficients in a reaction mechanism that contains key reaction pathways when carbon is exposed to CO2, H2O, and O2.17 The mechanism accurately characterizes the data obtained in all of the TGA environments in which mass loss measurements were made. For the char of each pure fuel, the mechanism correctly describes the increase in reactivity with increasing CO2 and H2O concentrations and increasing temperature. It also correctly describes the inhibiting effects of CO and H2 on char reaction rates when these species are present in the gaseous environment. 1710

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Figure 2. Wyodak coal char and corn stover char particle size distributions.

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RESULTS AND DISCUSSION Initial Char Properties. The measured size distributions of the chars are shown in Figure 2. On the basis of a Weibull distribution function, the mean size of the Wyodak coal char particles was ∼74 μm, about 14 μm smaller than that of the raw Wyodak coal particles. There was little swelling of the coal during devolatilization, but there seemed to be a fair amount of fragmentation. The size distribution of the corn stover char particles is described by a log-normal distribution function having a mean of 42 μm (the peak of a log-normal distribution is not the mean value), 16 μm less than the mean size of the raw corn stover particles. During devolatilization, the corn stover particles exhibited essentially no swelling. In comparison to the size distribution of the raw corn stover particles, there was significant fragmentation of the larger corn stover particles during heating and volatile release. Employing the tap density technique, the apparent densities for the chars produced from the pure coal and biomass components were found to be 0.56 g/cm3 for the Wyodak coal char particles and 0.26 g/cm3 for the corn stover char particles. The coal and biomass char particles had similar specific surface areas. Repeated adsorption measurements on the coal char yielded an average surface area of approximately 375 m2/g, with a test as high as 475 m2/g. Repeated measurements on the corn stover char showed an average surface area of approximately 410 m2/g, with a test as high as 450 m2/g. Mixed Char Specific Surface Area. Blend ratios for which specific surface areas were determined include 50:50% (coal/ biomass, raw basis), 70:30%, 85:15%, 90:10%, 95:5%, 98:2%, and 99:1%. Results reveal that the chars produced by co-firing the fuel blends possess a specific surface area that cannot be described by a weighted average of the specific surface areas of the pure coal and biomass chars; i.e., linear mixing rules do not apply. The majority of the blends tested yielded specific surface areas lower than any test on either pure fuel. As illustrated in Figure 3, as little as 1% (by weight) corn stover added to the coal appears to reduce the initial specific surface area of the mixed char, Sg,0. This trend continues until about 5−10% (by weight) corn stover, at which point the mixed char specific surface area begins to return to the value for pure corn stover char as the fraction of the biomass in the fuel blend increases. This trend was also observed with coal/torrefied corn stover blends.

Figure 3. Specific surface areas of the mixed chars produced from Wyodak coal/corn stover blends.

Because the only difference prior to the BET tests was the devolatilization process, the observed behavior is attributed to the fact that, when injected as a fuel blend, the two solid fuels do not devolatilize independently. Biomass particles devolatilize more quickly than coal particles when subjected to similar conditions,18−22 and as a consequence, the conditions in which the coal particles devolatilize are altered. Such an effect has been hypothesized and observed by other researchers.23−25 However, in those studies, devolatilization occurred in oxidizing conditions and the presence of the biomass particles resulted in an increase in the temperature as a result of oxidation of the biomass volatiles. Because particles undergo devolatilization in a reducing environment in this study, the volatiles released from the biomass particles react endothermically, resulting in a decrease in the temperatures experienced by the devolatilizing coal particles. Studies have shown that the initial specific surface area of a char particle varies with its temperature and heating rate during devolatilization;26,27 therefore, it follows that cofiring impacts char specific surface area and, consequently, char reactivity. In our approach, to predict the specific surface areas of coal/ biomass blends, it is assumed that the ability of biomass to affect coal devolatilization occurs on a per particle basis. The first biomass particles added to a coal stream have the ability to affect a certain number of coal particles, say N. While biomass particles are still relatively sparse, each additional biomass particle still affects N coal particles. This corresponds to a decrease in the specific surface area of the mixed char as the number of affected coal particles increases, which have a lower 1711

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Figure 4. Depiction of how each incremental biomass particle may affect a relatively constant number of coal particles for “biomass-lean” fuels (frames 1 and 2) but not affect any additional coal particles for high-biomass fuels (frame 3).

for the corn stover particles). Particles were assumed to be spherical. As noted, whenever there is less than 10%, by weight, corn stover in the raw fuel blend, there are always more coal particles than biomass particles. Frame 1 of Figure 4 is indicative of situations with less than about 2 wt % corn stover in the fuel blend, when there are considerably more coal particles than biomass particles, and frame 3 is indicative of situations with 15% or more biomass in the blend, when there are about the same or more biomass particles in the blend. In our mixed char specific surface area model, it is assumed that the specific surface area of the mixed char has contributions from the specific surface areas of the chars of the pure coal particles, the pure biomass particles, and the “affected” coal particles and that the fuel blend having the most affected coal char particles (i.e., the blend having the lowest specific surface area) can be used to estimate N, the number of coal particles affected by a single biomass particle. For blends with less biomass than this, each additional biomass particle affects N additional coal particles and, thus, reduces the specific surface area of the char mixture. For blends with more biomass than this, each additional biomass particle merely displaces an affected coal particle and the specific surface area of the mixed char approaches that of the pure biomass char. Note that there are no “affected” biomass particles, because they devolatilize before the coal particles. In reality, N would not remain constant but rather decrease as biomass is added. However, this piecewise approach is deemed to be sufficient. As noted by the specific surface area data shown in Figure 3, the fuel blend yielding the lowest specific surface area is around 5% biomass. On the basis of the information presented in Table 4, this corresponds to a mixed char that contains approximately 1.7%, by weight, corn stover char after devolatilization. Therefore, the affected coal char accounts for about 98.3% of the specific surface area of the mixed char at this minimum location. From Figure 3, the specific surface area of the affected coal char particles is approximately 150 m2/g. Also, the particle number ratio at this raw fuel blend mass ratio is 3.02 coal particles per biomass particle, rendering N = 3 for these conditions. The total mass of char (mtot) in the mixed char model is assumed to consist of the mass of the affected coal char particles (maffected WYchar) and the masses of the unaffected coal (mWYchar) and corn stover char (mCSchar) particles. Linear mixing rules are used to determine the specific surface area of the mixed char from the specific surface area (Sg) of each type of char and its mass fraction (mf). Thus

specific surface area. However, as the biomass content of the fuel blend increases, each additional biomass particle begins to affect some number of coal particles less than N. At some point, the number of coal particles affected by an additional biomass particle becomes small enough that the addition of more biomass particles raises the specific surface area of the mixed char, approaching that of the pure biomass char. This concept is illustrated in Figure 4. In frame 1, the first biomass particle introduced affects the devolatilization conditions of N particles; in this sketch, N is approximately equal to 4. The fuel blend with significantly more biomass particles but still “biomass-lean” is shown in frame 2. Although the affected zones of each biomass particle overlap slightly, each new biomass particle still affects approximately N coal particles. Shown in frame 3 is a fuel blend that has sufficient biomass that the devolatilization condition of nearly every coal particle is affected. Beyond this point, the addition of more biomass into the fuel blend does not increase the number of affected coal particles but instead only displaces them with biomass particles, thus returning the mixed char specific surface area back toward the pure corn stover value. It should be noted that the point at which this transition occurs is dependent upon not only the relative number density of corn stover and Wyodak coal particles but also the number of coal particles that each biomass particle has the capability to affect. The number of Wyodak coal particles per corn stover particle and the weight percent corn stover in the mixed char are shown in Table 4 for specified percentages of corn stover in the raw fuel blend. The information tabulated is based on the measured percentage mass loss during devolatilization of the pure fuels (57% for the Wyodak coal and 86% for the corn stover) and the measured mean particle sizes and apparent densities of the pure fuel particles (87 μm and 0.95 g/cm3, respectively, for the Wyodak coal particles and 58 μm and 0.51 g/cm3, respectively, Table 4. Wyodak Coal/Corn Stover Blends percent biomass in raw fuel blend (wt %)

number of coal particles per biomass particle

percent biomass in mixed char (wt %)

1 2 5 10 15 30 50 75 85

15.7 7.79 3.02 1.43 0.90 0.37 0.16 ∼0.05 ∼0.03

0.33 0.66 1.7 3.5 5.4 12.2 24.6 49.4 64.9 1712

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Energy & Fuels Sg,mixed char = mfaffected WYchar Sg,affected WYchar + mfWYchar Sg,WYchar + mfCSchar Sg,CSchar

if (1)

where the subscript WYchar denotes the unaffected coal char. In terms of the number of coal particles N that a biomass particle may affect as well as the apparent densities (ρ) and mean diameters (D) of the corn stover and Wyodak coal char particles, the mass fraction of corn stover particles that results in all coal particles being affected (mf CSchar * ) is given by

mfWYchar = 0

(2)

and the ratio of mass of corn stover to mass of Wyodak at this condition is given by

(3)

For a specified fuel blend (mcoal/mbiomass), the mass of coal char (affected plus unaffected particles) per mass of biomass char can be determined from the fractional mass loss during devolatilization, f vol. Thus, for the Wyodak coal/corn stover blend ⎛ mtotal WYchar ⎞ ⎛ mWY ⎞ (1 − fvol,WYcoal ) ⎜ ⎟=⎜ ⎟ ⎝ mCSchar ⎠ ⎝ mCS ⎠ (1 − fvol,CS )

(4)

The mass fraction of corn stover char in the mixed char for the specified fuel blend is therefore given by ⎛ ⎛m ⎞ ⎛ m ⎞ (1 − fvol,WYcoal ) ⎞ ⎟ mfCSchar = ⎜ CSchar ⎟ = 1/⎜⎜1 + ⎜ WY ⎟ ⎟ (1 − ) m f ⎝ mtot ⎠ ⎝ ⎠ CS ⎝ ⎠ vol,CS (5)

and the mass fraction of Wyodak char (affected plus unaffected particles) is given by ⎛ ⎛ mtotal WYchar ⎞ ⎛ m ⎞ (1 − fvol,WYcoal ) ⎞ ⎟ ⎜ ⎟ = 1 − 1/⎜⎜1 + ⎜ WY ⎟ ⎝ mtot ⎠ ⎝ mCS ⎠ (1 − fvol,CS ) ⎟⎠ ⎝ (6)

When the mass fraction of corn stover evaluated via eq 5 is below that dictated by eq 2, the mixture will have unaffected Wyodak coal particles because there were not enough corn stover particles to impact all of the coal particles. Thus m * if mfCSchar = CSchar ≤ mf CSchar : mtot m mfaffected WYchar = affected WYchar = mtot ⎛ ⎞ mCSchar mCSchar /⎜ ⎟ mtot ⎝ mWYchar ⎠ (7) all coal affected mfWYchar =

mWYchar m m = total WYchar − affected WYchar mtot mtot mtot

(9) (10)

Equations 1−10 represent the mixed char specific surface area model. The dashed line shown in Figure 3 was calculated using specific surface areas measured for the chars produced from the pure Wyodak coal and corn stover and the char produced from the 90:5% Wyodak coal/corn stover blend for the specific surface area of the affected coal char. As evidenced, the model adequately predicts the specific surface area of chars produced when co-firing Wyodak coal and corn stover particles into reducing environments. To implement the model, the mass fractions of the chars of the corn stover and affected and unaffected coal particles were calculated employing the measured mass loss during devolatilization, the measured apparent densities and mean particle sizes of the Wyodak coal and corn stover char particles, and N = 3, the value associated with 5% corn stover in the fuel blend. Note that this value for N is not meant to be universal to all types of fuel blends and devolatilization conditions, because these have the potential to change the number of coal particles impacted by a single devolatilizing biomass particle. For validation of the model, Wyodak coal blended with torrefied corn stover was tested. A sample of the size-classified corn stover particles was heat-treated in a N2-flushed oven at 200 °C until mass loss had ceased. Calculations indicate that 58% of the moisture and volatile matter contents of the corn stover was loss during heat treatment. The apparent density of the resulting torrefied corn stover particles was 0.30 g/cm3, and the average diameter was 46 μm. Because the torrefied particles are still a relatively high-volatile fuel (54% volatile matter, by mass) and because they will still devolatilize sooner than the coal particles, it follows that they should still be able to impact the devolatilization conditions of the coal particles. Presumably, because the torrefied biomass has a lower volatile content, each particle would likely be able to impact fewer coal particles. This means that N would decrease and the minimum specific surface area should occur at a higher particle fraction of biomass particles for torrefied corn stover than for raw corn stover. This is confirmed by specific surface area tests on char derived from the Wyodak/torrefied corn stover fuel blends, at mass percents of 90:10% (coal:biomass) and 50:50%. The torrefied corn stover particles did indeed have the potential to alter the coal devolatilization conditions, because both blends tested yielded specific surface areas lower than the two pure fuels (270 m2/g for chars produced with the 90:10% coal:biomass blend and 249 m2/g for the 50:50% blend). However, the specific surface area of the mixed char was reduced by a far lesser extent upon the addition of 10% torrefied corn stover, by mass. As noted, this is likely due to the fact that the biomass particles now have significantly less volatile matter and, consequently, cannot affect the coal particles as easily. It therefore takes significantly more torrefied corn stover particles before all Wyodak coal particles have been affected. Using the same model but with properties for the torrefied corn stover particles, the data were fit to obtain a value of N = 0.2, meaning that it now takes five biomass particles to affect a single coal particle. Such a change in the value for N is not surprising, given the significant reduction in the volatile matter of the torrefied biomass. This shows that

⎛ mCSchar ⎞ ρCS DCS3 * = ≡ mf CSchar ⎜ ⎟ ρCS DCS3 + NρWY D WY 3 ⎝ mtot ⎠all coal affected

⎛ mCSchar ⎞ ρCS DCS3 = ⎜ ⎟ NρWY D WY 3 ⎝ mWYchar ⎠all coal affected

mCSchar * > mf CSchar : mtot m mfaffected WYchar = total WYchar mtot mfCSchar =

(8)

When the fuel blend mass fraction of corn stover is above that dictated by eq 2, all coal particles will be affected; there are no unaffected coal char particles. 1713

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Figure 5. Normalized mass loss rates for co-fired Wyodak coal/corn stover fuel blends when exposed to various gasifying environments containing CO2, CO, H2O, H2, and N2. Data points are connected via lines for readability; the lines do not represent model fits.

To further test this theory, we used AspenPlus28 to estimate the amount of volatile matter necessary to cause such a decrease in ambient temperature. Conditions were modeled as an adiabatic flow reactor with two input streams at a specified temperature, pressure, and composition and a single exit stream in thermodynamic equilibrium at the adiabatic temperature. The primary input stream was set to the temperature and composition of the post-flame gases in our entrained laminar flow reactor [an equilibrium mixture of H2, CO, H2O, and CO2 in N2 at 1550 K (1277 °C) and 1 atm]. The second input stream was set to consist of a gas mixture containing equal moles of H2, CH4, C2H4, C6H6, and C6H6O (species taken to be representative of coal and biomass devolatilization species), also at 1550 K and 1 atm. The relative flow rates of the two streams were varied in our simulations, which indicated that, when the molar flow rate of the volatile gas stream is 2% of the total molar flow, the homogeneous reaction lowers the temperature of the product stream by approximately 200 K. Calculations based on the solids loading of our laminar flow reactor indicate that the corn stover volatiles could constitute as much as 4% (by mass) of the total gas in the vicinity of the

torrefying the biomass does decrease its ability to affect the coal at low biomass mass fractions but not necessarily so at high biomass mass fractions. To test our claim that the reductions in specific surface area of the coal char caused by co-firing are consequences of decreases in the temperature and heating rate (brought on by the endothermic reaction between the biomass volatiles and ambient gas), pure Wyodak coal char particles were produced in the entrained flow reactor when the maximum gas temperature in the reactor was approximately 1420 K (1147 °C), about 130 K lower than in the tests discussed above. The specific surface area of the char extracted from the reactor was determined to be 230 m2/g, about a 38% reduction in the specific surface area of the Wyodak coal char produced in the original flow reactor environment. This reduction in specific surface area is approximately 65% of the observed decrease when co-firing. While this result does not rule out the possibility of other causes contributing to the reduced specific surface area when co-firing, it does lend credence to the endothermic reactions associated with biomass volatile matter reacting with ambient gases as a potential source of this change. 1714

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Wyodak coal and corn stover particles. The trend is also exhibited in an inhibiting environment: 50% CO2 and 50% CO (lower left panel of Figure 5). In this environment at 900 and 1000 °C, both of the mixed chars exhibit similar reactivities, although, again, both are lower than the reactivities of the chars of the two pure fuels. Mixed char reactivities to steam were also studied. Conditions tested were nominally 20 and 10% H2O with N2 as a diluent at temperatures from 700 to 900 °C at atmospheric pressure. Also tested was mixed char reactivity to steam in the presence of H2, a steam gasification inhibitor, which consisted of exposing the chars to a gaseous environment containing 10% H2O and 2.7% H2, with the balance N2. For the chars of each of the pure fuels and for each mixed char, the TGA data indicated that, for a fixed H2O concentration, char reactivity increases with an increasing temperature and, for a fixed temperature, char reactivity increases with an increasing H2O concentration. In contrast to mixed char reactivity to CO2, mixed char reactivity to H2O either remained relatively unchanged in comparison to the reactivities of the two pure fuels or increased slightly at all of the temperatures investigated. This is evidenced in the top and middle right panels of Figure 5, in which data are shown for mixed chars exposed to both the 10 and 20% H2O environments. Note that the reactivity for pure corn stover char to H2O is significantly lower than that of pure Wyodak coal char. This was unexpected, given the similar reactivities to CO2. One potential explanation is the hypothesis that adsorbed hydrogen atoms can penetrate the loose char structure instead of staying on the surface and cause a change in char structure that corresponds to a drastically reduced reactivity. This has been observed directly by other researchers.30 The H2-containing environments (i.e., the inhibitory environments) indicated a more significant increase in reactivity for the co-fired fuel blend chars compared to the pure fuels (see the lower right panel of Figure 5). This trend appears to be diminished as the temperature increases, although the impact of inhibitory gases in general tends to decrease as the temperature increases. Predicting Mixed Char Reactivity. The underlying reasons for why the mixed chars exhibit reactivities lower than the reactivities of the chars produced from the pure fuels in environments containing CO2 and higher in environments containing H2O are unknown. Ash catalytic effects could be among the possible reasons; stronger inhibition effects of H2 and CO on coal char surfaces could also contribute. It is even possible that changes in char morphology cause changes in the rates of adsorption of the different reactive gases and rates of desorption of adsorbed species from carbonaceous surfaces; for example, more or less edge plane carbons may have been exposed in the char as a result of changes in devolatilization conditions. Regardless of the reason, the fact that the reactivities of the mixed chars lie outside the range of the reactivities of the chars produced from the pure fuels negates the use of a mixed char reactivity model that employs linear mixing rules and only the pure-fuel kinetic parameters. We investigated a mixed char reactivity model similar to the mixed char specific surface area model discussed above.31 It requires knowledge of the reactivity of the mixed char of coal particles that were affected by the devolatilizing biomass particles and is expressed as follows:

devolatilizing particles for the 50:50% coal:biomass blend, 2% for the 70:30% blend, and 1% for the 90:10% blend. This puts these results in line with the cooler flame tested. Because of the magnitude of the temperature change that the corn stover volatiles could potentially affect and the fact that temperature of devolatilization has been shown to correlate with the BET specific surface area of the char,26,29 we believe this effect to be the main cause of the observed decrease in the specific surface area of the mixed char when co-firing coal and biomass. It logically follows that, as the total particle loading decreases (or if a biomass fuel with less volatile content is used), the volatile matter to reduce the ambient gas ratio would decrease and, thus, the temperature reduction would decrease. The result could mean a less affected coal char. Mixed Char Reactivity. In addition to BET specific surface area measurements, reactivities of the mixed chars to several reactive gases were also measured. The tests were performed in a TGA at a constant temperature. The measured thermograms (mass versus time curves) were converted into normalized mass loss rate (1/mdm/dt) profiles and plotted versus conversion. Char reactivities were investigated under both dry gasification (CO2) and steam gasification (H2O) conditions. In selected cases, the respective product gases (CO and H2) were added to the input stream, because these gases are known to inhibit gasification reactivities. Chars produced from the pure fuels and the char produced from the blends were all subjected to the same test conditions. As noted previously, during reactivity tests in the TGA, char particles were spread thinly across the TGA balance pan. Not only did this help to minimize mass transport effects, but it also minimized any catalytic effects of biomass mineral matter on coal char during the char gasification process. It is unlikely that any mineral vapors released from the biomass char came into contact with the coal char during the reactivity tests, owing to the convective flow in the TGA that rapidly swept away product gases. The activation energies determined for the key adsorption reactions for the mixed char investigated are in the ranges that suggest that catalytic effects are not significant. The primary fuel blends tested in environments containing CO2 were prepared from Wyodak coal/corn stover mixtures containing 10, 30, and 50% biomass, by weight. For the chars of each of the pure fuels and for each mixed char investigated, the TGA data indicated that, for a fixed CO2 concentration, char reactivity increases with an increasing temperature and, for a fixed temperature, char reactivity increases with an increasing CO2 concentration. In each CO2-containing environment, the char produced from the pure corn stover had a higher reactivity than the char produced from the pure Wyodak coal, except at low temperatures in environments containing CO, where the char reactivities were comparable. As noted in the left panels of Figure 5, all fuel blends exhibited a decrease in reactivity to CO2 relative to the pure fuels, to varying degrees. This was the case for both pure CO2 (top left panel) and CO2 diluted in N2 (middle left panel). For pure CO2, the lowest reactivity was observed for the mixed char produced from the 90:10% coal/ biomass blend, which also registered the lowest specific surface area. The mixed char produced from the 70:30% blend had the next lowest reactivity, followed by the mixed char produced from the 50:50% blend. This trend was exhibited over nearly all temperatures tested in CO2 environments. At 1000 °C in pure CO2, the 70:30% co-fired char exhibited slightly higher reactivity than the 50:50% char, although both still exhibited lower reactivity than the reactivities of the chars of the pure 1715

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Figure 6. Comparisons of measured and calculated normalized mass loss rates for mixed chars produced from Wyodak coal/corn stover blends in selected environments containing CO2. Dashed lines represent model predictions.

Figure 7. Comparisons of measured and calculated normalized mass loss rates for mixed chars produced from Wyodak coal/corn stover blends in selected environments containing H2O. Dashed lines represent model predictions.

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Figure 8. Comparisons of measured and calculated normalized mass loss rates for mixed chars produced from Wyodak coal/corn stover blends in environments containing 6 vol % O2 (the balance N2) at 1 atm and selected temperatures.

⎛ 1 dm ⎞ ⎜ ⎟ = (SgR iC)mixed char = mfaffected WYchar ⎝ m dt ⎠mixed char

trend observed in the CO2 reactivity tests, i.e., that the mixed fuel chars tend to show lower reactivity than the pure fuel chars, and as temperature is increased, the trend begins to mimic that observed in the steam reactivity tests, i.e., that the mixed fuel chars tend to show higher reactivity than the reactivities of the pure fuel chars. This transition is likely to be caused by a regime change in the reaction pathways. In O2 environments at low temperatures, CO2 is the primary reaction product and its formation rate is limited by the oxygen adsorption on carbonaceous surfaces. This implies that the decrease in reactivity for the affected Wyodak coal char particles is due directly to the decrease in the specific surface area. However, at intermediate temperatures, CO is an important reaction product and the rate of the CO desorption reaction becomes important in limiting the char consumption rate. If the changes to the Wyodak coal char as a result of the altered devolatilization conditions affect the rate of this desorption reaction, the observed changes as a result of co-firing for both the CO2 reactivity tests and the H2O reactivity tests (and also the transition in the O2 reactivity tests) can be explained. The data and kinetic parameters determined for the chars of the pure fuels exposed to oxygen have been presented previously,17 and the kinetic parameters derived for the mixed char produced from the 90:10% Wyodak coal/corn stover blend are available in ref 31. As evidenced in this document, the oxidation mechanism and kinetic parameters yield predicted mass loss rates that accurately characterized the measured rates in all of the environments employed in the TGA tests undertaken. In implementing the mixed char models, the specific surface area and reactivity of the mixed char produced from the 90:10% Wyodak coal/corn stover blend served as a proxy for these properties of the affected Wyodak coal char. Comparisons between predicted and experimentally determined mass loss rates of chars produced from coal/biomass blends containing 30 and 50% corn stover, by mass, are shown in Figure 8. As evidenced, the agreement is good, further validating the mixed char reactivity model. In this study, char particles for testing were obtained by devolatilizing the fuels in a high-temperature, reducing environment that contained essentially no oxygen. In real entrained flow gasifiers, devolatilization occurs in an oxidizing environment, because oxygen is added to burn the volatiles that are released, providing energy to raise the temperatures of the char particles and drive the endothermic gasification reactions once the oxygen is consumed, a process referred to as

(SgR iC)affected WYchar + mfWYchar (SgR iC)WYchar + mfCSchar (SgR iC)CSchar

(11)

Here, RiC is the intrinsic chemical reactivity of the carbonaceous material, in g m−2 s−1. The chemical reaction mechanism determined from the TGA data along with kinetic parameters for the chars of the pure Wyodak and corn stover fuels is presented in the publication by Tilghman and Mitchell.17 Kinetic parameters were also derived for the mixed char derived from a 90:10% Wyodak coal/corn stover blend,31 and the reactivity of this mixed char served as a proxy for the reactivity of the affected Wyodak coal char. The mass fractions were determined via the equations presented above for each fuel blend but with N = 2 to better reflect the number of coal particles per biomass particle for the 90:10% blend. Comparisons between predictions and experimentally determined mixed char reactivities are shown in Figures 6 and 7 for mixed chars produced from 50:50% and 70:30% Wyodak coal/corn stover blends. As noted in Figure 6, the agreement is deemed to be quite good for mixed chars exposed to the environments containing CO2, CO, and N2. The agreement is also deemed to be quite good for mixed chars exposed to environments containing H2O, H2, and N2, as evidenced in Figure 7. The calculated behaviors, shown by the dashed lines, in reactivity late in conversion in the H2O environments correspond to when the altered coal char has mostly gasified, and the reactivity is defined by the slower reacting corn stover char. The variations in reactivity with the temperature and reactive gas concentrations are accurately captured by the model. The good agreement between predictions and measurements serves to validate the mixed char reactivity model in gasification environments. To further test the mixed char reactivity model, we exposed the chars produced from the pure Wyodak coal and corn stover and the chars produced from selected coal:biomass blends (90:10%, 70:30%, and 50:50%, by mass) to an oxidizing environment containing 6% O2, by volume, at selected temperatures that rendered kinetic-limited conversion rates. Comparisons of the mass loss rates obtained with the mixed chars to those determined with the chars produced from the pure Wyodak coal and corn stover chars indicate that, at the lower test temperatures, the mixed char reactivities follow the 1717

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ACKNOWLEDGMENTS Matthew B. Tilghman and Reginald E. Mitchell acknowledge the support of the United States Department of Energy (U.S. DOE), managed through the National Energy Technology Laboratory (NETL, DE-FC26-10FE0005372, Arun Bose, Project Manager). The authors also thank undergraduate student Brendan Freund for help collecting some of the experimental data.

autothermal gasification. During autothermal gasification, when co-firing coal/biomass blends, owing to the earlier release of volatile matter from the biomass particles compared to the coal particles, some (or all, depending upon the blend ratio) of the devolatilizing coal particles will experience higher temperatures than they would in the absence of the devolatilizing biomass particles. There will be “affected” coal char particles having specific surface areas and reactivities that differ from those of the pure coal chars. There will be a coal/biomass blend that yields the most affected char, as evidenced by the mixed char having the specific surface area that deviates most from linear mixing rule predictions employing pure fuel properties. A value of N can be determined for this coal/biomass blend employing the mean particle size and apparent density of the pure chars produced under the selected autothermal gasification conditions; the reactivity of the mixed char of this blend can be measured. With these properties, we expect for the mixed char specific surface area and reactivity models presented in this paper to describe accurately char conversion rates for all mixed chars produced under the selected autothermal gasification conditions.

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AUTHOR INFORMATION

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REFERENCES

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Co-firing coal and biomass particles into gasifying environments results in coal particles being affected during devolatilization, a consequence of gas-phase reactions between biomass volatiles and reactor gases altering the gaseous conditions that the devolatilizing coal particles experience. Owing to this impact, the specific surface areas and reactivities of the mixed chars are not bound by the specific surface areas and reactivities of the chars produced from the pure fuels. This negates the use of linear mixing rules to predict the reactivities of the mixed chars using only the properties of the pure fuels and their relative weight fractions in the fuel blend. However, a mixed char model has been developed that accurately describes mixed char specific surface area and reactivity. It requires specifying the specific surface area and reactivity of the chars produced from the pure fuels as well as these properties for the mixed char produced from the coal/biomass blend that yields about three coal particles per biomass particle, consistent with a blend containing about 5% corn stover, by mass. This coal/ biomass blend produced the mixed char having the measured specific surface area that deviated most from the predicted specific surface area had linear mixing rules applied using only specific surface areas of the pure fuel chars. As indicated with the Wyodak coal/torrefied corn stover blend, the number of coal particles affected during devolatilization by a single biomass particle decreases as the volatile matter content of the biomass employed in the blend decreases. The torrefied corn stover used in the blend had about 22% less volatile matter than the raw corn stover, and each torrefied corn stover particle affected the devolatilization behavior of roughly 0.2 coal particles.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1718

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