CO2 Gasification Reactivity of Char from High-Ash Biomass

Biomass char produced from pyrolysis processes is of great interest to be utilized as renewable solid fuels or materials. Forest byproducts and agricultural wastes are low-cost and sustainable biomass feedstocks. These biomasses generally contain high amounts of ash-forming elements, generally leading to high char reactivity. This study elaborates in detail how chemical and physical properties affect CO2 gasification rates of high-ash biomass char, and it also targets the interactions between these properties. Char produced from pine bark, forest residue, and corncobs (particle size 4–30 mm) were included, and all contained different relative compositions of ash-forming elements. Acid leaching was applied to further investigate the influence of inorganic elements in these biomasses. The char properties relevant to the gasification rate were analyzed, that is, elemental composition, specific surface area, and carbon structure. Gasification rates were measured at an isothermal condition of 800 °C with 20% (vol.) of CO2 in N2. The results showed that the inorganic content, particularly K, had a stronger effect on gasification reactivity than specific surface area and aromatic cluster size of the char. At the gasification condition utilized in this study, K could volatilize and mobilize through the char surface, resulting in high gasification reactivity. Meanwhile, the mobilization of Ca did not occur at the low temperature applied, thus resulting in its low catalytic effect. This implies that the dispersion of these inorganic elements through char particles is an important reason behind their catalytic activity. Upon leaching by diluted acetic acid, the K content of these biomasses substantially decreased, while most of the Ca remained in the biomasses. With a low K content in leached biomass char, char reactivity was determined by the active carbon surface area.

S2 S1. Adsorption isotherms. Figure S1 depicts the adsorption isotherms of the char samples produced from different pyrolysis temperatures and different biomasses.

Figure S1. Adsorption isotherms of char samples from (a) different temperatures and (b) different biomasses.
S2. Detail of experiment and calculation of gasification rate S2.1. The repeatability of the TG data. Figure S2 displays raw TG data obtained from 5 repetitions. It can be observed that the result contains great deviation due to the inhomogeneity of the feedstocks.

S2.2. Elimination of devolatilization from TG data.
According to the experiment measured under pure N 2 , the devolatilization part is fitted by using the first-order kinetic model where, is devolatilization conversion, . The terms and = ( -)/( 0 -) represent rate constant and time, respectively. Figure S3 shows normalized mass over time of devolatilization reaction evaluated by the model.

Figure S3. Mass loss rates of devolatilization.
The devolatilization part is then removed from the TG data of the measurement under CO 2 by using the following expressions = -(S2) Figure S4 shows an example of the thermogravimetric curve, which display a mass loss due to devolatilization and gasification.

Figure S5. Diffractograms from XRD analysis of char produced from (a) different pyrolysis temperatures and (b) biomasses.
S6. Deconvolution of Raman spectra. Three Gaussian profiles were introduced in the spectra.
The peak positions of the profiles were set at the position of G (1590 cm -1 ), D (1350 cm -1 ), and V (1450 cm -1 ) of the spectra with 5 cm -1 of allowable tolerance. The peak fittings progressed from the initial width at 0 cm -1 until the R-squared of 0.995 was achieved. Once the results were obtained, the height of assigned bands at G and D positions were checked to make sure they are at around the intensities of the spectra. Figure S6 shows an example of Raman deconvolution of B700. The full width at the half maximum (FWHM) was obtained by measuring the width at the half-height of the Gaussian bands. Figure S6. Deconvolution of the Raman spectrum from charcoal B700.

S6
S7. Annealing effect during gasification. An additional experiment was carried out to describe the thermal annealing effect in the gasification results. This part has been done by using the same TG instrument, i.e., TGA8000 coupled with gas mixing device GMD8000 from PerkinElmer Inc. The heating part is duplicated from the experiment described in Section 2.3.
Instead of switching the gas atmosphere from N 2 to 20% of CO 2 directly after the temperature reached 800 °C, the sample was held at the temperature under pure N 2 for 1 h before the gas switch to 20% of CO 2 .
LB700, LR700, and LC700 were selected in this investigation due to their minimal effect of inorganic elements on the reaction rate. Figure S7 represent the gasification rate as a function of conversion. The figure was plotted for comparison between the rates obtained from the original procedure and this additional procedure. LR700 did not show the decrease in the rate caused by thermal annealing, while LB700 still show a small effect at low conversion.
This could be attributed to the higher degree of thermal annealing that took place in LB700, similar to that shown in Figure 9. For LC700, thermal annealing did not appear and the gasification rate progress with conversion according to the pore development, which follows the random-pore model S1 .