Mass yield

Mass yield after torrefaction is defined according to Eq. (37) as the ratio of the amount of mass left after pretreatment to the original mass of the raw biomass.

Mass yield TB *100

RB

m m

 ₍₃₇₎

where mTB and mRB represent the mass of the torrefied biomass and raw biomass

respectively. Variation in mass yield under two environments, CO2(C) and N2 (N) is

shown below in Fig. 20 for mesquite (M) and juniper (J) on a dry ash free (DAF) basis. It can be seen that the mass loss was comparatively higher when using CO2 as the

and hemicellulose content when compared to juniper. Juniper with lower moisture and hemicellulose content showed similar mass losses under two torrefaction conditions at temperatures below 280˚C. At higher temperatures, the mass loss was much higher for both the species under study on using CO2 as the torrefaction medium.

Fig. 20. Mass retained after pretreatment in CO2 and N2. Graphs are presented for the dry ash free (DAF)

case. Mesquite size: 4-6 mm; Juniper size: 2-4 mm; 500 g sample [60].

Temperature (

0

C)

M

as

s

re

tai

n

ed

(D

A

F

%

)

200 220 240 260 280 300 65 70 75 80 85 90 95 M-C M-N J-C J-N

In addition to temperature, mass loss rate is affected by i) type of wood (hardwood or softwood), ii) batch size in grams, iii) particle size and iv) torrefaction medium used (CO2 or N2). Juniper with lower particle size showed higher VM loss when

compared to mesquite. Studies conducted on similar particle sizes (589-840 μm) for both the woody biomass (mesquite and juniper) using a TGA showed increased mass losses for mesquite samples when compared to juniper during the torrefaction process [51]. Softwood species (juniper) with lower hemicellulose content showed higher mass loss rate than mesquite which is a hardwood with higher hemicellulose. Hence, the effect of particle size on the mass loss behavior of the lignocellulosic samples was compared in the current study to understand the torrefaction process on different wood types. In the batch torrefaction reactor, the torrefaction medium permeates the samples. Thus batch size may not lead to temperature gradients. However the larger particle size will reduce the mass loss rate and hence will lead to higher mass retained for mesquite sample (4-6 mm).

CO2 had a minor effect on the softwood species when temperature was lower

than 280 °C compared to that of mesquite. Different phenomenon which can be accounted for such behavior of biomass under these pretreatment mediums include, a) higher specific heat of carbon dioxide when compared to that of nitrogen which results in some heat being removed by the pretreatment mediums during the heating process, b) reaction of the pretreatment medium (CO2) with the biomass fuels, c) effect of ash

contents in biomass which can catalyze the reaction between pretreatment medium CO2

can be concluded that the particle size can be altered in addition to using different torrefaction mediums to obtain desired mass loss from the samples.

The behavior of hemicellulose, cellulose and lignin content in both juniper and mesquite can be predicted from the TGA-DTA trace. A biomass sample which has higher percentage of hemicellulose will exhibit a hump at lower temperatures of around 200-300˚C during pyrolysis process [27, 88, 89, 116]. TGA-DTG (Thermogravimetric and Differential thermograms) curves obtained for mesquite and juniper pyrolysis under nitrogen environment is available elsewhere [51]. Lower amount of hemicelluloses in the softwood species juniper is evident from the smaller hump in the DTG curve (dotted line) when compared to that of mesquite. Since lower particle size of juniper was used in the current study, more hemicellulose is lost at the temperature range of torrefaction for juniper when compared to that of larger mesquite fuel particles. Though mesquite has larger amount of hemicelluloses, larger particle size has restricted the passage for the hemicelluloses at the center of the particle to escape. Studies conducted on similar particle sizes (589-840 μm) for both the woody biomass (mesquite and juniper) using a TGA showed increased mass losses for mesquite samples when compared to juniper during the torrefaction process due to difference in hemicellulose content [51].

Based on equilibrium concepts for reaction C+CO22CO, it has been shown

that the Boudouard reaction is thermodynamically favorable only at temperatures above 710˚C [58] i.e ∆G < 0 at T > 710˚C; called transition temperature which leads to an equilibrium constant value which is greater than 1. A higher value for the equilibrium constant indicates that the mole fraction of CO will be much higher than the mole

fraction of CO2 at temperatures above 710˚C. The effect of CO2 reacting with fixed

carbon in the biomass at temperatures used for the present study was considered to be negligible. In order to validate the temperature and time dependence, the Boudouard reaction kinetics was obtained from the literature [99, 120]. Assuming CO2 reacts with

the carbon in the biomass, the mass loss rate can be given by,

0 sm m exp dW E k p S W dt RT     _{ }   (38)

where ps is the partial pressure of species, Sm is the specific surface area of the particle

and W is the weight of the particle. The values of the constants m, k0 and E in the above

expression are available elsewhere [99]. The effect of the Boudouard reaction at lower temperatures and increased residence times (60 minutes) was studied. Curves obtained for the percent mass loss for different temperatures with respect to residence time shows an increasing trend in mass loss with increased residence times. Fig. 21 shows the results obtained from the Boudouard reaction kinetics for the case of coal char. Higher mass loss was observed when the temperatures were increased beyond the temperature range used for the current torrefaction studies indicating temperature and time dependent mass loss. It should be noted that the value of the constants used in Eq. (38) were derived for coal chars with higher surface area. Though the biomass undergoing torrefaction will not have a high surface area, the above model can be used as a reference to validate the time dependency of Boudouard reaction at the temperature range used for torrefaction study.

Fig. 21. Effect of residence time and temperature on the Boudouard reaction. Temperatures above 300˚C shows higher mass loss with respect to residence time.

Use of carbon dioxide as the pretreatment medium has an impact on the mass loss behavior of the biomass. A small amount of mass loss due to the reaction of carbon with carbon dioxide will cause a slight increase in the pore spaces available for the volatiles trapped within the biomass particle to leave the particle. A TGA unit was used to study the time dependency of CO2 reacting with the biomass. 10 mg of juniper sample

time (s)

M

as

s

los

s

(%

)

500 1000 1500 2000 2500 3000 3500 0 0.2 0.4 0.6 0.8 1 1.2 1.4 _T-200C T-220C T-240C T-260C T-280C T-300C T-350C

of particle size 300 micron was used for this study. The biomass was heated at a constant rate of 20˚C per minute from room temperature to 240˚C and the temperature was maintained at 240˚C for three different time periods (15 minutes, 30 minutes and 60 minutes). After the isothermal stage, the samples were heated again up to 1000˚C at a heating rate of 20˚C per minute. Two different mediums (N2 and CO2) were used to

study the mass loss behavior during the torrefaction stage (isothermal period). Fig. 22 shows the mass loss for different mediums at three different residence times.

It can be seen that lower residence time (15 minutes) did not have any impact on the mass loss upon using CO2 as similar mass losses were observed with both mediums.

However with increase in residence times (30 and 60 minutes), using CO2 resulted in

higher mass loss indicating a mild effect of the reaction of CO2 with biomass carbon at

higher residence times. Biomass treated with different mediums will have different kinetic parameters. Effect of the pretreatment mediums on the kinetic parameters of biomass will be presented later in the MVR results section.

In document Effect of Co-Firing Torrefied Woody Biomass with Coal in a 30 kWt Downfired Burner (Page 104-110)