Use of Alkali Carbonate Sorbents for Capturing Chlorine-Bearing

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Combustion

Capture of Chlorine-Bearing Gas Emissions from Corn Straw Torrefaction with Alkali Carbonate Sorbents Xiaohan Ren, Emad Rokni, Lei Zhang, Zhuozhi Wang, Yu Liu, and Yiannis Angelo Levendis Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01976 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Energy & Fuels

Use of Alkali Carbonate Sorbents for Capturing Chlorine-Bearing Gases from Corn Straw Torrefaction

3 4

Xiaohan Ren a, Emad Rokni b, Lei Zhangc, Zhuozhi Wangc, Yu Liua, * and Yiannis A.

5

Levendis b,*

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a Institute

of Thermal Science and Technology Shandong University, Jinan, Shandong Province 250061, China b Department of Mechanical and Industrial Engineering Northeastern University, Boston, MA 02115, USA c School of Energy Science and Engineering Harbin Institute of Technology, Harbin, Heilongjiang Province 150001, China

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Abstract

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Combustion of torrefied biomass for power generation has many advantages over combustion of raw biomass, one of which is lower emissions of chlorine-bearing gases. This is because partial evolution of these gases takes place during the torrefaction process; hence, the resulting torrefied biomass has a lower chlorine mass fraction than its raw biomass precursor. Research showed that during torrefaction of corn straw the predominant chlorinated species in the evolving gas (“torgas”) are CH3Cl and HCl. The former is more prevalent when torrefaction takes place at temperatures under 350 °C, whereas the latter is more abundant at higher temperatures. In this work, corn straw was torrefied at a furnace temperature of 300 °C for 20 min, under atmospheric pressure in an inert nitrogen flow. Under this condition corn straw lost nearly 40% of its original mass, along with 73% of its chlorine mass to the gas phase. To control the emissions of the chlorinated species, the torrefaction gas was heterogeneously reacted with beds of alkali carbonate sorbents kept at temperatures in the range of 25-500 ºC. It was determined that the sorbents captured HCl more effectively than CH3Cl, with amounts varying both with the type of sorbent and with the sorbent/gas reaction temperature. HCl removal effectiveness varied in the range of 26-80% whereas CH3Cl removal effectiveness varied in the range of 15-22% at the conditions implemented herein. Potassium carbonate (K2CO3) was found to be slightly more effective than sodium carbonate (Na2CO3) and significantly more effective than calcium carbonate (CaCO3), particularly at low reaction temperatures.

30 31

*Corresponding Authors:

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Yiannis A. Levendis: [email protected], Yu Liu: [email protected]

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Key Words: Biomass Torrefaction, HCl Emissions, CH3Cl Emissions, Alkali Carbonate Sorbents

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INTRODUCTION

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Renewable biomass is of a technological interest as it is considered a fuel that will help to address

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climate change, as its combustion is considered nearly carbon neutral 1, 2. However, there are issues

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with the use of raw biomass as a fuel 3. Its high moisture content and hydrophilicilty result in low

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specific internal energy (heating value) and high biodegradability. Its fibrous and tenacious nature

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results in poor grindability. Its high particle aspect ratios impair fluidization 4 . Its high chlorine

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and sulfur contents cause slagging, fouling and corrosion 5, 6, 7, particularly in high-temperature

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pulverized fuel boilers which are lined with metallic water tube heat exchangers.

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The aforementioned issues can be partially alleviated by thermal pretreatment of biomass in

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moderate temperatures and at the absence (or near-absence) of oxygen; this pyrolytic process has

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been termed torrefaction. Typically, torrefaction takes place in nitrogen at temperatures at or lower

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than 300 ºC, for less than 1 hr. It is a mild pyrolysis process and the resulting residue is somewhat

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“coal like” 3. It has less moisture and lower light volatile hydrocarbon mass fractions than its raw

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biomass precursor. It has improved biodegradability, hence longer storage life, better grindability,

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lower particle aspect ratios 8 and higher heating value.

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Torrefied biomass has lower sulfur and chlorine mass fractions than raw biomass 9, 10, 11, 12, 13, 14,

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thus it is less problematic to burn alone or blended with coal in pulverized-fuel boilers, where

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combustion temperatures are high and metallic surfaces abound. The lower emissions of the acid

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gases SO2 and HCl 9, 10, 11, 12, 13, 14 render their capture more manageable by the emission control

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equipment of power-plants and, consequently, the lower amounts released to the atmosphere are

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more environmentally acceptable 15.

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Partial release of the chlorine and sulfur to the gas phase during the torrefaction process is likely 3 ACS Paragon Plus Environment

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to be less problematic than during a subsequent combustion process. This is because the

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torrefaction furnaces can be refractory; thus, they should be relatively unaffected by corrosion or

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deposition. The predominant chlorine bearing gases evolving during torrefaction have been

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identified to be CH3Cl and HCl 16, 17, 18. Recent work by Ren et al. 18 torrefied corn straw and

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determined that CH3Cl was more prevalent when torrefaction took place at temperatures under

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350 °C, whereas HCl was more abundant at higher temperatures. For example, at the torrefaction

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temperature of 275 °C, CH3Cl accounted for 22.7% and HCl accounted for 23.1% of the mass of

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chlorine in the raw biomass. Another 41.1% remained in the ash and 13.1% was unaccounted for18.

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Hence, the amounts of CH3Cl and HCl in the torgas were similar and together amounted to nearly

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46% of the chlorine mass in the raw biomass.

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As nearly half the chlorine mass of raw corm straw was released to the atmosphere during

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torrefaction, under such conditions, its control is of technological interest for maintaining a safe

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environment. Therefore, this work focusses on the control of chlorine-bearing gases by

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heterogeneously reacting them with alkali carbonate sorbents. Based on recent experiences15 on

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controlling HCl emissions from combustion of corn straw and corn-based DDGS (Distillers Dried

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Grains with Solubles) using fixed beds of readily available alkali sorbents, this technique was also

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implemented herein to curtail chlorine bearing gas concentrations in the torgas. As torgas contains

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significant amounts of CH3Cl, in addition to HCl, the sorption efficiencies of the alkali carbonate

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sorbents for both species were examined. Moreover, since torrefaction is a moderate temperature

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process, it is sensible that the sorbent bed be also kept at moderate temperatures for practical

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purposes. Hence, the following low-to-moderate sorbent bed temperatures were explored herein:

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25, 300 and 500 ºC, the first two of which are deemed as the most practical for torrefaction

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processing plants. 4 ACS Paragon Plus Environment

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BIOMASS PREPRATION and CHARACTERIZATION – SORBENT SELECTION

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Corn straw, a biomass with high chlorine content (0.63%), was grown and harvested at the Harbin

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Province of China. Torrefaction of raw corn straw was carried out in a horizontal electrically-

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heated muffle furnace, depicted in Fig. 1, at the absence of oxygen for 20 minutes at 300°C.

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Batches of 0.5 g pulverized (75-150 µm) raw biomass were inserted in this furnace, preheated and

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purged with nitrogen, where they experienced a heating rate of ~ 60 °C/s.

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Figure 1. Schematic of the laboratory setup for torrefaction of corn straw in a horizontal fixed bed reactor and placement of sorbents in a separate vertical reactor.

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The proximate and ultimate analysis of the torrefied biomass are shown in Table 1. During

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torrefaction at this temperature, corn straw lost 39.6% of its mass by devolatilization. After

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torrefaction, corn straw was ground in a household blender and sieved to the size cut of 75-150

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µm. The Proximate and Ultimate analyses of the raw corn straw are given in Table 1. Proximate 5 ACS Paragon Plus Environment

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analysis was performed based on GB/T 212-2008 Chinese standard 19 in an electric oven (5E-

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DHG) and muffle furnace (5E-MF6000) both manufactured by Changsha Kaiyuan Instruments

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Co. Ltd. Ultimate analysis for carbon, hydrogen and nitrogen were carried out by an elemental

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analyzer (CHN2200, manufactured by Elementar Ltd.) according to GB/T 30733-2014 standard

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20,

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manufactured by Changsha Kaiyuan Instruments Co. Ltd) based on the GB/T 214-2007 standard

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21.

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manufactured by Changsha Kaiyuan Instruments Co. Ltd) according to GB/T 213-2008 22.

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whereas sulfur analysis was carried out in a Coulomb fixed sulfur analyzer (5E-8S

The heating value of the biomass fuels was measured with a calorimeter (5E-AC/PL,

Table 1. Proximate and ultimate analysis (wt%) and energy contents (MJ/kg) of raw corn straw Fuel Type

Corn Straw Raw

Corn Straw Torrefied at 300 °C for 20 min Proximate Analysis (Air dry basis)

Moisture (%)

6.18

2.31

Volatile matter (%)

71.21

50.34

Fixed Carbon (%)

16.12

38.83

Ash (%)

6.49

8.52

Ultimate Analysis (Air dry basis) Carbon (%)

45.84

53.19

Hydrogen (%)

5.11

4.98

Oxygen (%)

34.89

28.89

Nitrogen (%)

1.28

1.94

Sulfur (%)

0.21

0.17

Calcium (%)

0.48

0.58

Sodium (%)

0.01

0.02

Potassium (%)

0.64

1.25

Magnesium (%)

0.25

0.31

Chlorine (%)

0.63

0.28

K/Cl (molar ratio)

0.92

4.06

Ca/Cl2 (molar ratio) Heating Value (MJ/kg)

1.35

2.42

16.8

19.4


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Energy & Fuels

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The sorbents used in this work were: (i) calcium carbonate (CaCO3) reagent obtained from J.T.

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Baker Inc.; (ii) sodium carbonate (Na2CO3) obtained from EM Science; and (iii) potassium

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carbonate anhydrous (K2CO3) obtained from Fisher Scientific. All sorbents were used in powder

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form, as shown in Fig. 2. K2CO3

Na2CO3

CaCO3

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Figure 2. Fine powders of particles of the alkali-carbonate sorbents.

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The particle size distributions of the sorbent powders were obtained using a laser particle analyzer

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(Mastersizer 3000 by Malvern Panalytical) and results are included in Table 2. Results show that

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most sorbent particles were between 1 and 50 μm in diameter.

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Table 2. Alkali carbonate Sorbent particle size distributions CaCO3

K2CO3

Na2CO3

D10: Particles with diameters smaller than the shown values account for 10% of the total number of particles

2.3 μm

1.4 μm

5.1 μm

D50: Particles with diameters larger and smaller than the shown values account for 50% of the total number of particles

17.1 μm

10.8 μm

25.2 μm

D90: particles with diameters larger than the shown values account for 10% of the total number of particles

46.6 μm

41.9 μm

52.4 μm

The specific surface area of the sorbents, in their as received states, was measured according to the 7 ACS Paragon Plus Environment

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Brunauer Emmett and Teller (BET) method using an Autosorb-iQ Quantachrome instrument in

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nitrogen. The surface area of CaCO3 was measured to be 2.18 m2/g with a calculated corresponding

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average pore diameter of 19.1 nm. The surface area of the Na2CO3 was lower at 0.12 m2/g, whereas

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that of K2CO3 was yet lower, in fact below the resolution of the instrument.

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EXPERIMENTAL APARATUS AND PROCEDURE

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Torrefaction of batches of pulverized corn straw was performed in an electrically-heated horizontal

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fixed-bed furnace at 300 °C for 20 min in an inert nitrogen flow. Sorption was performed

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subsequently in a vertically-placed electrically heated furnace. Both furnaces, depicted in in Fig.

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1, were fitted with Pyrex glass tubes. The tube in the first furnace was 10 cm in diameter and 150

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cm in length, whereas the second furnace was 3 cm in diameter and 30 cm in length. In these fixed

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fuel bed experiments, the torrefaction of biomass happened first, then the effluent torgas passed

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through powders of sorbents placed in the second furnace. The temperature was of the secondary

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furnace was maintained at 25, 300, or 500 °C. The Pyrex glass tube of the secondary furnace was

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fitted with fritted glass disks at its entrance and exits. A small amount of alkali carbonate sorbents

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(0.5 g) was placed therein and filled only a small fraction of the length of the tube. The nitrogen

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flow rate was kept at 3 lpm, which fluidized the sorbent powders in this vertically-placed tube.

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The value of the weight hourly space velocity (WHSV), defined as the weight of torrefaction

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effluent (nitrogen plus the volatiles) flowing per unit weight of the sorbent per hour, was calculated

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to be in the order of 200 hr-1. The effluent of this apparatus was heated to 180 oC to avoid

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condensation of H2O, which can absorb HCl and, perhaps, CH3Cl. It then passed through a fiber

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filter to collect any condensed matter. Thereafter, to monitor the HCl emissions, a Fourier 8 ACS Paragon Plus Environment

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Energy & Fuels

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Transform Infra-Red (FTIR) spectroscopy (with a GASMET DX4000 instrument) was used,

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where the effluent of the furnace passed through a glass condenser, placed in an ice-bath, and it

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was subsequently channeled to the FTIR analyzer. All experiments were repeated three times and

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mean values and standard deviations were calculated.

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EXPERIMENTAL RESULTS AND DISCUSSION

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HCl emissions of biomass at the absence and presence of alkali carbonate sorbents

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Typical time-resolved profiles of HCl emissions from 300 °C torrefaction of biomass in the

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horizontal furnace, as monitored by the FTIR, are shown in Fig. 3. The first profile, shown in Fig.

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3a, corresponds to raw effluent at the absence of sorbents, whereas the ensuing three profiles, Fig.

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3b-3d, were recorded upon passing the effluent through calcium carbonate sorbent beds heated at

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the temperatures of 25, 300 and 500 °C, respectively. These experimentally-measured mass

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emissions were first integrated with time, they were subsequently normalized by the initial mass

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of biomass in each case and they are shown in Fig. 4a.


Energy & Fuels

(a) HCl release without sorbents

(b) HCl release upon CaCO3 sorption at 25 °C 15 mg/g of corn straw

mg/g of corn straw

15 12 9 6

12 9 6 3

3

0

0 0

300

600

152

0

900 1200 Time (s)

(c) HCl release upon CaCO3 sorption at 300 °C 15

15

12

12

9 6 3

600

900 1200 Time (s)

9 6 3

0

0 0

153

300

(d) HCl release upon CaCO3 sorption at 500 °C

mg/g of corn straw

mg/g of corn straw

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300

600

900 1200 Time (s)

0

300

600

900

1200

Time (s)

154 155 156

Figure 3. (a) Evolution of specific mass emissions of HCl in the torrefaction effluents at the absence of sorbents, and (b-d) at the presence of calcium carbonate sorbent beds kept at different temperatures.

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During torrefaction, at the absence of sorbents, nearly 28% of the chlorine mass in the raw biomass

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was released as HCl. At the presence of sorbents, a great deal of the HCl emissions was captured,

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and results are displayed in Figs. 4-6, for calcium carbonate, sodium carbonate and potassium

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carbonate, respectively. As shown in Fig. 4, the CaCO3 sorbent captured 26.5, 59.1, and 77.9% of

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the generated HCl at the 25, 300, and 500 °C sorbent bed temperatures respectively.

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Correspondingly, as shown in Fig. 5, the Na2CO3 sorbent captured 42.6, 71.4, and 78.2% of the

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generated HCl at 25, 300, and 500 °C respectively. Finally, as shown in Fig. 6, the K2CO3 sorbent 10 ACS Paragon Plus Environment

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164

captured 46.2, 72.3, and 80.2% of the generated HCl at 25, 300, and 500 oC, respectively. It is

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notable that increasing the sorbent bed temperature form 25 °C to 300 °C increased the sorption

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capacity of the sorbents more than by increasing the temperature further from 300 °C to 500 °C.

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This may be attributed to the fact that such sorption reactions are thermodynamically favorable at

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low temperatures but at increasing temperatures they become asymptotically less favorable, as

169

illustrated in an ensuing section of this manuscript as well as in Figure 5 in Ref. 15. Hence, the

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increase in sorbent sorption effectiveness may not justify the expense for heating the sorbent to a

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temperature higher than the torrefaction temperature itself, particularly so if in practical systems

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the torrefaction and sorbent beds share the same furnace. (b) HCl capture

(a) HCl specific mass emissions 100

2.4 2 1.6 1.2 0.8

1.81

77.9

80 59.1

1.33

%

mg/g of corn straw

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Energy & Fuels

60 40

0.74 0.4

20

0.4

26.5 0

0

0

No sorbents CaCO3 at 25℃

No sorbents CaCO3 at CaCO3 at CaCO3 at 25℃ 300℃ 500℃

CaCO3 at 300℃

CaCO3 at 500℃

173 174 175

Figure 4. (a) Integrated mass emissions of HCl (mg/g dry fuel) at the absence or presence of CaCO3 bed heated at different temperatures; (b) HCl emission reduction by passing the torrefaction effluent through CaCO3 sorbent beds.

176


Energy & Fuels

(a) HCl specific mass emissions

(b) HCl capture

1.81

100

1.6

71.4

80

1.04

1.2 0.8

78.2

%

mg/g of corn straw

2

60

0.52

42.6

40

0.39

0.4

20

0

0 No sorbentsNa2CO3 at Na2CO3 at Na2CO3 at 25℃ 300℃ 500℃

0 No sorbents Na2CO3 at Na2CO3 at Na2CO3 at 25℃ 300℃ 500℃

177 178 179

Figure 5. (a) Integrated mass emissions of HCl (mg/g dry fuel) at the absence or presence of Na2CO3 bed heated at different temperatures; (b) HCl emission reduction by passing the torrefaction effluent through Na2CO3 sorbent beds.

180 (a) HCl specific mass emissions 2

(b) HCl capture

1.81

100

1.6

72.3

80

80.2

%

mg/g of corn straw

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1.2 0.8 0.4

0.97

60 0.5

46.2

40 0.36

20 0

0

0 No sorbents K2CO3 at K2CO3 at K2CO3 at 25℃ 300℃ 500℃

No sorbents K2CO3 at 25℃

K2CO3 at 300℃

K2CO3 at 500℃

181 182 183

Figure 6. (a) integrated mass emissions of HCl (mg/g dry fuel) at the absence or presence of K2CO3 bed heated at different temperatures; (b) HCl emission reduction by passing the torrefaction effluent through K2CO3 sorbent beds.

184 185

Overall, these experiments showed that HCl capture can be realized by direct chlorination of the

186

carbonate sorbents, in line with the findings of Snow et al.23, who reported that the direct sulfation

187

of calcium carbonate (limestone) is not only possible, but it can also proceed to higher conversion

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than the sulfation of pre-calcined limestone. Thus, by analogy to the aforesaid direct sulfation

189

reaction, the direct chlorination reaction of limestone, as expressed below, appears to also proceed 12 ACS Paragon Plus Environment

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Energy & Fuels

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to substantial conversion, as proposed by Shemwell et al. 24:

191

2HCl(g) + CaCO3(s) → CaCl2(s) + CO2(g) + H2O(g)

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Under the conditions of these experiments the potassium carbonate sorbent proved to be the most

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effective (slightly better than Na2CO3) and the calcium-sorbent sorbent was the least effective for

194

HCl capture. This is supported by chemical equilibrium calculations which show that the

195

chlorination reaction of potassium carbonate is more favorable than that of sodium carbonate,

196

which in turn is more favorable than that of calcium carbonate, as illustrated in an ensuing section

197

of this manuscript and also in Figure 5 in Ref. 15.

198

Previous research

199

calcium-based sorbents with HCl gas and reported significant capture, depending on the amounts

200

of the sorbents, the flue gas temperature, the sorbent-gas contact time, the sorbent porosity, the

201

sorbent particle size, etc. Potassium carbonate and sodium carbonate have been identified to be the

202

most effective sorbents for HCl, while calcium carbonate has been reported to have lower

203

effectiveness 25, 31, 32, which is in line with the results reported herein.

25, 26, 27, 28, 29, 30

(1)

have also evaluated the reaction of potassium, sodium and

204 205

CH3Cl emissions of biomass at the absence and presence of alkali carbonate sorbents

206

In these torrefaction experiments, nearly 40 % of the biomass chlorine mass was released as CH3Cl.

207

At the presence of sorbents, a fraction of these emissions was captured, as illustrated in Figs. 7-9.

208

As shown in Fig. 7, using beds of CaCO3 sorbent heated to 25, 300 and 500 oC, as much as 12.4,

209

19.6, and 21.2% of the generated CH3Cl were captured respectively. While, as shown in Fig. 8,

210

using beds of Na2CO3 sorbent heated to 25, 300 and 500 oC, as much as 14.8, 21, and 21.4% of 13 ACS Paragon Plus Environment

Energy & Fuels

211

the generated CH3Cl were captured respectively. Finally, as shown in Fig. 9, using beds of K2CO3

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sorbent heated to 25, 300, and 500 oC, as much as 17.1, 21.7 and 21.9% of the generated CH3Cl

213

were captured, respectively. By comparing the CH3Cl sorption capacities at the temperatures of

214

300 oC and 500 oC, it can be seen that they are about the same. Therefore, as in the case of HCl, it

215

is notable that increasing the sorbent bed temperature form 300 °C to 500 °C only mildly increased

216

the sorption capacity of the sorbents for CH3Cl; therefore, again the expense for heating the sorbent

217

bed at the higher temperature may not be justified.

218

A comparison of the effectiveness of the three alkali carbonate sorbents for capturing CH3Cl

219

reveals that the potassium-carbonate sorbent was the most effective. The gap in performance

220

among the three sorbents was most pronounced at the lowest temperature examined herein (25 °C),

221

whereas as the temperature increases to 300 °C and, particularly, to 500°C the performance of the

222

sorbents converges to a similar value. This behavior is supported by thermodynamic calculations

223

which are outlined in an ensuing section of this manuscript (results are illustrated in Figure 11). (a) CH3Cl specific mass emissions

4

(b) CH3Cl capture 25

3.97 3.48

3.19

3.13

19.6

20

21.2

%

5 mg/g of corn straw

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3

15

2

10

1

5

0

0 No sorbents CaCO3 at CaCO3 at CaCO3 at 25℃ 300℃ 500℃

12.4

0 No sorbents CaCO3 at 25℃

CaCO3 at 300℃

CaCO3 at 500℃

224 225 226 227

Figure 7. (a) Integrated mass emissions of CH3Cl (mg/g dry fuel) at the absence or presence of CaCO3 bed heated at different temperatures; (b) CH3Cl emission reduction by passing the torrefaction effluent through CaCO3 sorbent beds.

228


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(b) CH3Cl capture

5

25 3.97

4

3.38

3.14

3.12

3

21.4

14.8

15

2

10

1

5

0

21

20 %

mg/g of corn straw

(a) CH3Cl specific mass emissions

0

0 No sorbents Na2CO3 at Na2CO3 at Na2CO3 at 25℃ 300℃ 500℃

No CaCO3 at CaCO3 at CaCO3 at sorbents 25℃ 300℃ 500℃

229 230 231 232

Figure 8. (a) Integrated mass emissions of CH3Cl (mg/g dry fuel) at the absence or presence of Na2CO3 bed heated at different temperatures; (b) CH3Cl emission reduction by passing the torrefaction effluent through Na2CO3 sorbent beds. (a) CH3Cl specific mass emissions 5 4

(b) CH3Cl capture 25

3.97 3.29

3.11

20

3.1

3

%

mg/g of corn straw

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

21.9

K2CO3 at 300℃

K2CO3 at 500℃

17.1

15

2

10

1

5

0

0

0 No sorbents K2CO3 at 25℃

CaCO3 at CaCO3 at CaCO3 at No sorbents 25℃ 300℃ 500℃

21.7

233 234 235 236

Figure 9. (a) Integrated mass emissions of CH3Cl (mg/g dry fuel) at the absence or presence of K2CO3 bed heated at different temperatures; (b) CH3Cl emission reduction by passing the torrefaction effluent through K2CO3 sorbent beds.

237

The aforementioned results show that all three alkali carbonates are more effective in capturing

238

HCl than CH3Cl. The reasons for this behavior may be based on chemical reaction

239

thermodynamics or kinetics and are discussed further in the following section.

240

241

Chemical equilibrium considerations for possible HCl and CH3Cl reactions 15 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

242

The overall heterogeneous reactions of HCl gas with the three alkali carbonate sorbents are shown

243

below, as outlined in past literature 15, 24:

244

2HCl (g) + CaCO3 (s) ↔ CaCl2(s) (s) + CO2(g) + H2O(g)

(1)

245

2HCl (g) + Na2CO3 (s) ↔ 2NaCl(s) (s) + CO2(g) + H2O(g)

(2)

246

2HCl (g) + K2CO3 (s) ↔ 2KCl(s) (s) + CO2(g) + H2O(g)

(3)

247

However, the author’s knowledge, there is paucity in the literature regarding the reactions of

248

CH3Cl with the alkali carbonate sorbents. Prior work on the thermal decomposition of CH3Cl in

249

the moderate temperature range of this investigation conducted by Won 33 showed that the thermal

250

stability of CH3Cl is good in this temperature range. Non-chlorinated decomposition products may

251

include CH4 and C2H6; smaller amounts of C2H4 were also detected, but mostly at higher

252

temperatures (T > 850 ºC). Koper and Klabunde

253

chlorinated hydrocarbons on calcium oxide at temperatures in the range of 300-500 ºC and they

254

detected carbon monoxide in the products. Based on those results, possible overall reactions of

255

CH3Cl gas with CaCO3, that produce CH4, CO and C2H6 may be the following two:

256

2CH3Cl(g) + CaCO3(s) ↔ CaCl2(s) + 2CO(g) + CH4(g) + H2O(g)

(4)

257

2CH3Cl(g) + CaCO3(s) ↔ CaCl2(s) + CO2(g) + C2H6(g) + 0.5O2(g)

(5)

258

Similarly, possible overall sorption reactions of CH3Cl with Na2CO3 sorbent are:

259

2CH3Cl(g) + Na2CO3(s) ↔ 2NaCl(s) + 2CO(g) + CH4(g) + H2O(g)

(6)

260

2CH3Cl(g) + Na2CO3(s) ↔ 2NaCl(s) + CO2(g) + C2H6(g) + 0.5O2(g)

(7)

261

Similarly, the overall sorption reaction of CH3Cl with K2CO3 sorbents are:

34

examined the destructive adsorption of


Page 17 of 23

262

2CH3Cl(g) + K2CO3(s) ↔ 2KCl(s) + 2CO(g) + CH4(g) + H2O(g)

(8)

263

2CH3Cl(g) + K2CO3(s) ↔ 2KCl(s) + CO2(g) + C2H6(g) + 0.5O2(g)

(9)

264

To evaluate the possibility that Reactions 4-9 take place during these experiments, the evolution

265

of CH4, CO and C2H6 were monitored in the torrefaction effluents both in the absence and in the

266

presence of one of the sorbents, the CaCO3. Experimentally-measured specific mass emissions are

267

shown in Fig. 10, where it can be observed that the concentrations of these products increased

268

consistently in the presence of the sorbent, as decomposition of CH3Cl takes place in this

269

temperature range. The specific emissions of CH4, C2H6 and CO increased mildly with increasing

270

temperature, as the amounts of CH3Cl decreased through the sorbent bed, as shown in Figs. 7-9.

271

This supports the proposed chemical mechanism and is in line with the observations of Koper and

272

Klabunde 34, regarding the CO generation from destructive adsorption of chlorinated hydrocarbons

273

on calcium oxide. (a') C conversion to released CH4

(a) CH4 specific mass emissions 2 1.6

1.561

1.602

1.619

0.5

1.653

1.2

0.4

%

mg/g corn straw

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.373

0.377

0.384

CaCO3 at 300℃

CaCO3 at 500℃

0.3

0.8

0.2

0.4

0.1

0

0.363

0 No sorbents CaCO3 at 25℃


274


Energy & Fuels

(b) C2H6 specific mass emissions 3.117

3.119

(b') C conversion to released C2H6

3.415

3.184

1.8

1.449

1.5

3

1.451

1.481

1.588

1.2

%

mg/g corn straw

4

2

0.9 0.6

1

0.3

0

0


No sorbents CaCO3 at 25℃

CaCO3 at 300℃

CaCO3 at 500℃

275 (c') C conversion to released CO (%)

(c) CO specific mass emissions 14.5 13.94

14 13.5

13.25

3.4

14.08

3.3

13.38

3.24

3.2

%

mg/g corn straw

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

13

3.1

3.08

3.27

3.11

3

12.5 12

2.9 2.8



276 277 278 279

Figure 10. (a, b and b) Integrated experimentally-determined mass emissions of CH4, C2H6, and CO (mg/g dry fuel) at the absence or presence of CaCO3 bed heated at different temperatures; (a’, b’ and c’) percent of biomass carbon converted to CH4, C2H6, and CO.

280

Thermodynamic chemical equilibrium constants, Kp, for Eqs. (1-9) are calculated based on the free

281

energy change as follows:

282

ln 𝐾𝑝 =

―∆𝐺𝑜𝑇

(10)

𝑅𝑇

283

Where

the

Gibbs

284

∆𝐺𝑜𝑇 = ∆𝐻𝑜𝑇 ―𝑇∆𝑆𝑜𝑇

free

energy

could

be

calculated

as

follows: (11)


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Energy & Fuels

285

Where, ∆𝐻𝑜𝑇, ∆𝑆𝑜𝑇, 𝑎𝑛𝑑 𝑇are the respective standard enthalpy change, standard molar entropy

286

change and the reaction temperature.

287

Generally, when the value of Kp is greater than 1000 (ln(Kp) > 6.9), the reaction can be considered

288

to proceed to completion

289

constants, shown in Fig 11, all reactions, with the exception of Reaction 5, have ln(Kp) > 6.9,

290

therefore they are favorable in this temperature range.

35.

Therefore, based on the calculated thermodynamic equilibrium

291 292

Figure 11. Equilibrium constant of Reactions 1-9.

293

Potassium carbonate was shown to be the most effective sorbent, followed by sodium and then by

294

calcium for capturing HCl and CH3Cl gases. This trend was observed both experimentally and

295

theoretically by the aforesaid chemical equilibrium calculations. The fact that the experimentally-

296

measured values for the reduction of HCl were higher than the corresponding reduction values of

297

CH3Cl, contrary to the thermodynamic predictions based on chemical equilibrium, may be

298

attributed to kinetic limitations for Reactions 4-9, as the residence times of the torrefaction effluent

299

gas (“torgas”) through the sorbent bed were very brief. To the contrary, reaction kinetics for HCl

300

sorption by alkali sorbents have been shown to be very fast, see References 24, 36, 37. Hence, as there

301

is a scarcity of kinetic studies for CH3Cl by alkali sorbents, this may be a suggested topic of future

302

research. 19 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

303

304

CONCLUSIONS

305

As nearly half of the chlorine mass of raw corm straw is released to the atmosphere during

306

torrefaction, control of emitted chlorinated gases is important. To achieve this goal, in this study

307

the torrefaction gas was heterogeneously reacted with beds of powdered alkali carbonate sorbents

308

(CaCO3, Na2CO3, K2CO3) kept at three different bed temperatures: 25, 300, or 500 oC. The results

309

are summarized as follows:

310

Concentrations of CH3Cl in the torgas were found to be higher than those of HCl at the torrefaction

311

temperature of 300 oC.

312

The removal efficiencies of HCl by the sorbents were found to be higher than the removal

313

efficiencies of CH3Cl at all sorbent bed temperatures tested.

314

The removal efficiencies of both HCl and CH3Cl by the sorbents increased with increasing bed

315

temperature. The increase was steep when the bed temperature was raised from 25 to 300 oC,

316

whereas it was milder when the temperature was further raised from 300 to 500 oC. Therefore, it

317

may be economically sensible to choose the sorbent utilization temperature to be in the former

318

range, i.e., analogous to the operating temperature of the torrefaction process.

319

The type of the alkali carbonate sorbent had small influence in this temperature range, particularly

320

so at the higher temperatures; although, the potassium-based sorbent was the most effective and

321

the calcium-based sorbent was the least effective at the lower temperatures.

322 323

ACKNOWLEDGEMENT 20 ACS Paragon Plus Environment

Page 20 of 23

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Energy & Fuels

324

This work has been supported by Fundamental Research Funds of Shandong University

325

(2018JC060), US National Science Foundation (Award Number 1810961), Harbin Institute of

326

Technology and Northeastern University in Boston.

327

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Use of Alkali Carbonate Sorbents for Capturing Chlorine-Bearing

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