LignoForce System for the Recovery of Lignin from Black Liquor

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The LignoForce System™ for the recovery of lignin from black liquor: feedstock options, odour profile and product characterization Lamfeddal Kouisni, Alain Gagne, Kirsten Maki, Peter Holt-Hindle, and Michael Paleologou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00907 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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ACS Sustainable Chemistry & Engineering

The LignoForce System™ for the recovery of lignin from black liquor: feedstock options, odour profile and product characterization Lamfeddal Kouisni1, Alain Gagné1, Kirsten Maki2, Peter Holt-Hindle2, and Michael Paleologou*1 1

FPInnovations, 570, Saint-Jean Boulevard, Pointe-Claire, Québec, Canada, H9R 3J9 Bio-Economy Technology Centre, FPInnovations, 2001 Neebing Ave, Thunder Bay, Ontario, Canada, P7E 6S3 2

*[email protected] ABSTRACT Lignin can be precipitated from kraft black liquor (BL) through the addition of an acidifying agent such as carbon dioxide or sulphuric acid. In most of the existing lignin precipitation processes that are using acid addition, sufficient acid is added to drop the pH of the black liquor from about 13-14 to about 9-10, followed by lignin particle coagulation, lignin slurry filtration and lignin cake washing with sulfuric acid and water. At pH values of less than 11, the potential exists for the generation of significant quantities of totally reduced sulphur (TRS) compounds and other volatile sulphur species. Such compounds which include hydrogen sulphide, methyl mercaptan, dimethyl sulphide, and dimethyl disulphide are strongly odorous compounds with well-known negative effects on human health and other forms of life. To address this problem, as well as other problems associated with existing lignin recovery processes, FPInnovations and Noram recently developed a new process called the LignoForce System™. This process employs a black liquor oxidation step to convert TRS compounds present in kraft black liquor to non-volatile species. This paper discusses the applicability of the LignoForce System™ to several feedstock black liquors (e.g. softwood, hardwood, and eucalyptus) as well as the sulphur compound outgassing potential from various stages of this process compared to a reference case in which the black liquor was not oxidized. In addition, , the emission of volatile sulphur and organic compounds from the two lignin products at different temperatures is discussed and compared. KEYWORDS: LignoForce™, Black liquor oxidation, Lignin, Sulphur compound emissions, VOCs. INTRODUCTION Softwoods (SW) and hardwoods (HW) are composed of 25-35 wt% and 15-25 wt% lignin, respectively. During the kraft pulping process, most of the lignin in the wood chips is chemically broken down after reaction with sodium hydroxide and sodium hydrosulphide (white liquor) at high temperatures and pressures. During this process, as ACS Paragon Plus Environment


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well as other kraft pulp mill operations, significant amounts of Totally Reduced Sulfur (TRS) compounds can be generated, namely: hydrogen sulfide (H2S), methyl mercaptan (MM), dimethyl sulfide (DMS), and dimethyl disulfide (DMDS). MM and DMS are formed through the reaction of hydrosulfide ions (HS-) and mercaptide ions (CH3S-), respectively, with lignin methoxyl groups (-OCH3), while DMDS is formed through the reaction of mercaptan with oxygen [1-8]. It is well known that these compounds produce unpleasant odours at concentrations as low as a few parts per billion in air (e.g. the odour threshold for hydrogen sulphide (when rotten egg smell is first noticeable) is 0.01-1.5 ppm) and might lead to serious environmental, health and safety issues if not properly collected and disposed of [9-10], as is the case in modern kraft pulp mills (e.g. the Threshold Limit Value (TLV) for hydrogen sulphide is 10 ppm – this is the timeweighted average exposure concentration for a normal 8-hour workday and a 40-hr workweek [10]). Following pulping, lignin and other residual chemicals are separated from the wood fibers through washing with water or evaporator condensates. The washing filtrate, usually referred to as weak black liquor (WBL), is subsequently concentrated from about 20 wt% to about 75 wt% solids through evaporation and then fired into the recovery boiler to produce steam, electricity and pulping chemicals for internal use. As many kraft pulp mills have been increasing pulp production over the last 30 years, the recovery boiler has, in many cases, become the production bottleneck. This is particularly true in the case of kraft pulp mills that were converted to dissolving pulp production since the yield at such mills is quite low (about 35-40%). A cost-effective way of offloading the recovery boiler with respect to calorific load is to remove a portion of the lignin from the black liquor. For every tonne of lignin that is removed, a typical kraft pulp mill can produce an additional tonne of pulp (assuming that no new bottlenecks are uncovered elsewhere in the mill). A number of processes exist for the recovery of lignin from black liquor. These include the Westvaco process developed over 60 years ago, the LignoBoost process, a process developed by STFI (now called Innventia) and licensed to Metso (now called Valmet), and the LignoForce System™ jointly developed by FPInnovations and NORAM (Figure 1) [11-15].

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Figure 1. The LignoForce System™ for lignin recovery from black liquor (a process developed by FPInnovations and NORAM, US Patent No. 8,771,464) For the purpose of evaluating lignin recovery from a wide range of black liquors (e.g. softwood, hardwood, Eucalyptus, high or low kappa number, high or low sulphidity), in 2012, FPInnovations and NORAM built a LignoForce™ lignin demo plant in Thunder Bay, Ontario, for the production of 100 kg of lignin per working day. In addition, in 2015, a commercial plant, using the LignoForce™ technology, was built by West Fraser at its Hinton pulp mill in Alberta, Canada, for the production of 30 tonnes of lignin per day. This system was recently started-up. A unique feature of the LignoForce™ process is that prior to the addition of carbon dioxide, the black liquor is oxidized, under controlled conditions with respect to oxygen charge, temperature and time. Under these conditions, the chemical requirements are reduced, lignin filterability is improved and purified, high-solids lignin is obtained [1113]. Another important advantage of the LignoForce™ process is the oxidation of malodorous sulphur compounds in black liquor to non-volatile species. Since most black liquors are highly alkaline (pH>13), totally reduced sulphur (TRS) compounds such as hydrogen sulphide and mercaptan, which possess ionisable sulphur groups, are totally in the sodium form, and, therefore, can not be easily volatilized. However, when such black liquors are acidified, as is the case in most lignin recovery processes, emissions of these two TRS compounds can become a significant problem. This is because the pKa of mercaptan is about 10.33 while that of hydrogen sulphide is around 7.04. This means that, at pH 10.33, 50% of mercaptan molecules are in the non-volatile sodium form (sodium mercaptide, CH3SNa) while the remaining 50% are in the easily volatilized acid form (mercaptan, CH3SH). Similarly, in the case of hydrogen sulphide, at pH 7.04, 50% ACS Paragon Plus Environment


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of hydrogen sulphide molecules are in the non-volatile sodium form (sodium hydrosulphide, NaHS) while the remaining 50% are in the easily volatilized acid form (hydrogen sulphide, H2S). It should be noted here that since DMS and DMDS do not possess any ionisable sulphur or other groups, they can be volatilized from black liquor or other media (e.g. condensates) at any pH at sufficiently high temperatures. The primary objectives of this work were: a) to evaluate the applicability of the LignoForce System™ to a wide range of feedstock black liquors (e.g. softwood, hardwood and eucalyptus black liquor) and b) to measure and compare the generation of volatile sulphur compounds, including TRS and sulphur dioxide, during the main steps of the LignoForce™ and conventional lignin recovery processes EXPERIMENTAL Black liquor characterization, lignin recovery and lignin characterization For the purpose of evaluating the recovery of lignin from a wide range of black liquors, the LignoForce™ pilot plant in Pointe-Claire, Quebec, Canada and the LignoForce™ demonstration plant at the Resolute pulp mill in Thunder Bay, Ontario, Canada, were used in this study. Black liquor samples were collected in different campaigns from different kraft pulp mills including SW, HW and Eucalyptus liquors. These samples were analyzed for total solids, residual effective alkali (REA), sulphide, ash, sodium, sulphur, organics, lignin, sugars, calorific value, soap, and elemental Carbon, Hydrogen, Nitrogen and Oxygen content. In the first step of the LignoForce System™, a sufficient amount of oxygen is added at high temperature (80ºC) to oxidize the BL to about 0 – 0.5 g/L residual sulfide. This oxidation step improves lignin filterability, reduces the chemical consumption and reduces the sulphur compound outgassing potential from all steps of the process. Possible oxidation reactions and products are summarized below: (1) (2) (3) (4)

2NaHS+ 2O2 → Na2S2O3 +H2O Na2S2O3 + 2 O2 + 2NaOH → 2Na2SO4 + H2O Organic TRS compounds + O2 → non-volatile sulphur compounds Sugars + O2 → Sugar acids

In the second step, a sufficient amount of carbon dioxide is added at relatively low temperature (65 to 75ºC) to reduce the pH of the oxidized liquor to about 9.5, at which point lignin comes out of solution in the colloidal form. In a third step, the lignin slurry is subjected to a coagulation/aging step, under gentle mixing and a lower temperature (60 to 65ºC), to improve its filterability.


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In a fourth step, the lignin slurry is filtered using a filter press. A lignin cake is thus obtained which is washed with sulphuric acid and water on the filter itself. Finally, the lignin cake is squeezed and air-blown to obtain a purified, high-solids lignin product. The recovered lignin products were analyzed for pH, total solids, ash, sodium, sulphur, lignin, and sugars content, functional groups, glass transition temperature, initial decomposition temperature, molecular weight, and calorific value. The procedures used are described elsewhere [12]. Impact of black liquor oxidation on sulfur gas emissions from the lignin recovery process To evaluate the sulphur compound outgassing potential from the LignoForce System™ as compared to conventional lignin recovery processes (in which no black liquor oxidation is employed) a bench-top setup was used. One liter of softwood black liquor was used for each experiment and three levels of black liquor oxidation were employed: a) no oxidation (conventional process), b) oxidation until a residual sulphide concentration of 0.5 g/L was reached, and c) oxidation until no residual sulfide was detected. In each case, four filtrate samples were collected in 0.5 L Nalgene bottles. Samples were collected during the following steps: : a) 1F: the filtration of the acidified BL (lignin slurry), b) 1AW: the first lignin cake acid wash, c) 2AW: the second lignin cake acid wash, and d) WW: the lignin cake water wash and air blow. Each Nalgene bottle was tightly connected to 1-L Tedlar sample bags in order to collect the corresponding sulphur compound-containing gas samples. Therefore, four sulphur compound-containing gas samples were collected during each one of the four lignin processing steps, namely: lignin slurry filtration, 1st acid lignin cake wash, 2nd acid lignin cake wash, and lignin cake water wash and air blow. At the end of each filtration/washing step, the gas bag was filled to its 1-L full capacity by blowing nitrogen, as needed, through the lignin cake. Hence, some gas stripping is expected to have taken place due to the blowing of nitrogen through the cake – this practice, is expected to have significantly increased the amount of gas collected in each case. Therefore, the results obtained in this study represent the worst-case scenario since no gas stripping would occur in commercial lignin plants except for the last step in which the lignin cake is air dried [16]. The collected samples were analyzed using an SRI 8610 gas chromatograph (GC). The samples were introduced into the GC via a 10-port gas sampling valve with a 1-mL sample loop. The various compounds in the injected samples were separated on a 60-m MXT-Volatiles column (Restek) and detected on-line by a sulphur-specific flame photometric detector. The speciated sulphur compounds (hydrogen sulphide, methyl mercaptan, dimethyl sulphide, dimethyl disulphide, and sulphur dioxide) were quantified using an external calibration curve prepared using standard sulphur-containing gases.



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The recovered filtrate samples were analyzed for dry solids, UV-lignin, sulphate, thiosulfate and total sulphur content, while the recovered lignin samples were analyzed for dry solids, UV-lignin, calorific value, carbohydrates, sodium, sulphur and ash content, weight average molecular weight (Mw) and number average molecular weight (Mn), functional groups, glass transition temperature, and initial and final decomposition temperatures. The procedures used are described elsewhere [12, 17-19]. Impact of black liquor oxidation on sulfur compound emission from the lignin product To evaluate the impact of black liquor oxidation on sulfur gas emissions from the final lignin product, about 0.05-0.1 grams of lignin produced from unoxidized or oxidized black liquor were conditioned at 60°C,100°C, 125°C, 150°C, 175 °C and 200 °C in a sealed 20-mL glass vial for 30 minutes prior to analysis. 1-mL headspace samples of each lignin were analyzed by gas chromatography (GC). Compounds were detected using a photoionization detector (PID). It should be noted here that the solids content of the lignins produced from unoxidized and oxidized black liquors was 65.5 and 62.2%, respectively. Impact of black liquor oxidation on volatile organic compound emission from the lignin product Lignins produced from unoxidized and oxidized black liquors were freeze-dried overnight and analyzed by Pyrolysis-GC/MS to evaluate the emission of volatile organic compounds after heating the lignin samples at different temperatures: 100, 125, 150, 175, 200, and 225°C. The following conditions were used: sample size: about 200µg; Pyrolyser: Frontier PY2020iD/AS1020E; Desorption time in pyrolyser furnace: 1min (using Helium); GC/MS: Varian 3900/2100Saturn;.Column used for the analysis of desorbed compounds: DB-17MS, 30 meters x 0.25mm x 0.25um; Flow rate: 1mL/min, split 50:1; TIC scan: 30-400 amu. RESULTS AND DISCUSSION Black liquor characterization The range of values obtained for various parameters of interest from the characterization of several softwood, hardwood and eucalyptus black liquors over the last few years is presented in Table 1. In this Table, the first and second lines show the total solids in black liquor (wt.%) and the pH, respectively, while all other values are expressed as a percentage of black liquor solids (BLS) except for calorific value which is expressed in units of BTU/lb of BLS.


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Table 1. Chemical analysis of several SW, HW and Eucalyptus black liquors Parameter

Softwood BL (from 6 pulp mill liquors)

Hardwood BL (from 4 pulp mill liquors)

Eucalyptus BL (from 3 pulp mill liquors)

27 – 35

24 – 40

42 – 46

12.9 – 13.3

13 – 14

13 – 14

Residual Effective Alkali, wt.%

2–4

3–6

2–4

Sulphide, wt.%

7 – 13

2–8

1–2

UV lignin, wt.%

34 – 36

27 – 38

39 – 50

Na, wt.%

17 – 20

18 – 21

18 – 21

S, wt.%

4–5

3–5

3–5

Total sugars, wt.%

2–4

1–4

1–6

0.2 – 0.4

0.3 – 0.7

0.05 – 0.7

5900 – 6600

5100 – 6300

5400 – 6000

Total Solids, wt.% pH

Soap, wt.% Calorific value, BTU/lb

As seen in Table 1, liquors from Eucalyptus kraft pulp mills were characterized by high total solids, high lignin content, and high calorific value, which is beneficial for the lignin recovery process in terms of yield and lignin filterability. In general, the higher are the total solids and the lignin content in the black liquor, the higher is the yield and the better is the lignin filterability. In addition, all liquors were characterized by low soap content. Usually, the lignin filtration resistance increases when the soap content is too high. Liquors from HW kraft pulp mills were characterized by high residual effective alkali (REA). As a result of the relatively high REA in these liquors, the carbon dioxide required to reduce the pH of these liquors to pH=9-10 was high. Since purchased carbon dioxide is the main operating cost for any given lignin plant (representing as much as 50% of the total costs), this will inevitably lead to higher operating costs in the case of HW mills. Liquors from HW kraft pulp mills were also characterized by low calorific value. Since the minimum black liquor calorific value required to operate a recovery boiler is around 5200 BTU/lb, in many cases, a limited amount of lignin can be taken out of the BL without recovery boiler operation being affected (e.g. recovery boiler blackouts). We previously demonstrated using modeling that about 150 tonne/day and 75 tonne/day of lignin can be taken out of typical Canadian 1000t/d SW and HW mills, respectively, before running into problems with operating the recovery boiler [20]. However, with increasing lignin production other problems could arise in other areas of the mill such as the evaporators and the power boiler. On the other hand, liquors from SW kraft pulp mills were characterized by high residual sulphide, thereby, increasing the volatile sulfur compound emissions if no black liquor ACS Paragon Plus Environment


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oxidation is applied. For this reason, we focused on studying the emission of volatile sulphur compounds from the conventional and LignoForce™ processes as applied to a softwood BL. Lignin characterization The range of values obtained from the characterization of lignins produced from several softwood, hardwood and eucalyptus oxidized black liquors using the LignoForce System™, is presented in Table 2. Table 2. Chemical and physico-chemical analysis of lignins recovered from several SW, HW and Eucalyptus BLs. Parameter

SW Lignins (from 6 pulp mill liquors)

HW lignins (from 4 pulp mill liquors)

Eucalyptus lignins (from 3 pulp mill liquors)

Total solids (as-pressed), wt.%

45 – 65

45 – 68

45 – 66

Total solids (air-dried), wt.%

94 – 99

95 – 98

87 – 96

pH @ 15% solids

1.7 – 5.9

1.9 – 3.8

1.8 – 3.9

Ash, wt.%

0.10 – 1.5

0.04 – 1.23

0.30 – 0.80

Na, wt.%

0.05 – 0.6

0.03 – 0.30

0.03 – 0.1

S, wt.%%

1.3 – 2.1

1.7 – 2.6

1.7 – 2.1

UV-lignin, %

92 – 98

96 – 97

95 – 97

Carbohydrates, wt.%

1.2 – 2.4

0.26 – 1.8

0.25 – 1.1

0.20 – 0.50

0.11 – 0.40

0.28 – 0.30

Aliphatic OH, mmol/g

1.4 – 2.1

1.3 – 1.6

1.1 – 1.2

Phenolic OH, mmol/g

1.3 – 1.8

2.2 – 3.2

3.1 – 3.3

Condensed OH units, mmol/g

1.1 – 1.6

0.31 – 0.5

0.33 – 0.38

Total OH groups, mmol/g

4.3 – 5.7

4.1 – 5.6

4.7 – 5.2

Glass transition temp., °C

162 – 185

133 – 152

118 – 128

Initial decomposition temp., °C

212 – 358

220 – 260

255 – 266

Molecular weight (Mw) using the UV detector

6000 – 12500

2635 – 6249

2100 – 2700

3.0 – 4.6

2.4 – 4.1

1.9 – 2.9

11200 – 11800

10200 – 11200

10800 – 11200

Carboxylic OH, mmol/g

Polydispersity index Calorific Value, BTU/lb

Particularly notable is the low ash content, high lignin content and high calorific value of all lignins. Hence, it appears that these lignins are sufficiently pure and rich in calorific ACS Paragon Plus Environment

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value to be used as a fuel in the lime kiln and the power boiler (or most other combustion processes) or in any one of several possible high-value applications. It is also worth noting here that, even though all liquors were diluted to about 30 wt.% dissolved solids prior to entering the lignin precipitation reactor, the solids content of the lignin cake recovered from the filter press was quite high in most cases. This suggests that the cost of drying the lignin to over 95% solids (if needed) will be relatively low as compared to the lignin cakes recovered from a belt filter which are usually much lower in solids (32-35% solids). Analysis of the lignins using Size Exclusion Chromatography (SEC) showed some differences in the Mw and the polydispersity index (Mw/Mn) of the three types of lignin. The Mw of lignin recovered from the HW and Eucalyptus black liquors was about half of the lignin recovered from the SW black liquors. This difference in Mw is also reflected in the thermal properties of the three lignins, with the softwood lignins having glass transition and initial decomposition temperatures that are higher by about 30 oC and 50oC, respectively, compared to the other two lignins. The thermal properties of lignin are very important in relation to the processability and thermal stability of lignin-polymer blends. Based on the data shown in Table 2, it appears that the glass transition temperature of all lignins is in the same range as the processing temperatures of most major common polymeric materials (e.g. polypropylene (PP)) while the first decomposition temperature is sufficiently high to allow, for example, for the conversion of these lignins to carbon fiber at high product yields. Based on prior work in our lab, it appears technically possible to incorporate as much as 20wt% kraft lignin into PP without any major impact on physical properties as long as suitable coupling agents and plasticizers are used. In Table 2, we also observe that the total number of hydroxyl groups in HW and Eucalyptus lignins is almost the same as that obtained in lignin recovered from SW black liquors. However, the distribution of hydroxyl groups (number of carboxylic acid groups, phenolic hydroxyl groups, aliphatic hydroxyl groups and condensed hydroxyl group units) was different between SW, HW and Eucalyptus lignins. The HW and Eucalyptus lignins contained a slightly lower amount of aliphatic but a much higher phenolic hydroxyl group content and less condensed unit hydroxyl groups as compared to the SW lignin. The latter observation is consistent with literature results indicating a lower number of C-C linkages in hardwood-derived lignins as compared to softwood-derived ones. The softwood lignins also contained a higher content of guaiacyl-type hydroxyl groups compared with the other two lignins suggesting better suitability for use as a phenol replacement in phenol formaldehyde resins.



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Impact of black liquor oxidation on lignin extraction process In order to evaluate the effect of BL oxidation on the lignin extraction process, lignin was recovered from several oxidized and unoxidized black liquors in our pilot plant in Pointe Claire, Quebec as well as our demonstration plant in Thunder Bay, Ontario. As expected, during the recovery of lignin from unoxidized BL, no operational problems were experienced, in our demo plant, in lowering the BL pH to 9.5 thereby inducing lignin to come out of solution. However, the samples taken for chemical analysis were very odorous and, frequently, set off an H2S detector (set at 10 ppm) placed over their headspace. In addition, the wash/press system generated significant fugitive H2S emissions. A headspace test was done again after the lignin slurry had coagulated for 2 hours and the sample immediately sent the personal H2S monitor into high-high alarm which was set at 15 ppm. As a result, it was not possible to process unoxidized liquor on the same day. Instead, it was decided to let the slurry ″degas″ overnight, to be run on the following day. When the slurry was tested again on the following day, it was still setting off the H2S alarm (set at 10 ppm) confirming that the ″degas″ time was not sufficient. Since our Thunder Bay demo plant is designed to collect all foul gases when closed up, we opted to run the skid with all inspection covers closed. Furthermore, the acid wash step was not successful since the filtration rate was very low. Therefore, we aborted the wash and opted to proceed with the squeeze and cake blow steps to dewater the poorly washed cake. Again, the process was operated very slowly, closing valves and stopping the process to let the venting catch up; again, at this time, the area H2S alarm, which was set at 10 ppm, was activated. While some of the vapour was captured in the hood, there was inadequate draw to prevent odours from escaping. In contrast, when the BL was oxidized, all steps went smoothly. The TRS emissions were negligible. A similar outgassing behavior was observed when a bench-top setup was used. In this case, the gases were collected and quantified as described in the experimental section of this paper. Table 3 shows the pH and dissolved sulphur compounds in the filtrates produced from different stages of the lignin recovery process under three different oxidation conditions (no oxidation, oxidation to 0.5g/L of residual sulfide, and oxidation to negligible levels of residual sulfide). In this study, only SW black liquor was used since such liquors are usually higher in residual sulphide (see Table 1).


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Table 3. pH and sulphur profile of the filtrates produced, under different oxidation conditions, from different stages of the lignin recovery process Oxidation level

Filtrates

pH

H2S

MeSH

DMS

DMDS

SO2

9.8

274

0.0144

0.163

0.00470

ND

9.5

1440

9.90

4.19

0.0168

ND

2 acid wash filtrate

1.3

2320

3.93

0.636

0.620

ND

Water wash filtrate

1.8

363

1.02

0.0908

0.0113

ND

1 lignin cake filtrate

9.6

0.800

0.00430

0.0626

0.0490

ND

1st acid wash filtrate

9.1

1.50

0.0353

0.274

0.179

0.0452


1.5

31.4

1.093

0.0342

0.0897

20.9

Water wash filtrate

st st

nd

1 lignin cake filtrate Unoxidized BL


st

BL oxidized to 0.5 g/l S2-

nd

2.0

16.2

0.731

0.0111

0.102

0.217

st

9.5

0.0

0.0

0.0126

0.0231

0.0183

st

8.9

32.4

0.0191

0.0390

0.109

5.54

nd


1.5

28.9

0.280

0.0102

0.0658

26.3

Water wash filtrate

1.6

6.8

0.0687

0.00560

0.0241

0.0550

1 lignin cake filtrate Fully oxidized BL

Dissolved sulphur compounds in the filtrates (as mg of S)


The pH values of the 1st lignin cake filtrate (pressate) and the 1st lignin cake acid wash filtrate ranged from 9.5 to 9.8 and 8.9 to 9.5, respectively, while the pH values of the 2nd lignin cake acid wash and the water wash filtrates ranged from 1.3 to 1.5 and from 1.6 to 2.0, respectively. It must be clarified here that the pH values presented in Table 3 are the values obtained for the total volume of filtrate collected in each experiment. However, in the two lignin cake acid wash steps, it is expected that the pH of the later fractions would be lower than the earlier fractions thereby being more likely to liberate pH-sensitive TRS compounds such as hydrogen sulphide and mercaptan. In contrast, in the lignin cake water wash step, the pH of the later fractions is expected to be higher than the earlier fractions, levelling off to a steady level once most of the residual acid is removed from the lignin cake. In addition, Table 3 shows the amounts of H2S, MeSH, DMS, DMDS, and SO2 dissolved in the four filtrates recovered during the lignin cake filtration, acid wash (2 stages) and water wash steps (expressed as mg of sulfur). As shown in this Table, the lignin cake filtrates produced from the unoxidized black liquor contained higher amounts of H2S and MeSH as compared to that obtained from the oxidized black liquors. The amount of DMS and DMDS, which are not pH sensitive, were low in all cases. In addition, while no SO2 was detected in the filtrates during the filtration and washing of the lignin produced from the unoxidized black liquor, small amounts were detected during the production of the lignin from oxidized black liquor. However, these amounts were much lower as



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compared to the amount of H2S and MeSH produced from the unoxidized black liquor. In addition to the sulfur compounds mentioned above, other soluble sulfur species were detected (but not shown here) such as SO32-, SO42-, S2O32- and S2-. The filtrates obtained from the oxidized black liquor contained higher amounts of S2O32- as compared to that obtained from unoxidized black liquor (results not shown here), which demonstrated the conversion of sodium hydrosulphide (NaHS) to sodium thiosulfate (Na2S2O3) during black liquor oxidation. The latter is converted (as shown below) to SO2 during the acid washing step. Figure 2 shows the amounts of H2S, MeSH, DMS, and DMDS generated in the headspace of the 0.5L Nalgene bottles containing the filtrates from the lignin cake filtration, acid wash (2 stages) and water wash steps.

Figure 2. H2S, MeSH, DMS, and DMDS emissions based on lignin final product from the four main lignin production steps: 1F – lignin cake filtration (pressing); 1AW – 1st lignin cake acid washing; 2AW – 2nd lignin cake acid washing and WW – water washing of the lignin cake. As seen in this figure, in the case of unoxidized BL, H2S was produced in all processing steps. About 5g of H2S/kg of lignin was generated during the first filtration step (pH=9.8) while about 26g of H2S/kg of lignin was produced during the first acid wash step (pH=9.5). The maximum amount of H2S (about 42.1g of H2S/kg of lignin) was detected during the 2nd acid wash step (pH=1.3). In addition, about 6.6g of H2S/kg of lignin was ACS Paragon Plus Environment

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detected in the water wash step (pH=1.8) indicating possible contamination of the lignin cake with H2S. However, negligible amounts of H2S were produced from all steps when the BL was oxidized to either 0.5g/L residual sulphide or no residual sulfide. This is likely to be due to the oxidation of Na2S to non-volatile compounds in the LignoForce System™ in the two latter cases. As compared to the H2S generated, only small amounts (below 0.3g of organic TRS/kg of lignin) were detected for MM, DMS and DMDS in all cases (with and without BL oxidation). However, the amounts of these gases were negligible in the case of the two LignoForce™ cases where the BL was oxidized to 0.5g/L and negligible levels of residual sulfide as compared to the conventional process where no BL oxidation was applied. As expected, in the case of unoxidized black liquor, the emission of DMS and DMDS did not demonstrate any clear dependence on pH with DMS emission peaking during the first acid wash while DMDS emission peaking during the second acid wash. The results shown in Figure 2 are consistent with the main sulphur species that are expected to be present in solution at any given pH in consideration of the pKa of hydrogen sulphide which is around 7.04 and that of mercaptan which is about 10.33 [21]. As mentioned before, in the case of mercaptan, at pH 10.33, 50% of mercaptan molecules are in the non-volatile sodium form (sodium mercaptide, CH3SNa), while the remaining 50% are in the easily volatilized acid form (mercaptan, CH3SH). Similarly, at pH 7.04, 50% of hydrogen sulphide molecules are in the non-volatile sodium form (sodium hydrosulphide, NaHS) while the remaining 50% are in the easily volatilized acid form (hydrogen sulphide, H2S). It is known that H2S can only be produced at pH levels below 10 as is the case with all filtrates shown in Table 3. Therefore, when no oxidation is applied before BL acidification, it is expected that H2S would be produced at every stage of the lignin recovery process (from acidification to final acid wash). As mentioned above, the pH values presented in Table 3 are the values obtained for the total volume of filtrate collected in each experiment. Therefore, since during the lignin cake acid washing steps, the pH of the later fractions was lower than the earlier ones, pH-sensitive TRS compounds such as hydrogen sulphide and mercaptan were liberated when the conventional process was used. The generation of TRS compounds in the case of the two oxidized black liquors was rather limited because these compounds were oxidized to nonvolatile sulphur species. The LignoForce benefits will be particularly pronounced when lignin does not undergo extensive washing. For a conventional process, less washing would result in an odorous lignin containing large quantities of reduced sulfur components. As a result of BL oxidation in the LignoForce System™, product odor will not be as much of a problem even if less extensive washing is performed.



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Figure 3 shows the amount of SO2 generated in the headspace of the 0.5L Nalgene bottles containing the filtrates from the lignin cake filtration (pressing), first acid wash, second acid wash and water wash steps.

Figure 3. SO2 emission based on lignin final product from the four main lignin production steps: 1F – lignin cake filtration (pressing); 1AW – 1st lignin cake acid washing; 2AW – 2nd lignin cake acid washing and WW – water washing of the lignin cake. As seen in this figure, small amounts of sulphur dioxide were detected in the samples that were collected during the filtration and washing of lignin recovered from the two oxidized black liquor samples (LignoForce System™) while no SO2 was detected in the case of unoxidized BL. The results shown in Figure 3 are consistent with the main sulphur species that are expected to be present in solution at any given pH in consideration of the pKa of of sulfur dioxide which is around 1.8 [21]. This means that, at pH 1.8, 50% of sulphur dioxide molecules are in the non-volatile sodium form (sodium bisulfite, NaHSO3) while the remaining 50% are in the easily volatilized acid form (sulphur dioxide, SO2). The emission of SO2 in the case of the two oxidized black liquors is due to the relatively mild oxidation conditions (low temperature, low degree of oxidation) used in these two experiments. Under these conditions, not all hydrosulphide ions were fully oxidized to sulphate. Instead, some other partially oxidized sulphur species such as thiosulfate were generated. It is well known in the literature that when thiosulphate is exposed to pH levels below 4 (e.g. during lignin sulphuric acid washing step), sulfur dioxide (SO2) could be generated according to the following reaction [22]: (5)

S2O32- (aq) + 2H+ (aq) → H2O + SO2 (g) + S (s) ACS Paragon Plus Environment

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Since significant amounts of sulfuric acid were used during the lignin cake acid washing steps, the pH of the acid filtrates was reduced to levels close to the pKa of bisulphite (HSO3-), leading to the generation of sulfur dioxide (SO2) during these two steps. As seen in Figures 2 and 3, the levels of sulphur compound emission from the LignoForce System™ are almost 2 orders of magnitude lower as compared to the levels of sulphur compounds produced in the conventional process where no BL oxidation was used (e.g. during the 2nd acid wash, 1.2 g/kg of lignin of SO2 was produced from the LignoForce System™ vs. 42.10 g/kg of lignin of H2S was produced from the conventional process). The low SO2 emissions from the LignoForce System™ are expected to be vented away from the lignin plant to the mill Non-condensible Gas (NCG) system. In addition, when Noram’s modified washing cycle is used, the SO2 emission levels will be further reduced since the pH of the filtrates will be higher. It is worth mentioning here that in this study, a significant amount of nitrogen was used in each step to fill the gas bags to their 1-L full capacity in order to make sure that all the gases produced in the system are captured in the bags. Therefore, an excess amount of sulfur-containing gas would have been stripped from the filtrates and/or the lignin cake during each step, thereby increasing the amounts of gases that were detected. Hence, much lower amounts of TRS and sulfur dioxide are expected in industrial installations. This was verified in our LignoForce™ Demo plant in Thunder Bay, Ontario as well as the first industrial installation of the LignoForce System™ at Hinton, Alberta, Canada. Impact of black liquor oxidation on volatile sulphur compound emissions from the lignin product Table 4 shows the emission of several volatile sulfur compounds such as hydrogen sulphide, mercaptan, dimethyl sulphide, dimethyl disulphide, and sulphur dioxide from softwood lignin expressed on a lignin weight basis (mg of sulphur compound/kg of lignin). These emission levels were established by conducting gas chromatographic analysis of the headspace in 20-mL glass vials after heating the lignins recovered from unoxidized and oxidized black liquors for 30 minutes at different temperatures ranging from 60°C to 225°C.



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Table 4. Total emissions of volatile sulphur compounds from lignin recovered from oxidized and unoxidized black liquors following heat treatment of the lignin samples for 30 minutes at different temperatures ranging from 60°C to 225°C (expressed on a lignin weight basis). Lignin

Lignin recovered from unoxidized black liquor

Lignin recovered from oxidized black liquor

Temp. (°C)

H2S

MeSH

DMS

DMDS

SO2

60

0.01

0.99

0.13

0.25

0.29

Total sulfur compound emissions on S basis (mg/kg) 1.05

100

1.04

4.42

0.20

2.82

0.15

5.90

2.22

4.27

0.41

9.16

4.32

13.6

9.74

20.0

0.11

21.32

43.6

58.9

175

34.7

50.4

0.89

84.62

94.8

171.8

200

50.0

67.7

1.23

100.5

132.7

227.6

60

0.01

0.54

LignoForce System for the Recovery of Lignin from Black Liquor

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