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|[ Article ]|
|Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 54, No. 6, pp. 94-105|
|Abbreviation: J. Korea TAPPI|
|ISSN: 0253-3200 (Print)|
|Print publication date 31 Dec 2022|
|Received 01 Dec 2022 Revised 20 Dec 2022 Accepted 22 Dec 2022|
|Characteristics of Fuel Pellets Made by Mixing Paper Sludge and Forest Byproducts|
Hyeong-Hun Park1 ; Jin-Hwa Park2 ; Chul-Hwan Kim3, † ; Chung-Ha Lee4 ; Ji-Su Lee4
|1The Lab. of Advanced Materials for Pulp and Paper, Gyeongsang National University, Full-time Researcher|
|2Department of Forest Products, Gyeongsang National University, Graduate Student|
|3Major of Environmental Materials Science, IALS, Gyeongsang National University, Professor|
|4Department of Environmental Materials Science, Gyeongsang National University, Undergraduates,|
|Correspondence to : †E-mail: email@example.com (Address: Major of Environmental Materials Science, IALS, Gyeongsang National University, Jinju, 52828, Korea)|
Funding Information ▼
In this study, for reusing the sludge generated in a fine paper mill as fuel, the characteristics of the pellets prepared by mixing auxiliary raw materials such as pulp mill chip rejects, camellia oilseed cake, Sancho pruning remnants, and mulberry pruning remnants were evaluated. The pellet yield decreased as more sludge containing about 50% of the inorganic fillers was mixed with each auxiliary material, but the camellia oilseed cake, unlike the others, was of great help in improving the pellet yield. The Van Krevelen diagram showed that, as more sludge was added to the woody biomass in preparing the pellets, they tended to have a similar elemental composition to that of coal or lignite. The sludge addition to the woody biomass had a negative effect on the calorific value of the pellets, but the lower calorific value of 2400-3000 kcal/kg was acquired even when more than 50% of sludge was added. The sludge pellets containing the camellia oilseed cake had a faster ignition point than those mixed with the other woody biomass, but the flame duration was shorter. This study showed that the sludge from fine paper mills containing about 50% of cellulose fibers could be used to produce economical solid fuels if mixed with other woody biomass in an appropriate ratio.
|Keywords: Paper mill sludge, woody biomass, solid fuels, pellets, calorific value
Although the demand for carbon-neutral fuel is continuously increasing, the domestic self-sufficiency rate is very low. In 2021, the self-sufficiency rate of wood pellets was only 17.2%, and about 3.2 million tons of wood pellets were imported. According to Korea Forest Service statistics, Korea’s wood self-sufficiency rate in 2020 is only about 16%. With this level of low self-sufficiency, it is impossible to supply the raw materials necessary for the demand for wood pellets. The Korea Forest Service is promoting a plan to actively utilize unused forest biomass scattered in forests to solve the problem of shortage of woody raw materials. To encourage the use of unused forest biomass, the Ministry of Trade, Industry and Energy of Korea imposes the weight of Renewable Energy Certificate (REC) of 1.5 for co-fired power generation of wood pellets made from unused forest biomass with coal and the weight of REC 2.0 for power generation by burning the pellets only.
It also determined the sunset for REC weights for imported wood pellets in 2025. On the other hand, unused forest biomass was left unattended to 3,578,000 m3 in 2017, but as the REC weight for the unused forest biomass was changed, its supply began to sharply increase from 220,000 tons in 2019 to 800,000 tons in 2021. For reference, the unused forest biomass refers to products that do not meet the log standards or are difficult to collect them, among products generated from domestic forest management activities, etc.
The amount of paper sludge generated at domestic pulp and paper mills amounts to about 1.44 million tons annually. Some of the sludges are incinerated in the mills own boiler or used for cement raw materials, steelmaking calming agents, etc., but they are still difficult to recycle. In accordance with the policy of prohibiting the dumping of paper mill sludge (PMS) to the sea in 2012, the paper industry in Korea has been striving to reduce the amount of paper sludge generated by improving processes and introducing new facilities.[3-6] The typical way to PMS disposal is landfill. However, environmental authorities in Korea estimate that more than half of the over 200 public landfill sites will reach saturation by 2030. Due to the shortage of landfills in the future, the cost of PMS disposal will continue to rise, which is likely to put a big burden on the paper industry. Accordingly, the Korean paper industry is struggling to come up with a new alternative for PMS treatment.
The chemical composition of PMS often exceedingly varies from mill to mill. The principal organic components are mostly cellulose fibers and the predominant inorganic components are calcium carbonate and clay. Lee et al. reported that cellulose fibers contained in PMS could be used for growing crops after composting with soil microorganisms.[ 3] Song et al. showed that PMS in paper mills to produce printing and writing paper could be used as a raw material for manufacturing biodegradable seedling pots. Tucker confirmed that it did not show phytotoxicity by using the fertilization technique of paper mill sludge mixed with fruit and vegetable waste to prepare fertilized soil. Camberato et al. confirmed that the fermentation of pulp and paper mill sludge positively affected organic matter, physical properties, nutrient sources, and pH in the soil. These studies can be regarded as empirical examples showing that PMS does not contain harmful substances to the soil environment.
In Korea, a large amount of unused lignocellulosic biomass, such as pruning remnants and leftovers after squeezing the oil of tree fruit, has been generated during forest management activities. Lee et al. confirmed that the bio-pellets using camellia fruit peels and toothache fruit peels left after squeezing oil had better effects than wood pellets in terms of initial ignition and combustion duration.
Therefore, in this study, solid fuels were prepared by mixing the above lignocellulosic byproducts with excellent properties in ignition and combustion with PMS generated in wood-free paper mills, and the fuel characteristics were analyzed. Since the fuel efficiency of PMS itself generated from woodfree paper mills was not good, it was expected that addition of these lignocellulosic byproducts would have a very positive effect on the production of PMS-based solid fuels.
Wood chip scraps and chip rejects from the hardwood wood chip yard provided by Moorim P&P Ltd. Co. in Korea were used as raw materials for manufacturing pellets. Hardwood chips are a 50:50 mixture of domestic oak and Vietnamese acacia. Paper mill sludge (P) was supplied by the same paper mill. The auxiliary raw materials used to make the pellets were camellia (Camellia japonica) oilseed cakes, sancho (Sapium japonicum) pruning remnants, and mulberry pruning remnants. The camellia oilseed cakes were provided by the Korea Camellia Research Institute in Tongyeong, Gyeongsangnam-do, and the sancho pruning remnants were supplied by sancho oil production corporation in Hadong, Gyeongsangnam-do, and the mulberry pruning remnants were provided from the academic forest of Gyeongsang National University. Table 1 shows the types of raw materials used to manufacture pellets.
It is vital to note that the particle size of the raw materials affects the required pressure of the pelletizer and its production efficiency. Therefore, to manufacture high-quality pellets, pretreatment must be carried out considering the performance of the pelletizer. H and C were ground to a particle size that passed through a 4 mm sieve using a laboratory blender, and no further screening was performed. S and M were pulverized into small pieces by a mobile wood crusher (refer to Fig. 1) and then ground to a particle size that passed through the 4 mm sieve using the laboratory blender.
In the earlier study, after preparing the pellets by mixing C, S, M, and H, the quality characteristics of the pellets were compared. Here, it was confirmed that the mixing ratio of C:H=7:3 (CH), S:H=3:7 (SH), and M:H=7:3 (MH) showed the best results. As shown in Table 2, P was mixed with CH, SH, and MH in proportions of 7:3, 5:5, and 3:7.
|Mixing ratios of raw materials|
|C:H = 7:3||S:H = 3:7||M:H = 7:3|
The pellets were made with a laboratory pelletizer equipped with a flat die (dia. 6 mm) capable of producing 500 kg of pellets per hour (refer to Fig. 2). The gap clearance between the roll and the die was adjusted to about 0.1-0.3 mm, so that a pressure of about 50 MPa was applied. Before preparing the pellets, the moisture content of the raw materials was adjusted to 12-15%.
* Indicates that pelletization was not successful.
The moisture content (MC) of the prepared pellets was measured using the automatic moisture analyzer (Ohaus, USA). The ash content (Ash) was measured according to ISO 18122. The percentage of volatile matter (VM) was determined according to ASTM D3175-20 (2020). A total of 2 g of the pulverized sample in a crucible with a lid cover was placed in a drying oven at 105±3℃ until a constant weight was reached. The sample was then kept in a furnace at a temperature of 950±13°C for 6 min, and then weighed after cooling in a desiccator. Three replicates were analyzed for each sample. VM was calculated using the following Eq.1:
The percentage of fixed carbons (FC) was calculated as in Eq. 2 as follows:
The total contents of carbon, hydrogen, sulfur, and nitrogen were determined according to ISO 16948 (2015) using the macro elemental analyzer (Vario MACRO, Germany).
The durability of the prepared pellets was measured according to ISO 17831-1. The pellets weighing around 500 g were placed in the rotary tester (KOS1, Korea) and rotated 500 times at a constant speed. The durability was determined by sieving the sample on a sieve with a hole 3.15 mm in diameter.
Fines contents of the prepared pellets were determined according to ISO 18846. Fines are defined as particles less than 3.15 mm.
The heating value was measured by burning the sample with the automatic bomb calorimeter (Calorimeter, Parr, Germany) following ISO 18125 (2017).
The pellets made from different raw materials were ignited with a portable gas torch (Prince, Japan) at about 10 cm, and the ignition onset time and burning duration of each pellet was measured using a digital timer and digital camera (Samsung Electronics, Korea). The ignition onset time of the pellet was set to the time when the flame stably revived without being extinguished, and the combustion duration was set to the time that the flame remained until it was extinguished after ignition.
Paper mill sludge (P) contains calcium carbonate of about 50% and cellulose fibers of about 50% not retained in the papermaking process. As more P was added to the other raw materials mixed with the camellia oilseed cake (C), the sancho pruning remnants (S), the mulberry pruning remnants (M), and the hardwood chip reject (H) in specific proportions, it had a negative effect on pelletization. Fig. 2 shows the images of the pellets prepared by mixing the different raw materials. When P over 50% was mixed with SH and MH, respectively, the pelletization were not performed properly, so the pellet yield dropped to less than about 15% or the input materials was discharged in the form of pulverized powder. For the CHP pellets, when P was mixed more than 70%, the forming ability was sharply deteriorated and the pellet yield fell to less than 40. Nevertheless, CHP showed a much higher pellet yield than SHP and MHP. It was considered that starch contained in the camelia oilseed cake among the raw materials of CHP had a very positive effect on pelletization. Starch or modified starch is widely used as an adhesive for pelletizing. When the starch is exposed to heat generated between the flat die and the forming roll of the pelletizer, it is gelatinized by the moisture of the lignocellulosic biomass and the starch themselves and develops adhesive strength.[13,16] Sarmah et al. reported that the camellia oilseed cake included 27-71% starch. In general, starch has been well known as a critical additive for forming pellets and improving their durability. Unlike the SHP pellets and the MHP pellets, most of the CHP pellets were found to be very polished on their surfaces.
Proximate testing of the pellets measures moisture content, ash content, volatile content, and fixed carbon. Fig. 3 compares the ash contents of the pellets prepared by mixing the different raw materials. When P is included as a raw material for pellets, it is bound to affect the increase in ash content. As P was added to the raw materials mixed with the camellia oilseed cake (C), sancho pruning remnants (S), and mulberry pruning remnants (M) and hardwood chip reject (H) in certain proportions, the ash contents of the pellets increased from a minimum of about 20% to a maximum of about 40%. In particular, CHP showed higher ash content than SHP and MHP, regardless of the mixing ratios. It was assumed that the starch included in the camellia oilseed cake (C) held more filler particles of P during pelletization.
Fig. 4 compares the volatile contents of the pellets prepared by mixing the different raw materials. Volatile matters are those components that can be easily burnt out under the presence of oxygen, except for moisture, as one of the key parameters tested in solid fuels. There was no significant difference in the volatile matters regardless of the types of mixed raw materials or the mixing ratio of different raw materials. However, as the ratio of the sludge P increased, the volatile matter decreased.
Fig. 5 compares the fixed carbon contents of the pellets prepared by mixing the different raw materials. The fixed carbon is the remaining residue in the char after volatile matters of solid fuels are driven off. On the other hand, total carbon from ultimate analysis includes some organic carbon that is removed with the release of the volatile matter during combustion. The solid fuel with lower fixed carbon content has a shorter burning duration with easier ignition. CH and SH without the sludge (P) showed higher contents of the fixed carbon than MH. However, the addition of P to CH and SH led to a sharp decrease in the fixed carbon contents. On the other hand, for CH, the fixed carbon content was greatly reduced only when more than 30% of P was mixed. It was estimated that the greater decrease in the fixed carbon content of the CHP pellets compared to the SHP and MHP pellets was due to the higher ash content of the CHP pellets up to about 9% (refer to Fig. 3).
Fig. 6 compares the moisture contents of the pellets prepared by mixing the different raw materials. As the sludge (P) was added, the moisture content of the pellets slightly increased compared to before the addition of P. It was thought that P, which contains about 50% of cellulose fibers, contributed to the increase in moisture content of CHP pellets. Furthermore, it was estimated that the starch and fiber components remaining in Camellia oilseed cake (C) also had an effect on the greatest increase in moisture content of the CHP pellets with C of more than 70%. In contrast, SH and MH, which are composed only of lignin and carbohydrates, were not remarkably affected by the change in moisture content even after the addition of P.
In Fig. 7, ultimate analysis determined the contents of carbon, hydrogen, nitrogen, and sulfur in solid fuels. The total carbon contents of the pellets include inorganic carbon and organic carbon which is partly removed along with the volatiles during combustion. As the sludge (P) was added, the carbon contents of all pellets constantly decreased regardless of the types of the mixed raw materials. Oxygen and hydrogen were also reduced with the increased addition of P. The increased O/C ratio.
On the other hand, the contents of nitrogen and sulfur were slightly increased as more P was mixed. Since the paper mill sludge had little nitrogen and sulfur components, it was confirmed that these elements did not increase remarkably even when P was mixed with the other lignocellulosic biomass.
Fig. 8 is a Van Krevelen diagram used to assess the maturity of various pellets according to their atomic ratios, e.g., the plot of the H/C ratio vs. O/C ratio. From the Van Krevelen diagram, it could be seen that the atomic ratios, i.e., the H/C and O/C ratios decreased with the increased addition of P. It is interesting to note that, as the paper mill sludge, P, was added to lignocellulosic biomass, it moved out of the realm of pure biomass and towards the realm of coal or lignite. It means that more carbon relatively appears in the sludgerich pellets.
Fig. 9 compares the durability and fines contents of the pellets manufactured by adding paper mill sludge (P) at designated ratios. The pellets exhibited excellent durability over about 95%, irrespective of the types of raw materials and the mixing ratios of P. Among them, the durability of CHP pellets mixed with camellia oilseed cake showed slightly higher durability despite P addition. Moreover, the fines contents of the pellets were mostly less than 0.25%, and it could be seen that fines were rarely generated. In the end, it was confirmed that the paper sludge particles, in which inorganic substances such as calcium carbonate composes the majority, could have a minor influence on the generation of fines during the transport and storage of pellets.
Fig. 10 compares the calorific values of the pellets manufactured by adding paper mill sludge (P) at designated ratios. As the proportion of P increased, the calorific values of the pellets decreased regardless of the types of raw materials. Before sludge addition, CH, SH, and MP all had low calorific values of about 3900–4000 kcal/kg. However, as the sludge was added, the calorific value began to decrease and ultimately fell to 2400-3000 kcal/kg. After all, it is desirable to avoid using the sludge discharged from fine paper mills independently because it contains an excessive amount of inorganic substances and thus reduces fuel efficiency.
Fig. 11 displays the flame images of the pellets prepared by mixing different raw materials before and after P addition. Fig. 12 compares the ignition point and the flame duration of the pellets prepared by mixing different raw materials before and after P addition. Regardless of the P addition, the pellets containing camellia oilseed cake (C) ignited within the shortest time with the brightest flames. However, the flame duration was rapidly shortened to less than 50 seconds as more than 50% of P was added. It was considered that this was caused by the reduction of lignocellulose biomass which contributes to the long duration of burning. On the other hand, SHP and MHP were composed of relatively more lignocellulosic biomass than CHP, so their initial ignition was delayed, but the burning lasted longer than CHP.
* Pellets are not formed.
Since the characteristics of a fuel varied depending on the types of biomass mixed with the sludge, it was confirmed that the mixing of woody biomass suitable for the characteristics of the sludge was very important.
In conclusion, the sludge from paper mills to produce wood-free paper contains about 50% ash, making it difficult to use it independently as a fuel. Therefore, it was ascertained that the sludge should be mixed with auxiliary raw materials such as unused lignocellulosic biomass to improve fuel efficiency.
To reuse the sludge generated in the paper mill as fuel, pellets were prepared by mixing it with pulp mill chip rejects, camellia oilseed cake, sancho pruning remnants, and mulberry pruning remnants as auxiliary raw materials. The pellet yield decreased as more sludge containing about 50% of inorganic fillers was added to the auxiliary raw materials, but the camellia oilseed cake was very helpful in improving the yield. When the sludge was added to the woody biomass in preparing the pellets, it had a tendency to have an elemental composition such as coal or lignite, which could be confirmed through Van Krevelen diagram. The addition of the sludge to the woody biomass had a negative effect on the calorific value of the prepared pellets, and the lower calorific value decreased to 2400–3000 kcal/kg when more than 50% of the sludge was added. The pellets mixed with the sludge and the camellia oilseed cake had a faster ignition point than those mixed with the other woody biomass, but their flame duration became shorter. Since the sludge from the fine paper mill contains about 50% fibers, it was confirmed that it could be used as an economical raw feedstock for producing solid fuels if mixed with other woody biomass in an appropriate proportion.
This work was supported by the Program for Forest Convergence Professional Manpower Promotion, funded by Korea Forest Service in 2021 (FTIS Grant No. 2020186A00-2022-AA02).
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