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A review of the valorization of paper industry wastes by thermochemical conversion Miloud Ouadi, Antzela Fivga, Hessam Jahangiri, Muhammed Saghir, and Andreas Hornung Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00635 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019

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Industrial & Engineering Chemistry Research

A review of the valorization of paper industry wastes by thermochemical conversion Miloud Ouadi1,2,*, Antzela Fivga1, Hessam Jahangiri1, Muhammed Saghir1, Andreas Hornung1,2,3 1

University of Birmingham, School of Chemical Engineering, Edgbaston, Birmingham, B15 2TT, United Kingdom 2 Fraunhofer UMSICHT, Fraunhofer Institute for Environmental, Safety, and Energy Technology, An der Maxhütte 1, 92237 Sulzbach-Rosenberg, Germany 3 Friedrich-Alexander University Erlangen-Nuremberg, Schlossplatz 4, 91054 Erlangen, Germany *Corresponding

Author Email: [email protected]. Keywords: Bioenergy; paper industry; waste; thermochemical; pyrolysis; gasification ABSTRACT The paper and pulp industry is the sixth largest consumer of energy in the UK. Furthermore, the industry produces a significant amount of fibrous sludge and reject waste material, containing high amounts of useful energy. Currently the majority of these waste fractions are disposed of by landfill, land-spread, or incineration. These disposal methods not only present environmental problems, but are also very costly. This review explores how paper industry wastes can be valorized into useful energy vectors via advanced thermal conversion routes thus providing not only a solution for waste disposal but also a means of producing useful sustainable energy at paper mill sites. The scope of this work explores the application of advanced thermal conversion methods (gasification and pyrolysis) for the conversion of secondary fibre paper mill wastes into energy vectors. The order of the paper follows a specific structure. Initially, a detailed description is given concerning which wastes are generated from secondary fibre paper mills. This is followed by a brief review of the state of the art in waste management and energy systems currently used by paper mills. Then a review on advanced thermal conversion pathways as a solution to the dual issue of waste management and energy generation for secondary fibre paper mills is given, including details regarding the feasibility of integrating them into the current mill infrastructure. Finally, a discussion of the challenges associated with the proposed conversion pathways is given. 1

INTRODUCTION

Since the industrial revolution, the world has been consuming ever-increasing amounts of energy from fossil fuels. This is now placing immense pressure on our planet’s natural resources and the CO2 produced from the combustion of fossil fuels is widely believed to be the principal cause of climate change. Furthermore, there are increasing concerns over the price and security of supply of petroleum and natural gas 1, 2. Therefore, any attempt to decrease the demand on fossil fuels is considered to be environmentally beneficial. The paper industry is a significant user of energy in the form of electricity and heat. Whilst paper is made on all continents. The US, Japan, Canada and China represent the largest producer countries. These countries make more than half of the world’s pulp and paper 3. The Confederation of Paper Industries claims that each tonne of paper produced in the UK resulted in 0.6 tonnes of fossil

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CO2 in 2016 4. The paper industry accounted for 6% of greenhouse gas (GHG) emissions from UK industries in 2016, as shown in Figure 1.

Figure 1. Greenhouse gas (GHG) emissions from UK industries. Reproduced with permission from ref. 5, Copyright © 2017, Elsevier.

In addition, the total energy input at typical paper mills accounts for approximately 30% of the total cost of paper production 6. As the cost for the required energy increases year upon year, many paper mills are finding it increasingly difficult to remain profitable, and this has led to the closure of lower tonnage operations that manufacture commodity grade paper and board products. It is extremely difficult to reduce the high energy demand from this industry without compromising the rate of production or quality of the paper making process itself. Therefore, there is increasing interest in alternative lower cost methods of providing the energy required. In recent years, there have been significant efforts by paper manufacturers to make their process more sustainable by increasing the use of recycled paper and board (secondary fibre) as a feedstock. The amount of recovered paper used by paper mills as a proportion of their total output stands at approximately 72%, which is a figure believed to be close to the maximum achievable 6. However, the use of recovered feedstocks results in large volumes of wastes, such as, rejected plastics, fibres, and other coarse materials, “stickies” (adhesive residues), and in the case of some mills, de-inking sludge (fibres, minerals and ink). These have hitherto been disposed of by landfill, land-spreading, or incineration (combustion). Landfilling waste is fast becoming an unacceptable method of disposal and in some countries such as Germany and the Netherlands the disposing of paper mill wastes in this way has already been prohibited with heavy penalties imposed on law breakers 7. Furthermore, landfilling costs are increasing year on year, making this disposal method less feasible economically 7. The Confederation of European Paper Industries (CEPI) has reported that the total paper production in Europe was around 92.2 million tonnes in 2018 8. The utilization of paper recycling was 48.5 million tonnes in 2018 which increased by 0.4% compared to 2017. Furthermore, it is estimated that the world paper and board production reached 417 million tonnes in 2018 8 and global paper market research, “World Paper Markets up to 2030”, reports that world paper production will grow to 482 million tons in 2030, resulting in a further increase of waste generation 3. This highlights the need for sustainable waste disposal methods.


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Table 1 shows the generation of waste from few European pulp and paper mills. The composition and quantity of waste produced from a mill that utilizes virgin materials as feedstock differs from a mill that uses secondary fibre as the raw material. Specifically, higher quantities of rejects are produced in a secondary fibre mill, due to the unrecyclable filler proportion in the raw material. Table 1. Generation of waste from European pulp and paper mills. Reproduced with permission from ref. 9, Copyright © 2015, Springer Nature. SCA 9.9 163 115 47 0.3

Mill production (millions of tonnes) Total waste generated (kg/ton product) Recovered waste (kg/ton product) Waste sent to landfill (kg/ton product) Hazardous waste (kg/ton product)

Norske Skog 4.8 163 138 16 1.5

Stora Enso 15.1 155 (dry) 22 0.3

Holmen 2.3 160 136 23 (wet) 0.2

Furthermore, the amount of waste produced depends on the type of mill. For example, the amount of waste may vary from 20% in a newsprint mill to 40% in a tissue mill, expressed on a dry mass basis 9. These differences are highlighted in Table 2, which shows the waste generated through production of different paper grades from recycled fibre. Table 2. Waste generated through production of different paper grades from recycled fibre. Reproduced with permission from ref. 9 Copyright © 2015, Springer Nature. Paper grade Packaging paper Newsprint Light-weight coated paper/super-calendered paper Tissue and market pulp

Solid waste (dry basis, kg/Adt) 50–100 170–190 450–550 500–600

Due to the significant amount of waste produced from secondary fibre paper mills and the difficulty of their disposal, this review will focus on the valorization of waste from secondary fibre paper mills. Thermochemical conversion technologies can provide a promising solution for the dual purpose of waste management and renewable energy production 10. Incineration 11, 12, gasification 13, 14, and pyrolysis 15,16,17 are the main thermochemical technologies to convert paper mill waste into useful energy vectors. The main emphasis of this review will be on advanced thermal conversion technologies, such as gasification and pyrolysis, since they provide significant advantages when compared with incineration for waste processing. These include higher electrical efficiencies 7, 18, 19, a greater degree of control over pollutants, and the ability to scale each unit to suit individual mill requirements 7, 20. Furthermore, both gasification and pyrolysis can receive extra government support in the form of carbon subsidies as opposed to traditional incineration systems 21. Such carbon incentives and government subsidies include the renewable heat incentives and renewable obligation certificates. This support comes as part of an essential government policy to incentivise the development of the most sustainable energy generating technologies. Incineration does not benefit from these incentives as it is not considered to be an advanced thermal conversion route. Traditionally, the efficiency of energy conversion and capture is poor if there is no economic use for the surplus heat produced. Waste heat usage accounts for only 2% of the UK’s Energy from Waste (EfW) schemes because market incentives, state subsidy and distribution infrastructure are weaker for heat than for electricity use. Heat is a valuable resource in paper mills as it is used to dry paper sheets,



therefore the added value for heat utilization onsite at a paper mill would be better than direct injection into district heating networks. An overview of the waste generated from secondary fibre paper mills paper is presented in the following section. Section 3 includes a brief review of the state of the art in waste management and energy systems currently used by paper mills. A review on advanced thermal conversion technologies as a solution to the dual issue of waste management and energy generation for secondary fibre paper mills is presented in Section 4, including details regarding the feasibility of integrating them into the current mill infrastructure. 2

SECONDARY FIBRE PAPER MILLS

Secondary fibre paper mills utilize recycled paper as a feedstock for the production of new paper sheets. The amount of wastes produced in paper mills based on recycled fibre depends mainly on the quality of recovered paper used as raw material. The paper manufacturing process at each mill employs essentially similar processing steps, although there exist two main aspects that differentiate each process. The first aspect is the initial quality and type of the recovered paper. A typical brown paper/board mill will only receive recovered fibre comprised of old corrugated containers (OCC) with lower amounts of contaminants, mainly stickies and plastic film (soft plastics), and unusable short length paper fibres 22,23. A tissue mill will receive mainly office grade paper, and a newsprint mill will receive a much broader range of waste paper grades, such as old newsprint paper (ONP), magazines, office grade paper, and packaging paper 24,25,26. Accepting a wider range of feedstock paper in this way means that the manufacturing process at a newsprint mill usually requires more rigorous paper conditioning to that at a board mill or tissue mill, as the removal of higher amounts of adhesives and stickie residues is required 27. Similarly, as the recovered fibre is mainly recycled by households, there is a higher potential for it to contain a broader range of other more generic contaminants. For example a newsprint mill will often see increased levels of glass, metals, textiles, mixed plastics (PE, PET, PP, HDPE), and other general household and business wastes (rubber, staples, paper clips, cardboard etc.) and the composition of these wastes will vary from day to day 3. Seasonal variation can also be observed, for example during festive periods, these mills may see a higher proportion of packaging and wrapping paper in their waste streams. The second aspect is that newsprint and tissue paper production requires extra de-inking and bleaching processing steps to remove ink from recovered paper pulp, increase the brightness and improve the overall appearance of the final paper or tissue product 3, 28. These steps are not necessary for packaging grade paper and so are not implemented at board mills 4. 2.1

Waste Generated in Secondary Fibre Paper Mills

Solid waste generated in secondary fibre paper mills are mainly of the following types; reject wastes, co-form rejects (dry and wet), de-inking sludge, and primary and secondary sludge. The details of each solid wastes will be discussed in the next section. Reject wastes and deinking sludge are produced in much larger quantities than primary and secondary sludge. The proportion and the specific types of mills of reject wastes and de-inking sludge that they are originated from are shown in Table 3.


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Table 3. Proportion of rejects and sludge production from different paper grades and papers. Reproduced with permission from ref. 9 Copyright © 2015, Springer Nature. Paper grade

Recovered paper grade

Market DIP (deinked pulp) Graphic paper Sanitary paper Liner and fluting Board

2.1.1

Wastes (% by dry mass) Total waste Rejects Heavy Rejects mass and and sludge coarse

Sludge Light mass and fine

Flotation de-inking

White water clarification

Office paper

32-46

70%

6-12 MJ/Kg



3

CURRENT STATE OF THE ART PAPER MILL ENERGY SYSTEMS

Secondary fibre paper mills require large quantities of energy in the form of both steam (heat) and electricity to run the paper manufacturing process. Steam is required mainly by the paper machines to dry paper sheets and electricity is used on-site to run drives, pumps, motors, computers, and other machinery. As the cost for importing both electricity from the grid and natural gas from energy suppliers increases, many mills have installed their own energy systems on-site to facilitate in providing some of the energy they require. A typical newsprint mill which produces approximately 500,000 tonnes per year of paper will require approximately 50 MW of electrical energy to run the process 3. The majority of this energy is usually supplied by a gas turbine which burns natural gas although some mills implement fluidized bed combustion systems to co-fire their sludge with natural gas to raise steam which generates electricity via a steam turbine 3. Part of the steam produced is often routed to the paper machine to dry paper sheets 3. Some mills operate natural gas fired CHP turbines which generate both heat and electricity for the mill and others have installed natural gas fired boilers to raise high pressure process steam usually at 15 bar gauge and temperature of 200 °C 4. Other onsite energy generating systems include anaerobic digestion plants, which convert the process wastewater streams into methane rich biogas to run either a CHP engine or boiler. If secondary fibre paper mills wish to further improve the sustainability of their process, then a cost effective waste management strategy that reduces dependency on fossil fuels and environmental impact must be implemented. One possibility is to utilize those waste streams that have a calorific value as a source of energy for generating combined heat and power (CHP), to contribute towards the energy demands of the mill. Of particular interest would be the possibility of using advanced thermal conversion technologies, such as gasification and pyrolysis, to take advantage of their enhanced efficiency and environmental performance, and their appropriate scale in the context of mill operations. 4

ADVANCED THERMAL CONVERSION PATHWAYS

The main thermal conversion techniques which could be used to process paper industry waste into useful energy vectors are incineration 11, gasification 32 and pyrolysis 33,34. Figure 2 summarises the main differences between the technologies, as well as, their products. Thermal conversion processing is well suited to the particular types and quantities of waste generated by secondary fibre mills. De-inking sludge, reject wastes, and blends of the two, can be converted into valuable gaseous or liquid fuels, by advanced thermal conversion processes, such as gasification and pyrolysis. These products can in turn be used in a gas turbine or engine base CHP system to generate energy. A detailed description of each technology follows in the next sections.


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Figure 2: Thermal technologies classification for treating paper wastes. Reproduced with permission from ref. 35, Copyright © 2013, Elsevier.

4.1

Incineration of Paper Industry Wastes

Incineration involves the complete oxidation of organic matter (combustion) at high temperatures (minimum 850 °C) into ash, flue gas, and heat. Within Europe incineration of paper industry waste for steam and power generation is one of the main pathways 36. This approach can be used for disposal of all types of sludge including secondary or biological sludge 36. The overall energy balance of the process can be deficient, due to the high moisture and ash content of paper industry derived waste. Incineration reactors are usually fluidized beds since they can achieve complete thermal oxidation and benefits from good heat transfer efficiencies 36, 37. Incineration as a waste management solution by the paper industry has resulted in a reduction of waste disposed to landfills by 80-90% 12. Currently, there are several approaches for the disposal of incineration derived ash which include landfill and ash reuse. The selection of the appropriate method depends on the properties of the ash, such as chlorine content and content of heavy metals. In order to remove or dilute the chlorine content, ash rinsing can be applied 12. Additionally the chlorine content of rejects can cause corrosion of downstream equipment and air contamination. The advantages and drawbacks of incineration are reported in Table 5. Europe has been strongly regulated by the air environment protection act since the 1970s. The Waste Incineration Directive (WID) (European Directive 2000/76/EC) is committed to reduce the human health risks associated with water, air, and soil pollution caused by incineration of wastes 38, 39. The directive requires measurements of CO, NOx, HCl, TOC, HF, SO2 and total dust/ particulates to be taken at continuous intervals and reported each year. Measurements for heavy metals (Tl, Cd, Hg, As, Sb, Pb, Co, Cr, Cu, Ni, Mn and V), furans, and dioxins should be carried out and reported at least twice a year 40. The fuel triangle for paper industry derived wastes is shown in Figure 3 38. It can be observed that there is a correlation between organic content/combustible, water, and ash content. Komilis et al. reported that a zone of self-combustion is defined for ash content lower than 60 wt.%, moisture content lower than 50 wt.%, and organics higher than 25 wt.% 41.



A detailed review of available technologies for processing paper wastes was carried out by Monte et al. 7. Currently, the method adopted by paper mills for processing paper wastes is incineration coupled to an energy recovery boiler to raise process steam, to either run a steam turbine for electricity generation or to be used directly in the paper making process for drying paper sheets 7. However, the utilization of steam for electricity production is an inefficient process especially for smaller mills, which generate small quantities of waste. Furthermore, it requires large capital investment and problems, such as, incineration emissions and boiler corrosion, are also of concern. It was noted by the authors that other options such as gasification pyrolysis, land-spreading, composting, and ash reuse for building materials, are used, although gasification and pyrolysis are still in the early research and development stages and further work is needed for their optimisation 7. Emphasis was placed on the uncertainty of the environmental impact of such processes, as well as, the economics associated with them. Table 5. Advantages and disadvantages of incineration. Reproduced with permission from ref. 42, Copyright © 2016, Elsevier.       

Advantages Decrease the waste volume Decrease the waste weight Decrease the landfilling demand Recovery of the energy (heat or electricity) Handle waste without pre-treatment Reduce the waste transportation cost Require minimum site

     

Disadvantages Bottom ashes (originate slags) Production of huge amount of flue gases with CO2 High operating cost and investment High maintenance cost Originate hazardous waste which needs safe disposal Require skilled people highly trained workforce

Figure 3. Fuel triangle of paper industry wastes. Reproduced with permission from ref. 7, Copyright © 2009, Elsevier.

4.2

Gasification of Paper Industry Wastes

Although gasification is not a new technology, its application in the pulp and paper industry is considered to be in the early stages of development. Gasification is defined as the partial combustion of organic matter into a fuel gas (product gas), which is composed mainly of CO, CO2, H2, CH4, H2O, N2, and CXHY that can be used in heat, power, or CHP applications 31,32. Air, oxygen, CO2, or steam can be used as oxidizing agents. There are three main types of reactor configuration suitable for the gasification of such types of wastes: (i) fixed bed gasifier (downdraft and updraft) 43,44, (ii) fluidized bed gasifier and (iii) entrained flow gasifier. Figure 4 illustrates the various reactor configurations 13, 45, 46.


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Figure 4. Different types of gasifiers (fixed bed, fluidized bed, and entrained flow gasifier). Reproduced with permission from ref. 44,47, Copyright © 2000 and 2009, Elsevier and Springer Nature, respectively.

Fixed bed gasifiers are the preferred configuration for small-scale distributed power generation at inputs 10 t/h)

The gasification chemistry is complex. The process can be separated into the following stages 50-54: 

Drying: The feedstock moisture content is reduced in this step and ranges from 5% to 35% at temperatures of around 100 °C. Pyrolysis (devolatilisation): In this stage, waste thermal decomposition occurs in the absence of oxygen. This results in production of pyrolysis vapours (condensable and non-condensable), as well as, bio-char. The condensable fraction of the vapours will form liquid tars. Oxidation: Occurs between solid carbonized feedstock, organic vapours and oxidizing agent, forming carbon monoxide. Furthermore, hydrogen is oxidized to produce water. Partial carbon oxidation may occur if oxygen is presented in sub-stoichiometric quantities and forms carbon monoxide. Reduction: Reduction reactions occur in the absence of oxygen at 800-1000 °C. The main reactions involved are as follows:





 I. II. III. IV.

Water-gas reaction: C + H2O → CO + H2 – 131.4 kJ/gmol

(1)

Bounded reaction: C + CO2 → 2CO – 172.6 kJ/gmol

(2)

Water gas shift reaction: H2O + CO → CO2 + H2 + 42.3 kJ/gmol

(3)

Methane reaction: 2H2 + C → CH4 + 75 kJ/gmol

(4)

Durai et al. investigated the potential of indirect steam gasification for electricity generation using paper mill wastes, mainly consisting of plastic rejects and short length fibres, from a recycled fibre mill 30. In this work, the authors performed gasification experiments at 950 °C, with a reaction time between 4-10 hours in a continuous steady state experiment, using a fluidized bed reactor fitted with an internal steam heated coil. This coil was claimed to give improved heat transfer and an even heat distribution throughout the fluidized bed. The fluidized bed consisted of CaCO3 (limestone) particles, used for the reduction of dioxin formation in the product gas. The fluidizing gas used was superheated steam generated from the downstream cooling of product gases. The particular feedstock used in the study consisted mainly of plastic reject material and short fibres expelled from a clarifier within a paper mill. Results showed


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that fluidized bed gasification performed well while handling feedstocks with high moisture, high plastic, and high ash content. It was reported that dioxin formation of the product gas was within emission limits. The product gas was composed of approximately 80% combustible gases, and had a calorific value of 13.7-15.2 MJ/Nm3. The author states that the main advantage of using a fluidized bed for gasification of paper mill wastes is the reduction in feedstock pretreatment costs, because pelletisation and excessive amounts of feed drying are not necessary. However, the low density of fibrous material in the fluidized bed caused high levels of entrainment and this increased the amount of tar and char produced. High levels of tar in the product gas can severely affect the performance of a downstream engine or gas turbine used for electricity production 55. Closer control of gas superficial velocities were the proposed solution for tar reduction. An important advantage of this process is that as the superheated steam needed for the process is raised from the subsequent gas cooling, there is no requirement for an external heat input 30, 56. Oudhuis et al. 57 studied pyrolysis followed by gasification of willow biomass and pelletized paper industry reject waste generated from a secondary fibre board mill (Rofire© pellets), in order to evaluate their potential use as fuels for electricity production. This system also integrated a solid oxide fuel cell (SOFC). Tests were carried out using the ECN two stage gasifier “Pyromaat” and the connected fuel cell was a downscaled Sulzer HEXIS 1 kWe SOFC stack. The full experimental set up consisted of a feed hopper, horizontal screw pyrolysis reactor, a secondary oxygen blown (tar cracker) gasifier, a series of cleaners for upgrading producer gas quality, consisting of a wet scrubber, dust filter, condenser, and filters (ZnO and carbon). This is followed by a 1 kWe solid fuel oxide stack containing 5 cells of 60-100 cm2 each with a state of the art electrolyte supported by 3 plate interconnects. The operating conditions were set up to test both Rofire and willow feedstocks at a feed rate of 3 kg/h. The process was carried out under atmospheric pressure, pyrolysis temperature of 550 °C, tar cracker temperature of 1300 °C, and a SOFC utilization of 80%. Results from this study showed successful operation for several days on both willow and Rofire feedstocks. H2, CO, and CO2 were by far the largest fraction of gases formed, with 36.7%, 33.3%, and 28.3%, by volume, respectively. Other gases were also detected such as CH4 and N2 but these were in much smaller volumes having a combined total volume of approximately 1.7%, by volume. The electrical efficiency of the SOFC was calculated and found to be between 35-41%; the authors outlined that at full scale the electrical efficiency would be comparable to a coal fired power plant although it is not clear if this would include the utilization of char generated from the process. Some negative aspects were encountered such as soot formation in the product gas, slight degradation of the SOFC over time, and decreased electrical efficiencies with the addition of 50 vol% steam. Overall, the objective to prove in principle that the use of secondary fibre paper mill rejects for electricity generation by pyromaat was achieved, although further work was required in order to optimize process efficiencies including the utilization of char generated from the process. Two main wastes produced from secondary fibre paper mills are de-inking sludge and rejects (composed mostly of fibres and plastics) which are generated from the pulping process during paper manufacture 58, 59. Ouadi et al. 32 studied the gasification of de-inking sludge and blends of pre-conditioned rejects pellets with mixed wood chips using a fixed bed downdraft reactor. Four different types of waste generated from three secondary fibre paper mills were used in this study. These were pulper rejects generated from Smurfit Kappa SSK’s brown paper mill at Nechells (SSK), pulper rejects and de-inking sludge generated from Aylesford Newsprint’s newsprint mill at Aylesford (AN), and co-form rejects (dry and wet wipes) and pulper rejects generated from Kimberly Clark’s tissue mill at Flint (KC). Blends of these materials were co-



gasified with mixed wood chips which were provided from a local UK based wood fuel supplier Midland Wood Fuel Ltd. Work was carried out in an Imbert type fixed bed downdraft gasifier with maximum feeding capacity of 10 kg/h. It was shown that as much as 80% of a brown paper mill’s rejects, which include 80 wt.% paper fibres and 20 wt.% mixed plastics, could be gasified in a blend with 20 wt.% mixed wood chips successfully. The average composition of the gas produced was 23.34% CO, 16.25% H2, 5.21% CH4, 12.71% CO2, and 42.49% N2 by volume with a higher heating value of 7.3 MJ/Nm3. After the tar removal and water condensate removal, the gas had sufficient calorific value and flow rate to power a 10 kWe gas engine at 100% operational capacity on the syngas. Furthermore, some blends using rejects from other mill types were unsuccessful due to their plastic content, which caused agglomeration within the reactor bed. It was concluded that the most efficient application of this technology would be on the reject streams from secondary fibre paper mills manufacturing brown paper for the corrugated board industry. Wood chips as a co-gasification fuel may be necessary, however it may be possible without them if the rejects are pre-sorted to reduce their plastic content. Rivera et al. 60 studied the gasification of de-inking sludge and rejects using a circulating fluidized bed (CFB) reactor. The waste was provided from Holmen Paper, which is located in Fuenlabrada area close to Madrid. A blend of de-inking sludge (5%) and rejects (95%) was chosen for gasification to reduce the overall ash content. The overall energy consumption was 407 kW h/t for the material preparation process. Of this 339 kW h/t of energy was consumed for grinding, 56 kW h/t for pressing pellets, and 12 kW h/t for drying the material in a solar dryer. The feedstock was injected as pellets (8 mm of diameter) using a feeding rate of 100 kg/h. During the continuous gasifier operation, no formation of bridge within the hoppers was observed. Pressure around the loop, temperature, gas composition, and tar content were analysed during the tests. The most efficient conditions were achieved at equivalence ratio (ER) of 0.3 and at a reaction temperature of 850 °C, which yielded a net calorific value of 5 MJ/Nm3 and a total tar content of 11.44 g/Nm3. After inspection of the bed and ash, no agglomeration was detected. A summary of the gasification work with secondary fibre paper mills waste is presented in Table 7.


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Table 7. Summary of gasification work with secondary fibre paper mills waste. Residue Plastic rejects and short length fibres

Manufacture From a recycled fibre mill

Equipment The fluidized gasifier

Paper industry reject waste

From a secondary fibre board mill (Rofire pellets)

ECN two stage gasifier “Pyromaat”

550 °C and 1300 °C Feed rate of 3 kg/h Atmospheric pressure SOFC utilization 80%

Pulper rejects

Generated from Smurfit Kappa SSK’s brown paper mill at Nechells (SSK) Generated from Aylesford Newsprint’s newsprint mill at Aylesford (AN)

Fixed bed downdraft gasifier

800–1000 °C

Pulper rejects and de-inking sludge

Co-form rejects (dry and wet wipes) and pulper rejects Blend of these materials with wood chips

Generated from Kimberly Clark’s tissue mill at Flint (KC)

De-inking sludge and rejects

From paper industry wastes (Holmen Paper)

bed

Conditions 950 °C

Feeding capacity of 10 kg/h

Results Combustible gases 80% Calorific value of 13.7-15.2 MJ/Nm3 High levels of tar and char Average gas compositions: 36.7 vol% H2 33.3 vol% CO 28.3 vol% CO2 1. 7 vol% Other gases (CH4 and N2) Electrical efficiency of the SOFC was 35-41% 80 wt% of a brown paper mill’s rejects (containing of 20 wt.% mixed plastics and 80 wt.% paper fibres) could be gasified in a blend with 20 wt.% mixed wood chips successfully

Ref. 30

57

32

Average gas compositions: 16.25 vol% H2 5.21 vol% CH4 23.34 vol% CO 12.71 vol% CO2 42.49 vol% N2

Generated from a local UK based wood fuel supplier Midland Wood Fuel Ltd

HHV of 7.3 MJ/Nm3 circulating fluidized bed (CFB) gasifier

850 °C 100 kg/hr

A blend of de-inking sludge (5%) and rejects (95%) was chosen for gasification experiment due to the low amount of ash. The most efficient condition: ER of 0.3 Tar content of 11.44 g/Nm3 Net calorific value of 5 MJ/Nm3


60


4.3

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Pyrolysis of Paper Industry Wastes

Pyrolysis is defined as the thermal decomposition of organic matter in the complete absence of oxygen. The pyrolysis process consists of a set of complex chemical reactions involving the formation of radicals. These reactions are largely endothermic, so a heat input is required. As the temperature increases chemical bonds in the feedstock are broken and unstable radicals form, which then react with each other to form more stable low molecular weight molecules 61. Pyrolysis takes place in the temperature range 280-850 °C 61 depending on the nature of the feedstock, the desired products, and the particular pyrolysis process employed. Similarly, pressures can range from hypobaric to atmospheric to elevated pressure 62. In general, pyrolysis processes can be broken down into three main types: slow, intermediate, and fast pyrolysis. The main differences between the three processes are heating rates and solid residence times involved, reaction temperatures, and the relative yields of solids, liquids and gaseous products formed. The characteristics of the three processes are summarized in Table 8. Table 8. Characteristics of Fast, Intermediate, and Slow Pyrolysis. Reproduced with permission from ref. 62, Copyright © 2012, Elsevier. Mode Fast Intermediate Slow

Conditions ~500 oC, solid residence time ~ Seconds ~400 oC, solid residence time ~ Minutes ~300 oC, solid residence time ~ Hours/Days

Liquid 75 wt.% 40 wt.% 30 wt.%

Product Profile Solid Gas 12 wt.% 13 wt.% 30 wt.% 30 wt.% 35 wt.% 35 wt.%

Fast pyrolysis involves heating at very high heating rates at a reaction temperature of around 500 °C with very short solid/vapour residence times followed by rapid quenching in order to achieve maximum liquid yields (“bio-oil”) 48. Slow pyrolysis is the opposite with slow heating rates, long solids residence times, and low reaction temperatures, favouring the production of high yields of solid char. Intermediate pyrolysis applies reaction conditions which are inbetween fast and slow pyrolysis thus favouring intermediate yields of solids, gases and liquids. Pyrolysis has the ability to produce high yields of pyrolysis oil 48; this liquid can contain a high energy density per unit volume and thus can be easily transported and stored as an energy carrier. However, pyrolysis oil can contain large amounts of high molecular weight compounds that also contain oxygen. This can lead to highly viscous bio oils, which may cause polymerization over time; these ageing effects lead to storage and handling problems, and the need for further liquid upgrading. Slow pyrolysis requires lesser amounts of heat energy to achieve the desired pyrolysis temperatures; however much longer solid residence times are required 48. The product distribution favours high solid yields, with little or no pyrolysis liquids formed. Having a process tailored towards the production of maximum quantities of solids is not suitable for feedstocks with high ash contents and relatively small amounts of fixed carbon, such as de-inking sludge. Typical pyrolysis reactors are bubbling fluidized beds 63, circulating fluidized beds (CFB) and transported beds 64, rotating cone 65, ablative 66, and screw, kiln or auger 67, 68 and are illustrated in Figure 5. The ablative reactor operates by pressing the feedstock against a heated surface causing rapid disintegration of the biomass. The system requires larger feedstock particles and a heat transfer gas is not required. Similarly, the rotating cone system delivers the particles on a high-speed rotating cone along with hot sand, these fast pyrolysis systems apply rapid heating rates. The vacuum system has slower heating rates, with the quick removal of the pyrolysis products. The circulating fluidized bed, transported bed, and bubbling fluidized bed, provide high heat transfer rates by using gas–solid heat transfer mediums, where heat is transferred from the fluidized gas to a hot bed containing feedstock particles. The screw (auger) or kiln reactor are reactors which transfers the feedstock particles


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using a screw or conveying system. The key features of each reactor type are summarized in Table 9.

Figure 5. Different types of pyrolysis reactors. Reproduced with permission from ref. 69, Copyright © 2010, John Wiley and Sons. Table 9. Characteristics of pyrolysis reactor types. Reproduced with permission from ref. 70, Copyright © 1999, Elsevier Reactor type Ablative

Circulating fluidized bed

Bubbling fluidized bed

                   

Characteristics Accepts large size feedstocks High mechanical char abrasion from feedstock Compact design Problematic heat supply Heat transfer gas is not required High heat transfer rates High char abrasion from feedstock and char erosion leading to high char in product Solid with heat carrier separation is required Solids recycle required Increased complexity of system Maximum particle sizes up to 6 mm Possible liquids cracking by hot solids Possible catalytic activity from hot char Greater reactor wear possible High heat transfer rates Heat supply to fluidizing gas or to bed directly Limited char abrasion Very good solids mixing Particle size limit < 2 mm in smallest dimension Simple reactor configuration



Screw, kiln or auger

   

Accepts large size feedstocks in the form of pellets Compact design Lower solids and vapours residence times Heat transfer gas is not required

Lou et al. 71 investigated the pyrolysis characteristics of de-inking sludge for potential energy generation. The main aim of this study was to investigate the application of de-inking sludge through a pyrolysis technique as an energy resource and thermally characterize the properties of the pyrolysis products produced from de-inking sludge. The de-inking sludge utilized for this study was obtained from the deinking process of an old newsprint pulping and papermaking mill and the sample was air-dried, ground, and oven dried at 105 °C for 6 hours. The particles were maintained at 40-60 mesh. Three different techniques of pyrolysis were applied in this study. Thermo-gravimetric analysis (TGA) at 800 °C using nitrogen, Py-GC/MS at 800 °C under a helium atmosphere, and pyrolysis in a static tubular furnace using nitrogen. The experimental results revealed that de-inking sludge contained 41.46% ash with a low gross heating value of 10.35 MJ/kg (dry basis). The proximate analysis of the de-inking sludge and its calorific value was compared to other feedstocks and its combined volatiles and fixed carbon content were very similar to maize straw (comparison with lignocellulosic biomass of similar composition). This comparison shows that maize crop residues and de-inking sludge possess similar volatiles and fixed carbon contents. From the TGA runs it was found that the pyrolysis process of de-inking sludge is made up of 3 steps; evaporation, devolatization, and decomposition of residues. The evaporation occurred at 71 °C, pyrolysis peaked at 361°C and the combustion of fixed carbon and decomposition of calcium carbonate was observed to peak at 772 °C. When the Py-GC/MS was used for the experiment, it was found that different temperature ranges favoured different pyrolysis products. Benzene and its derivatives peaked at 85.23% when the temperature was 800 °C. When experiments were carried out in a tubular furnace a three phase product mixture consisting of incondensable gas (29.78 wt.%), condensed liquid (24.41 wt.%), and solid (45.81 wt.%) were found to be the products. The gaseous products were composed of 31.6% CO, 21.53% CO2, 19.54% CH4, 9.62% C2H4, and 17.71% H2, by volume. The decomposition of CaCO3 contributed to the CO2 in the gaseous products. The solid product obtained was composed of mainly inorganic material, mainly CaCO3, and the pyrolysis oil produced was made up of mainly benzene, toluene, ethylbenzene, and xylene (constituents of gasoline). The authors concluded that the oil and gas produced can be used as an energy fuel and the CaCO3 can be reused as paper filler. Ouadi et al. 2 investigated the intermediate pyrolysis of de-inking sludge for the production of a sustainable liquid fuel. The de-inking sludge samples used were from manufacturers tissue products (Kimberly–Clark Flint – KC) and a mill which manufactures newsprint (Aylesford Newsprint– AN). Thermo-gravimetric analysis (TGA) at 900 °C using nitrogen was implemented on the samples. The KC sample was found to contain 43.6 wt.% ash, while the AN sample produced 51.5 wt.%, both expressed on dry feedstock basis. TGA revealed that pyrolysis peaked between 350 °C to 400 °C and the combustion of fixed carbon and decomposition of calcium carbonate was observed to peak approximately at 800 °C. The higher heating value for the KC and AN samples, were 7 MJ/kg and 6.4 MJ/kg, respectively. This study explored further the potential of de-inking sludge by applying intermediate pyrolysis of pre-conditioned de-inking sludge pellets in a patented 20 kg/h intermediate pyrolysis reactor (The Pyroformer©). The reactor temperature was set at 450 °C and 15 kg/h feeding rate. The de-inking sludge utilized for this study was dried to a moisture content of less than 3 wt.% using an industrial scale rotary drum dryer at a drying temperature of 100 °C. The dry material was pelletized to form pellets at 6 mm diameter by 15 mm length. The pyrolysis oil produced


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contained a higher heating value of 36–37 MJ/kg and was composed of aromatic hydrocarbons (benzene, toluene, ethylbenzene), phenolic compounds, and fatty acid methyl esters. This is in agreement with Lou et al. 71. The oil produced was found to have improved fuel physical properties when compared to traditional pyrolysis oil produced from the fast pyrolysis of woody biomass, and was comparable to biodiesel due to the presence of FAME in the oil. It should be mentioned that biochar from pyrolysis of de-inking paper sludge shows potential to be used as treatment of polluted soil. Mendez et al. investigated the influence of biochar from de-inking paper sludge pyrolysis in a nickel-polluted soil 72. Results showed that biochar obtained at different pyrolysis temperatures affected differently the polluted soil. The biochar that was produced at a pyrolysis temperature of 500 °C, reduced the quantity of mobile, leached, and bioavailable nickel. A reduction was also observed in soil CO2 emissions. For the biochar that was produced at a pyrolysis temperature of 300 °C, an increase was shown in soil biological activity, but no effect was observed on mobile, leached, and bioavailable nickel. Mendez et al. 73 studied the pyrolysis behavior of a range of paper mill waste materials using SEM, FTIR, DRX, and TGA techniques. In general, paper mill sludges from recycled paper showed high CaCO3 and clays contents, as well as, lower values of organic matter, and this was more profound in wastes obtained from the deinking process (CaCO3 content of 44-46.9 wt.%). This is due to the removal of inorganic fillers from recycled paper. FTIR and DRX of the primary sludge and reject wastes, which derived from a paper mill producing paper from virgin wood, revealed an elevated content of cellulose fibres when compared with recycled paper sludge. TGA at 900 °C with heating rates of 10 °C/min using nitrogen was implemented on the samples. TGA indicated that the degradation of cellulose, as well as, the presence of ash, lowered the starting temperature for weight loss. For the recycled paper sludge samples weight loss continued at temperatures higher than 500 °C due to kaolinite dehydration and carbonates decomposition. Ouadi et al. carried out two studies, one with pulper rejects acquired from a board mill manufacturing brown paper for cardboard and packaging applications 68, and the other on coform rejects obtained from a secondary fibre paper mill, which manufactures tissue and hygiene products 74. Pulper rejects were composed of 16 wt.% plastic (mainly soft plastic wrap and film) and 84 wt.% paper fibres, while co-form rejects of approximately 30 wt.% polypropylene and 70 wt.% wood pulp fibres. An intermediate pyrolysis Thermo-Catalytic Reforming (TCR©) reactor was used to process the wastes, which is an auger reactor followed by a second stage post-reformer (fixed bed). The pyrolysis and reformer reactor temperature was set at 450 °C and 700 °C, respectively. It should be noted that the pyrolysis vapours were catalytically cracked in the reformer due to the reforming reactions that occur between char and pyrolysis gases. Both feedstocks were successfully processed using the TCR. Pyrolysis product yields for the bio-oil, char, and gas were 8, 24, and 53 wt.% for pulper rejects, and, 12, 8, and 71 wt.% for the co-form rejects, respectively. Results showed that the bio-oil produced from both wastes has thermal chemical properties suited for blending with fossil fuels. Another important finding was that char combustion is a viable valorization route due to the low levels of inorganics. Specifically for the pulper rejects, the produced pyrolysis gas contained an average hydrogen content of 29.33%, by volume, which is attractive for hydrogen separation for hydrogen fuel cells PEM or SOFC’s, process heating, and other high value applications. Both studies were able to demonstrate the advantage of the TCR technology as a promising route for valorization of pulper rejects and co-form rejects to produce energy vectors, as the TCR was successful in processing high plastic content feedstocks.



Frederick et al. 75 carried out work, on the gasification and pyrolysis of primary sludge generated from a recycled fibre paper mill. In the experiments, the authors focused on producing a fuel gas for energy and reusing the residual ash residue as an admixture mineral for Portland concrete. The authors performed the gasification experiments in a batch furnace over a temperature range of 500-900 °C, using air as the gasifying medium with equivalence ratio of 0.58. Higher temperatures above 900 °C were reported to be effective in the calcination of kaolinite and MgCO3 compounds, thus the overall yields of ash inorganic residues after gasification were reduced to approximately 45% of the total dry sludge feed input. The ash was later tested for its properties as a concrete admixture mineral and was found to be slightly outside the required specification; however, the authors reported that slight alterations to the temperature of the gasification reaction would remedy this, although this was not demonstrated. The authors carried out a material and energy balance of the total gasification system. The thermal efficiency of the process, which was defined as the heating value of the product gas plus the energy available from steam generation divided by the total energy input, was 69.8% (at a reference temperature of 25 °C). It was stated that there was enough residual heat from the product gas cooling to provide heat energy for both drying the sludge and pre-heating the air used for gasification. The product gas contained 17.1% hydrogen and 5.4% carbon monoxide, by volume; the remaining composition of the gas was not stated. The gas produced had a gross heating value of 2.64 MJ/Nm3 (dry basis) and the authors discussed that using oxygen-enriched air for the gasification reaction would help quite considerably to increase this value as it reduces the N2 present. However, using a gasifying agent with high oxygen purity would in turn incur higher operating costs of the gasification process. For the pyrolysis reactions, the authors 75 focused on producing a fuel gas for energy and then subsequently gasified the residual char solids to increase the overall efficiency of the process. Pyrolysis of sludge was performed in a laminar entrained flow reactor (LEFR) at high heating rates and at temperatures of between 500 °C and 900 °C. The optimal pyrolysis temperature for conversion into useful hydrocarbon fuel gases was reported as 900 °C. This produced the smallest amount of residual char; it also gave a conversion of approximately 60% of carbon in the solid feed into a useful fuel gas, of which the main components were 35% CO and 13% C2H4O, by volume. Performing pyrolysis at these higher temperatures was stated to result in the successful calcination of CaCO3 and MgCO3, which makes up a large fraction of the solid sludge initial composition. Negligible levels of NOx were formed at higher pyrolysis temperatures and less than 1% of residual metals from the feed were found in the post cyclone filter. The char produced from the pyrolysis runs were subsequently gasified and it was reported that the residual fixed carbon in the char gasified readily. However, no further data was collected on product gas composition or the total remaining ash yields. Further work was performed using high ash (HAPWS) and low ash (LAPWS) paper waste sludge, supplied by the Kraft pulp mill and the Kimberly Clark Enstra recycle tissue paper mill, respectively 76. Ridout et al. used a bubbling fluidized bed reactor with a feed capacity of 1 kg/h to conduct fast pyrolysis experiments at 300, 425 and 550 °C for LAPWS, and 290, 340 and 390 °C for HAPWS. The highest yields of bio-oil were produced at 400 °C, giving high maximum bio-oil yields of up to 44.5 daf. wt.%, for LAPWS, and 340 °C with maximum biooil yields of up to 59.9 daf. wt.% for HAPWS. The higher heating values of the produced pyrolysis oils ranged from 17.4 to 22 MJ/kg for LAPWS, and 17.0 to 20.0 MJ/kg for HAPWS. The higher heating values are significantly lower than the values reported by Ouadi et al. for deinking sludge 2. This could be due the high concentration of Ca in de-inking sludge that acts as a catalyst resulting in bio-oil upgrading, as well as, the different reactor configuration.


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Work was also conducted by Strezov et al. with recycled paper sludge (RPS) which was collected from a paper manufacturing industry located in Sydney 77. A batch packed bed reactor was used with 1 g of sample packed to a density of 400 kg/m3. The experiment was conducted at a heating rate of 10 °C/min up to 700 °C. Maximum pyrolysis oil yields of 40 wt.% expressed on dry paper sludge basis were achieved at 500 °C. GC/MS analysis of the oil revealed that it was composed mainly of organic acids and specifically linoleic acid. A summary of the pyrolysis work with secondary fibre paper mills waste is presented in Table 10. The review on current research has demonstrated the potential of pyrolysis as a solution for waste processing and energy generation. It can be suggested that there are several feasible configurations for integrating a pyrolysis process into a CHP system, but the preferred route for secondary fibre paper mills would be to condense the pyrolysis liquids and then subsequently utilize both these and the permanent gases (which also have a calorific content) in a dual fuel engine. These suggestions are based on experimental works as reviewed earlier by multiple authors 15,69,71,72,77. The remaining solid char can be combusted to create additional heat for the process. A proposed system for the process of paper mill residues at industrial scale would consist from the following stages: waste pre-treatment (sorting, sizing and drying); pyrolysis to generate pyrolysis oil, char, and permanent gases; then a dual fuel engine in parallel with a char combustor or gasifier.


Industrial & Engineering Chemistry Research 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

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Table 10. Summary of pyrolysis work with secondary fibre paper mills waste. Residue De-inking sludge De-inking sludge

Rejects, Primary and Secondary sludge, De-inking sludge

Manufacture Old newsprint pulping and papermaking mill (Guangzhou, China) Tissue products (Kimberly–Clark Flint – KC) and a mill which manufactures newsprint (Aylesford Newsprint – AN)

Equipment Tubular furnace batch pyrolysis

Conditions 800 °C

Intermediate pyrolysis reactor

450 °C 15 kg/h

Results Solid 45.81 wt.% Gases 29.78 wt.% Bio-oil 24.41 wt.% Pyrolysis oil higher heating value of 36–37 MJ/kg

Ref. 71

2

Composed of aromatic hydrocarbons (benzene, toluene, ethylbenzene), phenolic compounds and fatty acid methyl esters.

Paper mill producing paper from virgin wood; recycled cellulose without deinking process; paper from recycled cellulose without deinking process; newspaper; paper mill that recycled paper not recycled previously

SEM, FTIR, DRX and TGA

Board mill manufacturing brown paper for cardboard and packaging applications Secondary fibre paper mill, which manufactures tissue and hygiene products

Intermediate pyrolysis ThermoCatalytic Reforming (TCR) reactor

450 °C (Pyrolysis) 700 °C (Reformer)

Primary sludge

Primary sludge generated from a recycled fibre paper mill

500 and 900 °C

Hydrocarbon fuel gases

75

Paper waste sludge

High ash (HAPWS) and low ash (LAPWS) paper waste sludge, supplied by the Kraft pulp mill and the Kimberly Clark Enstra

Laminar entrained flow reactor (LEFR) Bubbling fluidized bed reactor

300, 425 and 550 °C (LAPWS) 290, 340 and 390 °C (HAPWS)

Pyrolysis oils ranged from 17.4 to 22 MJ/kg for LAPWS, and 17.0 to 20.0 MJ/kg for HAPWS

76

Pulper rejects Co-form rejects

900 °C with heating rates of 10 °C/min (TGA)

Approximately 9 wt.% condensable organic vapours (pyrolysis oil), 1 wt.% aqueous phase, 15 wt.% permanent gases with the remaining 75 wt.% being the solid inert residues, mainly calcium based. Paper mill sludge from recycled paper showed high CaCO3 content when compared with sludge derived from virgin wood.

73

44-46.9 wt.% of CaCO3 content for deinking sludge. Primary sludge and reject wastes, which derived from virgin wood, revealed an elevated content of cellulose fibres when compared with recycled paper sludge. Char 24 wt.% Gases 53 wt.% Bio-oil 8 wt.% Char 8 wt.% Gases 71 wt.% Bio-oil 12 wt.%


68

74

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Recycled paper sludge (RPS)

recycle tissue paper mill Recycled paper sludge (RPS) which was collected from a paper manufacturing industry located in Sydney

Batch packed bed reactor

Heating rate of 10 °C/min up to 700 °C

Oil was composed mainly of organic acids and specifically linoleic acid.


77


CONCLUSIONS The UK paper industry accounts for 6% of GHG emissions with total paper production of 91 million tons in Europe in 2016, with 50-600 kg of waste per ton of paper produced of various qualities; this presents a huge challenge for waste management. At the same time, these paper mills are big energy consumers for paper production. There are various types of paper industry wastes, which include waste rejects, de-inking sludge, and wastewater sludge (primary and secondary) wastes. These varieties of wastes have shown to be suitable for energy conversion. Low calorific paper mill wastes provide an excellent opportunity to produce much needed energy by using advanced thermochemical conversion technologies. However, in general, the low calorific value of paper industry wastes ( 60 wt.%), and ash content (> 45 wt.%) make the energy conversion process somewhat challenging in combustion plants. Many benefits can be realized by using this waste material onsite at paper mills to reduce GHG emissions associated with waste transport and disposal. By applying advanced thermochemical conversion (gasification and pyrolysis), one can increase this waste resource efficiency, reduce landfill costs, and help to mitigate against climate change. Processing paper industry wastes by means of gasification and pyrolysis converts this waste into various products such as syngas, bio-oils, and chars which can be subsequently combusted in downstream engines or turbines for energy production. The application of such technologies presents many advantages in terms of scale, efficiency and emissions when compared to traditional incineration. During gasification, variable quality and calorific value syngas can be produced. The low calorific value syngas by air gasification can be applied for combustion in engines and boilers. Whereas, the high calorific value syngas through oxygen enriched gasification with ultra-low tar content can be used in SOFC and chemical synthesis. The high alkaline mineral content in the ash can also have an influence on reducing tars and has been shown to have in-situ catalytic effect for the production of appropriate range of hydrocarbons for liquid and gaseous fuels during gasification and pyrolysis. The diversity of products during pyrolysis (bio-oil, syngas, and char) lends itself to flexible end use. The calorific values of biooils can be as high and comparable to conventional bio diesel fuels and the calorific value of syngas (< 10 MJ/Nm3) enables its use as a gaseous fuel for boiler engines and solid oxide fuel cell (SOFC) applications. Furthermore, by using the waste heat for drying the high moisture containing material conversion and utilization efficiency can be enhanced by more than 40%. The char fraction is found to be useful for multiple applications such as energy recovery through combustion or gasification to satisfy the heat demand of pyrolysis. Furthermore, the ash and char from gasification and pyrolysis processes also have the potential to be used as paper fillers, and admixtures for the cement process. In future, there is a huge potential for deinking sludge to become a useful feedstock or fuel resource for the paper industry. Other mill wastes such as pulper rejects from board mills and co-form rejects from secondary fibre paper mills with varying plastics content up to 30 wt.% have shown great potential for conversion into value added products through TCR pyrolysis technology. The gaseous products from TCR also show a good potential for hydrogen production with hydrogen yields up to 30%. All these paper industry wastes show good potential for energy production and utilization for energy production. Advanced thermal conversion presents an excellent opportunity for paper industry wastes to be valorized into useful energy vectors. However, one should conduct a detailed techno-economic analysis of each process to help understand their economic viability as alternatives to incineration at secondary fibre paper mills.


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A review of the valorization of paper industry ... - ACS Publications

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