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Mitigation of global warming, energy security and industrial competitiveness urge the energy-intensive pulp and paper industry (PPI) to transform energy use practices. This study investigates how the PPI has responded to the need for the energy transition in the 2000s. Finland and Sweden as forerunners of energy-efficient operation and decarbonization of the PPI are used as target countries. Understanding of changes in energy consumption is complemented using decomposition analysis (Logarithmic Mean Divisia Index Method) and the energy efficiency index approach. Analysis of companies’ investments in energy technologies is used for explaining changes in energy production. Evidence of significant development towards the more sustainable operation of the PPI was found. Energy consumption per produced unit has decreased, i.e., energy efficiency has improved. Fossil fuels have been partially replaced with bio-based alternatives. Thus, the CO2 intensity has decreased substantially. The generation of renewable electricity has increased in both countries. Examples of Finland and Sweden indicate that the PPI has great potential to contribute to CO2 emission reduction worldwide in the future as energy efficiency can be further improved, and the share of fossil fuels can be decreased increasing the use of biofuels and self-generated green electricity at least in kraft pulp mills.

This study evaluates the potential of the oxidative fast pyrolysis (OFP) of eucalyptus wood residues (EWR) for producing bio-oil to replace fossil fuels in the lime kilns of the pulp industry. OFP is an alternative to inert-atmosphere fast pyrolysis where separate heat addition stage is not required. OFP was studied by characterizing the fuel using proximate and elemental chemical analyses, thermogravimetric analysis and heating value. Then, OFP experiments in a pilot-scale autothermal fluidized reactor were done with EWR. Pyrolysis products were gases, bio-char and bio-oil (heavy and light). The gases were burnt, and the energy used for heating the fluidization air. Heavy bio-oil energy yield of 30% and 21.4 MJ kg−1 lower heating value indicate good potential for fuel applications. The results were used to model and evaluate industrial-scale cases. Integration with the pulp mill recovery boiler and steam cycle allows easy recovery of the considerable waste heat from the process itself, as well as the combustion of solid and gaseous residues. Economic analysis indicates profitability for OFP of fine EWRs from the mill. A higher net present value, but longer payback period, was obtained for a larger OFP plant using purchased feedstock. Stand-alone production was found unprofitable. Publishers version

In this work, the production viability, physical, chemical and mechanical properties of briquettes produced from mixtures of coffee shrub residues and pinewood, were evaluated. The densification was carried out under constant operating conditions (temperature of 120 °C, pressure of 8.27 MPa) in a piston-press type laboratory-scale briquetting machine. Coffee shrub residues were mixed with pinewood in ratios of 25%, 50% and 75%. In addition, reference briquettes of pure pinewood and of each type of coffee shrub residue were produced. To characterize the raw material, ash content, volatile matter, fixed carbon together with the calorific value of produced samples, were measured. To characterize the suitability of the briquettes produced: apparent density, energy density, tensile strength, and equilibrium moisture content were determined. The highest values of energy density (19133–19899 MJ m−3), tensile strength (415–569 kgf), apparent density (1107–1163 kg m−3) and favorable values of equilibrium moisture content (9–11 wt %) were obtained from a mixing ratio of 75% of pinewood. The novel contribution of this research was to develop briquettes with appropriate physical and mechanical parameters from new raw materials that could serve as sustainable fuel sources for local firing systems.

This work aims to assess the adequacy of the often made two dimensional mesh simplification in ash deposition models. Little information is available regarding its validity due to the heavy computational costs that a proper three-dimensional grid model would entail. We have implemented a case study (a deposition probe in a kraft recovery furnace) with 2D and 3D mesh models in order to compare their results regarding the ash deposition and the fluid flow. An additional simulation has been carried out to compare the results between URANS and DES turbulence models. For the particular case studied in this article, the two-dimensional simplification is justified as the results did not vary notably whereas entailing remarkably smaller computational costs. Nonetheless, the usage of DES turbulence model yielded moderately different results, qualitatively closer to deposit observations, justifying perhaps the three-dimensional approach when accuracy is needed for the deposition of fine particles on the lee edges of the tubes. Publisher's version

Chemical quantitative characterization of biomass is relevant for waste to energy recovery technologies. In the present work, selected agroindustry solid residues from coffee crops – parchment and coffee shrub, i.e., stem, branches and leaves – were characterized. Properties such proximate, ultimate and biochemical composition, energy content, and thermogravimetric analysis, were evaluated. Results showed high values of higher heating value and volatile matter content. The silica contents are small for all samples. Additionally, the high content of extractives and lignin, reveal that these residual biomasses are more suitable for charcoal than cellulose pulp production. The extensive residue characterization provided valuable data that helped in outcome of the evaluation of different conversion technologies as being an environmentally friendly alternative, contributing to sustainable, reliable, carbon-neutral form of modern energy and upgrade the large quantity of waste generated by the coffee industry into energetically valued residues, by improving their management.

The goal of the study was to develop an optimization methodology combining a cost model, a 2-D heat transfer model, and load variation, executable in a feasible time in a personal computer. The objective function to maximize is the annual net cash flow. For this purpose, the cogeneration plant performance is determined at different load points as a function of the condenser performance using IPSEpro process simulation software. This data is then used to implement the calculation process to obtain the objective function value. Due to the computationally heavy heat transfer model, and each objective function evaluation requiring multiple runs, one per load point, improving optimizer performance was given particular attention. A novel hybridization of cuckoo search and a greedy differential evolution strategy was created for this; the new algorithm is shown to perform better than either of its parent algorithms or other benchmarks, finding optimal solution reliably, and within approximately half the number of function evaluations as required by the parent algorithms.

The study analyzes a potential to improve the local waste and wastewater management in a Finnish community. The Lappeenranta wastewater treatment plant (WWTP) is used as a case study. Two different technological setups are considered for improving the wastewater treatment plant. These are used to construct four alternative wastewater reclamation scenarios. The mass and energy balances for the considered scenarios are developed and used as input for the profitability evaluation. The utilization of the sewage sludge from the WWTP and municipal solid waste fraction that cannot be recycled for the generation of heat and electricity at the CHP plant is investigated with the aim to improve the economic performance of the wastewater treatment facility. The studied scenarios are initially compared based on their investment and operational costs. The cost of water treatment increases significantly in the case of the investigated tertiary treatment systems: higher amounts of chemicals and electricity are needed to improve the water quality. At the same time, the study indicates that the profitability of a WWTP integrated with a CHP plant can be reasonably high in a wide range of likely price scenarios for alternative wastewater purification systems. The results of the analysis showed a significant potential for the investigated wastewater reclamation systems to improve the efficiency of solid waste and wastewater management in the community.

Russia accounts for approximately 20% of the world's forest cover. More than 59% of the total territory of the Russian Federation is classified as forest land. However, the forest sector contributes only 1.3% to Russian gross domestic product and the Russian forest industry accounts for less than 4% of the world's forest products trade. This paper analyzes the most significant challenges facing Russian forest management including example of wood chips export to the EU and will suggest specific policy proposals that could be made in order to improve the forest management and forest transportation in Russia. Russian forestry has a lot of challenges such as lack of proper forest infrastructure, inadequate forest fire protection, unqualified personnel and unclear legislation. Most of these challenges are linked to one another and solving one problem can lead to the solution of others. The improvement of Russian forest governance is difficult and hard for policy, but without significant changes in current Russian forest management practices, forest degradation in Russia will continue to increase. The degradation of Russian forests could lead to a loss of global resources in general.

Spent coffee grounds (SCGs) and tea leaf residues are investigated as a potential source of bioenergy and biochemicals and compared to several woody biomasses. The effect of initial processing (coffee roasting and tea oxidizing) on feedstock composition and thermal behavior is assessed experimentally and analytically. In addition to proximate and ultimate analyses, the materials are examined by combustion tests coupled with Fourier-transform infrared spectroscopy and thermogravimetric analysis coupled with mass spectrometry in oxidizing and inert atmospheres. The biggest differences in composition of the samples are found in the high elemental N and S contents of SCGs (2.9 wt% of N and 0.1 wt% of S on average) and especially tea leaf residues (4.5 wt% of N and 0.2 wt% of S on average). The elevated N and S values lead to NO and SO2 release during combustion. Overall, the obtained levels of reactivity and heating values (with a maximum higher heating value of 24 MJ kg−1 for the Arabica-Robusta coffee blend) confirm the significant energy potential of both the coffee and tea samples. Mass spectrometry analysis of the studied materials reveals higher heterogeneity of macromolecular composition of coffee and tea samples compared to woody biomasses. Pyridine, pyrazine, phenols and acetic acid are the most abundant among detected species and have demonstrated beneficial characteristics for a broad spectrum of human activities. The composition characteristics and reactivity test results presented in this work promote more effective valorization and usage of these currently little used biomass residues.

Torrefied biomass has considerable potential as a biomass fuel to replace coal in energy and process heat production. The aim of this paper is to evaluate the potential of torrefied biomass in different industries, both power and non-power generation industries, and considers the impact of such use on the international biomass market. The power generation sector has been so far the leader in testing torrefied biomass use with other industrial demand lagging behind. There are promising technical possibilities for greater torrefied biomass use in a number of other areas such as the steel, non-metallic minerals, as well as the pulp and paper industries. Although a large increase in torrefied biomass consumption by industry is not immediately foreseeable, industrial use by actors outside the energy generation sector could increase demand for torrefied biomass in general and, as a result, stimulate development of global torrefied biomass markets. Results show that the torrefied biomass demand significantly depends on the bioenergy markets. It seems that despite of the challenges, the growth of torrefied biomass demand will have a large progress in coming years. Post-print / final draft

Iron oxide pellets are currently one of the main feedstocks in iron-making processes, and increasing over the years. They are mostly produced in moving-grate induration furnaces. Several mathematical models describing the pellet induration process have been published over the last 50 years. Despite being a topic of research for a long period, a relative small amount of journal papers is available in the main academic databases. In addition, the model assumptions have not changed considerably over time, regardless the development of faster computational tools. The purpose of this paper is to provide a review regarding the available mathematical models for travelling-grate iron ore induration furnaces, pointing at some opportunities and aspects to improve the accuracy of these models.

Mathematical models of circulating fluidized bed (CFB) combustion systems vary from simple lumped models to full-scale 3D models with multi-phase flow fields. Models help to predict the behavior of the boiler under new operating conditions and to understand the underlying phenomena. Is it more important to make experiments or models? The answer is both. The real values can be assessed with the help of experiments and refined models. Is a complex model always better than a simple one? A simple model can be more easily modified and better adapted to the actual use. A 1,5-dimensional model of a CFB furnace is the simplest possible model that takes into account the most important heat transfer and flow features. Of these solids circulation is the most important factor that determines the amount of heat transfer at the furnace walls. Consequently, regulating the solids circulation is the fundamental means of load control in CFB furnaces. One dimensional model takes into account only the vertical flow direction, but 1,5-dimensional model considers solids circulation inside the furnace as well. The internal circulation is up to 2 times greater than the circulation around the solids separator and return in CFB combustors. 1,5-dimensional model is also called the core-annulus model. The furnace is considered as a cylinder with an annular space around it. The hot solids flow upwards along the cylinder and downwards along the annular space. In this study, a core-annulus model is implemented using a commercial IPSEpro software. The developed model consists of several modules. The mathematical principles of each module is described. The software is also presented briefly. The new model is applied to study the behavior of a large biomass boiler. The model inputs include mass flows of fuel and air, fuel type and parameters regarding the solids amount, size and distribution. In addition to inputs for the design operation, other scenarios are considered such as partial load and burning of different fuels. Strengths and weaknesses of the model are also assessed and pathways of future research are reviewed.

This work is concerned with the research field of ash deposition phenomena. An unsteady CFD model of the convective section of a 100-kW PC pilot-scale combustor is implemented aiming to simulate and study the ash deposition trends. Emphasis is placed at analyzing how particles of different sizes may contribute differently to the deposit buildup. A discrete particle tracking and a rebound–stick submodel are used. Four simulation cases are implemented to investigate the effect of the inlet flow velocity and the fouling conditions (clean vs. fouled tubes). The sticking efficiencies decreased with the particle diameter d p after being constant and higher than 95% for d p 1 μ m. It also seemed to decrease with the flue gas inlet velocity. The normalized particle arrival rates to the tubes were rather constant up to particle sizes of d p = 8 μ m. The studied cases of faster flow inlet velocity showed higher values for both the deposition and arrival trends as a consequence of a higher overall particle Stokes’ number and the total ash mass input to the model. The cases with fouled tubes presented higher deposition rates (approximately, 7.91% more for the fast velocity case) than what is observed for clean tubes.

The buildup of ash deposits represents a major challenge in the operation of industrial boilers since it entails significant losses in heat transfer performance. Therefore, the research of predictive tools is of great value in boiler operation. A CFD model for ash deposition and fouling growth prediction of industrial furnaces is presented. Considerations are taken regarding grid resolution and accuracy requirements available in literature. A 2D transversally-periodic bundle of four in-line tubes of a kraft recovery boiler bank is modeled with the gas laden with discrete solid fume ash particles. A sticking-rebound submodel determines whether to account the mass of a hitting particle for the deposit growth or to make it rebound, re-entraining it back into the flow. A dynamic mesh modifies periodically the model grid, simulating the expansion of deposits. 0.7 μm diameter particles showed round and uniform deposits caused by thermophoresis. The deposit thickness grew up to about 3 mm after 100 min of fouling. 3.62 μm particle showed more irregular distributions with some high local peaks, but lower deposition rates in average. For this particular case modeled, the constant particle sticking probability approach showed no major differences with respect to the particle sticking submodel.

To increase the understanding of hydrothermal carbonization (HTC) of lignocellulosic biomass residues, four feedstocks: giant bamboo, coffee wood, eucalyptus, and coffee parchment, were studied. The effect of operating conditions on the products in terms of yield, composition and energy densification were quantified. Each feedstock was treated for 3 h at temperatures of 180, 200, 220 and 240 °C. For all samples, the higher heating value (HHV), fixed carbon content and energy density increased with increasing reaction severity, while volatile matter content and mass yield decreased. The HHV of hydrochar samples obtained at temperatures ≥220 °C were in the range of 24.6–29.2 MJ kg−1 and indicated the high potential of these materials for fuel applications. The mass yields varied in the range of 46.5–56.9%, with the exception for coffee parchment, where the lower values of 34.4–46.0% were obtained. The fixed carbon varied from 33.8% to 53.0%. The HTC liquor had pH values of 2.9–4.4 due to organic acids. The results were used to model and evaluate different industrial-scale HTC simulation cases. The overall efficiency was similar within all studied biomasses. The integration with a bio-fired power plant allows simplification of the process while also bringing efficiency gains. All studied biomasses appear to be suitable for energy and value-added products generation through HTC treatment. Coffee residues, which have received little research consideration previously, responded well.

Coffee production in Brazil creates significant amounts of residues. The goals of this study are to evaluate the characteristics of these residues (parchment, husk, pulp, spent grounds, silverskin and defective beans); to discuss their potential for conversion to improved biofuels via thermochemical methods; and to develop mass and energy balances of the processes to help determine the value of residues for direct combustion, fast and slow pyrolysis, gasification, hydrothermal carbonization and torrefaction. Particularly the pulp, but also husk and parchment, are characterized by high moisture, as well as high contents of cellulose (41–64%) and hemicellulose (27–35%). These residues are suitable for several conversion routes, albeit with the drawback of drying need for the dry methods. The ash of these also creates a risk of fouling, corrosion and agglomeration with high-temperature and fluidized bed technologies. The silverskin and some of the defective beans are available at lower moisture. The spent coffee grounds appear a particularly advantageous residue for energy use: while moisture varies, the roasted product dries easily and has the highest heating value of the residues. For defective beans, little thermochemical treatment data is available. Among the technologies, for wet feedstocks hydrothermal carbonization has the advantage of post-conversion drying. Gasification appears advantageous for parchment with a high syngas yield and heating value. Fast pyrolysis of biomass suffers from the oxygen content of the liquid, requiring additional treatment; slow pyrolysis may be more appropriate. In conclusion, coffee residues have potential as feedstocks for a number of thermochemical conversion processes.

Wood-fired combined heat and power (CHP) plants are a proven technology for producing domestic, carbon-neutral heat and power in Nordic countries. One drawback of CHP plants is the low capacity factors due to varying heat loads. In the current economic environment, uncertainty over energy prices creates also uncertainty over investment profitability. Hydrothermal carbonization (HTC) is a promising thermochemical conversion technology for producing an improved, more versatile wood-based fuel. Integrating HTC with a CHP plant allows simplifying the HTC process and extending the CHP plant operating time. An integrated polygeneration plant producing three energy products is also less sensitive to price changes in any one product. This study compares three integration cases chosen from the previous paper, and the case of separate stand-alone plants. The best economic performance is obtained using pressurized hot water from the CHP plant boiler drum as HTC process water. This has the poorest efficiency, but allows the greatest cost reduction in the HTC process and longest CHP plant operating time. The result demonstrates the suitability of CHP plants for integration with a HTC process, and the importance of economic and operational analysis considering annual load variations in sufficient detail.

This work presents a CFD model of a transversally-periodic bundle of four in-line tubes of a kraft recovery boiler bank for ash deposition calculations. The flue gas was laden with discrete solid ash particles. Particle contact and sticking–rebound mechanics were used. The ash deposition parameters (arrival rate, sticking efficiency, deposit rates) were computed locally, studying the dependency on the particle diameter. Emphasis was put regarding grid resolution, unsteady flow solving, and the differences observed among different tubes and locations. Thermophoresis was responsible for 94.1% of the total deposition rates for submicron (0.7 μ m ) particles, becoming markedly less significant for coarser particles. These small particles showed a much higher sticking efficiency (above 95%) than other studied particles. The particles with the largest diameter studied (18.7 μ m ) had much higher arrival rates to the cold surfaces since they did not follow the flue gas flow paths (inertial impaction). Due to their low sticking efficiency (about 15%), they formed more irregular deposition distributions. Particles with an intermediate diameter (3.62 μ m ) showed the smallest arrival rates among the three particle sizes considered.

Power-to-gas (PtG) is recognised as potential option to benefit from periods of low electricity prices. Temporary low prices will probably be more common in the future due to increasing share of solar and wind energy in power systems. When producing methane by electrolysis and synthesis with CO 2 , significant amounts of oxygen and steam are produced as by-products. It is possible to store oxygen and utilise it in biomass fired or co-fired combined heat and power (CHP) plants to temporarily increase production during peak prices. Together with utilisation of the by-product steam, integration of PtG with biomass fired CHP plant offers an attractive concept for future energy system. In this paper, the feasibility of this concept is analysed in several market scenarios. As main results, annual operational costs, profits and payback times are presented. In addition, the impacts on CO 2 emissions are discussed.

This article contains data related to the research article entitled “A survey analysis of the wood pellet industry in Finland: Future perspectives" (S. Proskurina, E. Alakangas, J. Heinimö, M. Mikkilä, E. Vakkilainen, 2016) [1]. The dataset include information about the importance of wood pellets on the global bioenergy development and role of wood pellets on the Finnish bioenergy development. Data leads to an expansion of knowledge and discoveries of new possibility for wood pellets industry analysis. Publishers version

The Russian wood pellet business has expanded rapidly in the last decade. The first Russian pellet production plant was built in 2003 and within ten years, Russia has become an important exporter of pellets, mainly to the Nordic and Baltic area. The aim of this paper is to present an overview of the current status of the Russian pellet market with particular emphasis on pellet exports. In addition to reviewing the state of the pellet market in Russia, the paper addresses challenges facing the Russian wood pellet business, both domestically and in key export markets. The paper is based on a review of published scientific literature, trade magazines, the latest Russian official documents and reports, as well as interviews with experts and the materials of IEA Bioenergy Task 40. Attempts have been made to reconcile the varying and conflicting data given in different Russian documentation sources, thus permitting a more accurate representation of the Russian pellet business, an important player in the global pellet trade. Despite its importance, there is limited work available on the Russian pellet industry and this paper thus contributes to greater understanding of the Russian pellet business. Based on an analysis of the market situation, challenges and opportunities are presented for future scenarios of Russian pellet exports. The study further demonstrates the important role that Russia plays in the global pellet market. This paper finds that although the Russian pellet business developed very rapidly during 2004–2009, the industry now faces considerable challenges. Due to an oligopolistic market structure, inadequate infrastructure and lack of foreign investments, it is expected that, despite its obvious potential, the Russian wood pellet industry may not develop significantly in the near future and exports to Europe could even decline. Post-print / final draft

CFD tools are of great value in the design and operation of boilers. One particular aspect that can be modeled by CFD is the ash deposition and plugging of heat transfer surfaces of boilers. Fouling and slagging are the most typical causes of unscheduled boiler shutdowns. Consequently, appropriate predictions of deposit geometries and rates are of considerable interest. CFD multiphase flow simulations are capable of modeling particle-laden streams and constitute a suitable tool for study of material deposition. Fouling phenomena have a complicated and multidisciplinary nature involving thermo-fluid mechanics, sticking/rebounding of particles, sintering, etc. If the deposit growth rate has been accurately calculated by a multiphase model appropriate for particle-laden flows, CFD dynamic mesh techniques are able to move the interphase fluid-deposit according to the growth rate. This work develops and presents a CFD model for prediction of deposition shapes in a classical boiler bank of a Kraft Recovery Boiler by combining a multiphase and a dynamic mesh model. The effect of tube transversal spacing is also analyzed. The paper highlights the features and effects of the dynamic mesh model.

The ore fines' pelletizing is an important part of iron mining, which ensures better use of natural resources and increases the blast furnace efficiency. However, this process consumes high amounts of non-renewable energy, such as natural gas (NG) and coke. Due to fossil fuel scarcity and global warming issues, at least partial substitution for renewable energy is desirable. Biomass gasification projects are being successfully developed in Northern Europe and large-scale circulating fluidized bed (CFB) biomass gasifiers have been commissioned. This work compares different technologies, such as CFB air and oxygen gasification, dual fluidized bed (DFB) steam gasification, and bio-synthetic natural gas (bio-SNG) production, focusing on the use of the product gas in an iron ore pelletizing process located in the Southeast of Brazil. The main parameters evaluated were flame temperature and gaseous flow rates. Economical evaluation was also performed. Air gasification provided a product gas with the lowest energy content but it was the most attractive investment. Oxygen gasification is apparently the best option as it provides a product gas with higher heating value at almost the same cost as air gasification. In both cases changes to the burners would be required. Bio-SNG could be utilized without any adaption in the indurating machine; however, it is still more expensive than NG.

Restrictions of energy production emissions set new challenges to combustion facilities, and new methods, such as High Temperature Air Combustion (HiTAC) are considered to meet these challenges. In HiTAC, the flue gas is recirculated to the combustion region while preheating the combustion air. The HiTAC combustion is an environmentally friendly and energy-efficient method, but it requires special burner arrangements and additional equipment for air preheating. This work investigates the feasibility to obtain low emissions without preheating the combustion air. Experimental work showed that in this case the applicable flue gas recirculation rates were lower than with conventional HiTAC. Numerical analysis was performed to analyze flow behavior in the combustion chamber. The main contributing factor for combustion stability was found to be pronounced internal recirculation. The flame was forced aside towards the side walls by a back flow in the chamber centerline, which kept the flame stable and attached to the burner. The results suggest that the advantages of HiTAC can be partly achieved without the preheating of combustion air and with moderate flue gas recirculation. This enables a simplified and more economical construction, applicable for instance in small-scale boilers.

The implementation of wood extraction prior to pulping (pre-hydrolysis), with subsequent recovery of hemicellulose, is expected to affect the operation of a conventional kraft pulp mill. The magnitude of impacts will depend especially on the extraction conditions. In this specific work, the consequences of integrating the auto-hydrolysis process are studied using a detailed mill balance. A softwood pulp mill in Finland was used as a reference. With 14.1% of wood extracted, the wood demand increased by 15.5% and the steam generation in the recovery boiler by 13.5%. The removal of approximately 17% of lignin from black liquor would put the flue gas side of the boiler back to the original required capacity. In turn, this would enable the simultaneous recovery of lignin and hemicellulose. To make this process economically feasible, the extra revenue from the sales of lignin and hemicellulose products would need to compensate for the additional operating costs in the pulp mill.

This paper is the first in a series of two reporting on results from an extensive laboratory-scale corrosion study where tailor-made well-characterized synthetic alkali salt deposits were used for corrosion testing of several steel materials used in or aimed for recovery boiler superheater tubing. The corrosion testing was done in temperatures ranging from 450 to 600 °C. The synthetic alkali salt deposits, containing sodium, potassium sulfates and chlorides, were composed in such a way that their first melting temperature, T 0 , and the amount of melt formed at this temperature, varied for each salt mixture. The results showed on one hand that an increased amount of melt in the salt deposit increased the corrosion of the steel material markedly. The results showed, however also, that corrosion could take place at temperatures clearly below any melting of the salt deposits if the composition was suitable. This took place with salts that contained chlorine. Already a very low amount of chlorine in the salt caused corrosion at temperatures typical for superheaters in the recovery boiler. These effects are qualitatively well-known from earlier but it was surprising that already a very small amount of chlorine caused significant increase in corrosion. To stress the importance of the deposit layer on the corrosion we introduce two new terms: (1) sub- T 0 corrosion, indicating corrosion taking place below any melting of the deposit and (2) super- T 0 corrosion, indicating corrosion taking place when the deposit contains melt.

In this paper, we present results from a new detailed physical model for single black liquor droplet pyrolysis and swelling, and validate them against experimental data from a non-oxidizing environment using two different reactor configurations. In the detailed model, we solve for the heat transfer and gas phase mass transfer in the droplet and thereby, the intra-particle gas–char and gas–gas interactions during drying and devolatilization can be studied. In the experimental part, the mass change, the swelling behaviour, and the volume fraction of larger voids, i.e. cenospheres in the droplets were determined in a non-oxidizing environment. The model gave a good correlation with experimental swelling and mass loss data. Calculations suggest that a considerable amount of the char can be consumed before the entire droplet has experienced the devolatilization and drying stages of combustion. Char formed at the droplet surface layer is generally consumed by gasification with H 2 O flowing outwards from the droplet interior. The extent of char conversion during devolatilization and the rate of devolatilization are greatly affected by swelling and the formation of larger voids in the particle. The more the particle swells and the more homogeneous the particle structure is, the larger is the conversion of char at the end of devolatilization.

A novel mechanism for gasifying a char is described. For thermally large particles (i.e., Bi > 0.1) the temperature distribution is non-uniform. Because different temperature regimes exist in the particle, the stages of drying, devolatilization (or pyrolysis), and reaction of the char may overlap. At some point the particle’s surface is fully devolatilized, while the particle’s interior is still undergoing drying and devolatilization. As H2O and CO2 flow out from the particle they pass through the hot surface layers of char. If the temperature is high enough, the char may be gasified. Black liquor was used here as a sample fuel. It has desirable properties for such auto-gasification; thermally large particles, a high initial water-content and a very porous and highly reactive char. Detailed numerical simulations suggest that 30 to 40% of the char may be converted simultaneously with devolatilization by auto-gasification. The larger the particle and the higher the temperature, the larger is the fraction of char gasified. For coal and peat, a typical particle size is too small for this mechanism to play any role when fired in pulverized fuel or fluidized bed furnaces. For burning wooden logs, the particle size is large and the pyrolysis time is ≈10 min., so then auto-gasification might be important.

To understand recovery boiler ash deposit formation, it is necessary to have detailed information on the ash particle properties in the flue gas. Since coarse particle deposition is orders of magnitude more efficient than fine particle deposition, the coarse particles were studied in detail. In an industrial recovery boiler, ash particles were extracted directly from the flue gases with a settling probe, a double cyclone–filter system and a thermophoretic sampler in the superheater area. The particles were analysed for their bulk chemical composition with ion chromatography. The individual particles were studied for their morphology and elemental composition with a high-resolution scanning electron microscope connected to an energy dispersive X-ray analyser. Measured coarse particle mass fraction of the total particle concentration was about 40%, which is significantly larger than proposed earlier. Five coarse particle types were observed at the superheater area: (i) partially sintered large agglomerates formed from fine fume particles that had entrained from the heat exchanger surfaces, (ii) extensively sintered irregular particles that had entrained from the surfaces by soot blowing, (iii) spherical particles that appeared highly porous inside, (iv) dense spherical particles, and (v) intermediate irregular particles of non-process mineral matter. At the boiler exit, the coarse particles were mainly partially sintered large agglomerates.

The strongest driving force in the pulp industry is the environment. New process equipment has been installed to reduce emissions and chlorine usage. Recently, more attention has been paid to mill closure. Some of the promising new technologies are waste-water evaporation, green-liquor clarification and thermal-heat-treatment of black liquor. Black-liquor gasification requires more materials technology research. The best way to new meet demands is to modernize the basic process step by step, one unit at a time.