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Sustainability and efficient use of resources are becoming increasingly important aspects in the operation of all industries. Recently, some biomass-fired boilers have been equipped with increasingly complex condensing back-end heat recovery solutions, sometimes also using heat pumps to upgrade the low-grade heat. In kraft recovery boilers, however, scrubbers are still mainly for gas cleaning, with only simple heat recovery solutions. In this paper, we use process simulation software to study the potential to improve the power generation and energy efficiency by applying condensing back-end heat recovery on a recovery boiler. Different configurations are considered, including heat pumps. Potential streams to serve as heat sinks are considered and evaluated. Lowering the recovery boiler flue gas temperature to approximately 65°C significantly decreases the flue gas losses. The heat can be recovered as hot water, which is used to partially replace low-pressure (LP) steam, making more steam available for the condensing steam turbine portion for increased power generation. The results indicate that in a simple condensing plant, some 1%–4% additional electricity could be generated. In a Nordic mill that provides district heating, even more additional electricity generation, up to 6%, could be achieved. Provided the availability of sufficient low-temperature heat sinks to use the recovered heat, as well as sufficient condensing turbine swallowing capacity to utilize the LP steam, the use of scrubbing and possibly upgrading the heat using heat pumps appears potentially useful.

Bioenergy with CO2 capture and storage (BECCS) is a promising negative emission technology (NET). When using sustainably produced biomass as fuel, BECCS allows the production of power and heat with negative CO2 emissions. The main technical challenges hindering the deployment of BECCS technologies include energy penalties associated with the capture process. This work evaluates the performance of an advanced CO2 capture technology, chemical looping with oxygen uncoupling (CLOU), replacing a conventional fluidized bed boiler in the power boiler role in a large, modern integrated pulp and paper mill. Results from a MATLAB/Simulink reactor model were incorporated in a plant and integration model developed in a commercial process simulation software to quantify the performance of the CLOU-integrated cogeneration plant. The results show that in this specific application, the typically already low efficiency penalty of CLOU-based carbon capture and storage (CCS) systems could be eliminated entirely, and actually even a very small efficiency gain could be obtained. The highly efficient operation is possible due to the high moisture and hydrogen contents of the biomass and the separation of combustion products and excess air streams in the CLOU process; this provides an opportunity to recover a significant amount of heat by flue gas condensation at a higher temperature level than what is possible in a conventional boiler. Together with abundant low-temperature heat sinks available at the pulp and paper application allows freeing a considerable amount of low-pressure steam for expansion in the condensing turbine. The resulting increase in gross generator output proved enough to not only match, but very slightly exceed the approximately 18 MW parasitic load introduced by the CLOU system in comparison to the conventional boiler.

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

A modern pulp mill generates a variety of different by-products and waste streams, some of these can be recycled, refined, sold, or used on-site for energy production. However, some, such as chemical and biological sludges produced in wastewater treatment cannot be reused or disposed of easily, mainly due to their high moisture content and poor drying characteristics. Tightening legislation regarding waste disposal as well as the growing need to increase the process efficiencies of pulp mills act as driving forces to find environmentally friendly and energy-efficient techniques for pulp mill sludge treatment. This study summarizes the current methods for pulp mill sludge handling and evaluates the potential of hydrothermal carbonization (HTC), a conversion process through which wet organic substrates can be transformed into a carbonaceous material (hydrochar). Depending on the process parameters, the material’s structure is modified, enabling hydrochar use in energy, soil conditioning and adsorption applications. The sludges were hydrothermally carbonized at 180, 200, 220 and 240 °C for 3 h. The hydrochar and liquid products’ main properties were analyzed. Their potential applications were also evaluated. The effective treatment of sludges from the pulp industry with HTC could transform energy-demanding waste into a value-added source of materials.

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.

Upgrading biomass waste streams can improve economics in wood industries by adding value to the process. This work considers use of a hydrothermal carbonization (HTC) process for the residual feedstock after lignin and hemicelluloses extraction. Batch experiments were performed at 200&ndash, 240 &deg, C temperatures and three hours residence time with an 8:1 biomass to water ratio for two feedstocks: Raw spruce and spruce after lignin extraction. The proximate analysis and heating value showed similar results for both feedstocks, indicating that the thermochemical conversion is not impacted by the removal of lignin and hemicelluloses, the pretreatment processing slightly increases the heating value of the treated feedstock, but the HTC conversion process produces a consistent upgrading trend for both the treated and untreated feedstocks. The energy yield was 9.7 percentage points higher for the treated wood on average across the range temperatures due to the higher mass yield in the treated experiments. The energy densification ratio and the mass yield were strongly correlated with reaction temperature, while the energy yield was not. Lignocellulosic composition of the solid HTC product is mainly affected by HTC treatment, the effect of lignin extraction is negligible.

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.

Wood-fired small- and medium-scale combined heat and power (CHP) plants are a proven technology for producing domestic, carbon-neutral heat and power. Hydrothermal carbonization (HTC) is a promising conversion technology for producing an improved, more versatile wood fuel. By integrating the HTC and CHP processes, the HTC process can be notably simplified. This study compares six different integration schemes to non-integrated plants. The overall energy conversion efficiencies were similar in all six schemes, but there were differences in power output and in plant complexity, and thus likely differences in investment cost and operability. The most promising cases were evaluated at varying HTC temperatures. This comparison showed that temperatures over 220 °C became problematic for the simpler heat recovery schemes, which resulted in poor efficiencies.

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.

Biomass conversion via thermal processes to generate energy will be an important part of the future energy landscape. The objective of this study was to determine the kinetic parameters of five types of eucalyptus wood derived from different clones, sugarcane bagasse, elephant grass, bamboo and fibers of coconut fruit. The framework to describe the kinetic pyrolysis consists of a fuel model including four constituents, namely hemicelluloses, cellulose, lignin and extractives. Each pseudo-component was converted via two competing reactions into volatile and char. A statistical fit was achieved with mass loss rate data, obtained from the pyrolysis modeling and the thermogravimetric analysis, providing satisfactory statistical variance. In the end of the kinetic parameters optimization, the activation energies for reaction of hemicelluloses, cellulose and lignin were obtained as 179.98, 130.0 and 40 kJ mol−1, respectively, whereas the decomposition of the pseudo-components resulted in relatively similar values of pre-exponential factor for all biomasses evaluated. The experimental results and kinetic parameters provide useful data to improve design of thermochemical conversion units.

The composition, heating value, and reactivity of different municipal and industrial solid waste materials were studied and compared with biomass and coal samples. Reactivity characterization was performed in a vertical tube reactor by dropping a pelletized sample on a porous grid and monitoring the conversion of the sample by Fourier-transform infrared spetroscopy. Samples were tested in ambient oxygen atmosphere at three temperatures typical to incinerator applications, and in reduced oxygen atmospheres, that better represents conditions near the fuel feeding points in incinerators. Thermogravimetrical analysis was used to analyze selected samples and reactivity information was compared with the vertical tube reactor results. This research provides information on the material composition and reactivity for utilization of munical solid waste and sewage sludge in incineration applications. Comparison of two characterization methods illustrates that the characterization method has a significant effect on the reaction rate results, with the implication that the reaction characterization method should be selected to represent the actual conditions of the application. Post-print / Final draft

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.

Sustainability and energy efficiency have become important factors for many industrial processes, including chemical pulping. Recently complex back-end heat recovery solutions have been applied to biomass-fired boilers, lowering stack temperatures and recovering some of the latent heat of the moisture by condensation. Modern kraft recovery boiler flue gas offers still unutilized heat recovery possibilities. Scrubbers have been used, but the focus has been on gas cleaning, heat recovery implementations remain simple. The goal of this study is to evaluate the potential to increase the power generation and efficiency of chemical pulping by improved back-end heat recovery from the recovery boiler. Different configurations of heat recovery schemes and different heat sink options are considered, including heat pumps. IPSEpro simulation software is used to model the boiler and steam cycle of a modern Nordic pulp mill. When heat pumps are used to upgrade some of the recovered low-grade heat, up to +23 MW gross and +16.7 MW net power generation increase was observed when the whole pulp mill in addition to the boiler and steam cycle is considered as heat consumer. Combustion air humidification proved to yield a benefit only when assuming the largest heat sink scenario for the pulp mill.

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.

Concerns about climate change and other issues mostly related to the reduction of fossil fuel usage have increased the demand for renewable energy sources. The possibility of using lignocellulosic biomass for energy generation is gaining interest in many countries worldwide. The current paper presents the analysis of physicochemical characteristics of nine lignocellulosic biomasses: five types of eucalyptus wood chips, sugar cane bagasse, elephant grass, bamboo, and coconut husk. Selection of a thermal conversion technology depends on the knowledge of important biomass characteristics in relation to thermal conversion: density and productivity, proximate and ultimate analysis, heating value, ash and polysaccharides composition, and thermogravimetric analysis. With regard to the annual energy potential and density, it was suggested that Eucalyptus urophylla (Flores IP) × E. urophylla (Timor) and bamboo have the greatest potential for energy application, which reduces transport and storage costs. Moreover, ...

Biomass torrefaction is a pre-treatment technology with high potential to convert biomass into a valuable commodity. The heat integration of torrefaction and combined heat and power (CHP) plant was investigated in previous work (Sermyagina et al., 2015). The aim of the present study is to assess possible economic benefits from integration. Three most promising integration concepts from the previous work were studied in terms of seasonal operational changes of district heating demand and varying ambient conditions. The performance of two integration concepts were evaluated together with stand-alone and co-located plants. The integration leads to a higher utilization of the CHP boiler capacity during part-load operation, possible increase of the operation time and growth of electricity generation as a result. The total efficiencies of the integrated cases (around 72% in higher heating value terms) are slightly higher than the stand-alone CHP plant (69%) or the co-located option (71%). The integration requires 40% more capital investments than the stand-alone CHP. On the other hand, the total capital investments of the integration cases are 20% lower than in co-located plants, and a profitability evaluation shows that lower investment costs may make integration schemes advantageous over the non-integrated plants. Feedstock price and investment costs are the main economic drivers affecting the profitability of the integrated options. An integration case which uses back pressure steam to account for the torrefaction heat demand showed the highest profitability due to a longer annual operating time, resulting in a growth of electricity and DH production over the stand-alone CHP plant. Post-print / Final draft

An important factor for industrial-scale implementation of torrefaction – a thermal pre-treatment technology that enhances the fuel characteristics of biomass – is the requirement for a low-cost source of heat energy. Significant benefits can be achieved if torrefaction is integrated with a large heat producer. To study this possibility, a new model for the energy and mass balances of the torrefaction process was developed on the basis of available experimental data. The torrefaction model was then integrated into steam power plant simulations, and the performance of different integration schemes was evaluated. To investigate the effect of plant size and operating mode, the same configurations were studied for both large and small combined heat and power (CHP) plants. Large plant operates at full boiler load, and capturing a portion of heat from the plant reduces the power output for all integration cases. At the same time, higher trigeneration efficiencies in comparison with non-integrated case together with fuel consumption decrease due to torrefaction gas co-firing indicate that integration of torrefaction and CHP plant at full load could be beneficial. In case of small plant, free boiler capacity at reduced-load conditions allows to cover the torrefaction heat demand and simultaneously increase the electricity generation for almost all integration cases. Trigeneration efficiency is changing within a relatively narrow range for most of the cases, while the solid fuel consumption is higher than in design case. This work shows that integration method together with process temperature and plant operation mode are the important factors that could have an effect on CHP plant integration with torrefaction process. Post-print / Final draft

Hydrothermal carbonization of biomass presents a promising way to improve fuel characteristics of biomass without preliminary drying. In this process, feedstock is subjected to heating with water at temperatures between 180 and 250 °C during a certain period of time. This paper investigates the effect of process conditions (temperature, time, and the ratio between biomass and water) on the hydrothermal carbonization of coniferous biomass. Three sets of experiments were carried out with coniferous wood chips. Mass and energy yields together with proximate analysis measurements were used for the carbonization process evaluation. Dependencies of hydrochar mass and energy yields from the operation parameters could be described with mathematical correlations. Post-print / Final draft

Current paper presents the results of proximate analysis and heating value measurements for several different municipal solid waste (paper, cardboard, waste wood, textile, juice box carton and sewage sludge) and industrial waste fractions (bark, sawmill residue and industrial sludge). The values are compared with the fuel samples (forest residue, eucalyptus wood chips, peat and coal). Municipal solid waste fractions exhibit quite similar composition, comparable on dry basis with the industrial wood wastes, woody fuels and peat. Post-print / Final draft