A study on torrefaction of various biomass materials and its impact on lignocellulosic structure simulated by a thermogravimetry

Torrefaction processes of four kinds of biomass materials, including bamboo, willow, coconut shell and wood (Ficus benjamina L.), were investigated using the thermogravimetric analysis (TGA). Particular emphasis is placed on the impact of torrefaction on hemicellulose, cellulose and lignin contained in the biomass. Two different torrefaction processes, consisting of a light torrefaction process at 240 °C and a severe torrefaction process at 275 °C, were considered. From the torrefaction processes, the biomass could be divided into two groups; one was the relatively active biomass such as bamboo and willow, and the other was the relatively inactive biomass composed of coconut shell and wood. When the light torrefaction was performed, the results indicated that the hemicellulose contained in the biomass was destroyed in a significant way, whereas cellulose and lignin were affected only slightly. Once the severe torrefaction was carried out, it further had a noticeable effect on cellulose, especially in the bamboo and willow. The light torrefaction and severe torrefaction were followed by a chemically frozen zone, regardless of what the biomass was. From the viewpoint of torrefaction application, the investigated biomass torrefied in less than 1 h with light torrefaction is an appropriate operation for producing fuels with higher energy density.     

Torrefaction and co-torrefaction characterization of hemicellulose, cellulose and lignin as well as torrefaction of some basic constituents in biomass

Torrefaction is a thermal pretreatment process for biomass where raw biomass is heated in the temperatures of 200-300 °C under an inert or nitrogen atmosphere. The main constituents contained in biomass include hemicellulose, cellulose and lignin; therefore, the thermal decomposition characteristics of these constituents play a crucial role in determining the performance of torrefaction of lignocellulosic materials. To gain a fundamental insight into biomass torrefaction, five basic constituents, including hemicellulose, cellulose, lignin, xylan and dextran, were individually torrefied in a thermogravimetry. Two pure materials, xylose and glucose, were torrefied as well for comparison. Three torrefaction temperatures of 230, 260 and 290 °C, corresponding to light, mild and severe torrefactions, were taken into account. The experiments suggested the weight losses of the tested samples could be classified into three groups; they consisted of a weakly active reaction, a moderately active reaction and a strongly active reaction, depending on the natures of the tested materials. Co-torrefactions of the blend of hemicellulose, cellulose and lignin at the three torrefaction temperatures were also examined. The weight losses of the blend were very close to those from the linear superposition of the individual samples, suggesting that no synergistic effect from the co-torrefactions was exhibited.     

Thermogravimetric analysis of microalgae combustion under different oxygen supply concentrations

Recently, studies of microalgae in China have increased a lot because of their obvious advantages over other biological fuels. In this paper, the combustion behavior of Chlorella vulgaris (a genus of unicellular green microalgae) was investigated in a thermogravimetric analyzer (TGA) from room temperature to 800 °C in O₂/N₂ atmospheres. The effects of different oxygen concentrations (20, 50, 60, 80 vol.%) and different heating rates (10, 20 and 40 °C min⁻¹) on the combustion processes of C. vulgaris had been studied. The results indicated that the combustion processes of C. vulgaris could be divided into three stages. The oxygen concentrations and heating rates had important effects on the main combustion processes of C. vulgaris. The iso-conversional method involving Flynn–Wall–Ozawa (FWO) and the Kissinger–Akahira–Sunose (KAS) methods were used for the kinetic analysis of the main combustion process. The results indicated that, when the oxygen concentration varied from 20 to 80 vol.%, the value of activation energy increased respectively from 134.03 to 241.04 kJ mol⁻¹ by using FWO method and from 134.53 to 242.33 kJ mol⁻¹ by KAS method. Moreover, the optimal oxygen concentration for C. vulgaris combustion was 25–35 vol.%_.     

Isothermal torrefaction kinetics of hemicellulose, cellulose, lignin and xylan using thermogravimetric analysis

In recent years, torrefaction, a mild pyrolysis process carried out at the temperature range of 200-300 °C, has been considered as an effective route for improving the properties of biomass. Hemicellulose, cellulose, lignin and xylan are the basic constituents in biomass and their thermal behavior is highly related to biomass degradation in a high-temperature environment. In order to provide a useful insight into biomass torrefaction, this study develops the isothermal kinetics to predict the thermal decompositions of hemicellulose, cellulose, lignin and xylan. A thermogravimetry is used to perform torrefaction and five torrefaction temperatures of 200, 225, 250, 275 and 300 °C with 1 h heating duration are taken into account. From the analyses, the recommended values of the order of reaction of hemicellulose, cellulose, lignin and xylan are 3, 1, 1 and 9, respectively, whereas their activation energies are 187.06, 124.42, 37.58 and 67.83 kJ mol⁻¹, respectively. A comparison between the predictions and the experiments suggests that the developed model can provide a good evaluation on the thermal degradations of the constituents, expect for cellulose at 300 °C and hemicellulose at 275 °C. Eventually, co-torrefaction of hemicellulose, cellulose and lignin based on the model is predicted and compared to the thermogravimetric analysis.     

Quantitative method applicable for various biomass species to determine their chemical composition

A quantitative method applicable for various biomass species to determine their chemical constituents was explored. The widely used wood analytical method was found to be not entirely applicable to different biomass species. It was then demonstrated that by incorporating protein and starch determinations, by ash-correcting the Klason lignin and holocellulose and also by protein-correcting Klason lignin and holocellulose of high protein content species, reliable summative results that enable comparison between different types of biomass materials were achieved. Thus, an analytical method with starch and protein determinations as well as ash and protein corrections was proposed for quantitative assay of chemical composition of various biomass species.     

Physical characterisation and chemical composition of densified biomass fuels with regard to their combustion behaviour

With respect to the use of densified biomass fuels in fully automatic heating systems for the residential sector a high quality of these fuels is required. Several European countries already have implemented standards for such fuels. In other countries such standards are in preparation or planned. Furthermore, in some countries also standards from associations are existing (e.g. from the Austrian Pellets Association). In addition to these national standards, European standards for solid biomass fuels are under development. For producers of densified biomass fuels, especially for pellet producers, it is therefore very important to produce high-quality fuels keeping the limiting values of the standards addressed. However, in this context it has to be considered that as a high fuel quality as is necessary for the combustion of densified biomass fuels in automatic small-scale furnaces is not necessary if these fuels are used in larger industrial furnaces as they are equipped with more sophisticated flue gas cleaning, combustion and process control systems. Two pellet qualities, one for industrial and one for small-scale consumers seem to be more meaningful.

Within the framework of the EU-ALTENER-project “An Integrated European Market for Densified Biomass Fuels (INDEBIF)” a questionnaire survey of European producers of densified biomass fuels was performed. In this connection the possibility was offered to the producers to participate in an analysis programme with their fuels. An overview was obtained of the qualities of densified biomass fuels offered in the European market, covering pellets and briquettes from Austria, Italy, Sweden, Spain, Norway and the Czech Republic.

The parameters analysed were the dimensions of the fuels, the bulk and the particle density, the water and the ash content, the gross and the net calorific value, the abrasion, the content of starch (as an indication for the use of biological binding agents), the concentrations of C, H, N, S, Cl, K as well as of the heavy metals Cd, Pb, Zn, Cr, Cu, As and Hg. These parameters have been chosen following the Austrian, German, Swiss and Swedish standards for densified biomass fuels.

The results showed that a majority of the participating producers produce fuels of high quality. However, wood pellets of some producers show a high abrasion, one of the most important quality parameters for pellets. An increased amount of fines often causes failures in the feeding systems used in the residential heating sector. In order to decrease abrasion, the addition of small amounts of biological binding agents (e.g. maize or rye) is possible. This kind of additive is most common in Austria.

Moreover, some producers obviously use not only chemically untreated raw materials or additives, which increase the content of pollutants. Such fuels cause problems regarding emissions, deposit formation and corrosion. Emission problems are expected due to increased contents of N, Cl, S as well as heavy metals. Increased concentrations of heavy metals additionally contaminate the ash, increased Cl concentrations raise the risk of corrosion. Moreover, an increased content of K has a negative effect on the ash melting behaviour and causes higher aerosol formation, which enhances deposit formation and particulate emissions.     

An evaluation on rice husks and pulverized coal blends using a drop tube furnace and a thermogravimetric analyzer for application to a blast furnace

To evaluate the potential of pulverized coals partially replaced by rice husks used in blast furnaces, thermal behavior of blends of rice husks and an anthracite coal before and after passing through a drop tube furnace (DTF) was investigated by using a thermogravimetry (TG). For the blends of the raw materials in the TG, fuel reaction with increasing temperature could be partitioned into three stages. When the rice husks were contained in the fuel, a double-peak distribution in the first stage was observed, as a consequence of thermal decompositions of hemicellulose, cellulose and lignin. A linear relationship between the char yield and the biomass blending ratio (BBR) developed, reflecting that synergistic effects in the pyrolytic processes were absent. This further reveals that the coal and the rice husks can be blended and consumed in blast furnaces in accordance with the requirement of volatile matter contained in the fuel. After the fuels underwent rapid heating (i.e. the DTF), a linear relationship from the thermogravimetric analyses of the unburned chars was not found. Therefore, the synergistic effects were observed and they could be described by second order polynomials. When the BBR was less than 50%, varying the ratio had a slight effect on the thermal behavior of the unburned chars. In addition, the thermal reactions of the feeding fuels and of the formed unburned chars behaved like a fingerprint.     

Thermogravimetric analysis of the relationship among calcium magnesium acetate,calcium acetate and magnesium acetate

Thermal decomposition characteristic of calcium magnesium acetate (CMA), calcium acetate (CA) and magnesium acetate (MA) are investigated through thermogravimetric (TG) analysis at the heating rates of 5 K min⁻¹, 7.5 K min⁻¹, 10 K min⁻¹ and 15 K min⁻¹. After dehydration, the evaporation of carboxylic radical and carbon dioxide of CMA and CA exist in two separate segments, but for MA, this occurs together in just one segment without clear borderline. The curves of calculated CMA (C-CMA) and the deduced characteristic parameters illustrate the different characteristic of CA and MA from the corresponding components in CMA which may be the reason for the different performances of these sorbents in SO₂ and NO_x reduction. Also, the kinetic parameters of activation energy and reaction order of the three sorbents are calculated through Vyazovkin method and Avrami theory, respectively.     

Determining the most sustainable lignocellulosic bioenergy system following a case study approach

The paradigm shift from fossil to renewable energy sources is driven, largely, by a growing sustainability awareness, necessitating more sophisticated measurements in terms of a wider range of criteria. Technical efficiency, financial profitability, environmental friendliness and social acceptance are some of the aspects determining the sustainability of renewable energy systems. The resulting complexity and sometimes conflicting nature of these criteria constitute major barriers to the implementation of renewable energy projects.The Worcester biomass procurement area in the Western Cape Province, South Africa, is used as a case study. It provides a blueprint for measuring the impacts of lignocellulosic bioelectricity systems – using life-cycle assessment (LCA), multi-period budgeting (MPB), geographic information systems (GIS) and multi-criteria decision-making analysis (MCDA), among others – and for prioritising the relevant criteria to determine the most sustainable technological option.Following the LCA approach, 37 plausible lignocellulosic bioenergy systems were assessed against five financial-economic, three socio-economic and five environmental criteria. On translating the quantitative performance data into a standardised ‘common language’ of relative performance, an expert group attached weights to the considered criteria, using the analytical hierarchy process (AHP). Assuming the prerequisite of financial-economic viability, the preferred option comprises a feller-buncher for harvesting, a forwarder for biomass extraction, mobile comminution at the roadside, secondary transport in truck-container-trailer combinations and an integrated gasification system for the conversion into electricity. This approach illustrates how to internalise externalities as typical market failures, aiding decision makers to choose the most sustainable bioenergy system.     

Thermal pretreatment of wood (Lauan) block by torrefaction and its influence on the properties of the biomass

The purpose of this study is to investigate the torrefaction behavior of woody biomass (Lauan) blocks and its influence on the properties of the wood. Three different torrefaction temperatures of 220, 250 and 280 °C, corresponding to light, mild and severe torrefactions, and four torrefaction times of 0.5, 1, 1.5 and 2 h were considered. After analyzing the torrefied woods, it was found that the torrefaction temperature of 280 °C was able to increase the calorific value of the wood up to 40%. However, over 50% of weight was lost from the wood. The grindability of the torrefied wood could be improved in a significant way if the torrefaction temperature was as high as 250 °C and the torrefaction time longer than 1 h. Therefore, the torrefaction temperature of 250 °C along with the torrefaction time longer than 1 h was the recommended operation to intensify the heating value and grindability as well as to avoid too much mass loss of the wood. This study also suggested that over 50% of the reacted wood was converted into condensed liquid. The main components in the liquid were monoaromatics; little amount of heterocyclic hydrocarbons were also obtained from the torrefactions, especially at the torrefaction temperature of 280 °C.     

An evaluation on improvement of pulverized biomass property for solid fuel through torrefaction

The improvement on physical and chemical properties of pulverized biomass from torrefaction is investigated to evaluate the potential of biomass as solid fuel used in boilers and blast furnaces. Three biomasses of bamboo, banyan and willow are considered. The results indicate that when the torrefaction temperature is relatively low such as 230 and 260 °C, the weight loss of biomass depends significantly on the temperature, as a result of consumptions of hemicellulose and cellulose. However, once the torrefaction temperature is as high as 290 °C, the weight losses of various biomass materials tend to become uniform. The decreased O/C ratio in biomass from torrefaction can be explained by intensified lignin content in that the O/C ratio in lignin is low compared to that in hemicellulose and cellulose. Furthermore, the enriched element C in torrefied biomass results in an increase in the calorific value of the torrefied materials. However, the enlarged higher heating value (HHV) of biomass from torrefaction cannot keep up with the weight loss; this leads to the decrease in total energy of biomass as the torrefaction temperature rises. The conducted correlation in predicting the HHV of raw biomass can also be utilized for torrefied biomass. The raw pulverized biomasses are characterized by agglomeration in the regime of smaller particle size. Once the biomasses undergo torrefaction, the dispersion of powder is improved, thereby facilitating the injection of biomass powder. This enhances the applications of pulverized biomass in boilers and blast furnaces.     

An investigation of thermal behaviour of biomass and coal during co‐combustion using thermogravimetric analysis (TGA)

The thermal behaviour and kinetic analysis of biomass (cypress wood chips and macadamia nut shells) and Australian bituminous coal during combustion were studies using the thermogravimetric technique with four different heating rates under an air atmosphere. Each type of biomass was blended with coal at mass ratios (biomass:coal) of 95:5, 90:10, 85:15 and 80:20 to investigate the effect of coal as a supplementary fuel on thermal behaviour during the combustion process. Combustion of the individual samples and the blends took place in three steps comprising dehydration, devolatilisation and char oxidation. During co‐combustion, the thermal decomposition behaviour of the blends followed that of the weighted average of the individual samples in the blends. In kinetic analysis, thermal decomposition of biomass and coal appeared to take place independently, and thus, the activation energy of the blends can be calculated from that of the two components. No evidence for any significant synergetic effects or thermal interaction was found between either type of biomass and the coal during co‐combustion based on the lack of deviation from expected behaviour of the blends. Copyright © 2013 John Wiley & Sons, Ltd.     

Application of MLP-ANN as a novel predictive method for prediction of the higher heating value of biomass in terms of ultimate analysis

In the recent years, the energy issue is known as one of the main entries for economic and social development of human. So the biomass fuels as one of the approaches for supplying energy become the attractive topic for investigation. The higher heating value (HHV) is a key parameter for evaluation of energy of biomasses; so in the present study, a novel work was done to predict HHV as a function of ultimate analysis by utilization of multi-layer perceptron artificial neural network (MLP-ANN). To this end, a total number of 78 actual data were extracted from reliable references for training and validation of the model. The predicted HHVs were compared with the experimental data graphically and statistically, and the obtained results expressed that the MLP-ANN has a great potential for estimation of HHV of biomasses; so this approach can be used as a simple and accurate tool for forecasting HHV in terms of ultimate analysis. Based on the obtained results, this approach becomes one of the applicable softwares in industries.     

A Life Cycle Analysis on a Bio-DME production system considering the species of biomass feedstock in Japan and Papua New Guinea

This paper describes the performance and/or CO₂ intensities of a Bio-DME (Biomass Di-methyl Ether) production system, considering the differences of biomass feedstock. In the past LCA studies on an energy chain model, there is little knowledge on the differences of biomass feedstock and/or available condition. Thus, in this paper, we selected Papua New Guinea (PNG) which has good potential for supply of an energy crop (a short rotation forestry), and Japan where wood remnants are available, as model areas. Also, we referred to 9 species of biomass feedstock of PNG, and to 8 species in Japan.The system boundary on our LCA consists of (1) the pre-treatment process, (2) the energy conversion process, and (3) the fuel transportation process. Especially, since the pre-treatment process has uncertainties related to the moisture content of biomass feedstock, as well as the distance from the cultivation site to the energy plant, we considered them by the Monte Carlo simulation.Next, we executed the process design of the Bio-DME production system based on the basic experimental results of pyrolysis and char gasification reactions. Due to these experiments, the gas components of pyrolysis and the gasification rate under H₂O (steam) and CO₂ were obtained. Also, we designed the pressurized fluid-bed gasification process. In a liquefaction process, that is, a synthesis process of DME, the result based on an equilibrium constant was used. In the proposed system, a steam turbine for an auxiliary power was assumed to be equipped, too. The energy efficiencies are 39.0–56.8 LHV-%, depending upon the biomass species.Consequently, CO₂ intensities in the whole system were 16.3–47.2 g-CO₂/MJ-DME in the Japan case, and 12.2–36.7 g-CO₂/MJ-DME in the PNG one, respectively.Finally, using the results of CO₂ intensities and energy efficiencies, we obtained the regression equations as parameters of hydrogen content and heating value of a feedstock. These equations will be extremely significant when we install the BTL (biomass-to-liquid, ex. Bio-DME) energy system in the near future, in order to mitigate CO₂ emissions effectively, and to estimate the energy’s efficiency.     

Fatty acids composition as a means to estimate the high heating value (HHV) of vegetable oils and biodiesel fuels

High heating value (HHV) is an important property which characterises the energy content of a fuel such as solid, liquid and gaseous fuels. The previous assertion is particularly important for vegetable oils and biodiesels fuels which are expected to replace fossil oils. Estimation of the HHV of vegetable oils and biodiesels by using their fatty acid composition is the aim of this paper. The comparison between the HHVs predicted by the method and those obtained experimentally gives an average bias error of −0.84% and an average absolute error of 1.71%. These values show the utility, the validity and the applicability of the method to vegetable oils and their derivatives.     

Detailed study of the impact of co-utilization of biomass in a natural gas combined cycle power plant through perturbation analysis

Co-utilization of fossil fuels and biomass is a successful way to make efficient use of biomass for power production. When replacing only a limited amount of fossil fuel by biomass, measurements of net output power and input fuel rates will however not suffice to accurately determine the marginal efficiency of the newly introduced alternative fuel. The present paper therefore proposes a technique to determine the marginal biomass efficiency with more accuracy. The process simulation model for co-utilization of natural gas and a small perturbing fraction of biomass in an existing combined cycle plant (500 MW_th Drogenbos, Belgium) is taken as case study. In this particular plant, biomass is introduced into the cycle as fuel for a primary steam reforming process of the input natural gas.     

RDEA: A recursive DEA based algorithm for the optimal design of biomass supply chain networks

The optimal design of supply chain networks is often examined based on one or more economic or other criteria (e.g., cost, profit environmental impact, danger, time). However, the efficiency of the derived solutions is often ignored. In this work, a recursive DEA (RDEA) algorithm is presented, which introduces a different way of designing a supply chain network. The selection of possible installed facilities is based on minimum cost and maximum efficiency, through a MILP model. Optimal supply chain structure is obtained when the termination criterion is met, yielding only the efficient solutions, while simultaneously reducing the overall cost. An application of this RDEA algorithm to a biomass supply chain is examined. A comparative study is also presented, demonstrating the results obtained when solving the MILP without the proposed algorithm and with the use of an RDEA.     

A new method for simulating the combustion of a large biomass particle—A combination of a volume reaction model and front reaction approximation

Combining a volume reaction model and front reaction approximation is proposed to simulate the combustion of a large biomass particle. Two intraparticle processes—drying and char oxidation—are simplified as front reaction because they are transport controlled. The other intraparticle process—pyrolysis—is described as the volume reaction because it is controlled by both heat transfer and kinetics. A new numerical method based on the basic mechanism of the process is applied to mitigate oscillations of the solution of the front reactions. To compare the calculation results with the experimental results presented in the literature, combustion of cubic wood particles between 5 and 25 mm is chosen to test the new method. Drying, pyrolysis, char oxidation, vapor condensation, shrinkage of the process, heat transfer via conduction, diffusion, convection, radiation and mass transfer via diffusion, and convection inside particle are taken into account. Finite volumes attached to solid materials are used to discretize the domain and explicit method with variable time step is used to calculate the process. A program was written and the calculation showed that the conversion of a particle is almost independent of computational mesh from 10 cells on. However there is significant instability in the mass loss rate curve when the number of cells is less than 20. Predictions for different particle sizes, furnace temperatures and moisture contents were compared with measurements and they agree reasonably well. The results highlight the significance of pyrolysis kinetics on prediction. Thus, the front reaction model of pyrolysis assuming a constant reaction temperature of 773 K is sometimes inadequate. The proposed method also showed that moisture content and pyrolysis reactivity significantly affect the thickness of devolatilizing fuel.     

Sunflower as a biofuels crop: An analysis of lignocellulosic chemical properties

Four accessions of cultivated sunflower (Helianthus annuus) and silverleaf sunflower (Helianthus argophyllus), were each grown in three locations (Georgia, British Columbia, and Iowa) at different planting densities and phenotyped for biomass-related traits and wood biochemistry. In most environments, H. argophyllus produced significantly more biomass than H. annuus. Cell wall chemistry for a subset of plants grown in Georgia and Iowa was assessed using analytical wet chemistry methods to measure lignin and sugar content/composition. The analysis of lignin and the S/G-lignin ratios for a larger number of samples (n > 250) was also assessed by high-throughput pyrolysis Molecular Beam Mass Spectrometry. Average pyMBMS estimated lignin content (i.e., dry weight fraction) for 60 °C dried basal stem samples of H. annuus and H. argophyllus was 29.6% (range, 24.0%–34.6%) and 28.6% (range, 24.6%–33.3%), respectively when averaged across all environments. The average S/G lignin mass ratio was 1.5 (range, 1.0–2.0) for H. annuus and 1.7 (range, 1.0–2.4) in H. argophyllus. Stem samples from these two species only differed statistically for a few cell wall chemistry traits; however, accession level differences within each species were apparent. Cell wall chemistry in both species was significantly affected by both location and planting density, thus demonstrating the need to select for these traits in the environment for which the crop will be produced. Overall, these results show that cultivated sunflower and silverleaf sunflower both possess the necessary phenotypic diversity to facilitate the development of a hybrid sunflower with improved lignocellulosic biofuels traits, namely increased biomass, decreased lignin, and increased glucan.     

Application of an empirical model in CFD simulations to predict the local high temperature corrosion potential in biomass fired boilers

To gain reliable data for the development of an empirical model for the prediction of the local high temperature corrosion potential in biomass fired boilers, online corrosion probe measurements have been carried out. The measurements have been performed in a specially designed fixed bed/drop tube reactor in order to simulate a superheater boiler tube under well-controlled conditions. The investigated boiler steel 13CrMo4-5 is commonly used as steel for superheater tube bundles in biomass fired boilers. Within the test runs the flue gas temperature at the corrosion probe has been varied between 625 °C and 880 °C, while the steel temperature has been varied between 450 °C and 550 °C to simulate typical current and future live steam temperatures of biomass fired steam boilers. To investigate the dependence on the flue gas velocity, variations from 2 m·s⁻¹ to 8 m·s⁻¹ have been considered. The empirical model developed fits the measured data sufficiently well. Therefore, the model has been applied within a Computational Fluid Dynamics (CFD) simulation of flue gas flow and heat transfer to estimate the local corrosion potential of a wood chips fired 38 MW steam boiler. Additionally to the actual state analysis two further simulations have been carried out to investigate the influence of enhanced steam temperatures and a change of the flow direction of the final superheater tube bundle from parallel to counter-flow on the local corrosion potential.