Biomass Char Research Articles

Reaction of Substituted Phenols with Lignin Char: Dual Oxidative and Reductive Pathways Depending on Substituents and Conditions.

Biomass chars are known to be intrinsically redox-reactive toward some organic compounds, but the mechanisms are still unclear. To address this, a char made anoxically at 500 °C from dealkaline lignin was reacted either in the fresh state or after 180-day aging in air with p-nitrophenol (NO2-P), p-hydroxybenzaldehyde (CHO-P), phenol (H-P), or p-methoxyphenol (MeO-P). The reactions were carried out under oxic or anoxic conditions. Degradation occurred in all cases. Both oxidation and reduction products were identified, with yields dependent on the presence or absence of air during reaction or storage. They included oligomers, amines, and ring-hydroxylated compounds, among others. Exposure to air suppressed sorption, annihilated reducing sites, and provided a source of reactive oxygen species that assisted degradation. Sorption suppression was due to the incorporation of hydrophilic groups by chemisorption of oxygen, and possibly blockage of sites by products. Fresh char has comparable electron-donating and accepting capacity, whereas there is a preponderance of electron-accepting over donating capacity in aged char. Under anoxic conditions, both oxidation and reduction occurred. Under oxic conditions or after aging in air, oxidation predominated, and linear free energy relationships were found between the rate constant and the Hammett or Brown substituent electronic parameter or the standard electrode potential of the phenol. The results demonstrate that chars possess heterogeneous redox activities depending on reaction pairs, reaction conditions, and aging.

A biomass-based small-scale power generation system with energy/exergy recuperation

Small-scale biomass power generation will play an important role in ensuring the regional power supply by using the local biomass resource and protecting the environment in the future. In this study, a small-scale high-efficient combined heat and power generation system with a separated-type biomass gasification process combining the energy/exergy recuperation is proposed for the first time. The spatial subdivision of the processes for the biomass pyrolysis, char combustion, tar reforming and catalyst regeneration is adopted by using a separated-type biomass gasifier design to realize the optimization of each conversion step and improve the whole system performance. To obtain the maximum power generation efficiency, the energy flow and exergy flow in the system are analyzed in details and the operating condition of the gasification system is optimized. The results demonstrate that the relatively low temperature as well as low steam/carbon ratio in the tar reformer should be conducive to the improvement of energy and exergy efficiencies. In the optimum operation condition, the biomass input of 548.86 kW (higher heating value) could generate 263.65 kW of electrical power with the total energy and exergy efficiencies of 37.9% and 43.2%, respectively, in which 153.44 kW of energy could be recuperated back to the gasification process by air and steam with 136.56 kW of energy obtained from gas turbine exhaust to enhance the whole power generation efficiency. It is expected to provide a new design concept for the development of high-efficient small-scale biomass gasification system for the combined heat and power generation.

Investigations on NO reduction with biomass char: Char structural changes during the heat treatment in N2 and subsequent NO/O2 gasification

NO reduction with biomass char was studied from considering its structural properties. Raman, XPS and XRD were employed to characterize the char structure. Three types of biomass chars were used for NO reduction at a fixed bed reactor in NO/O2 at 900 ℃. The results showed that the heat treatment led to an obvious reordering of structures in all chars. There were significant differences between NO and O2 reaction in terms of the char structural changes in aromatic systems, surface groups and crystalline developments. New O-containing groups were formed and could be present as C-O and –COO groups in both the NO and O2 reactions. Two different pathways for char-NO and char-O2 reaction were given, respectively. It is proposed that the reaction channel of NO would be blocked as carbon molecular was firstly surrounded by O2 during NO/O2 reaction. The denitration performance of biomass char should be linked to the structural features of char-HT and the behaviors of char structural changes in the subsequent gasification in NO/O2.

A pore-structure dependent kinetic adsorption model for consideration in char conversion – Adsorption kinetics of CO2 on biomass chars

This study presents a pore-structure dependent kinetic adsorption (PSK) model to describe experimentally observed adsorption processes. The model describes loadings and adsorption kinetics as a function of time, pressure and temperature, considering pore size distributions derived from non-local density functional theory (NLDFT) calculations. Time-resolved mass transfer coefficients, effective diffusion coefficients and adsorption flow rates can be derived from the model. For the integration of mass transfer or effective diffusion coefficients into carbon conversion models, the time-resolved description is more meaningful than an averaged coefficient from the established linear driving force model or the Fickian diffusion models. Using the PSK model, adsorption effects for different pore regimes can be calculated separately, which helps to understand the occurring diffusion phenomena. The PSK model has been validated with data for CO2 adsorption on two different biomass chars at temperatures from (298.15 to 398.15 K) and pressures from (46 to 540) kPa.

Commissioning, performance benchmarking, and investigation of alkali emissions in a 10 kWth solid fuel chemical looping combustion pilot

Chemical looping combustion of biomass-sourced fuels (bio-CLC) is a novel bio-energy with carbon capture and storage (BECCS) technology for power and heat generation with net negative CO2 emissions. In this study, a new 10 kWth CLC pilot designed for high-volatiles biomass fuels was commissioned with ilmenite oxygen carrier and five different biomass fuels of varying volatile and alkali content fractions. The system was tested for its ability to convert high and low volatile content biomass, while achieving high carbon capture efficiency. The new pilot achieved carbon capture close to 100% for high-volatiles biomass, and >94% for low-volatiles biomass char fuels. Furthermore, due to the implementation of a volatiles distributor, the new pilot demonstrated an improvement of up to 10 percentage points of gas conversion efficiency for high-volatiles biomass vs. the previous generation reactor. Gaseous alkali emissions were measured with a surface ionization detection system. Flue gas alkali release levels were found to rise with higher fuel alkali content. Alkali emissions were found to be approximately similar in the AR and the FR for all but the straw pellet mixture fuel (highest alkali content fuel). For the straw pellet mixture, gaseous alkali release levels in the AR were up to seven times higher than those of the FR. In all cases, over 96% of the fuel’s alkalis were absorbed by the ilmenite bed material. Ilmenite’s strong alkali absorption characteristics were concluded to be the key determinant of gas-phase release of biomass alkali in the conducted experiments.

Single-use LDPE - Eucalyptus biomass char composite produced from co-pyrolysis has the properties to improve the soil quality

The co-pyrolysis of Single-use low density polyethylene (LDPE) and Eucalyptus biomass (EuBm) can be considered as a sustainable waste management technique to produce viable byproducts. This study elucidates the effects of variable temperatures (300–600 °C), residence times (90–150 minutes), and proportions of LDPE (0.25, 0.33 (w/w)) on physicochemical characteristics of LDPE - EuBm char composites. The interference of liquified polymer coating on the surface with degradation of biomass could be the reason for low nutrient extractability of chars synthesized at 300 and 400 °C. These chars were rich in volatile matter (> 68 %) and their pores were filled with partially pyrolyzed products. Interestingly however, substantial changes in properties were observed at 500 °C due to the likely synergetic effect between the feeds. The highest plant-extractable concentrations of major nutrients (Na, K, Ca, Mg, NO3−, PO43-), electrical conductivity (4.73 mS/cm), and cation exchange capacity (50.5 Cmolc/kg) of char were observed at this temperature. The optimization through regression modeling identified 524 °C, 118 min, and 31 % (w/w) of LDPE as optimal process parameters to obtain char suitable for application in soil. Soil incubation test fortified the benefits of char to soil with 3.5 times improvement in soil fertility index at 5 % (w/w) rate of application.

Kinetic analysis of CO2 gasification of biochar and anthracite based on integral isoconversional nonlinear method

AbstractThe nonisothermal thermogravimetric analysis was implemented for gasification of sawdust char (SD-char), wheat straw char (WS-char), rice husk char (RH-char), bamboo char (BB-char) and anthracite coal (AC) in the presence of CO2. The dependence of activation energy upon conversion for different biochars and AC was obtained by the integral isoconversional nonlinear (NL-INT) method which is a model-free method. Based on the activation energy values from the NL-INT method, a model-fitting method called random pore model (RPM) was used to estimate the kinetic parameters including the preexponential factor and pore structure parameter from the experimental data. The results are shown that the gasification reactivity of different samples from high to low can be sorted as that of WS-char, SD-char, BB-char, RH-char and AC. In the early stage of gasification, the activation energy values of biochars increase generally with an increase in the conversion degree, whereas the value of AC decreases. Thereafter, the activation energy values remain almost unchanged when the conversion is up to some extent. When the conversion degree varies between about 0.3 and 0.9, these carbon materials can be sorted in the order of average activation energy from low to high as WS-char, SD-char, AC, RH-char and BB-char, respectively, 134.3, 143.8, 168.5, 184.8 and 193.0 kJ/mol. It is shown that a complex multistep mechanism occurs in the initial stage of gasification, while a single-step gasification mechanism exists in the rest of the gasification process. The RPM is suitable for describing the gasification of biomass chars and AC except the initial gasification. Additionally, it is found that the kinetic compensation effect (KCE) still exists in the gasification reactions of biochars and AC. However, the AC deviates markedly from the KCE curve. This may be caused by the similarity of carbonaceous structure of biochars and the difference in reactivity between biochars and AC.

Particle temperature and potassium release during combustion of single pulverized biomass char particles

This work investigated the combustion characteristics of single pulverized biomass-derived char particles. The char particles, in the size range 224–250 µm, were prepared in a drop tube furnace at pyrolysis temperatures of 1273 or 1473 K from four types of biomass particles – wheat straw, grape pomace, kiwi branches and rice husk. Subsequently, the char particles were injected upward into a confined region of hot combustion products produced by flat flames stabilized on a McKenna burner, with mean temperatures of 1460, 1580 and 1670 K and mean O2 concentrations of 4.5, 6.5 and 8.5 vol%. The data reported include particle temperature, obtained using a two-color pyrometry technique, and potassium release rate, measured using a laser-induced photofragmentation fluorescence imaging technique. In addition, particle ignition delay time and burning time, obtained from the temporal evolution of the thermal radiation intensity of the burning char particles, are also reported. The results indicated that ignition of the char particles occurs simultaneously with the starting of the potassium release, then the particle burning intensity increases rapidly until it reaches a maximum, after which both the particle temperature and the potassium release rate remain approximately constant until the end of the char oxidation process. The char ignition process is temperature controlled, and the char oxidation process is oxygen diffusion controlled, with the total potassium release being independent of the oxygen concentration and the temperature of the combustion products. The combustion behavior of the chars studied is more affected by the char type than by the conditions used to prepare them.

Experimental and Kinetic Investigation on NO Reduction by Rice Husk Char and Catalytically with CO

Biomass char is accepted as an environmentally friendly energy resource, and is promising for NO removal in industrial furnaces. NO reduction with biomass char (rice husk) was investigated in a fixed-bed quartz reactor. The influence of CO concentration, reaction temperature, initial NO concentration, and char loading on the NO reduction were evaluated. The results showed that the overall NO reduction reaction was dominated by the direct reaction of NO with char without CO. Increasing CO concentration resulted in an enhanced catalytic reaction between NO and CO over the char surface and a decreased carbon consumption of char. With decreasing temperature, the influence of CO concentration on NO reduction was strengthened. Over the test temperature range, char provided a surface to catalyze the NO–CO reaction at excess CO/NO ratio. Increasing the initial NO concentration led to an increasingly significant influence on NO reduction with the enhanced CO/NO ratio. With increased char loading, the enhancement of NO reduction became unobservable for excess CO/NO ratio. Reaction orders of NO and activation energy for NO reduction by char were 0.71 and 60.21 kJ·mol−1, respectively, against 0.82 and 34.60 kJ·mol−1, respectively, for NO reduction by CO, catalyzed by char.

Study of the Molecular Structure and Elemental Mercury Adsorption Mechanism of Biomass Char

Based on a comprehensive study of the pyrolysis characteristics, pore structure, surface chemical functional groups, organic carbon framework structure, microcrystalline morphology and lattice char...

Investigation of Pulverized Biomass and Coal Char Emissivity

Current work presents an optical setup, its calibration and reference process and the first results from single particle emissivity measurements of pulverized biomass and coal fuel particles. In contrast to earlier attempts, the setup offers the possibility of emissivity measurements during the whole particle burn-off. A laser ignites a single particle, placed in the center of the setup. Two photomultipliers observe the emitted particle radiation in the visible range (550 nm and 700 nm) for temperature calculation, using two-color pyrometry. An InSb-detector records the emitted particle radiation between 2.4 µm and 5.5 µm, which is later used to calculate particle emissivity in this range. The conclusion of multiple particle measurements lead to decreasing particle emissivity with increasing temperature. For coal particles the emissivity decreases from 0.45 at 2300 K to 0.03 at 3400 K. Biomass char shows a similar trend with a decrease from 0.18 (2100 K) to 0.03 (2900 K).

High-temperature pyrolysis of biomass pellets: The effect of ash melting on the structure of the char residue

The formation of ash during pyrolysis of biomass pellets (corn straw and rice husk) was studied, at high temperature conditions where the formation of tars is minimized. In particular, the influences of the melting ash on the physical and chemical properties of the biomass chars were examined. Cylindrical pellets (9 by 19 mm) were pyrolyzed at three temperatures (1200, 1300, and 1400 °C) and three resident times (10, 20, and 30 min), at moderately high heating rates. The biomass behavior during pyrolysis and the properties of the formed chars were assessed by a variety of experimental techniques. Results show that the char yields decreased with increasing pyrolysis temperature and increasing residence time. The char yields from rice husk were significantly higher than those from corn straw. The ash on the surface of corn straw pellets melted and polymerized, and balls of ash formed and agglomerated, especially at the highest temperature. Molten ash of corn straw contained Ca, O, Al, Mg and Si, forming mainly CaAl2Si2O8, quartz, and cristobalite. In contrast, the surface morphology of rice husk pellets remained intact and the molten ash on the surface of chars consisted mainly of quartz and cristobalite. In the case of corn straw, the melting ash blocked the pore structure and decreased the accessible surface area. To the contrary, in the case of rice husk, the effect of ash melting on its pore structure was negligible and the accessible surface area increased upon pyrolysis. As the pyrolysis temperature increased from 1200 °C to 1400 °C, the carbon structure of the biomass chars became more orderly. Overall, increasing the pyrolysis temperature and duration increased the amount of pyrolysis gas, decreased the char yields and decreased the extent of ash melting and polymerization on the surface of the chars.

Influence mechanism and kinetic analysis of co-gasification of biomass char and semi-coke

In order to clarify the reaction mechanism of co-gasification of biomass char and semi-coke, the gasification properties of bamboo reed char (BR), semi-coke (SC) and their blends were systematically studied by the non-isothermal thermogravimetry. The physicochemical properties of BR and SC were characterized in detail, the main factors affecting the co-gasification reaction were studied, and the gasification kinetics under CO2 environment were further investigated. The results show that the gasification reactivity of SC is lower than BR, which is due to the higher order degree of carbonaceous structure and the lower alkali metal content in the ash, and the addition of BR can significantly improve the gasification reaction performance. Meanwhile, the synergistic effect between BR and SC was evidenced by the experimental and theoretical results. The gasification process of different chars was described with three typical kinetic models: unreacted core model (URCM), volumetric model (VM) and random pore model (RPM). The findings show that the performance of RPM is better than URCM and VM, and there is a significant kinetic compensation effect in the co-gasification reaction of BR and SC through kinetic fitting.

Effect of process conditions on the surface properties of biomass chars produced by means of pyrolysis and CO2 gasification

Biomass char is defined as the carbonaceous by-product of biomass pyrolysis or gasification and is an undesired by-product that is disposed as a waste, usually at a very high cost. Chars have interesting characteristics like high carbon content and high porosity. These characteristics can make attractive the utilization of biomass char for alternative and innovative applications, e.g. as precursors for activated carbons or as filtering mediums. In this framework, this present work focuses the attention on the surface properties of char which is a key parameter for the evaluation of its applicability in adsorption applications. The combination of three main variables has been considered: the carrier gas (nitrogen or carbon dioxide), the temperature (set at 500°C, 700°C and 900°C) and the initial heating rate (10°C min−1, 30°C min−1 or 50°C min−1). Maximum heating values are observed at 700°C for the case of inert atmosphere (pyrolysis) and the values reduce slightly for the case of CO2 gasification. On the contrary, CO2 gasification has a positive effect on the porosity of the biomass chars. This present research has the scope to provide a first insight on the effect of operating conditions on the optimal char characteristics for the utilization of biomass char as a cheap alternative for filters and adsorbents. An interesting future potential transition of existing biomass CHP facilities would be the evolution into biorefineries, in accordance with the new mandates about Circular Economy.

Role of inorganics on the biomass char gasification reactivity: A review involving reaction mechanisms and kinetics models

This review article presents the effects of the inorganic elements and compounds on gasification and their mechanisms of action. The influence of inorganics on the gasification reaction have been described in the literature. However, the specific phenomena involved and the mechanisms of action are not stated with consensus. Therefore, the present review gathers the various mechanisms proposed by authors. Some have been demonstrated to be unlikely such as the mechanisms involving intercalation compounds or mechanical actions. In order to give more practical applications of this work, the review also gathers the gasification kinetic models that take these inorganic effects into account.

Towards understanding the role of K during biomass steam gasification

Steam gasification is known to be catalyzed by K. However, the exact mechanisms are not identified to this day. In the present study, the catalytic influence of K on biomass steam gasification and its interaction with Si was investigated. The study was conducted through thermogravimetric analysis (TGA) on rice husks as a Si-rich biomass and K2CO3 as a model K-compound. Experiments were conducted in three different configurations: biomass or inorganic compound alone; mixtures of biomass and inorganic powders; biomass and inorganic compound without contact in a two-compartment crucible. It was shown that K has a catalytic effect on both the pyrolysis and the gasification steps. These effects occur even when the K-compound is not in direct contact with the biomass. Therefore, it demonstrates that the mechanisms involve the gas phase through the volatilization of a K-compound, most likely KOH(g). However, the observed catalytic effect on rice husks was weaker than the effect obtained in the case of beech wood, a Si-poor and more generally ash-poor biomass, under the same conditions. It shows that there is a competition between i) the influence of K on the biomass pyrolysis and char gasification kinetics and ii) the interaction of K with SiO2 inherent to the biomass.

Effect of gasification reactions on biomass char conversion under pulverized fuel combustion conditions

The effect of gasification reactions on biomass char conversion under pulverized fuel combustion conditions was studied by single particle experiments and modelling. Experiments of pine and beech wood char conversion were carried out in a single particle combustor under conditions of 1473-1723 K, 0.0-10.5% O2, and 25-42% H2O. A comprehensive progressive char conversion model, including heterogeneous reactions (char oxidation and char gasification with CO2 and H2O), homogeneous reactions (CO oxidation, water-gas shift reaction, and H2 oxidation) in the particle boundary layer, particle shrinkage, and external and internal heat and mass transfer, was developed. The modelling results are in good agreement with both experimental char conversion time and particle size evolution in the presence of oxygen, while larger deviations are found for the gasification experiments. The modelling results show that the char oxidation is limited by mass transfer, while the char gasification is controlled by both mass transfer and gasification kinetics at the investigated conditions. A sensitivity analysis shows that the CO oxidation in the boundary layer and the gasification kinetics influence significantly the char conversion time, while the water-gas shift reaction and H2 oxidation have only a small effect. Analysis of the sensitive parameters on the char conversion process under a typical pulverized biomass combustion condition (4% O2, 13% CO2, 13% H2O), shows that the char gasification reactions contribute significantly to char conversion, especially for millimeter-sized biomass char particles at high temperatures.

NH4Br-Modified Biomass Char for Mercury Removal in a Simulated Oxy-fuel Atmosphere: Mechanism Analysis by X-ray Photoelectron Spectroscopy

Mercury from recycled oxy-fuel gas needs to be removed because it can damage the aluminum devices in the system. In this work, low-cost NH4Br-modified rice husk char (RHCBr) was prepared as the biosorbent for mercury removal in an oxy-fuel atmosphere. RHCBr was characterized by scanning electron microscopy, Brunauer–Emmett–Teller, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS) technologies to study the physical and chemical properties of the prepared sorbents. Effects of O2, HCl, SO2, and HCl/SO2 synthesis on mercury removal in a simulated oxy-fuel atmosphere were investigated in a fixed-bed reactor. XPS technology was applied to explore the mercury adsorption species and deduce the potential mercury heterogeneous oxidation mechanism in an oxy-fuel atmosphere. It was found that enriched CO2 in an oxy-fuel atmosphere could facilitate the mercury removal process. Generally, O2 and HCl could promote the mercury removal efficiency in an oxy-fuel atmosphere, while SO2 could inhibit it. However, a low concentration of SO2 could promote it with the presence of HCl and O2. The mercury adsorption species on RHCBr were mainly HgSO4, HgCl2, and HgBr2 in the presence of acid gas components (HCl and SO2). HCl could actively facilitate the generation of C–Cl groups, which were also active sites for mercury removal, while SO2 could generate active site cover NH4HSO4, causing the deactivation of active sites. When HCl/SO2 synthesis was added, active site cover HSO4– would be removed by HCl, facilitating the mercury removal process. Meantime, HgCl2 species could be further converted to the strong-bonded species HgSO4 in the presence of O2 and SO2, which was also beneficial for mercury removal.

Effect of atmosphere of SO2 coexisted with oxidizing gas on mercury removal under oxy-fuel condition

The effects of oxidizing gas and SO2 in oxy-fuel combustion on mercury adsorption by modified corn stalk char were studied. Results showed that after 1% NH4Cl modification, the number of surface pores and oxygen-containing functional groups increased, which contributed to the improvement of adsorption capacity. O2 could promote the adsorption of mercury by heterogeneous oxidation and oxygen-containing functional groups. The addition of HCl could obviously improve the adsorption effect of adsorbent as the result of the chlorine-containing function groups on the surface of biomass char and the effect of Cl2 produced by Deakin reaction. SO2 could be oxidized to SO3 by oxygen atoms on the surface of adsorbent, which competed with the active sites and inhibited adsorption. SO2 could also directly react with Hg0 to produce HgSO4. The increase of O2 would alleviate the inhibition effect of SO2. SO2 could react with O2 by some ways and thus resulted in the decrease of oxidation rate owing to the consumption of O2. The existence of SO2 would weaken the oxidation effect under HCl through the reaction with Cl2 produced by HCl and O2, and SO2 would compete with HCl for the active sites on the surface of adsorbent.

Combustion kinetics and mechanism of biomass pellet

The effect mechanism of biomass pelletization on the property of bio-char, combustion kinetics and mechanism of pellet remain unclear. Therefore, in this work combustion behavior and kinetics of typical biomass pellets were investigated using TGA and Distributed Activation Energy Model. The bio-chars, from the pellets and raw biomasses, were also characterized in detail, to deeply understand the combustion mechanism of biomass pellet. The results showed that the activation energies of devolatilization stage of the pellet combustion were 118.1–230.2 kJ/mol, higher than that of raw biomasses, however, the activation energy decreased to 201.4–222.4 kJ/mol, for char combustion stage of pellet combustion, lower than the raw biomass. The compact physical structure of the biomass pellet caused by pelletization suppressed the mass and heat diffusion and thus enhanced the activation energy of devolatilization stage. The pelletization also suppressed the deoxygention, polymerization and aromatization reactions during the devolatilization stage, which caused higher reactivity of the pellet char than raw biomass char. This was the main reason for the lower activation energy of the pellet char combustion stage. Thus, the controlling factor of biomass pellet combustion is mass and heat diffusion for devolatilization stage, on the contrary, it was chemical reactions for char combustion stage.