Char Types Research Articles

Isothermal CO2 Gasification Reactivity and Kinetic Models of Biomass Char/Anthracite Char

Gasification of four biomass chars and anthracite char were investigated under a CO2 atmosphere using a thermo-gravimetric analyzer. Reactivity differences of chars were considered in terms of pyrolysis temperature, char types, crystallinity, and inherent minerals. The results show that the gasification reactivity of char decreased with the increase of pyrolysis temperature. Char gasification reactivity followed the order of anthracite coal char (AC-char) ˂ pine sawdust char (PS-char) ˂ peanut hull char (PH-char) ˂ wheat straw char (WS-char) ˂ corncob char (CB-char) under the same pyrolysis temperature. Two repesentative gas-solid models, the random pore model (RPM) and the modified random pore model (MRPM), were applied to describe the reactive behaviour of chars. The results indicate RPM performs well to describe gasification rates of chars but cannot predict the phenomenon that there appears to exist a peak conversion for biomass chars at a high conversion rate, where the MRPM performs better.

Effects of different biochars and digestate on N2O fluxes under field conditions

Field studies that have investigated the effects of char materials on the emission of nitrous oxide (N2O) are still scarce. Therefore, we conducted a field trial with bio- and hydrochars and measured N2O emissions for one whole year. It was hypothesised that the incorporation of chars reduces the emissions of N2O. Chars were produced by pyrolysis and hydrothermal carbonisation (HTC) using either maize silage or wood residues as feedstock. In addition, after production chars were post-treated with digestate in order to accelerate the ageing process of the chars. Chars and digestate were applied to the soil to raise the C content. Emissions of N2O were measured weekly and soil samples for inorganic nitrogen (N) and soil water-content were taken once a month. Additionally, the abundance of functional marker genes from denitrification (nosZ) was determined in October 2012 and in June 2013. The treatment with pure digestate emitted the most N2O compared to the control and char treatments. However, this was significant only in one case. There were no great differences between the char treatments due to high spatial variability and gene abundance of nosZ did not differ between treatments. Overall, emissions of N2O were relatively low. This was attributed to the heterogeneous distribution of the chars and the sandy soils that did not favour the production of N2O. To conclude, the emissions of N2O were mainly influenced by temperature and precipitation and to a minor extent by the type of char and post-treatment.

Application of mercury temperature programmed desorption (HgTPD) to ascertain mercury/char interactions

This work investigates the scope of a mercury temperature programmed desorption (HgTPD) technique for identifying mercury species in solids. The specific objective of this study was to clarify the mechanism of mercury retention by chars used as sorbents in coal combustion in air and oxy-combustion atmospheres based on the identification of the mercury species retained. Different mercury species were identified by HgTPD depending on the flue gas composition and the type of char. The results led to the conclusion that depending on these conditions the main mechanism of mercury retention will be the interaction of mercury with organic matter, or the interaction of mercury with sulfur to form HgS. In a few particular cases Hg2(NO3)22H2O was produced on the char surface. It was found that HgTPD is a highly useful technique for investigating the different mechanisms of mercury/char/gas interactions.

Effect of Torrefaction on Physical Properties and Conversion Behavior of High Heating Rate Char of Forest Residue

In the work reported here, both forest residue (FR) and torrefied forest residue (TFR) were devolatilized in a drop tube reactor at 1473 K at a heating rate greater than 104 K/s. The physical properties of parent fuel particles and their corresponding char particles were examined by using a scanning electron microscope and a granulometer. After the same milling and sieving process, the TFR particles had a smaller size and smaller aspect ratio than the FR particles. The char particles consisted of two types of particles with different sizes and morphologies: a small particle mode (presumably char fragments) and a large particle mode. The volume fraction of char fragments in the TFR char was considerably less than for the FR char. Both types of char were converted in a drop tube reactor under oxidation and gasification conditions at 1473 and 1573 K, respectively. The total organic mass loss and release of individual organic elements during char conversion were determined using a tracer method. Calcium, manganese, barium, and magnesium were found to be suitable for use as tracers. The fractional carbon conversion rate of TFR char was found to be slower than that of FR char under both oxidation and gasification conditions. The fractional release rate of hydrogen was much higher than that of total organic mass loss, while the corresponding oxygen release was lower for both types of char and for different reactive environments.

Impact of hydro-/biochars on root morphology of spring wheat

Despite suggested benefits of hydrochars and biochars, little is known about their effects on root development and root-char interactions. To compare effects of two types of biochar (Pyro and Pyreg) and one hydrochar (hydrothermal carbonization (HTC)) on root growth of spring wheat, two rhizobox experiments were set up where physical contact of roots with chars was prevented using nylon gauze. Rhizoboxes were filled with unamended soil as a control or with three different soil-char mixtures (Pyro, Pyreg and HTC). Shoots and roots were harvested before flowering and at tillering in the first and second experiment, respectively. Chemical soil properties (Nt, K, Ct, pH value) were affected differently by the different chars. Both shoot and root dry matter were influenced by chars. Pyro-char had positive effects on root dry matter in both experiments. At tillering, HTC-char affected root length, root surface and number of root tips negatively. Our findings suggest that nutrients released from chars may affect root morphology of spring wheat. The comparison of different types of chars showed different effects on root growth, shoot growth and soil changes depending on feedstock, production process and the varied amount of chars.

Petrographic controls on combustion behavior of inertinite rich coal and char and fly ash formation

The inability to yield heat up to the expected and desired level by a few of the coal fired thermal power plants can primarily be attributed to the ineffectual combustion of coal. In an intensive endeavour to understand the role of petrographic characteristics in combustion behavior, the authors have collected inertinite rich feed coal and fly ash from six different power plants in India. The technological characteristics, petrographic make up (maceral – and microlithotype composition) and vitrinite reflectance of the feed coals were studied in details. The variation in char types were identified in the various fly ashes. The chars from different feed coals were prepared under controlled laboratory conditions. The burn out level and the unburnt carbon amount in fly ash appear to be controlled by rank and inertinite content. The good correlation between the high-density chars and inertinite dominated microlithotype suggests that high-density chars are mainly derived from these related microlithotypes. The presence of a new type of char, named vitrosphere is reported. The fly ash with little unburnt carbon is predominantly comprised of plerospheres.

Gasification characteristics of coal char under CO2 atmosphere

The char gasification characteristics and the composition of evolved gases in a CO2 environment have been studied using a thermogravimetric analyzer (TG) coupled with a mass spectrometer. Three types of coal char were studied: lignite (TXL), sub-bituminous (PRB), and bituminous (KYB). TG results showed that the reactivities of TXL and PRB were higher than that of KYB, and the reactivity of TXL was higher than that of PRB. The characterization of the chars implied that the mineral content in the char plays an important role in the reactivity and that the surface area and pore volume may accelerate the reactivity of chars. The evolved gases from three chars were mainly CO and SO2. SO2 was slightly delayed by CO during gasification of TXL and PRB chars, but for KYB, SO2 and CO formed in the same temperature range, but at higher temperatures compared with TXL and PRB. The CO production of KYB was the best, 0.98 mg mg−1; and SO2 was the least, 0.031 mg mg−1. PRB and TXL chars had similar CO production, but SO2 in TXL was higher.

Mass transfer studies during CO2 gasification of torrefied and pyrolyzed chars

A comparative study on CO2 gasification kinetics of flax straw char produced by two different processes, namely, torrefaction and pyrolysis was investigated using a TGA (thermogravimetric analyzer). The objective of this research was to investigate the diffusion and surface reactions that occur during the CO2 gasification of the torrefied and pyrolyzed char particles. Gasification experiments were carried out at four different gasification temperatures from 750 to 900 °C and for different particle sizes (<90 μm–925 μm). The effects of temperature, particle size, and the char type on the rate of CO2 gasification were determined. The 50% conversion rate showed an increase in the reactivity values, along with an increase in the temperature, and decreased with an increase in the particle size for both the char types. The diffusional parameters such as effective diffusivity, effectiveness factor, and the DLI (diffusion limitation index) were calculated from the experimental data for both the char types. BET (Brunauer-Emmett-Teller) analysis and SEM (Scanning Electron Microscopy) analysis were performed to determine the surface area and to study the structural morphology of the char types. Torrefied char had a greater surface area than the pyrolyzed char.

Evaluation of the changes induced by gasification biochar in a peat-sand substrate

A b s t r a c t. Gasification biochar represents one of the bio char types tested for agricultural needs. The aim of this study was to clarify the physico-chemical and biological changes occurring in a peat-sand substrate amended with hardwood-derived gasifica tion biochar in the rates of 2, 4 and 20 g l -1 . The pH(H 2 O) of the substrate with 4 g l -1 and 20 g l -1 biochar was increased from 5.6 to 6.2 and 6.7, respectively. The testing of the substrate in the respirometry device showed that the increase in the biochar rate led to a decrease in the amount of CO 2 evolved at the maximum pressure drop. The continuous decrease in pressure observed in the respirometry bottles filled with pure biochar allows explaining this effect by biochar sorption activity. Addition of 2 and 4 g l -1 biochar to the peat-sand substrate stimulated the growth of cucumbers in an 18-day pot vegetation experiment. An increase in the number of root tips and root volume with a decreasing average root diameter was shown in the presence of biochar. Stimulation of plant growth on the background of low rates of biochar requires a further study with emphasis on the specific combination of bio char, soil type, plant species, and climatic conditions. K e y w o r d s: gasification biochar, plant growth, CO 2 evolution, soil, buffering capacity

Influences of heating rate during coal char preparation and AAEMs on volatile–char interaction with different sources of biomass volatile

Volatile–char interaction is important for the operation of co-gasification of coal and biomass. Biomass generally produces large amount of volatiles while char is mainly produced from coal. Hence, in this study, the interaction between coal char and volatiles from biomass samples (xylan, cellulose and rice straw) was examined using a rapid-heating thermobalance reactor. Three types of coal char, consisting of slow pyrolyzed-coal char (Ex-char), fast pyrolyzed-coal char (In-char), and acid washed-coal char (Ac-char), were employed to reveal influences of heating rate during char preparation and retention of alkali and alkaline earth metallic (AAEM) species on char surface. The steam gasification rates of all coal chars in both cases with and without volatile contacting were investigated from the gas evolution rate and remaining char analyses. The results indicate that the steam gasification rates of Ex-char and In-char were significantly diminished by the volatiles derived from all sources, especially rice straw. However, in the case of Ac-char, the reduction of steam gasification rate was less important. Moreover, the Ex-char exhibited the catalyst for the decomposition of biomass volatiles, resulting in the increase in gas production rate. Porous structure and the AAEM over char play crucial roles on the volatile–char interaction.

Can biochar and hydrochar stability be assessed with chemical methods?

Field application of biochar is intended to increase soil carbon (C) storage. The assessment of C storage potential of biochars lacks methods and standard materials. The reactivity of biochars and hydrochars may be one possible means of evaluating their environmental stability. The aim of this study was to evaluate the reactivity of biochar produced by gasification (GS) and hydrochar produced by hydrothermal carbonisation (HTC). The approach included analysis of the two different char types produced from the same three feedstocks. Moreover, we analysed the reactivity of Holocene charcoal (150 and 2000yr old) to evaluate whether or not their use as standard materials to represent stable biochar is meaningful. We assessed carbon loss following oxidation with acid dichromate as well as hydrolysis with HCl. Our results showed that chemical reactivity is not a straightforward approach for characterising the stability of biochar and hydrochar. Acid hydrolysis showed little difference between HTCs and GSs, despite the contrasting elemental composition. Using acid dichromate oxidation, we determined that GSs contained ca. 70% of oxidation resistant C while the proportion for HTCs was<10%. The different feedstocks had a slight, but significant, influence on the reactivity of GSs and HTCs. The content of oxidation resistant C decreased in the order 100yr old charcoal=GSs>2000yr old charcoal>HTCs>feedstock and was related to elemental composition. This shows that acid dichromate oxidation may allow differentiation of the reactivity of modern biochars but that there is not necessarily a relationship between reactivity and age of Holocene charcoals. As the chemical reactivity of biochars may change with exposure time in soil, it is poorly suited for assessing their environmental residence time.

Thermogravimetric analysis of gasification reactivity of coal chars with steam and CO2 at moderate temperatures

Comparative study on the gasification reactivity of the three types of Chinese coal chars with steam and CO2 at 850–1050 °C was conducted by isothermal thermogravimetric analysis. The effects of coal rank, pore structure, ash behavior, and gasification temperature on the gasification reactivity of coal chars were investigated. It is found that the gasification reactivity difference between different coal chars changes with reaction degree and gasification temperature, and has no immediate connection with coal rank and initial pore structure. Ash behavior plays an important role in the char reactivity, and changes with gasification temperature and reaction degree due to the variation in the compositions and relative amount. The influence of pore structure is more noticeable during a relatively moderate reaction process. The relative reactivity ratio of steam to CO2 gasification generally decreases with the increasing temperature, and is related with the catalytic effect of inherent minerals. The characteristic parameters of the chars were analyzed, finding that the value of half reaction specific rate is approximate to the average specific rate under the same conditions. The nth-order distributed activation energy model is proposed to describe the coal char gasification process, and the results show that the activation energy increases with the increasing carbon conversion.

A demonstration of the high variability of chars produced from wood in bushfires

Vegetation fires play a major role in global C cycling through the addition of inert carbon (char) to the environment. The objective of this study was to compare and contrast the chemical composition of 53 natural chars collected from the soil surface 6–32years after a natural fire. In order to achieve this, we applied a recently developed nuclear magnetic resonance (NMR) technique that gauges the degree of condensation of aromatic structures within chars. Our results show that the degree of condensation varied considerably among char samples (n=4–5), collected from burnt-out tree stumps at each of the 11 fire sites. This demonstrates that there is a great degree of variability in the composition of the char produced in natural fires, which is likely to be reflected in widely varying rates of char decomposition. This highlights a major difficulty in quantifying the effects of vegetation fires on global C cycling. Importantly, no differences could be discerned between chars of different ages, indicating that ageing of this type of char in this environment is slow on the decadal timescale. Finally, this study demonstrates that although char samples appear to preserve a record of fire conditions, great care must be taken when interpreting this record to account for the high degree of heterogeneity in char composition. Bulking of char samples would alleviate this problem to some extent; however, bulking would mask the inter-particle heterogeneity clearly evident in this study.

Leaching of major and trace elements from paper–plastic gasification chars: An experimental and modelling study

The control of soluble metal species in the sub-product leachate generated in electricity production processes is of great concern from an environmental and health point of view. Unlike fly ash, the leaching behaviour of char materials has received little attention. Yet, these solids are captured together with fly ashes in the particle control devices of power plants and are emitted in the same way as by-products. The present study was carried out using two char samples: (i) a raw char and (ii) the same type of char employed in a previous study so that it could serve as a sorbent for mercury species in gas phase. The char samples were by-products (residues) that had been generated during the gasification of plastic and paper waste. The leachates were analyzed for the following elements: Al, Ca, Si, Mg, Ba, Cu, Ni, Pb, Zn, Mo and Hg. In addition, geochemical modelling of the leaching test results was employed to identify the underlying chemical processes that led to the release of toxic elements. The results showed that at alkaline pH values, sorption on the solid surfaces of the char was negligible due to the inorganic complexation of cations in the solution. When the char was used as mercury sorbent slight changes occurred on the reactive surface resulting in the modification of the binding of some elements. As the pH increased, complexation with dissolved organic matter played a more important role in the case of some elements such as Cu because of the greater concentration of dissolved organic matter in solution.

Investigation on the NO Reduction with Coal Char and High Concentration CO during Oxy-fuel Combustion

Oxy-fuel combustion is one of the near-zero emission clean coal technologies which can realize large-scale CO2 capture, low NOx emission, and easy removal of SO2. This paper focuses on the reduction mechanism of NOx during oxy-fuel combustion. The influence of high concentration CO on the NO reduction with coal char, ash, and various metal materials has been studied in a laboratory-scale quartz reactor. Experimental results indicate that the presence of high concentration CO plays a crucial role in reducing NO on the coal char surfaces. Effect of NO concentration varies with different reaction conditions and different char types, which may be attributed to the different kinetic reaction orders. Moreover, both the reaction temperature and char prepared temperature have a pronounced effect on the char–NO–CO reaction. The active components in coal ash exhibit a clearly catalytic effect on the NO reduction with high concentration CO. Furthermore, the catalytic effect of various metal oxides, as well as coal a...

Kinetics of coal oxy-combustion by means of different experimental techniques

A large number of papers reported measures of coal char conversion rate under combustion and gasification conditions. Measures involved a variety of coal and char types, as well as different experimental techniques, ranging from TGA to wire mesh reactors and drop tube furnaces. For oxy-combustion with flue gas recirculation the impact of CO2 on the rate of combustion of char combustion is still controversial.In the present paper the rate of carbon conversion in O2/CO2 mixtures typical of oxyfuel conditions has been investigated by conventional as well as innovative experimental techniques, respectively isothermal/non-isothermal thermogravimetric analysis and reaction in an aerodynamic levitator particle reactor (ALPR).Results show that the ALPR provides reliable results also at values of temperature and oxygen concentration where thermogravimetric analysis fails. Experimental results have been worked out to obtain simple rate expressions for oxy-fuel combustion. Commonly used kinetic laws, however, proved unable to fit the experimental results in the whole range of operating conditions.

Fuel characteristics of wheat-based Dried Distillers Grains and Solubles (DDGS) for thermal conversion in power plants

For many years, the sole use of Dried Distillers Grains and Solubles (DDGS) has been as a feed for livestock, due to its high protein and fiber content. Recently there has been interest in DDGS as a power station fuel. In this study, DDGS produced from wheat has been studied for its fuel properties. Pyrolysis studies were conducted via Thermogravimetric Analysis (TGA) and pyrolysis gas chromatography–mass spectrometry (py GC–MS): the former was used to calculate the apparent first order kinetics while the latter helped to characterise pyrolysis products such as methoxyphenols and long chain fatty acids and esters. Fast heating rate chars were made, and nitrogen and sulphur partitioning between volatiles and chars are reported. In addition the chars were studied for their combustion behavior via Simultaneous Thermogravimetric Analysis with Mass Spectrometry detection (STA–MS) to study the conversion of char nitrogen and sulphur to different nitrogen and sulphur-containing species. In addition DDGS was combusted in a 50 kW fluid bed furnace, and the nitrogen species also monitored. DDGS is predicted to have a high alkali index that predicts high fouling tendency due to the high potassium and sodium contents in the ash. The phosphorus content in DDGS ash is also very high. Pyrolysis studies showed some oil evaporation/decomposition occurring at lower temperatures compared to decomposition of the lignocellulosic material. Nitrogen and sulphur partitioning showed that approximately 91% N and 94% S were given off as volatiles. The char was enriched with nitrogen but depleted in sulphur. Char combustion took place in two distinct stages indicating the presence of two types of chars, which yield selective evolution of S and N species during their combustion. Fluid bed combustion studies showed release of both HCN and ammonia during initial stages, when the bed is heating and the fuel undergoes devolatilisation. Steady-state combustion gave NO and N 2O emissions of 290 and 30 ppm respectively, with HCN and NH 3 emissions dropping to below 1 ppm. Agglomeration of the bed was avoided by keeping the combustion temperature below 900 °C

Influences of multiwalled carbon nanotubes and plant residue chars on bioaccumulation of polycyclic aromatic hydrocarbons by Chironomus plumosus larvae in sediment

Carbonaceous materials (CMs), including carbon nanotubes (CNTs), and black carbon have been suggested as potential remediation materials for hydrophobic organic contaminants (HOCs) in sediments or soils. However, the concentration-dependent and potential effects of CMs on the decrease in HOC bioavailability are not well understood. In this research, the effects of two types of multiwalled CNTs (MWNT-1 and MWNT-2) and chars (char-stalk produced from stalk and char-wood from wood) on the bioaccumulation of polycyclic aromatic hydrocarbons (PAHs), including phenanthrene, pyrene, and chrysene, in the benthic organism Chironomus plumosus larvae were studied. When CM content was 1.5% or less in sediments, biota-sediment accumulation factor (BSAF) values for PAHs decreased sharply as CM increased. However, when char and MWNT-1 content was greater than 1.5% in sediments, reduction rates of BSAF were slight. Furthermore, when MWNT-2 content was greater than 1.5%, BSAF values were elevated. This indicated that the MWNT-associated PAHs may have been absorbed by larvae through particle ingestion, and suggested that some CNTs may not be suitable for the remediation of HOC-contaminated sediments because they probably could increase the exposure risk of PAHs to benthic organisms, possibly because of their unique structure.

Gasification reactivity of char with CO2 at elevated temperatures: the effect of heating rate during pyrolysis

ABSTRACTIntegrated coal gasification combined cycle (IGCC) technology is being promoted to make better use of energy resources, and an entrained‐flow gasifier is a key item of equipment in the process due to its high gasification efficiency, smooth discharge of molten ash, etc. The temperature and heating rate under entrained‐flow gasification conditions are very high, and these two parameters can influence the gasification reactivity of a char with CO2. Therefore, the clarification of characteristics and mechanisms of char gasification under conditions close to entrained‐flow gasification is very important. To obtain a valid kinetics of char gasification, it is necessary to clarify the effect of heating rate during pyrolysis on char reactivity. The effect of heating rate during pyrolysis on the gasification reactivity of three types of chars in CO2 was investigated experimentally at elevated temperatures in a novel fluidized bed. It was shown that even at elevated temperatures, the heating rate has significant influence on the char reactivity during gasification. A higher heating rate during pyrolysis leads to higher char reactivity of gasification, however it has only limited effect on the activation energy of char gasification with CO2. The char reactivity is closely related to its specific surface area and pore volume. The high reactivity of a char derived from pyrolysis at a high heating rate is most likely due to its enhanced porous structure as a result of the rapid release of volatile matter at a high heating rate. The effect of heating rate during pyrolysis on char reactivity is pronounced at low temperatures, and is very different for various coal types, i.e. very pronounced for a low‐rank coal. The gasification reactivity of a char is related to both the char pore structure and temperature, which suggests that even at elevated temperatures the gasification of a char with CO2 is controlled by both chemical reaction and diffusion inside the particles. Our results demonstrate that it is necessary to derive the gasification kinetics of a char at a high heating rate to obtain valid kinetic equations for an entrained‐flow gasifier. Copyright © 2010 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Textural properties of chars as determined by petrographic analysis: Comparison between air-blown, oxygen-blown and oxygen-enriched gasification

In this study, the textural properties of chars generated from a vitrinite, high ash coal in a fluidised bed gasifier under air-blown, oxygen-blown and oxygen-enriched conditions were determined by detailed petrographic analysis. The char samples were assessed in terms of their microscopic characteristics (reflectance properties, carbon rich forms (char types) and basic forms of visible minerals). The chars formed under oxygen-blown conditions exhibited the lowest in overall mean reflectance value and also possessed the lowest proportion of chars with reflectance values above 4% RoVr relative to those in the air-blown and oxygen-enriched samples. The oxygen-blown sample also possessed the lowest total carbon forms (53%) whereas the oxygen-enriched and air-blown chars had higher proportions of organic matter (58% and 60% respectively). Melted “slag” minerals were more common in the oxygen-blown char, totalling 31%, compared with 24% in the other two samples.These results suggest that a greater degree of consumption of carbon took place under the oxygen-blown conditions, possibly with higher particle temperatures, and that under these conditions, the highly reflecting porous thin-walled chars would have been consumed first and fastest. This would have left the higher proportion of partially consumed chars and inertinites behind which, in turn, would have lead to the occurrence of lower reflectance readings.These conditions also resulted in the presence of higher proportions of melted “slag” minerals in the oxygen-blown sample. The oxygen-enriched and air-blown chars appear to have undergone reduced levels of carbon consumption and at lower particle temperatures relative to the oxygen-blown sample. Of these two samples, the air-blown sample had the lowest rate of consumption of all three conditions. This, in turn, implies that the temperatures operating under these conditions were the lowest of all. In summary, it would appear that the conditions under which the most rapid rates of consumption would occur, in decreasing order of reactivity and particle temperature, would be (i) oxygen-blown, (ii) oxygen-enriched and (iii) air-blown.These results have important implications for the future, namely, (i) to ensure the highest optimum efficiencies in future gasification processes by selecting the most appropriate gaseous environments and (ii) to prevent or minimise the occurrence of slagging in future gasification processes. This is specifically applicable to operations involving fluidised bed technologies and may well be of value in countries in which high ash coals will be the primary energy source in such technologies.