Transformation of nitrogen during hydrothermal carbonization of sewage sludge: Effects of temperature and Na/Ca acetates addition

This study reports the effects of temperature and Ca/Na acetates addition on the transformation of nitrogen during hydrothermal carbonization of sewage sludge at 160–250 °C. The nitrogen species in the hydrochar, aqueous, oil and gas products from sludge hydrothermal carbonization at different temperatures are well characterized, with a focus on the amino acid species in various products. Temperature is found to greatly affect the nitrogen transformation during sludge hydrothermal carbonization. At 160 °C, 47.3% of nitrogen is transformed into the aqueous product. When the temperature increases to 250 °C, only 27.1% of nitrogen is retained in the hydrochar, while 69.2 and 6.7% of nitrogen is present in the aqueous and oil products, respectively. During hydrothermal carbonization, the protein-N is first converted into the polypeptide-N in the aqueous product, followed by its further decomposition into the NH+ 4-N. This leads to a high content of the NH+ 4-N in the aqueous product, especially at increased temperatures. The labile protein-N is also transformed into the heterocylic-N (especially the pyrrole-N) in the hydrochar as the temperature increases. Among all nitrogen species in the aqueous product, the polypeptide-N consisting of amino acids with the alkyl group is the most stable. Moreover, the addition of NaAc and CaAc₂ reduces the nitrogen retention in the hydrochar, mainly due to enhanced hydrolysis of the protein-N. While for CaAc₂ addition, the deamination of the polypeptide-N is also enhanced, leading to a higher NH+ 4-N in the aqueous product. Our results show that the type of amino acid in protein is important to determine the nitrogen transformation pathways, and acetate addition is an important strategy for enhancing nitrogen removal in the hydrochar during hydrothermal carbonization.     

NOx and N2O precursors (NH3 and HCN) from biomass pyrolysis: Co-pyrolysis of amino acids and cellulose, hemicellulose and lignin

Nitrogen in biomass is mainly in forms of proteins (amino acids). Glycine, glutamic acid, aspartic acid, leucine, phenylalanine and proline are the major amino acids in agricultural straw. The six amino acids were pyrolyzed individually at 800 °C in a tubular reactor in an argon atmosphere. Each amino acid sample was then pyrolyzed individually with cellulose, hemicellulose or lignin with 1:1 mixing ratio by weight under the same condition. The emissions of HCN and NH₃ were detected with a Fourier transform infrared (FTIR) spectrometer. The extent of interaction between the amino acids with cellulose, hemicellulose or lignin was determined by comparing the yields of HCN and NH₃ from co-pyrolysis with those from single amino acid pyrolysis under the same condition. The results indicate that the structure of the amino acid has a significant effect on the nitrogen transformation during pyrolysis. The mixtures undergo solid-state decomposition reactions during co-pyrolysis. The extent of interaction between the amino acids with cellulose, hemicellulose or lignin depends on the amino acid types and the components in biomass. Although single proline and leucine form no char, they give a significant amount of nitrogen-containing char when co-pyrolyzed with cellulose, hemicellulose and lignin. HCN and NH₃ yields and nitrogen conversion pathway from amino acid pyrolysis are influenced by cellulose, hemicellulose and lignin.     

Insight into the calcium carboxylate release behavior during Zhundong coal pyrolysis and combustion

In this paper, the dynamic behavior of calcium carboxylate release during Zhundong coal pyrolysis and combustion is studied via reactive molecular dynamics (ReaxFF MD) simulation. The molecular structure model of Zhundong coal is constructed based on the combination of the classic Hatcher coal model and experimental characterizations. Pyrolysis simulations on the coal model are performed at different temperatures ranging from 2000 K to 2800 K. The pyrolysis experiments are also carried out to validate the ReaxFF simulation. The results show that most of the calcium are released into the volatiles by the thermal decomposition of CM-Ca (coal/char matrix with calcium bonded) after releasing CO₂. The distributions of the calcium bonded to gas, tar and inorganics as well as the atomic calcium in the volatiles are quantitatively classified. The thermal cracking of tar fragments are significant at high temperatures leading to the conversion of calcium from tar into the organic gas. Furthermore, the nascent char model is constructed to study the release behavior of calcium in char combustion stage. The calcium is initially released in the form of oxidized calcium and atomic calcium. With increasing temperature, the oxidized calcium trends to convert to the organically bonded calcium. By using the Arrhenius expression, the kinetic parameters for the release of calcium into various species during pyrolysis and char combustion stages are quantitatively determined.     

Fundamental investigation into characteristics of particulate matter produced from rapid pyrolysis of biochar in a drop-tube furnace at 1300 °C

This paper reports the emission characteristics of leaf and wood biochar (LC500 and WC500) pyrolysis in a drop tube furnace at 1300 °C in argon atmosphere. The char yields at 1300 °C are ～ 65% and ～ 73% respectively for LC500 and WC500. Over 60% Mg, Ca, S, Al, Fe and Si are retained in char after pyrolysis at 1300 °C. The retentions of Na and K in the char from LC500 pyrolysis are lower than those in the char from WC500 pyrolysis due to release via enhanced chlorination as a result of much higher Cl content in LC500. Particulate matter (PM) with aerodynamic diameter of < 10 µm (i.e. PM₁₀) from LC500 and WC500 pyrolysis exhibits a bimodal distribution with a fine mode diameter of 0.011 µm and a coarse mode diameter of 4.087 µm. The PM₁₀ yield for LC500 pyrolysis is ～ 8.2 mg/g, higher than that of WC500 pyrolysis (～2.1 mg/g). Samples in PM_1-10 (i.e. PM with aerodynamic diameter 1 µm – 10 µm) are char fragments that have irregular shapes and similar molar ratio of (Na+K + 2Mg+2Ca)/(Cl+2S+3P) as the char collected in the cyclone. In PM₁ (i.e. PM with aerodynamic diameter < 1 µm), the main components in sample are inorganic species, and carbon only contributes to ～5% and ～8% the PM₁ produced from rapid pyrolysis of LC500 and WC500, respectively. Na, K and Cl are main inorganic species in PM₁, contributing ～ 98.8% and ～ 97.5% to all inorganic species. Na, K and Cl from rapid pyrolysis of biochar have a unimodal distribution with a mode diameter of 0.011 µm. In PM_1–10, Ca is the main inorganic specie, contributing to ～71.2% and ～65.3% to all inorganic species in PM_1–10 from pyrolysis of LC500 and WC500, respectively.     

Characteristics of chars formed during rapid pyrolysis of biomass model components at high temperature

This paper reports char formation and inherent inorganic transformation during rapid pyrolysis of various biomass model components under simulated pulverized fuel (PF) conditions at 1300 °C. A drop-tube furnace with a novel double-tube configuration was deployed to achieve direct determination of char yield. The results show that rapid pyrolysis of xylan and water-washed lignin (W-L) under the conditions results in char yields of 3.4 wt.% and 12.6 wt.%, respectively, while no char was founded during rapid pyrolysis of water-washed cellulose (W-C). After loading K₂CO₃ into the W-C (i.e. KW-C) and W-L (i.e. KW-L), the char yields increase to 2.1 wt.% and 15.6 wt.%, respectively. The retentions of Na and S are low in chars after pyrolysis. After rapid pyrolysis, W-L and KW-L chars have higher retentions of AAEM species than xylan, W-C and KW-C chars. Micromorphology analysis shows char particles formed after rapid pyrolysis of all biomass components have a cenospheric structure and a rough surface with many bubbles and pores, demonstrating strong melting processes. For xylan and KW-L, the abundant inorganics accelerate char formation with swelling and reduce the extent of particle shrinkage, resulting in char particles with apparent sizes bigger than the parent feedstock particles. Oppositely, for KW-C and W-L that have low contents of inorganic species, the pyrolyzing particles experience significant shrinkage, resulting in formed char particles with apparent sizes that are much smaller than feedstock particles.     

A reactive molecular dynamics study of NO removal by nitrogen-containing species in coal pyrolysis gas

Coal splitting and staging is a promising technology to reduce nitrogen oxides (NOx) emissions from coal combustion through transforming nitrogenous pollutants into environmentally friendly gasses such as nitrogen (N₂). During this process, the nitrogenous species in pyrolysis gas play a dominant role in NOx reduction. In this research, a series of reactive force field (ReaxFF) molecular dynamics (MD) simulations are conducted to investigate the fundamental reaction mechanisms of NO removal by nitrogen-containing species (HCN and NH₃) in coal pyrolysis gas under various temperatures. The effects of temperature on the process and mechanisms of NO consumption and N₂ formation are illustrated during NO reduction with HCN and NH₃, respectively. Additionally, we compare the performance of NO reduction by HCN and NH₃ and propose control strategies for the pyrolysis and reburn processes. The study provides new insights into the mechanisms of the NO reduction with nitrogen-containing species in coal pyrolysis gas, which may help optimize the operating parameters of the splitting and staging processes to decrease NOx emissions during coal combustion.     

Rapid pyrolysis of biochar prepared from slow and fast pyrolysis: The effects of particle residence time on char properties

This paper investigates the evolution of char properties with particle residence time during rapid pyrolysis of biochar under conditions pertinent to pulverized fuel (PF) applications. Two biochar samples were considered, prepared via slow (S-BC) and fast (F-BC) pyrolysis of mallee wood (150–250 µm) at 500 °C and two different heating rates (10 °C/s and ∼400 °C/s), respectively. The biochar samples were then subjected to rapid pyrolysis at 1300 °C using a novel drop-tube furnace (DTF), which enables direct determination of char yield experimentally. The evolution of char yield, the release of alkali and alkaline earth metallic (AAEM) species, and particle size and shape during rapid pyrolysis are investigated as a function of particle residence time (0.45 s to 1.4 s). The results show that char yields decrease from ∼77% to 75% when particle residence time increases from 0.45 s to 1.4 s. Rapid pyrolysis of F-BC has slightly higher char yields, due to the higher ash content of F-BC. More Cl in F-BC facilitates the release of Na during rapid pyrolysis, leading to the lower retention of Na in FC than in SC. Nevertheless, the retentions of K (∼90%), Mg (∼85%), and Ca (∼90%) are higher in FC, which can be ascribed to its higher contents of oxygen after rapid pyrolysis. The investigation of particle size and shape shows that biochar particles exhibit little changes after rapid pyrolysis, indicating their strong resistance to shrinkage and deformation even at high temperature.     

High-temperature pyrolysis of petroleum coke and its correlation to in-situ char-CO2 gasification reactivity

The understanding of the pyrolysis behavior of petroleum coke (PC), a by-product of oil refining, is very critical to its energy utilization. Different from most previous work, this investigation particularly focused on PC pyrolysis at high temperatures > 1273 K. The CO₂ gasification reactivity of in-situ char, rather than quenched char, was evaluated and correlated to PC pyrolysis behavior at different temperatures. A Chinese PC with sizes below 100 μm was tested on a high-temperature thermogravimetric analyser (TGA). Three sets of tests were carried out for different purposes. The first set was to simultaneously obtain sample mass loss and gas evolution data during pyrolysis through thermogravimetry-mass spectrometry (TG-MS). The second set aimed to collect char samples for subsequent analyses by scanning electron microscopy (SEM) and Raman spectrometry. The third set was to evaluate in-situ char-CO₂ gasification reactivity through the TGA. The results showed that, in addition to the commonly-observed primary pyrolysis stage at low temperatures, there was a secondary PC pyrolysis stage at high temperatures > 1300 K. In this process, the gases such as HCN, CO₂ and SO₂ were significantly released. The observed changes of char morphology suggested a four-staged thermoplastic transformation of the PC during pyrolysis, which has little been discussed previously. At different stages, i.e. softening, plasticizing, resolidification and graphitization, the rate of carbon ordering was different. The in-situ char-CO₂ gasification reactivity was found to first increase, then decrease and finally increase again with increasing temperature. Such changes coincided with the thermoplastic state of the pyrolyzed char, but not with the changes of char surface area or carbon ordering. The obtained knowledge is new and highlights the potentially important roles of char thermoplastic state in determining its reactivity towards CO₂.     

Predicting nitrogen release during coal tar decomposition

This paper develops a detailed phenomenological reaction mechanism for N-species transformations throughout tar decomposition, including tar-N sequestration into soot. It expands the previously validated mechanism for tar decomposition based on FLASHCHAIN^® theory to cover N-transformations during pulverized fuel firing. Tar-N transformations are described by two distinctive features: (1) An elimination reaction that produces HCN governs the decay in the average moles of nitrogen per aromatic nucleus throughout tar decomposition; and (2) Empirical observations determine the fraction of tar-N incorporated into soot. Validation cases represent heating rates of at least several thousand degrees per second; temperatures from 600 to 1100 °C; tar contact times from 75?ms through 2?s; and coal ranks from subbituminous through low volatile bituminous. The predicted partitioning of coal-N into tar-N, HCN, soot-N, and char-N was within measurement uncertainties for all coals for simulated p. f. firing conditions, including the variation in fractional char-N levels from 0.4 to 0.8 across this domain. Since primary tar-N levels are directly proportional to fractional primary tar yields, and since ultimate soot-N levels account for one-third of tar-N with any coal type, the ultimate coal-N partitioning for CFD furnace simulations can be accurately described with two analyses: (1) A primary devolatilization mechanism to predict primary tar yields under rapid heating conditions; and (2) A submechanism to predict HCN release from char throughout devolatilization up to the point of char ignition. Dynamics may be resolved with either global reactions or the full tar decomposition mechanism, depending on the impact of the lag between tar decomposition and soot production in the subject application.     

The transformation and fate of sulphur during CO2 gasification of a spent tyre pyrolysis char

The transformation and fate of sulphur (S) in a spent tyre pyrolysis char during CO₂ gasification were studied by following the S species and contents using X-ray photoelectron spectroscopy (XPS). The spent tyre pyrolysis char (particle size fraction ≤150 µm), without and with 1 M HCl acid washing to remove inorganic S, were gasified in a fixed bed reactor. The effect of temperature (850, 950, 1050 °C), reaction time (1, 2, 3, 6 h) and CO₂ concentration (33.3, 50.0, 66.7 vol% in N₂) on the S species in the char samples were investigated. The main S species in the spent tyre pyrolysis char were ZnS and aliphatic sulphide. After CO₂ gasification, aliphatic sulphide, thiophene, sulphoxide and sulphone became the dominant organic S while ZnS and CaSO₄ were the main inorganic S. The percentage of total S increased with increasing gasification temperature, time and CO₂ concentration. The content of organic S increased with increasing gasification temperature and time, while, the content of inorganic S decreased. Increasing CO₂ concentration had negligible effect on the content of organic S but led to significant reduction in the content of inorganic S since ZnS reacted with CO₂ to produce ZnO and SO₂. Aliphatic sulphide, sulphoxide and sulphone were shown to have transformed to more stable thiophene. ZnS decomposed to release S_X at > 900 °C while CaSO₄ reacted with CO and carbon to produce COS. Both S_X and COS reacted with the organic matrix in the char to form sulphoxide and sulphone.     

褐煤热解与燃烧时矿物转化和细灰形成的CCSEM研究

在沉降炉中进行了一种典型中国褐煤的热解与燃烧实验,热解气氛为N2,燃烧气氛为O₂/N₂=21:79,采用CCSEM分析原煤、煤焦与煤灰。CCSEM分析结果表明,铁氧化物、石英、黄铁矿、伊利石和高岭土是煤中主要的矿物成分,同时也是主要的外在矿成分,褐煤中57.26%的矿物粒径小于10μm。在热解与燃烧过程中,煤中主要矿物发生了明显转化。富Si矿物和硅铝酸盐在热解和燃烧过程中可能发生了破碎;而富Fe矿物部分明显破碎生成细小矿物,部分外在矿直接转化,未发生明显破碎。细灰少量来自于细小富硅矿、石英和铁氧化物等矿物的直接转化,70%以上的细灰由Ca、Fe含量很高的混合硅铝酸盐组成。     

Measurement and kinetics of elemental and atomic potassium release from a burning biomass pellet

Combining polarizing-filtered planar laser-induced fluorescence (PLIF) with simultaneous laser absorption, quantitative laser-induced breakdown spectroscopy (LIBS) and two-color pyrometry, the potassium release during the combustion of biomass fuels (corn straw and poplar) has been investigated. The temporal release profiles of volatile atomic potassium and potassium compounds from a corn straw show a single peak. The woody biomass, poplar, produces a dual-maxima distribution for potassium and potassium compounds. For both biomass samples, the highest concentrations of released atomic potassium and potassium compounds occur in the devolatilization stage. The mass ratios between volatile atomic potassium and potassium compounds in the corn straw and poplar cases are 0.77% and 0.79%, respectively. These values agree well with chemical equilibrium predictions that 0.68% of total potassium will be in atomic form. A two-step kinetic model of potassium release has been developed, which gives better predictions during the devolatilization stage than the existing single-step model. Finally, a map of potassium transformation processes during combustion is developed. Starting with inorganic and organic potassium, there are eight proposed transformation pathways including five proposed release pathways that occur during the combustion. The pathways describe the transformation of potassium between the fuel volatile matter, char, and ash. Potassium release during the devolatilization stage is due to pyrolysis and evaporation; during the char burnout stage, potassium release is due to char oxidation and decomposition; and during the ash cooking stage, potassium release is caused by reactions between the ash and H₂O in the co-flow.     

Characterization of nitrogen and fluorine co-doped titania photocatalyst: Effect of temperature on microstructure and surface activity properties

Nitrogen and fluorine co-doped titania photocatalyst samples to be used for air purification were prepared by spray pyrolysis using a mixed solution of TiCl₄ and NH₄F. Droplets of the mixed solution formed by nebulizer passed through a ceramic tube furnace under a suction of an aspirator and a titania-based powder was formed at temperatures in the range from 700 to 1000 °C. The resulting nanopowders were characterized by electron microscopy, X-ray diffraction, temperature programmed desorption of NH₃, methods of thermal analysis, particle size, surface area and porosity determination by nitrogen adsorption. Morphology and surface activity of the samples prepared at various conditions were compared. The thermal behavior of the samples characterized by TG, DTA and ETA under air heating conditions is discussed considering the differences in samples preparation. A high photocatalytic activity for acetaldehyde decomposition in a visible region of spectrum depended on the spray pyrolysis temperature and can be ascribed to a synergetic effect of nitrogen and fluorine doping.     

Experimental and kinetic modeling studies of phenyl acetate pyrolysis at atmospheric pressure

Phenyl acetate (CH₃COOC₆H₅, PA) shares a similar aryl acetate group with vitamin E acetate, which is thought to be responsible for producing pulmonary toxic ketene in e-cigarettes. Hence, PA is reported to be a model compound of vitamin E acetate in producing ketene. To better understand the pyrolysis chemistry of vitamin E acetate, pyrolysis of PA in a jet-stirred reactor was investigated by using synchrotron vacuum ultraviolet photoionization mass spectrometry at atmospheric pressure and at temperature range of 700 – 1025 K. Several key products such as acetylene, ethylene, carbon monoxide, formaldehyde, carbon dioxide, vinyl acetylene, 1,3-butadiene, 1,3-cyclopentadiene, benzene, phenol, etc., and especially, ketene, were identified and measured. By extending the phenyl formate pyrolysis model, a detailed PA pyrolysis model containing 735 species and 3365 reactions was constructed and validated against the current experimental results of PA pyrolysis. Rate of production analysis and sensitivity analysis show that the main reaction pathways of PA pyrolysis are the unimolecular decomposition forming phenol and ketene, followed by the C–CH₃ bond cleavage forming phenoxycarbonyl and methyl. The corresponding products of these two reactions and of the subsequent reactions, including phenol, ketene and carbon monoxide, etc., are demonstrated to be the key products in PA pyrolysis. Toxic aromatic compounds, such as benzene, toluene and ethylbenzene, etc., also have relatively high mole fractions in PA pyrolysis.     

Pyrolysis of sewage sludge under conditions relevant to applied smouldering combustion

Pyrolysis of sewage sludge under conditions relevant to applied smouldering combustion was carried out in this study to investigate the influences of gas flow rate, oxidative atmosphere, and inert porous medium involvement on the properties of products. The experiments were carried out at 300–600 °C under atmospheres of N₂, 5% O₂/95% N₂, 10% O₂/90% N₂, and 15% O₂/85% N₂, with Darcy flow rates of 1.0 and 3.5 cm/s, respectively, with dried sewage sludge loaded individually or as a mixture with sand. As a result, both the increment of gas flow rate and involvement of sand leaded to lower yields of char and higher yields of bio-oil and gas under N₂ at temperature of ≤500 °C, due to the enhanced efficiency of pyrolysis reaction and gas transportation. However, when temperature increased to 600 °C, the influencing trends on product distributions changed due to the mechanisms of secondary cracking reaction and volatile-char interaction. The involvement of oxygen in fraction of ≤15 vol% at temperatures of 400–500 °C would lead to the intense decreasing yields of char and bio-oil, and increasing yield of the gaseous (dominated by CO₂ and CO), due to the involved oxidation reaction during pyrolysis. Both increment of temperature and oxygen fraction would lead to the delay of ignition and the increase of activation energy of the produced char, except for that of char produced at 400 °C under 5% O₂/95% N₂, whose calculated activation energy was lower and volatile content was higher compared to that of char produced from pyrolysis at 400 °C under N₂. The bio-oil from pyrolysis under N₂ was dominated by aliphatic acids, phenols, steroids, amides, and indoles, etc., and the involvement of partial oxidation would lead to the weakened formation of aromatics, phenols, and S/Cl/F-containing compounds in bio-oil.     

Effect of pyrolysis conditions on gasification reactivity of woody biomass-derived char

The effect of pyrolysis conditions on char reactivity has been studied using Raman spectroscopy. This paper reports on the relationship between the properties of biomass char and the gasification rate. The gasification kinetics of biomass char have been revealed by measuring the rate of weight loss during its reaction with CO₂ as a function of temperature. First-order kinetic rate constants are determined by fitting the weight loss data using a random pore model. The relationship between the char structure and CO₂ gasification reactivity was investigated in the range of 15–600 °C/min at a constant pyrolysis pressure (0.1 MPa), and 0.1–3.0 MPa at a constant heating rate (15 °C/min). The experimental results reveal that the reactivity of biomass char is determined by the pyrolysis condition. The CO₂ gasification rates in char generated at 0.1 MPa exhibited approximately twice the values as compared to those obtained at 3 MPa. This is because the uniformity of the carbonaceous structure increases with the pyrolysis pressure. The uniformity of carbonaceous structures would affect the CO₂ gasification reactivity, and the decreasing uniformity would lead to the progression of cavities on the char surface during the CO₂ gasification process. The gasification rate of biomass char increases with the heating rate at pyrolysis. This is due to the coarseness (surface morphology) of biomass char and rough texture, which increases with the heating rate.     

Heterogeneous fixation of N2: Investigation of a novel mechanism for formation of NO

Formation of NO initiated by heterogeneous fixation of N₂ during pyrolysis is investigated experimentally and theoretically. The experiments were conducted with beech wood as well as with the pure biomass components cellulose, xylan, and lignin. The NO formation during char oxidation was recorded as function of pyrolysis atmosphere (N₂ or Ar), pyrolysis temperature (700–1050 °C), and oxidizing atmosphere (O₂ in N₂ or Ar). The results confirm earlier reports that biomass char may be enriched in N during pyrolysis at 900 °C and above. The N-uptake involves re-capture of N-volatiles as well as uptake of N₂. During char oxidation, the captured N is partly oxidized to NO, resulting in increased NO formation. The NO yield from oxidation of beech wood char made in N₂ increases with pyrolysis temperature, and is about a factor of two higher at 1050 °C than the corresponding yield from chars made in Ar. The experiments with pure materials show that the lignin char has the strongest ability to form NO from uptake of N₂, while xylan char forms only small amounts of NO from N₂. Density Functional Theory (DFT) calculations on model chars have revealed a number of chemisorption sites for N₂, many of which are weakly bound and therefore expected to have a short half-life at the higher pyrolysis temperatures. However, the chemisorption of N₂ across a single ring of the armchair surface was found to have an activation energy of 344 ± 30 kJ mol⁻¹ and form a stable, exothermic product with cyano groups. This demonstrates that at least one channel exists for the high-temperature incorporation of N₂ into a char which could give rise to the observed increase in NO release in subsequent char oxidation.     

Catalytic cracking of tar derived from the pyrolysis of municipal solid waste fractions over biochar

The tars derived from the pyrolysis of four typical municipal solid waste fractions at 600 °C, namely pine wood (PW), tryptone (TP), polyethylene (PE) and polyvinyl chloride (PVC), were characterized and then catalytically cracked by activated biochar catalyst (ABC) at 700–900 °C. The ABC was produced from the pyrolysis of pine wood at 800 °C for 1 h, then activated by CO₂ at 900 °C for 20?min. The results showed that O-containing species, N-containing species, chain hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) were the main products in the raw tar from the pyrolysis of PW, TP, PE and PVC, respectively. The tar cracking efficiency by ABC was ordered as PW>TP>PE>PVC, which indicated that the biomass tars were easier to be converted by ABC than plastic tars. The highest tar conversion of 98.7% was achieved for PW at 900 °C. Besides, N-containing tars were more stable than O-containing tars. The coke deposition on the ABC was more serious after the cracking of plastic tars (PE and PVC) than that of biomass tars (PW and TP). After the catalytic cracking of TP and PVC tars at 900 °C, the nitrogen and chlorine contents in ABC increased by 3 times and 10.5 times, respectively.     

造纸污泥在回转窑中热解的试验研究

本文在自行设计的小型外热式回转窑试验装置上进行了污泥热解试验,考察了污泥在不同热解连温下三种形态热解产物的产率,分析了污泥在热解过程中热解气体的成分和热值的变化特性以及热解半焦的品质。采用表观动力学模型对热解气体的试验数据进行计算,结果表明用该模型描述热解气体的析出是合理的。     

Detailed modeling of hydrogen release and particle shrinkage during pyrolysis of inhomogeneous wood

Hydrogen release during pyrolysis of woody biomass is studied considering anisotropicity and inhomogeneity of wood structure. A new anisotropic shrinkage model is proposed based on the decomposition of main wood constituents, i.e., cellulose, hemicellulose, and lignin. The new shrinkage model can predict the temporal evolution of the wood structure, and the differences between axial and radial shrinkage during pyrolysis. The model agrees very well with several experimental data from the literature. Based on particle temperature during conversion, the pyrolysis is partitioned into four stages, and the hydrogen release and H${}_2$ formation from each stage are investigated. Stage (IV) of pyrolysis, from 1000 to 1273 K, is found to be efficient for H${}_2$ production owing to the production of considerable mass of H${}_2$ with a minimal amount of tar species. Furthermore, the char quality is found to be different at the end of stages (II), (III), and (IV), where around 67.7, 80.5, and 93.4% wt. of solid residue is made of carbon, respectively. The model is also used to explain how the heating rate affects the temperature distribution inside the particle and how it shifts the peak of hydrogen release. Finally, the pyrolysis of two inhomogeneous wood samples — a beech twig with bark and a beech dowel with growth rings — are investigated. The bark can affect the pyrolysis rate, products, and flow pattern inside the particle. The growth rings do not have a considerable effect on the pyrolysis rate and products, but they have a significant impact on the flow pattern. This has an important implication for char conversion studies where the internal surface area and porosity field distribution have a significant effect on the gasification and oxidation rates.