Some aspects of the role of coal thermoplasticity and coke structure in coal gasification: 3. The effect of rank,pitch and sodium carbonate on Brabender plastometry parameters

The thermoplastic properties of coal are a significant factor in determining gasification behaviour in most gasification processes. This investigation has involved the use of constant shear rate (Brabender) plastometry to study the plastic properties of a wide range of coals varying from 101 to 902 in the NCB Classification. The results obtained show more complex behaviour during the carbonization process than reported previously and are compared with ASTM Gieseler plastometry and BS dilatometry results. Some coals have two peaks in the caking region of the plastometry curve. This has been attributed to an initial peak corresponding to softening of the coal followed by caking due to active decomposition. The phenomenon was observed for coals with ranks 301a to 602, the temperature difference between the peaks being largest for the prime coking coals. It has been shown that this technique produces much additional information about the initial phase of the carbonization process. The effect of pitch and sodium carbonate addition on the plastometry parameters has also been studied. The results show that small amounts of these additives can radically change the plastometry curves. Pitch increases the plastic range whereas sodium carbonate reduces it. The addition of sodium carbonate tends to modify the plastometry curves so that they are similar to coals of lower rank whereas pitch addition, in general, tends to increase fluidity and, in the case of low-rank coals, produces plastometry curves similar but not identical to higher-rank coals.     

Effect of alkali treatment on the caking properties of coal

Three weakly caking and non-caking high volatile bituminous coals from India were converted into good caking derivatives by treatment with alcoholic alkali at 250–300 °C and 22–25.5 MPa. These derivatives showed high Gieseler fluidities with wide plastic range and may be useful as additives to coals for oven charges. During alcoholic alkali treatment, the organic material of coal undergoes extensive degradation into relatively low molecular weight substances which possess sufficient thermal stability to maintain a fluid state on carbonization. Increasing the severity of alkali treatment produces a pitch-like material from coal, with a potential use as a binder for formed coke, carbon electrodes etc.     

The volatilization behavior of chlorine in coal during its pyrolysis and CO2-gasification in a fluidized bed reactor

The volatilization behavior of chlorine in three Chinese bituminous coals during pyrolysis and CO₂-gasification in a fluidized bed reactor was investigated. The modes of occurrence of chlorine in raw coals and their char samples were determined using sequential chemical extraction method. The Cl volatility increases with increasing temperature. Below 600 °C the Cl volatility is different, depending on the coal type and the occurrence mode of Cl. Above 700 °C, the Cl volatilities for the three coals tested are all higher than 80%. About 41% of the chlorine in Lu-an coal and 73% of that in Yanzhou coal are organic forms, and most of them are covalently-bonded organic chlorine, which shows high volatile behavior even at low pyrolysis temperatures (below 500 °C), while the inorganic forms of chlorine in two coal samples are hardly volatilized even at low pyrolysis temperatures (below 400 °C). The restraining efficiency of addition of CaO on chlorine volatility is greatly dependent on pyrolysis temperature. The optimal restraining efficiency can be obtained at temperature range from 450 to 650 °C during pyrolysis of Lu-an coal. The volatile behavior of Cl is mainly dependent on temperature. Above 700 °C high volatility of Cl is obtained in both N₂ and CO₂ atmospheres.     

Influence of coal forced oxidation on technological properties of cokes produced at laboratory scale

The effect of coal oxidation in air at 140 °C on the technological properties of cokes obtained at laboratory scale from two medium volatile bituminous coals has been studied. The proximate and ultimate analyses do not show important changes with coal oxidation time. However oxidation clearly has a strong effect on the plastic properties of the coals in view of the fact that the Gieseler fluidity eventually disappears. From this point variations in plastic properties can still be detected by FSI. Other changes, such as a shortening of the length of the saturated fragments of the aliphatic chains, a decrease in the aliphatic hydrogen content and an increase in the oxygen-containing groups are detected by PA-FTIR. It was also found that the main coke quality indices (mechanical strength and reactivity to CO₂) of both coke series are impaired with coal oxidation. A close relationship between reactivity to CO₂ and the micropore specific surface area of the cokes has been corroborated.     

Influence of coal forced oxidation on technological properties of cokes produced at laboratory scale

The effect of coal oxidation in air at 140 °C on the technological properties of cokes obtained at laboratory scale from two medium volatile bituminous coals has been studied. The proximate and ultimate analyses do not show important changes with coal oxidation time. However oxidation clearly has a strong effect on the plastic properties of the coals in view of the fact that the Gieseler fluidity eventually disappears. From this point variations in plastic properties can still be detected by FSI. Other changes, such as a shortening of the length of the saturated fragments of the aliphatic chains, a decrease in the aliphatic hydrogen content and an increase in the oxygen-containing groups are detected by PA-FTIR. It was also found that the main coke quality indices (mechanical strength and reactivity to CO₂) of both coke series are impaired with coal oxidation. A close relationship between reactivity to CO₂ and the micropore specific surface area of the cokes has been corroborated.     

Effectiveness of high temperature pyrolysis in sulfur removal from coal

A series of Polish hard coals of different rank that contained sulfur in the quantity from 0.37 to 4.90 wt% was investigated in this study. The coals were subjected to pyrolysis at 1000°C in an atmosphere of the gases evolved during the pyrolysis process. Results have shown a decreasing trend in the degree of sulfur removal by high temperature pyrolysis with increasing coal rank. The effectiveness of pyrolysis in sulfur removal appears to be related to the proportion of the non-thiophenic sulfur to the total organic sulfur in the coal. These are linear correlations between the total sulfur content in the coke and total, pyritic and organic sulfur contents in the initial coal.     

The removal of mercury from coal via sub-critical water extraction

The transformation behavior of mercury in two Chinese coals (WJP and HYS coal) during sub-critical water treatments was studied in a semi-continuous apparatus. Float–sink method, demineralization and sequential chemical extraction were used to study the occurrence mode of mercury in raw coals and extracted samples. The results show that with increasing temperature, pressure, water flow and extraction time, the removal of mercury increases. During sub-critical water treatment, the content of mercury associated with sulfates, monosulfides, disulfides, organic material, and insoluble forms decreases. The removal efficiency of mercury is almost 100% at 410 °C, 15 MPa, 0.58 l/h, and 60 min for HYS coal and 96.7% at 380 °C for WJP coal. Under the same temperature and pressure the mercury removal through pyrolysis is less than that through sub-critical water extraction which is an efficient method to remove most mercury from coal.     

Studies of inorganics added to low-rank coals for catalytic gasification

Inorganic complexes were added to low-rank coals by step-wise pH adjustment of a mixture of inorganic solution and brown coal while avoiding the formation of precipitates. The nature and amounts of the added inorganic depend on the pH of the coal/solution mixture and concentration of inorganic salts. The amount of hydroxide added for high loading of iron to coal is consistent with added multinuclear complexes. Computer generated models of brown coal with multi-iron species account for observed OH/Fe ratios. X-ray photoelectron spectroscopy (XPS) data for these samples are consistent with multi-iron species in coal. Computer molecular modelling of two brown coal models with added inorganics shows monodentate carboxyl coordination to metals is sterically favoured. Mononuclear Fe(III) with bidentate carboxyl coordination form distorted structures and are energetically unfavoured. Modelling indicates significant reductions in partial charges on metal centres, consistent with a redistribution of electron density on complexation. Low temperature pyrolysis of brown coals with added inorganics provides increased yields of gases, no detectable tar and lower char, compared with acid washed coals. Gasification of these coals using a nitrogen/oxygen mix at 150–200 °C yields CO, while steam gasification at 250–450 °C yields CO₂, CO and CH₄. Iron oxide/carbonate complexes are postulated during the pyrolysis and gasification.     

Coal flash pyrolysis: 2. Polymethylene compounds in low temperature flash pyrolysis tars

Pyrolysis of coals at low temperatures (< 600 °C) produces tars containing the precursors of the low molecular weight aliphatic hydrocarbons, such as ethylene and propylene, observed on flash pyrolysis of the coals at higher temperatures (700–800 °C). This is shown by further pyrolysis of these low temperature tars at high temperatures. Various methods, including isolation by h.p.l.c. were used to confirm the presence of straight chain paraffin and olefin pairs (C₁₄C₂₆ and above) in the low temperature tars. Pyrolysis of pure paraffins and olefins in this molecular weight range at temperatures > 700 °C produce ethylene, propylene and other cracking products similar to those obtained on flash pyrolysis of coal.     

Studies on artificial coal. 3. Polycondensates from aromatic hydrocarbons

Artificial coals (AC) or coal-like model substances were prepared by the polycondensation of polynuclear hydrocarbons. Anthracene and pyrene gave caking models while phenanthrene (under severe conditions) produced a non-caking model substance. These polycondensates were subjected to solvolysis, thermosolvolysis, thermogravimetry, Gieseler plastometry and differential thermal analysis. The principal conclusions drawn are:

1. (1) The role of the starting aromatic hydrocarbon in deciding the nature of the polycondensate was important for a given set of conditions of polycondensation.

2. (2) The three models behaved differently in yields of chloroform extract by soxhlet extraction: 35.7% (anthracene) and only 0.9% (pyrene), and 3% (non-caking, phenanthrene). Also, in solvolysis and thermosolvolysis the anthracene model behaved like the caking AC and the phenanthrene model behaved like the non-caking AC prepared by hydrothermal carbonification. The preheating of all these models except that from pyrene caused a progressive fall in the subsequent yield of chloroform extract, unlike the other models or AC but like natural caking coal.

3. (3) The anthracene and pyrene models had strikingly high maximum fluidity (>27 000 dial div./min), thus resembling the highly fluid sawdust AC. They also showed low softening temperatures. The stability of chloroform extract as shown by the thermosolvolysis studies was found to be of greater significance than the maximum fluidity or the characteristic Gieseler temperatures in correlating with the caking properties of coal models.

4. (4) Thermograms of these models displayed predominantly exothermic behaviour during pyrolysis, as with other AC and natural coals.

     

Char morphology and behaviour of Australian vitrinites of various ranks pyrolysed at various heating rates

This study concerns the relative importance of rank and heating rate on the development of plasticity during pyrolysis of several Australian vitrinites. Dispersed vitrinite particles, 100 μm in diameter, were heated to 1000 °C in nitrogen at linear heating rates ranging from 10^?1 to 10⁴ °Cs^?1 in a special electrical strip furnace. When pyrolysed at 10⁴ °C s^?1 all vitrinites became plastic and vesiculated except for vitrinite from anthracite, and gelinite from brown coal. The greatest plasticity developed in bituminous-rank vitrinite. Brown coal textinite and the lower-rank sample of the two subbituminous-rank vitrinites behaved similarly, whereas the behaviour of the higher-rank subbituminous coal resembled that of the semi-anthracite sample. At heating rates of 10^?1 °C s^?1 all the vitrinites retained their original morphologies. Cenospheres began to form in the vitrinites within the heating rate range of 1 °C s^?1 (for bituminous rank) to 10² °C s^?1 (for brown coal and semi-anthracite ranks). During pyrolysis, the differences in plastic behaviour attributable to rank could largely be eliminated by changing the heating rate by two orders of magnitude. The effects attributable to plasticity related to increasing heating rates reach a limit within five orders of magnitude of heating rate (for the conditions used in this study).     

Pyrolysis of South Wales coals: 1. Preliminary thermogravimetric studies

Thermogravimetry (TG) has been applied, in a preliminary investigation, to the pyrolysis of seven samples of South Wales coals varying in rank from bituminous to anthracite. Coal samples (4–10 mg) were investigated within the temperature range 150–900 °C at a heating rate of 80 °C min⁻¹, under a nitrogen atmosphere (40 cm³ min⁻¹). The chart recorder method of collecting data, was replaced by a BBC Model B microcomputer system which included a monitor screen, dual disc drive and a printer. The results indicate a significant increase in the sensitivity, compared with the chart recorder method. In addition, the technique shows promise as an aid in the characterisation of coals, due to the distinct pattern of differential thermogravimetric (DTG) peaks for each sample. It is possible these patterns may be of use in predicting qualities of coal important in gasification processes.     

Flash pyrolysis of coals: behaviour of three coals in a 20 kg h−1 fluidized-bed pyrolyser

Flash pyrolysis of Loy Yang brown coal, and Liddell and Millmerran bituminous coals has been studied using a fluidized-bed reactor with a nominal throughput of 20 kg h^?1. The apparatus and its performance are described. The yields of tar and hydrocarbon gases are reported for each coal in relation to pyrolysis temperature, as also are analytical data on the pyrolysis products. The peak tar yields for the dry, ash-free Loy Yang and Millmerran coals were respectively 23% wt/wt (at ≈ 580 °C) and 35% wt/wt (at $\text{\$}\text{?}$600 °C). The tar yield from Liddell coal was 31% wt/wt at ≈ 580 °C. Hydro-carbon gases were produced in notable quantities during flash pyrolysis; e.g. Millmerran coal at 810 °C gave 6% wt/wt (daf) methane, 0.9% wt/wt ethane, 6% wt/wt ethylene, and 2.5% wt/wt propylene. The atomic $\text{H}\text{C}$ ratios and the absolute levels of hydrogen in product tars and chars decreased steadily with increasing pyrolysis temperature.     

Influence of additives of various origins on thermoplastic properties of coal

Two coking coals of different rank were chosen in order to assess the influence of various additives on their thermoplastic properties. Six additives of different origin and characteristics were selected: two non-coking coals, together with a commercial coal tar pitch, a residue from the bottom of the benzol distillation tower and two residues from the tyre recycling industry. The effect of the additives on coal thermoplastic properties was assessed by means of the Gieseler test. The additives were pyrolyzed to a final temperature of 550 °C and the tar characterized by means of Fourier transform infrared spectroscopy (FTIR) and gas chromatography (GC). The influence of the additives on coal thermoplasticity is related to the volatile matter content of the additive, its evolution profile with temperature and the composition of the tar obtained during the pyrolysis of the additives.     

Use of stable sulphur isotopes to monitor directly the behaviour of sulphur in coal during thermal desulphurization

A method has been developed using stable sulphur isotope analyses to monitor the behaviour of sulphur forms in a coal during thermal desulphurization. In this method, the natural stable isotopic composition of the pyritic and organic sulphur in coal is used as a tracer to follow their mobility during the desulphurization process. This tracer method is based on the fact that the isotopic compositions of pyritic and organic sulphur are significantly different in some coals. Isotopic results of pyrolysis experiments at temperatures ranging from 350 to 750 °C indicate that the sulphur released with the volatiles is predominantly organic sulphur. The pyritic sulphur is evolved in significant quantities only when pyrolysis temperatures exceed 500 °C. The presence of pyrite seems to have no effect on the amount of organic sulphur evolved during pyrolysis. The chemical and isotopic mass balances achieved from three different samples of the Herrin (No. 6) coal of the Illinois Basin demonstrate that this stable isotope tracer method is quantitative. The main disadvantage of this tracing technique is that not all coals contain isotopically distinct organic and pyritic sulphur.     

Pyrolysis of brown coals. 1. Decomposition of acid groups in coals containing carboxyl groups in the acid and cation forms

The effect of low-temperature pyrolysis (up to 300 °C) on the acid groups of two low-rank coals (a brown coal from Victoria, Australia, and a lignite from Texas, U.S.A.) has been studied for samples in both the acid and cation forms. A preliminary study at temperatures above 300 °C was made on the brown coal. The carboxyl groups of coals in the acid form decompose to give one mole of carbon dioxide for each equivalent of carboxyl content. Cation-form coals yield more carbon dioxide on pyrolysis than can be accounted for by the carboxyl groups present. Water is evolved in proportion to the carbon dioxide evolved from both acid- and cation-form coals, but the ratios differ. Findings have been interpreted as indicating that some other oxygen-containing group is associated with the carboxyl group. In the case of the acid-form coal this group decomposes to give water. When the carboxyl group is in the cation form, decomposition of the associated groups gives carbon dioxide as well as water. Phenolic groups appear to be stable, at least to 300 °C.     

Characterization of chars made of solvent extracted coals

Thermal extraction of a sub-bituminous coal (Roto south) using 1-methylnaphthalene solvent has produced ash-free coals successfully. The extracted (EC) and residual coal (RC) as well as its parent coal (PC) were pyrolyzed at 300–900 °C and then the carbonized products were characterized. The extracted coal (EC) contained lower molecular weight components than PC and RC, showing much higher fuel ratio after the pyrolysis. EC is expected to be advantageous over PC and RC when applied to coal gasification and reforming, because EC is readily decomposed and volatized. The heating value of EC chars (7,610–8,120 kcal/kg) was independent of the pyrolysis temperature and was higher than those of PC and RC chars, especially for the chars carbonized below 600 °C. The oxygen content of PC chars at T≤600 °C was mostly at least twice that of EC/RC chars, pointing out the difference in the chemical composition. 13CNMR and FT-IR spectra revealed the release of aliphatic hydrocarbons and reactive functional groups with increasing temperature, in agreement with ultimate/proximate analysis results.     

Evolution of organic oxygen bonds during pyrolysis of coal

Five Czech coals of different rank (70.2, 71.5, 74.5, 75.0 and 83.3 wt% carbon) were heated at different temperatures in the range 250–850 °C and the contents of oxygen-containing functional groups (OH, COOH, CO, OCH₃) were determined in the coals and carbonization residues. The coals and residues were subjected to pyrolysis chromatography at 740 °C and the yields of phenol produced were measured. The dependence of the oxygen functions and the phenol yield on temperature and coal rank is discussed. In the coals studied, the most resistant oxygen groups appear to be hydroxyl and methoxyl; carboxyl and carbonyl oxygen are eliminated below 550 °C.     

Ambient-pressure thermogravimetric characterization of four different coals and their chars

Ambient-pressure thermogravimetric characterization of four different coals and their chars was performed to obtain fundamental information on pyrolysis and coal and char reactivity for these materials. Using a Perkin-Elmer TGS-1 thermobalance, weight loss as a function of temperature was systematically determined for each coal heated in helium at 40 and 160 °C/min under various experimental conditions, and for its derived char heated in air over a temperature range of 20 to 1000 °C. The results indicate that the temperature of maximum rate of devolatilization increases with increasing heating rate for all four coals. However, heating rate does not have a significant effect on the ultimate yield of total volatiles upon heating in helium to 1000 °C; furthermore, coupled with previous data⁹ for identical coal samples, this conclusion extends over a wide range of heating rate from 0.7 to 1.5 × 10⁴ °C/s. Using the temperature of maximum rate of devolatilization as an indication of relative reactivity, the devolatilization reactivity differences among the four coals tested that were suggested by this criterion are not large. For combustion in air, the overall coal/char reactivity sequence as determined by comparison of sample ignition temperature is: N. Dakota lignite coal ≈ Montana lignite coal > North Dakota lignite char > III. No. 6 bituminous coal ≈ Pittsburgh Seam bituminous coal > Montana lignite char > III. No. 6 bituminous char > Pittsburgh Seam bituminous char. The reactivity differences are significantly larger than those for devolatilization. The reactivity results obtained suggest that coal type appears to be the most important determinant of coal and char reactivity in air. The weight loss data were fitted to a distributed-activation-energy model for coal pyrolysis; the kinetic parameters so computed are consistent with the view that coal pyrolysis involves numerous parallel first-order organic decomposition reactions.     

Studies on artificial coal. 2. Thermogravimetry, plastometry and differential thermal analysis

The artificial coals (AC) or coal models were prepared from sawdust, lignite and cork by the method of hydrothermal carbonification. The specific techniques used to study their pyrolytic behaviour were thermogravimetry, Gieseler plastometry and differential thermal analysis (DTA). The principal conclusions drawn are:

1. (1) The initial stage of pyrolysis of AC can be considered to be governed by an overall reaction of first order, as in the case of natural coals.

2. (2) All caking types of AC showed quite low softening points (75–210 °C), the values for sawdust AC being lower than those for the cork AC. The latter showed fairly sharp peak values of the temperature of maximum fluidity, but the sawdust AC showed only a range of temperatures of maximum fluidity, the upper limit being less than the maximum fluidity temperature of the cork AC. However the temperatures of resolidification were consistently high, namely 425 °C and above, thus bringing in a significant point of similarity with natural caking coals.

3. (3) The value of maximum fluidity was very high for one AC sample, exceeding 27000 dial div./min. Such a high value is rare for natural caking coals. With the latter, no correlation could be observed between the caking properties and maximum fluidity. Hence the role played by the stability of the plastic mass may be the more dominant factor in the caking process.

4. (4) From the comparison of the softening temperatures and the temperatures of active decomposition of the AC, it is inferred that the process of initial softening of coal may be physical in nature.

5. (5) The AC indicated an overwhelming exothermicity in their primary pyrolysis, thus behaving like the natural coals studied under similar experimental conditions.