Kazimierz Tomków

Formation of porous structures in activated brown-coal chars using O2, CO2 and H2O as activating agents

Two brown coals, xylitic and earthy, carbonized at 1173 K were activated with water vapour, carbon dioxide and oxygen, each producing a different distribution of porosity. In the xylitic coke, activated in the range of burn-offs from 1 to 70%, the action of water vapour results in the development of pores of all dimensions. At the highest burn-off the product has an effective surface area of 920 m2 g−1 and a total sorptive pore volume of 0.83 cm3 g−1, 33% of which is in micropores. Carbon dioxide creates, from the xylitic coke at the burn-off of 70%, a highly microporous adsorbent with about the same surface area (890 m2 g−1) as the corresponding water-vapour activated product. The pore volume of the carbon dioxide sample is lower (0.49 cm3 g−1) but these contain 63% of micropores, which amounts to a contribution of 92% of these pores to the effective total surface area. The activation of the xylitic coke with oxygen leads to a high development of porosity at low burn-offs, but becomes ineffective on continuation of the process to medium and high burn-offs. This is thought to be due to a blocking of the entrances of the micropores by surface oxygen complexes formed on the surface of the coke. Oxygen gives, at a high burn-off, a product with the lowest total adsorptive volume (0.45 cm3 g−1) and surface area (650 m2 g−1). All the activated products obtained from the xylitic coke can be regarded, when effective surface areas are considered, as microporous adsorbents. With the earthy coke a total adsorptive pore volume (consisting mainly of wide mesopores) is developed which is higher than with the corresponding xylitic coke, but this result is difficult to reproduce, because the earthy coke samples are easily influenced by temperature in the process of activation, especially that by oxygen.

Development of porosity in brown-coal chars on activation with carbon dioxide

Abstract Two petrographic types of Tertiary brown coals, xylitic and earthy, were carbonized, and activated with carbon dioxide between 1123 and 1273 K. The development of porosity in the activated chars was studied by adsorption of benzene and carbon dioxide at 298 K and by mercury porosimetry. The type of brown coal exerts a dominant influence on the properties of the activated chars. The xylitic brown-coal, when compared with the earthy brown-coal, yields products with a higher pore volume and better sorptive properties. Activated chars from the xylitic brown-coal reach a surface area of 800 m 2 g −1 , contained principally in micropores and very narrow mesopores (radius below 3.0 nm). Dimensions of pores in the activated chars from the earthy brown-coal are less uniform, the mesopores are broader (an important part of them has a radius between 5.0 and 100.0 nm), and micropores are present to a smaller extent; the surface area of these products is between 200 and 350 m 2 g −1 . Activated chars from both types of brown coals have a well developed system of macropores.

Activation of brown-coal chars with oxygen

Abstract Semicokes and cokes prepared respectively at 773 and 1173 K from brown-coals, xylitic and earthy, from Polish coal seams, were activated with gaseous oxygen (10% oxygen and 90% argon) in a thermogravimetric apparatus to different burn-offs. With increasing temperature of oxygen activation a constant decrease of the sum of micropores and mesopores is observed, but probably as a result of chemisorption of oxygen the micropore volume passes through a maximum at 663 K. There is a strong influence of the temperature of carbonization of the char on the formation of porosity in the products of oxygen activation: activated cokes have better adsorptive properties than activated semicokes. The highest value of surface areas (benzene adsorption) are, for semicokes and cokes respectively, 520 and 700 m2 g−1. These differences can be attributed to the uniform microporosity in the non-activated coke as distinct from the wide range of the micropore diameters in the non-activated semicoke, and also to the lack of ultramicropores in the former sample. The earthy type of brown coal yields products with a less developed porosity than the corresponding products from the xylitic coal. For the xylitic semicoke as well as for the coke, after continuing the process of activation to burn-offs higher than 50%, a lowering of adsorptive properties is observed.

Influence of the oxygen content of low-rank coals on the development of porosity during carbonization

Abstract The development of porosity in the course of carbonization of a flame coal, original and pre-oxidized, was studied by means of the adsorption of benzene and carbon dioxide. The results were compared with corresponding data for cokes from a xylitic brown coal. The influence of coal oxygen content on the formation of coke porosity and its thermal dependence is discussed.

Capillary structure and reactivity of chars from brown coal humic acids containing Ca, Fe, Mn and Na

Abstract Salts of Ca, Fe, Mn and Na were added to humic acids extracted from brown coal. The influence of the added inorganic substances on the course of carbonization and gasification with H 2 O, CO 2 and 10% O 2 was studied.

Multi-stage activation of brown-coal chars with oxygen

Activation of xylitic brown-coal coke XBC 900 with water vapour and carbon dioxide, when modified by partial replacement of the basic activating agent with 10% oxygen at a lower temperature, results in products with an increased microporosity. Thus, oxygen as activating agent for xylitic coke develops, preferentially, micropores, and this property is more strongly pronounced for oxygen than for the carbon dioxide and water vapour. A drawback to the process of activation with oxygen, i.e. blockage of initially formed micropores by chemisorbed oxygen, can be eliminated by removal of the chemisorbed oxygen by heat treatment in argon (multi-stage oxygen activation). This increases the micropore volume of the xylitic brown-coal coke XBC 900 activated with oxygen to 70% total burnoff, from about 0.2 cm3 g−1 to almost 0.5 cm3 g−1. The increase of the total adsorptive volume (micropores and mesopores) of these samples is from 0.45 cm3 g−1 to over 0.6 cm3 g−1 and the surface area SBET in benzene increases from 650 m2 g−1 to over 1200 m2 g−1. These last values are close to the limiting conditions for 70% activation obtainable for this material. Temperature of carbonization of the brown-coal char has a strong effect on the possibility of pore development through further activation. Multi-stage oxygen activation of xylitic brown-coal semicoke XBC 500 produces a material with a smaller micropore volume and a lower surface area than that of xylitic brown-coal coke XBC 900 similarly activated.

Evaluation of Microporosity in Steam Activated Brown Coal Humic Acids Chars

Abstract Different approaches were tried to evaluate nitrogen sorption data on steam activated humic acids chars in terms of microporosity. Special attention was payed to CO 2 adsorption at temperatures from 195 K to 298 K.

Adsorption of Carbon Dioxide, Benzene, Nitrogen and Argon by Microporous Carbons: Interpretation of Isotherms and Enthalpies of Adsorption and Immersion

Abstract Adsorption isotherms, together with microcalorimetrie data comprising differential enthalpies of adsorption of carbon dioxide at 311 K and of nitrogen and argon at 77 K, as well as enthalpies of immersion in benzene and other organic liquids differing in shape and molecular size, were used to evaluate the microporosity of a series of carbon, characterized by systematic changes of their capillary structure (steam activated brown coal humic acids chars). In chars of low burn-offs the entire microporosity is contained in primary micropores. With increasing burn-offs secondary microporosity appears and the molecular sieve properties of the chars diminish. The mechanism of carbon dioxide adsorption at room temperature seems to differ from that of other adsorbates: for burn-offs between 10 and 75% the micropore volumes calculated from carbon dioxide adsorption are almost constant (0.21 – 0.25 cm 3 .g −1 ), while respective values from adsorption of benzene are 0.13 – 0.50 cm 3 .g −l , and from nitrogen, also argon, 0.14 – 0.53 cm 3 .g −l.