2009, Volume 12, Issue 23+24
Obtaining and Chemical Activation of Biomass Chars to Increase Their Capacity for CO2 Capture
1 National Institute for Research and Development for Cryogenics and Isotopic Technologies - ICIT Rm. Valcea, Uzinei Street no. 4, P.O. Box Raureni 7, 240050, Ramnicu Valcea, Romania
2 Research Station for Fruit Growing Valcea, Calea lui Traian, no. 464, 240263, Rm. Valcea, Romania
3 University of Piteşti, Research Center for Advanced Materials
Because of worldwide increasing environmental problems and sticter regulations set by governments in both industrialized and developing countries, the demand for activated carbon will continue to increase. Accordingly, in this paper, a novel thermal process of producing high-yield activated carbons from waste biomass chars is presented. Rape seed oil cakes and walnut shells as agricultural by-products are employed as raw material in this process. It used a mixture consisting of 50% rape seed oil cakes and 50% walnut shells.
Raw materials, both rape seed oil cakes and walnut shells undergo a series of pretreatments and posttreatments before forming the final products - activated carbons. Pretreatments included pyrolysis in a laboratory reactor, in which high yield carbons were obtained, and then followed by high temperature carbonization process at atmospheric pressure with nitrogen surroundings. The activated carbons were obtained by activation in temperature range 350-750°C. They were investigated to determine their adsorption properties (adsorption capacity, adsorption/desorption rate, and adsorption selectivity) and porosity(specific surface area, pore volume, and pore size distribution), as well as the effects of activation temperature and processing time. The increase in activation time resulted in a continuous steady rise of the mesopore area and volume, while the micropores and total pore area and volume reach a maximum at 3 h. The surface areas go through a maximum with increasing solid yields. The activated carbons were investigated regarding with CO2 adsorption capacities. The CO2 capture results did not show a linear relationship with the surface area. The sample with highest CO2 adsorption capacity (64.5mg CO2/g-adsorbent) was the carbon activated at 700OC for 2 h., whose surface area was only 620m2/g. Similarly, the carbon presenting the highest surface area (1070m2/g, 750°C for 3h) has a CO2 capacity of only 42 mg CO2/g-adsorbent. This is probably due to a relationship between microporosity and CO2 physisorption processes, only certain size pores being effective for CO2 adsorption.
Several surface treatment methods, including ammonium (NH3) heat treatment and aqueous monoethanolamine (MEA) impregnation, were used to modify the surface properties of the activated carbons in an attempt to increase their CO2 capture capacity at higher temperatures. The influence of temperature and type of chemical reagents on the porosity development was investigated and discussed. The surface treatment methods investigated change the porous structure and surface chemistry of carbon, and therefore affect their CO2 capacities. NH3 was found more effective than (MEA) as a chemical reagent under identical conditions in terms of both porosity development and yields of the activated carbons. The NH3 treatment increases the surface area of the activated samples, especially at lower temperatures (600°C).The chemical impregnation with aqueous monoethanolamine (MEA) results in a decrease of the surface area of the activated carbon, probably due to pore blockage and surface coverage by (MEA). Both the NH3 treatment and aqueous monoethanolamine impregnation can increase the CO2 capture capacity of the activated carbons at higher adsorption temperature, due to the introduction of alkaline nitrogen groups on the surface of carbons, that are selectivly to CO2 adsorption.
Biomass char, pore structure, functional group, CO2 adsorption.
Tag search Biomass char pore structure functional group CO2 adsorption