Activated carbon can be characterized by dozens of tests, many of which are highly application specific. For the purposes of flue gas sorption the primary considerations are; BET surface area, pore volume, pore size distribution, ash and trace metals content. These properties combine to indicate how effectively a carbon will attract and hold a sorbate, such as mercury (Hg) in its internal structure. Surface area indicates how much space the carbon has to produce an attractive force against a sorbate. Pore volume indicates how much of a sorbate could be contained while pore size distribution indicates how many of the pores which develop the surface area can be utilized. Ash content and trace metals indicate what chemical compounds are available in the carbon to exert an attractive, electrostatic force against a sorbate.
An activated carbon is defined, initially by the properties of the parent feedstock it was produced from; lignite based carbons have a certain structure, hard coal carbons have another, and biomass derived carbons have properties defined by the type of biomass used. While lignite, the principal feedstock used to produce gas phase mercury sorbents, is fairly uniform in structure and composition, there is a tremendous variety of different species of biomass available for use in the production of activated carbon. The variety of biomass allows selection from a wide range of species which yields an almost infinite array of characteristics to produce the optimal traits for mercury removal for a given customer.
Using careful analysis of available materials and the desired qualities of a sorbent product, enviraPAC™ uses blended feedstocks to produce an engineered carbon with an optimal pore size distribution and ash composition that will outperform the lignite/coal based.
Along with blending biomass species to tune the performance of a carbon product, enviraPAC™ can add one or more of several chemical modifying agents during processing to tune the carbon for the capture of specific pollutants, including mercury. Elemental impregnations of sulfur, bromine, and chlorine in a carbon are common for sorbent manufacture. These methods are effective, sometimes essential, but frequently costly. If the selection of a high sulfur or high chlorine feed stock is not enough, we have several alternatives to elemental impregnation using proprietary, low cost chemical additions to produce highly active mercury sorbents.
Chemical activation can be used to modify the pore structure of a carbon during activation, increasing meso-porosity by changing how carbon bonds form in activation. While both coal and biomass based carbons can be modified in this way, biomass is better suited and will produces better results because the chemical modification can begin before initial carbonization of the biomass, resulting in a more profound and effective modification to the material.