Preliminary Evaluation of Coal- and Biomass-Derived Chars as Beneficial Soil Amendments.

Increased development and use of gasification technologies in the power generation sector using coal and biomass mixtures could result in greater quantities of by-product char, and increase the need for innovative options for its use and disposal.  Partial combustion of organic matter produces synthesis gas (syngas), which can be used in gas turbines as well as converted into liquid diesel fuel.  The process creates less total CO₂ than is produced by total combustion in contemporary coal-fired power plants, and the by-product char may serve as a useful amendment to agricultural and “green roof” soils, simultaneously serving as a long-term repository of carbon.  In this study, we used a soil quality test kit, developed by the U.S. Department of Agriculture, to compare the effects of char amendments derived from Mississippi lignite gasification, coal-derived activated carbon, coconut-shell derived activated carbon, and several hardwood charcoal sources on overall soil quality.  The kit is assembled from readily available materials and measures numerous physical, chemical and biological components in soil, such as water infiltration, bulk density, aggregate stability, soil slaking, pH, soil respiration, and other parameters.  Additional measures of microbial activity (plate counts, community level physiological profiling, etc.) were used to evaluate this component of soil health.  Coal-derived chars contained higher concentrations of potentially toxic heavy metals than biomass-derived chars, but the operationally-defined bioaccessible fraction of this pool appeared small in tested samples.  Addition of biochar material resulted in the general improvement of soil quality indicators such as water-holding and cation exchange capacities.  The potential advantages and disadvantages of the use of these materials as agricultural and green roof soil amendments are discussed.