energies Article Comparison of Novel Biochars and Steam Activated Carbon from Mixed Conifer Mill Residues Nathaniel Anderson 1, *, Hongmei Gu 2 and Richard Bergman 2   Citation: Anderson, N.; Gu, H.; Bergman, R. Comparison of Novel Biochars and Steam Activated Carbon from Mixed Conifer Mill Residues. Energies 2021, 14, 8472. https:// doi.org/10.3390/en14248472 Academic Editor: Mejdi Jeguirim Received: 6 November 2021 Accepted: 10 December 2021 Published: 15 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 United States Forest Service, Rocky Mountain Research Station, 800 East Beckwith Avenue, Missoula, MT 59801, USA 2 United States Forest Service, Forest Products Laboratory, 1 Gifford Pinchot Drive, Madison, WI 53726, USA; hongmei.gu@usda.gov (H.G.); richard.d.bergman@usda.gov (R.B.) * Correspondence: nathaniel.m.anderson@usda.gov Abstract: There is increasing demand in environmental remediation and other sectors for specialized sorbents made from renewable materials rather than hard coals and minerals. The proliferation of new pyrolysis technologies to produce bio-based energy, fuels, chemicals, and bioproducts from biomass has left significant gaps in our understanding of how the various carbonaceous materials produced by these systems respond to processes intended to improve their adsorption properties and commercial value. This study used conventional steam activation in an industrial rotary calciner to produce activated carbon (AC) from softwood biochars made by three novel pyrolysis systems. Steam was injected across four heating zones ranging from 816 ◦ C to 927 ◦ C during paired trials conducted at calciner retention times of 45 min and 60 min. The surface area of the three biochars increased from 2.0, 177.3, and 289.1 m 2 g −1 to 868.4, 1092.9, and 744.8 m 2 g −1 , respectively. AC iodine number ranged from 951 to 1218 mg g −1 , comparing favorably to commercial AC produced from bituminous coal and coconut shell. The results of this study can be used to operationalize steam activation as a post-processing treatment for biochar and to expand markets for biochar as a precursor in the manufacture of specialized industrial sorbents. Keywords: activated carbon; biochar; woody biomass; pyrolysis; steam activation; adsorbent 1. Introduction 1.1. Background Biochar is an anthropogenic pyrogenic carbonaceous material used as a soil amend- ment. It can be made from biomass using a wide range of methods, from wooden ricks, earth mound kilns, and small portable flame-cap kilns to large-scale industrial pyrolysis systems. In all cases, it is the product of the thermal decomposition of biomass in an inert or low oxygen atmosphere, which removes volatile compounds and leaves a high-carbon char. Depending on the circumstances, amending soils with biochar can improve soil properties and plant growth by enhancing nutrient retention, water holding capacity, organic matter content, bulk density, biological activity, and other soil characteristics and processes [1]. Other benefits of biochar, not directly related to improving soils for plant growth, include carbon sequestration and waste management as well as benefits from the renewable en- ergy and fuel products that are coproduced by some pyrolysis systems [2]. Compared to conventional charcoal manufacturing, which is often focused on making solid fuels for combustion, many of the advanced pyrolysis systems that produce biochar are designed to simultaneously meet soil, waste management, climate, and energy objectives [3]. A variety of recent review papers have described biomass feedstocks [4], pyrolysis of biomass [5,6], and biochar production and end use [7,8]. Given the diversity of feedstocks, conversion processes and postprocessing techniques for various end uses, biochar exhibits wide variation in its physical and chemical properties [9]. Table 1 summarizes some of the main characteristics of biochar according to standards established by the International Energies 2021, 14, 8472. https://doi.org/10.3390/en14248472 https://www.mdpi.com/journal/energies