James Weifu Lee, A. C. Buchanan III, Barbara R. Evans, and Michelle Kidder

Abstract This chapter reports a technological concept for producing a partially oxygenated biochar material that possesses enhanced cation-exchanging property by reaction of a biochar source with one or more oxygenating compounds in such a manner that the biochar material homogeneously acquires oxygen-containing cation-exchanging groups. This concept is based on our recent experimental finding that the O:C atomic ratio in biochar material correlates with its cation-exchange capacity. The technology is directed at biochar compositions and soil formulations containing the partially oxygenated biochar materials for soil amendment and carbon sequestration.

1 Introduction

Photosynthesis captures more carbon dioxide (CO2) from the atmosphere than any other process on Earth. Each year, land-based green plants capture about 403 giga — tons (Gt) of CO2 (equivalent to 110 Gt C y-1) from the atmosphere into biomass [1]. However, only about й of the primary photosynthesis product (110 Gt C y_1) becomes plant tissue (biomass), the other half is respired directly from photosyn­thetic sugars; furthermore, since biomass is not a stable form of carbon material,

J. W. Lee (*)

Formerly, Oak Ridge National Laboratory, P. O. Box 2008, Oak Ridge, TN 37831, USA

Department of Chemistry and Biochemistry, Old Dominion University,

Physical Sciences Building 3100B, Norfolk, VA 23529, USA

Whiting School of Engineering, Johns Hopkins University,

118 Latrobe Hall, Baltimore, MD 21218, USA e-mail: JWLee@odu. edu; JLee349@jhu. edu

A. C. Buchanan III • B. R. Evans • M. Kidder

Oak Ridge National Laboratory, P. O. Box 2008, Oak Ridge, TN 37831, USA

J. W. Lee (ed.), Advanced Biofuels and Bioproducts, DOI 10.1007/978-1-4614-3348-4_4, 35

© Springer Science+Business Media New York 2013

a substantial portion of the biomass decomposes in a relatively short time to CO2. As a result, increased biomass production (i. e., by increased tree growth) is of limited utility for carbon sequestration since the resulting biomass soon returns the absorbed CO2.

Unlike untreated biomass, carbonized biomass (i. e., charcoal or “biochar”) contains carbon in a highly stabilized state, i. e., as elemental carbon. The inertness of elemental carbon results in its very slow decomposition to CO2 . Typically, at least several 100 years are necessary for the complete decomposition of biochar to CO2. Through a 14C labeling study, the mean residence time of pyrogenic carbon in soils has now been estimated in the range of millennia [2]. As a result, there is great interest in producing biochar as a means for mitigating atmospheric CO2 production. There is particular interest in incorporating produced biochar into soil (i. e., as a soil amendment) where the biochar functions both as a CO2 sequestrant and as a soil amendment [3].

Biochar production and incorporation into soil has been practiced since ancient times. Of particular relevance is the recent discovery of biochar particles in soils formed by pre-Columbian indigenous agriculturalists in Amazonia, i. e., the so-called Terra Preta soil [4].

The capacity of carbon sequestration by the application of biochar fertilizer is estimated to be quite significant. The amount of biochar materials that could be placed into soil could be as high as 10% by weight of the soil [5]. Accordingly, in the first 30-cm layer of US cropland soil alone, 40 Gt of carbon could be sequestered in the form of biochar particles. The worldwide capacity for storing biochar carbon in agricultural soils could exceed 400 Gt of carbon. A conversion as low as 8% of the annual terrestrial photosynthetic products (110 Gt C y_1) into stable biochar material would be sufficient to offset the entire amount (over 8 Gt C y-1) of CO. emitted into the atmosphere annually from the use of fossil fuels.

Significant amounts of biochar are currently being produced as a byproduct in biomass-to-biofuel production processes. The most common biomass-to-biofuel production processes include low temperature and high temperature pyrolysis (i. e., gasification) processes [6, 7). Pyrolysis operations generally entail combusting biomass in the substantial absence of oxygen. Biofuels commonly produced in low temperature pyrolysis operations include hydrogen, methane, and ethanol. Gasification processes are generally useful for producing syngas (i. e., H2 and CO).

An important property of biochar is its cation-exchanging ability. The cation­exchanging ability or lack thereof of a biochar is evident by the magnitude of its cation exchange capacity (CEC). It is known that biochar which has an increased CEC generally possesses a greater nutrient retention capability. These biochars with greater CEC generally possess a significant amount of hydrophilic oxygen-containing groups, such as phenolic and carboxylic groups, which impart the greater cation exchange ability [7] .

The CEC is defined as the amount of exchangeable cations (e. g., K+, Na+, NH4+, Mg2+, Ca2+, Fe3+, Al3+, Ni2+, and Zn2+) bound to a sample of soil. CEC is often expressed as centimoles (cmol) or millimoles (mmol) of total or specific cations per kilogram (kg) of soil. A substantial lack of a cation-exchanging property is generally considered to be reflected in a CEC of less than 50 mmol kg-1 . A moderate CEC is typically considered to be within the range of above 50 and at or less than 250 mmol kg-1. An atypical or exceptionally high CEC would be at least 250 mmol kg-1.

Though biochar is generally considered useful for CO2 sequestration, the types of biochar found in ancient soils or produced as an industrial byproduct are highly variable in their physical and property characteristics, e. g., chemical composition, porosity, charge density, and CEC. One of the most common production processes of biochar is the practice since ancient times of burning biomass in open pits. Such uncontrolled processes generally produce significant quantities of oxide gases of combustion, such as CO2 and CO, generally in amounts significantly greater than 20% by weight of the carbon content of the biochar source. In addition, the resulting biochar is highly nonuniform in composition, e. g., substantially nonoxygenated portions particularly in the interior portions of the biochar pit and moderately oxygenated portions at the outer peripheral portions of the biochar pit. Furthermore, the uncontrolled process generally results in significant batch-to-batch variability. Moreover, by the uncontrolled process, the characteristics of the resulting biochar are generally unpredictable and not capable of being adjusted or optimized.

Though biochar materials possessing moderate cation exchange capacities are known, such biochar compositions are not typical, and moreover, are found sporadi­cally and in unpredictable locations of the world. Therefore, there is a need for a method that produces oxygenated biochar compositions which have at least a moderate CEC, and more preferably, a CEC significantly higher than found in known soil deposits. Such biochar materials would have the advantage of more effectively retaining soil nutrients, and thus, functioning as superior fertilizing/soil amending materials as well as sequestering carbon.

In order to make such superior biochar materials readily available for widespread soil application, the biochar must be reproducibly manufactured with low batch-to — batch variation in one or more characteristics of the biochar (e. g., CEC, particle size, porosity, C:O ratio, and the like) and should be substantially uniform in its characteristics, such as oxygen-to-carbon ratio, CEC, and chemical composition. Further, the production method needs to able to be appropriately modified in order to obtain adjustment or optimization in one or more properties dependent on desired application and source feed stock. In the remainder of this chapter, we discuss several technology concepts to achieve these goals.