As illustrated in Figs. 1 and 2 , photosynthesis captures more CO2 from the atmo­sphere than any other process does on Earth. Each year, land-based green plants capture about 440 gigatons (Gt) CO2 (equivalent to 120 Gt C y-1) from the atmo­sphere into biomass [16]. However, biomass is not a stable form of carbon material with nearly all returning to the atmosphere in a relatively short time as CO2. Because of respiration and biomass decomposition, there is nearly equal amount of CO2 (about 120 GtC y-1) released from the terrestrial biomass system back into the atmo­sphere each year [17]. As a result, using biomass for carbon sequestration is limited. Any technology that could significantly prolong the lifetime of biomass materials would be helpful to global carbon sequestration. The approach of smokeless biomass pyrolysis can provide such a possible capability to convert the otherwise unstable biomass into biofuels, and, more importantly, biochar which is suitable for use as a soil amendment and serve as a semipermanent carbon-sequestration agent in soils/ subsoil earth layers for hundreds and perhaps thousands of years [4].

Biomass pyrolysis is a process in which biomass, such as waste woods and/or crop residues (e. g., cornstover), is heated to about 400°C in the absence of oxygen and converted to biofuels (bio-oils, syngas) and biochar—a stable form of solid black carbon (C) material (Fig. 3). Although its detailed thermochemical reactions are quite complex, the biomass-pyrolysis process can be described by the following general equation:

Biomass(e. g., lignin cellulose) ^ biochar + H2O + bio — oils + syngas

The yield and characteristics of biochar produced varies widely depending on the feedstock properties and pyrolysis conditions, including temperature, heating rate, pressure, moisture, and vapor-phase residence time. Typically, biochar contains mainly carbon (C) with certain amounts of hydrogen (H), oxygen (O), and nitrogen (N) atoms, plus ash. As reported in one of our previous studies [4], a typical compo­sition of biochar on an ash — and nitrogen-free basis can be 82% C, 3.4% H, and 14.6% O. Biochar has been produced throughout recorded history in energy-wasteful earthen pits, kilns, and steel drums releasing smoke. Advances in technology allow the organic volatiles (bio-oils) and syngas (CO, CO, , and H, , etc.) from biomass

More fabric, paper,
wooden furniture,
houses, and buildings

pyrolysis to be used as a biofuel for clean energy production. Typically, through a 400°C slow-pyrolysis process, about 50% of the dry biomass C (carbon) can be converted into biochar while the remaining 50% C in the biofuel portion provides part of its energy to sustain the process.

Terrestria Biomass

Biochar Geologic Storage

As Carbon Energy Reserves

(Capacity: Limitless)

Fig. 2 The global carbon cycle and envisioned “carbon-negative” biomass-pyrolysis energy tech­nology concept for biofuel and biochar production, and carbon dioxide capture and sequestration (reproduced from ref. [5])

In perspective of the global carbon cycle, as shown in Fig. 1, this biochar-producing biomass-pyrolysis approach essentially employs the existing natural green-plant photosynthesis on the planet as the first step to capture CO2 from the atmosphere; then, the use of a pyrolysis process converts biomass materials primarily into bio­fuel and char—a complex but stable form of solid carbon material that can enhance plant growth when used as soil amendment. The net result is the removal of CO2 from the atmosphere through the process of capturing CO2 from the atmosphere and placing it into soils and/or subsoil earth layers as a stable carbon (biochar) while producing significant amount of biofuel energy through biomass pyrolysis. Therefore, this is a “carbon negative” energy production approach.

The world’s annual unused waste biomass such as crop stovers is about 6.6 Gt of dry biomass [18], which is equivalent to about 3.3 GtC y-1 since dry biomass typically contains C as 50% of its total mass. If this amount of biomass (3.3 GtC y-1) is processed through controlled low-temperature pyrolysis assuming 50% conversion of biomass C to stable biochar C and 35% of the biomass energy to biofuels, it could produce biochar (1.65 GtC y-1) and biofuels (with heating value equivalent to 3,250 million barrels of crude oil) to help control global warming and achieve energy independence from fossil fuel. By using 1.65 GtC y-1 of biochar as a beneficial soil amendment, it would offset the 8.5 GtC y-1 of fossil fuel CO2 emissions by 19%. Therefore, the envisioned biochar-producing biomass-pyrolysis approach (Figs. 1 and 2) should be considered as an option to mitigate the problem of global green­house-gas emissions.

The capacity of carbon sequestration by the application of biochar fertilizer in soils could be quite significant since the technology could potentially be applied in many land areas, including croplands, grasslands, and also a fraction of forest lands. The maximum capacity of carbon sequestration through biochar soil amendment in croplands alone is estimated to be about 428 GtC for the world. This capacity is estimated according to: (a) the maximal amount of biochar carbon that could be cumulatively placed into soil while still beneficial to soil environment and plant growth; and (b) the arable land area that the technology could potentially be applied through biochar agricultural practice.

Using charcoal collected from a wildfire, Gundale and DeLuca recently showed that the amount of charcoal that can be applied can be as much as 10% of the weight of soil to increase plant Koeleria macrantha biomass growth without negative effect [19]. The composition of a biochar material depends on its source biomass material and pyrolysis conditions. Typically, biochar material produced from low-temperature (about 400°C) pyrolysis contains about 70% (w) of its mass as the stable carbon (C) and the remainder as ash content, oxides, and residual degradable carbon (such as bio-oil residue). The density of bulk soil is typically about 1.3 tons m-3. With this preliminary knowledge, we calculated that the maximum theoretical biochar seques­tration capacity is about 303.8 ton C per hectare (1 ha=2.47 acres; 123 ton C per acre) in a 30-cm soil layer alone. From the size of the world’s arable land (1,411 million hectares), the worldwide potential capacity for storing biochar carbon in agricultural soils was estimated to be 428 GtC (Table 1). This estimate (428 GtC), which is somewhat higher than that (224 GtC) estimated by Lehman et al. [20] ,

probably represents an upper limit value. The maximal amounts of biochar carbon that could be cumulatively placed into soil while still beneficial to soil environment and plant growth are probably dependent on a number of factors, including the specific biochar properties, topography, soil type, weather, and plant species. Worldwide biochar soil field tests are needed to obtain more accurate information on the capacity of biochar carbon sequestration in soils.