Since this technology concept of smokeless biomass pyrolysis with biochar soil application is still in its early developing stage, much more research and development work is needed before this approach could be considered for practical implementation. To achieve the mission, a number of technical issues still need to be addressed. First, as pointed out previously, the process technology must be smokeless (emission — free, clean, and efficient) to avoid negative impact on air quality with such a large (GtC)-scale operation. Therefore, it is essential to fully develop a smokeless bio­mass-pyrolysis process to achieve the mission. As illustrated in Fig. 4, one of the possible productive ways to achieve the smokeless (emission-free, clean, and efficient) feature is by converting the pyrolysis syngas “smoke” into clean energy, such as liquid biofuels. Currently, there are a number of Fischer-Tropsch process­ing technologies that might be helpful for conversion of biomass-derived syngas into advanced (drop-in-ready) liquid biofuels, such as biodiesel, to replace petroleum — based transportation fuels. However, significant R&D efforts are needed to fully develop and demonstrate this approach since the currently existing refinery technologies cannot cost-effectively convert biomass-derived syngas and/or bio-oils into liquid transportation biofuels.

Second, there are significant R&D opportunities to improve biofuels products in relation to proper utilization of the biomass-pyrolysis-derived syngas and bio-oils that currently cannot be used as a liquid transportation fuel. For example, one of the problems here is that the biofuel fractions (syngas and bio-oils) from biomass pyrolysis are not in a desired form that could be used as a transportation fuel for cars. In other words, although the amounts and the heating value of the syngas and bio-oils from the envisioned biomass pyrolysis approach could potentially be very large (with heating value equivalent to that of about 3,250 million barrels of crude oil for the world), it is not clear how much they could really replace crude oil because the efficiency for conversion of the syngas and bio-oils into the advanced liquid biofuels has not been fully established. Additional refinery technology is needed to convert the biomass-derived syngas and/or bio-oils into certain desirable advanced liquid biofuels such as biodiesel for use in cars in order to replace the petroleum-based transportation fuels. It is possible to catalytically convert the biomass-derived syngas into liquid transportation fuels through the Fischer-Tropsch synthesis of hydrocar­bons [21-23]. It is also possible to convert the liquid bio-oils by different refinery processes, such as catalytic processing into biodiesel for use as a transportation fuel. Therefore, it is probably worthwhile to explore the use of a proper refinery process, such as the Fischer-Tropsch process, to couple with a continuous biomass pyrolysis for production of advanced liquid biofuels and biochar. However, since most require expensive catalysts, other viable options (such as using bio-oils as a heating oil and noncatalytic fuel production) remain to be examined to determine trade-off between energy efficiency and costs. Significant research efforts are needed to develop inno­vative technologies, such as catalytic bio-oils-hydroprocessing and/or an updated Fischer-Tropsch process, to convert the syngas into advanced (drop-in-ready) liquid biofuels to replace petroleum-based transportation fuels. This also reflects the need of developing tools for optimizing the entire process in relation to feed­stock, energy, greenhouse gases, and economics.

Third, significantly more research efforts are needed on the aspects of biochar production and biochar soil applications. Biochar typically is a spectrum of sub­stances produced from biomass pyrolysis. In order for biochar to serve as an effective soil amendment and carbon sequestration agent, the biochar product must possess certain required properties and quality standards, such as its cation exchange capacity and stability. More research is needed to further optimize the biochar cation exchange capacity [24] while still retaining its carbon stability. It has also been reported that biochar occasionally shows inhibitory effects on plant growth, especially, when biochar soil application exceeds about 5-10% by weight [25-27]. So far, very little is known about the true identity of the biochar inhibitory factors. If biochar were to be globally used as a soil amendment and carbon sequestration agent at GtC scales, the release of potentially toxic compounds into soil and associated hydro­logic systems might have unpredictable consequences in agroecosystems. These characteristics of biochar, including both its beneficial features (e. g., cation exchange capacity) and possible harmful factors need to be systematically studied, since they are directly related to its application impacts (benefils and risks) on soil and the environment. Rigorous biochar-soil studies at large scales across the world are needed to systematically assess the effects of biochar applications on soil fertility, including plant growth, on soil carbon storage (sequestration) and on the associated soil and hydrological ecosystems. Additional work is needed also on how to employ this approach in helping to utilize various waste biomass materials, including (but not limited to) dead leaves, waste woods, and various agriculture stovers, such as cornstovers and rice straws to produce both biochar and biofuels in a distributed manner across the world. It is also worthwhile to explore beneficial utilization of certain sewage solid waste, such as, on what fraction of sewage sludge could be added to waste biomass to produce a higher nutrient dense slow-release biochar and safely recycle the valuable nutrients currently lost in waste processing. With these studies and added knowledge, it should be possible to produce “fil-for-purpose” biochars to address specific soil and environmental constraints and maximize the benefits of biochar soil application at the national and international scales.

Fourth, systematic lifecycle impact-assessment on energy, carbon, land, water, air, and environmental health, including toxicology and ecology studies, must be carefully conducted to fully evaluate the potential benefits and possible risks in rela­tion to biomass pyrolysis and biochar application as soil amendment for global carbon sequestration at GtC scales.

Furthermore, the maximum capacity of carbon sequestration through biochar soil amendment in world agricultural soils (1,411 million hectares) is estimated to be about 428 GtC, assuming maximally 10% biochar C by soil weight for the first 30-cm soil layer alone. To verify this potential capacity and demonstrate its feasibility, soil pot studies and field trials of biochar applications in relation to soil fertility and carbon sequestration are needed in all of the world regions and different soil types. For the immediate future, biochar should be used to revitalize barren degraded land.

This will improve the world’s capacity for growing biomass thus naturally removing more CO2 from the atmosphere.

Another question to answer is whether it is possible to provide a global carbon “thermostat” mechanism to control the atmospheric CO2 concentrations by creating large reservoirs underground (and/or above ground) for any biochar not used for soil restoration. Since the capacity of biochar storage reservoirs could be so large (limit­less), the envisioned photosynthetic biomass production and biochar-producing biomass-pyrolysis approach (Figs. 1 and 2) could be used for many years to reduce the atmospheric CO2 concentrations to any desired levels if the world population is mobilized to implement the approach. This is different from the application of bio­char as a soil amendment where biochar particles are mixed with soil particles in such a diluted manner (such as 10% by soil weight) that recovering of biochar mate­rials from the mixed soils would be very difficult. The biochar materials in reser­voirs are preferably in a pure and concentrated form so that they could be readily retrieved at any time when needed for use of its energy by combustion. Therefore, global use of biochar reservoirs in a regulated manner could provide a global carbon “thermostat” mechanism to control global warming (climate change) in a desirable manner as well.

As a conclusion, this smokeless biomass-pyrolysis “carbon-negative” energy — production approach merits a major program support for serious research and devel­opment worldwide. With further research and development, this approach could provide more efficient and cleaner biomass pyrolysis technology for producing bio­fuels and biochar from biomass as a significant arsenal to help achieve indepen­dence from fossil energy and to control climate change for a sustainable future on Earth.