Could we have enough biomass to feed the world?
There is no doubt that the production of food is more important than the production of energy and materials. Prior to the green revolution, the production of food was the first priority for human beings for several thousand years. For example, the former Soviet Union and United States investigated the production of single-cell proteins from crude oil. When the food supplies are abundant, the prices of food decrease greatly and the prices of crude oil soars, the production of liquid transportation fuels (i.e., ethanol and biodiesel) from food sources is in practice, especially the United States and Brazil. However, it is discouraged or even prohibited to expand the production capacity of first generation biofuels in most countries, such as China and the European Union, mainly due to the concern of food security.
How to meet increasing food needs is becoming a global challenge [10, 43, 11, 44]. Because the production of 2.5 billion tons of food has utilized ~30% arable lands and ~70% freshwater withdrawals, it is difficult to greatly increase agricultural lands and increase water withdrawals. Therefore, a group of scientists  suggests a variety of solutions to address food security: (i) closing yield gaps on underperforming lands, (ii) increasing agricultural resource efficiency, and (iii) increasing food delivery by shifting diets and reducing food waste, while halting agricultural land expansion. For example, several studies find that about one-third to one half of food is never consumed [45, 46]. For example, developing countries usually lose more than 40% of food postharvest or during processing, while industrialized countries often lose more than 40% of food at the retail or consumer levels . On the other hand, some plant biologists, big plant companies, and policy makers promoted the genetically modified (GM) crops as a future solution . However, long-term impacts of GM cereals on human health are not clear and their wide application is in heated debates [48-50, 47, 51].
Here, a paradigm-shifting solution is proposed – enzymatic biotransformation of cellulose to synthetic starch in next generation cellulosic biorefineries . Via biomass fractionating , a variety of multiple products could be produced from major lignocellulosic components: cellulose, hemicellulose, and lignin (Fig. 2). I demonstrate simultaneous enzymatic biotransformation and fermentation (SEBF) that can transform cellulosic materials to starch, ethanol, and single-cell protein in one vessel in the presence of cascade enzymes isolated from bacterium, fungus, and plant sources, and a typical ethanol-producing yeast. Our data showed that up to 30% of the anhydroglucose units in cellulose were converted to synthetic starch; the remaining units were hydrolyzed to glucose suitable for yeast fermentation that can produce ethanol. This cellulose to starch biotransformation could be scaled up by increasing the stability of two key enzymes – cellobiose phosphorylase and starch phosphorylase because this process does not involve any labile coenzymes (e.g., CoA and NAD[P]); no glucose is wasted; neither energy nor costly reagents is added. The stability of both cellobiose phosphorylase  and starch phosphorylase  can be enhanced greatly by protein engineering. Also, starch production from cellulose mediated by enzymes rather than GM organisms may avoid potential negative impacts of GM cereals and prevent bioethics debate.
Figure 2. Next generation biorefineries based on fractionated lignocellulosic components for the production of multiple products for meeting different needs from biofuels to biochemicals to food/feed.
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Cellulose resource is approximately 40 times the starch produced by cultivated crops. Every ton of cereals harvested is usually accompanied by the production of at least two tons of cellulose-rich crop residues, most of which are not utilized . In addition to the use of agricultural and forest residues (e.g., straws, corn stover, and wood dust), growing dedicated bioenergy crops could greatly increase biomass availability. Dedicated bioenergy crops usually have much higher productivities (e.g., approximately 40–80 ton/ha/y [57-59]), have much higher water utilization efficiency, require less energy-related inputs, such as fertilizers, insecticides, and herbicides, tolerate harsher environments, and could not require annual seedling, compared to cultivated starch-rich crops. Dedicated bioenergy crops can grow on low-quality arable land.
The Department of Energy (DOE) of the United States has summarized three distinct goals associated with potential bioenergy crops: (i) maximizing the total amount of biomass produced per hectare per year, (ii) producing sustainable biomass with minimal inputs (e.g., pesticides, fertilizers, seeds, and harvesting), and (iii) maximizing the amount of biofuels that can be produced per unit of biomass . A yield of ca. 50 dry tons per hectare per year may be considered as a reasonable target in an area with adequate rainfall and good soil , which is about 15–25 times average yields of cultivated cereals. In addition to well-studies bioenergy crops, such as switchgrass, poplar, and Miscathanus [61, 62, 59], this study recommends two new promising bioenergy plants – bamboo and common reed. Although both of them have been cultivated and harvested in some areas, they are often ignored by most. Bamboos are giant woody, tree-like, perennial evergreen grasses . They have been cultivated in East Asia and South East Asia . Phyllostachys pubescens (Moso bamboo) grows in a subtropical monsoon climate but it can withstand as low as −20°C in winter. It can be cultivated in marginal lands, such as mountain valley, foot of mountain, and gentle slope. The bamboo productivity is highly dependent on soil, water, and climate conditions. The highest average yearly biomass productivity during 10-year plantation is approximately 76 tons of dry culms/ha/y, which can be easily collected . Phragmites australis (common reed) is a widespread perennial grass that grows in wetlands or near inland water ways . Although it is harvested for thatched roofs, ropes, baskets, and pulping feedstock, the common reed is more typically considered an invasive weed due to its vigorous growth and difficulty of eradication. Common reed could be used as a bioenergy crept due to three unique features: (i) high biomass productivity (e.g., ca. 45–71 tons/ha/y), (ii) low inputs needed for planting, such as water, fertilizers, and pesticides, and (iii) removal of phosphorus- and nitrogen-containing pollutants in water ways .
Intensive irrigation for cultivating dedicated bioenergy crops could not be recommended. Since it consumes approximately three and one orders of magnitude water based on energy content more than the production of oil from traditional oil drilling and advanced oil recovery, respectively , the production of biomass is believed to increase usage of freshwater [65, 66]. This issue has raised concerns about the increase in water stress, particularly in countries that are already facing water shortage . Therefore, cultivating future dedicated bioenergy crops must take in account water consumption.
In a word, the cost-effective transformation of nonfood cellulose to starch could not only revolutionize agriculture by promoting the cultivation of plants chosen for rapid growth rather than those optimized for starch production [68-70] but also could maintain biodiversity and minimize agriculture's environmental footprint . Also, wide implementation of cellulosic biorefineries would decrease postharvest food loss, especially for developing countries, so to increase overall food/feed availability .
What powertrain and fuel will become the dominant transport means in the future?
A number of scenarios (Fig. 1) can and could bridge between renewable primary energy and transportation energy demand through four powertrain systems: (i) ICEs and/or hybrid electric vehicles (HEVs) that burn liquid biofuels and compressed methane [73, 19], (ii) BEVs that run on electricity stored in rechargeable batteries, where electricity can be generated from sun radiation, tide, geothermal, wind, and nuclear energy , (iii) hydrogen FCVs that run on stored hydrogen through proton exchange membrane (PEM) fuel cells and electric motor , and (iv) sugar fuel cell vehicles (SFCVs) that run on stored sugar as a high-density hydrogen carrier based on FCVs . Powertrain systems for vehicles must meet all of the following criteria: high energy storage capacity in a small container, high power output, economically competitive fuel, affordable vehicle, fast charging or refilling of the fuel, and high safety .
Table 3 compares the gravimetric energy densities of liquid fuels, stored hydrogen, rechargeable batteries, and capacitors, as well as kinetic energy output densities on wheels through different powertrain systems. The energy storage densities in a decreasing order are diesel, gasoline, butanol, ethanol, methanol, sugar, stored hydrogen, rechargeable batteries, and capacitors. Liquid gasoline and diesel plus their respective ICEs have kinetic energy output densities of 6.50 and 8.32 MJ/kg, respectively. When ICE's energy efficiencies are increased through hybrid electric systems, HEV-gas, and HEV-diesel can drive farther. Conventional hydrogen storage means have lower energy storage densities from 5.0 to 9.3 MJ/kg or even lower, resulting in shorter driving distance of FCVs compared to vehicles based on ICEs if the same weight fuel tank is used. Therefore, the DOE strongly encourages to develop novel high-density hydrogen storage means and provides the H-prize cash award . Rechargeable batteries have at least one order magnitude lower energy storage densities than liquid fuels and stored hydrogen (Table 3). As a result, BEVs have very short driving distances. The energy densities of capacitors are very low, limiting its application in the transport sector.
Table 3. Gravimetric energy densities of stored energies and kinetic energy released through different powertrain systems 
|Name||Gravimetric energy density (MJ/kg)||Kinetic energy output (MJ/kg)||Powertrain (efficiency, %)|
|H2 without container||143||NA||NA|
|8% H2 mass including container||11.4||5.13||PEMFC/Motor (45%)|
|Cryo-compressed H2 including container||9.3||4.19||PEMFC/Motor (45%)|
|Compressed H2 (700 bars) including container||6.0||2.70||PEMFC/Motor (45%)|
|4% H2 mass including container||5.7||2.57||PEMFC/Motor (45%)|
|Compressed H2 (350 bars) including container||5.0||2.25||PEMFC/Motor (45%)|
|Lithium ion rechargeable battery||0.56||0.381||BEV (68%)|
|NiMnH rechargeable battery||0.36||0.245||BEV (68%)|
|Lead acid rechargeable battery||0.14||0.095||BEV (68%)|
Battery electric vehicles will not be a dominant future transport means. For example, the International Energy Agency and several studies predict that BEVs will play a minor role in the future [75, 74]. Rechargeable lithium (Li) batteries have energy densities of approximately 150 Wh/kg (i.e., 0.56 MJ/kg), resulting in very short driving distances for BEVs [76, 77]. If the energy densities of lithium batteries were increased by 5–10-fold [78, 79], other issues, such as safety, recharging time, and lifetime, could still prohibit their wide use in personal vehicles. In reality, future energy densities of rechargeable lithium batteries are expected to increase by twofold in next decades [76, 77] rather than 5–10 times by considering the configuration of Li batteries and its combustion energy (i.e., 43.1 MJ/kg lithium) . Although developing lithium-air batteries are expected to have very high energy densities but the regeneration of lithium oxidize to lithium by electricity is energy intensive. Therefore, metal-air batteries are not suitable in the transport sector.
In addition to low energy densities of Li batteries, BEVs have other weaknesses. First, the recharging cycles and lifetime of high-density lithium batteries is approximately 1000 time and 2–3 years, respectively. Both are much shorter than requirement of the major car components lasting at least 10 years. (Think of lithium ion batteries in cellphones and laptops.) Second, lithium ion batteries are still costly for vehicles although its production costs could be decreased by several-fold. It is not realistic to believe that battery costs would be drastically decreased following Moore's Law because it is impossible to exponentially both decrease material consumption in batteries and increase battery performance according to the basic physical limits of materials. Third, Li batteries require a long recharging time. Although ultra-fast charging batteries have been developed , these capacitor-like batteries are made at the cost of decreasing energy storage densities . Fourth, a huge infrastructure investment could be needed to upgrade the electrical grid, install sockets for fast recharge, and build power stations . Fifth, disposing and recycling a large number of used rechargeable batteries could be another environmental challenge . Sixth, the energy density loss rates of rechargeable batteries depend on temperature; for example, standard loss rates per year are 6% at 0°C, 20% at 25°C, and 35% at 40°C . Seventh, whether there is enough low-cost lithium for BEVs is not a certain thing. Goodenough, a pioneer of lithium batteries, pointed out that the principal challenges facing the development of rechargeable batteries for BEVs are cost, safety, energy density (voltage × capacity), rate of charge/discharge, and service life . Due to BEVs' unique features such as cleanness and quietness, BEVs will still be popular in some special markets, for example, in golf courts. In a word, a complete switch to all battery electric cars is utterly unrealistic  by considering the above problems and the likelihood that better competing technologies will appear and mature.
This study suggests another paradigm-shifting solution for the future vehicles – SFCVs. Based on FCVs, carbohydrate (shorthand, CH2O) is suggested to be a high-density hydrogen carrier so that its use could address hydrogen storage, distribution, and safety issues [83, 40, 84, 85]. In the hypothetical SFCV, an on-board biotransformer containing numerous thermoenzymes and (biomimetic) co-enzymes that can achieve the reaction of CH2O + H2O 2H2 + CO2 [86, 87]. Because enzymes are 100% selective, work under moderate reaction conditions, and generate highly pure hydrogen, carbohydrates have a gravimetric density of 8.33 H2 mass% for the carbohydrate/water slurry [25, 16]. During the past several years, we have increased enzymatic hydrogen generation rates to approximately 160 mmole H2/L/h by nearly 800-fold (in preparation for publication). We anticipate to increase reaction rates by another 30-fold within next several years so that the on-board biotransformer will be small enough to store in a SFCV [16, 40].
In a word, HEVs based on ICEs are believed to be a short- and middle-term solution before FCVs . SFCVs could be a good solution to address the problems of FCVs from hydrogen production, storage, distribution, infrastructure, and safety. SFCVs could have several advantages over BEVs: much higher energy storage densities, faster refilling rates, better safety, and less environmental burdens [19, 40].
Could we have enough extra biomass source to drive vehicles and feed the world?
As shown in Table 1, two irreplaceable applications of biomass resource are food/feed (1.33 TW) and wood for materials (1.28 TW). Compared to all terrestrial biomass resource (65 TW), the current biomass utilization efficiency is 6.32% and it is expected that biomass utilization efficiency will be increased to up to 12.3% . This value is also partially supported by the DOE and USUA's a billion ton report .Two liquid fuels used for land transportation are gasoline (1.2 TW) and middle distillates (1.79 TW). Since the global average ICE-gas and ICE-diesel have fuel-to-wheel efficiencies of approximately 14% and 23%, respectively , the global kinetic energy output on wheels is 0.58 TW.
When we increase biomass utilization efficiency from 6.32% now to 12.3% in 2050, this study provides quantitative predictions for the worst, best, and most likely scenarios for the year 2050 based on different assumptions. In the worst scenarios, food/feed needs, wood consumption, and biomass for burning could increase by 100%, 50%, and 50%, respectively. At the same time, total biomass resource could be constant. Therefore, the remaining biomass source that could be collected and utilized will be 1.17 TW. The land transportation energy in terms of kinetic energy could increase to 0.85 TW from 0.58 TW based on an annual growth rate of 1%.
In the best scenarios, food/feed needs and wood consumption could increase by 50% and 20%, respectively. Slow growth in wood consumption could be attributed to less use of papers in affluent countries and better recycling. Biomass for burning could be decreased to half due to an increase in burning efficiency in developing countries . At the same time, total biomass resource could increase to 94.9 TW at an annual growth rate of 1% due to (i) rising CO2 levels in the atmosphere that fertilizes plant productivity [19, 38] and (ii) dedicated high-yield bioenergy crops . Therefore, the biomass resource will be 7.52 TW. The land transportation energy in terms of kinetic energy could increase to 0.70 TW based on an annual growth rate of 0.5%.
In the most likely scenarios, food/feed needs, wood consumption, and biomass for burning could increase by 70%, 35%, and 0%, respectively. Food/feed production from cultivated cereals could increase to 1.66 TW; the remaining food/feed need (0.60 TW) could be supplemented with synthetic starch made from biorefineries. At the same time, total biomass resource could increase to 78.6 TW at an annual growth rate of 0.5%. Therefore, the remaining biomass resource will be 4.84 TW. The land transportation energy in terms of kinetic energy could increase to 0.76 TW based on an annual growth rate of 0.7%.
The last uncertainty is the biomass-to-wheel (BTW) efficiency of future land transport means. The worst scenario is based on current ICE-gas (ethanol) system (BTW = 7%), while the best could be SFCVs (BTW = 27%). Several transitional powertrains could be HEV-gas (BTW = 20.7%), HEV-diesel (BTW = 24.8%), and FCV (BTW = 22%). In the 2050 market, it is likely that the transport sector could constitute different transportation means so that an average BTW efficiencies could range from 11% to 20%.
Table 4 presents the analysis for the future biomass and biofuels roles. In the worst scenarios, biomass could play a significant role in replacing approximately 10–25% transportation fuel need. On the contrast, in the best scenarios, biomass could be sufficient to meet all land transportation energy need plus a large surplus. In the most likely scenarios, biofuels made from biomass could replace at least 50% to nearly 100% land transportation fuel need. The above analysis suggests that (i) we must increase powertrain system efficiency so to decrease biomass consumption, (ii) we must develop next generation biorefineries because it not only produce biofuels but also could produce food/feed and biochemicals, and (iii) we must utilize agricultural and forest residuals and then grow dedicated water-saving bioenergy crops by spatial segregation of food/feed and energy-producing areas by continuing producing food on established and productive agricultural land while growing dedicated energy crops on marginal land .
Table 4. Scenarios of the roles of biomass for the production of food/feed, wood, heating, and land transportation fuels in 2050
|Worst 2050||Best 2050||Highly possible 2050|
|Name||Power (TW)||Assumption||Name||Power (TW)||Assumption||Name||Power (TW)||Assumption|
|Food/Feed||2.66||100% gain||Food/Feed||2.00||50% gain||Food/Feed||2.26||70% gain|
|Food/Feed crops||2.66||100% gain||Food/Feed crops||1.86||40% gain||Food/Feed crops||1.66||35% gain in crop|
|New food/Feed||0.00||NA||New food/Feed||0.13|| ||New food/Feed||0.60|| |
|Wood||1.92||50% gain||Wood||1.54||20% gain||Wood||1.66||30% gain|
|Burning||2.25||50% gain||Burning||0.75||50% decrease||Burning||1.50||No change|
|Total land biomass||65||No change||Total land biomass||94.87||1% gain/year||Total biomass resource||78.56||0.5% gain/year|
|Available biomass||1.17||12.3% biomass use||Available biomass||7.52||12.3% biomass use||Available biomass||4.84||12.3% biomass use|
|Land kinetic energy||0.85||1% gain/year||Land kinetic energy||0.70||0.5% gain/year||Land transportation use||0.76||0.7% gain/year|
|Scenario||Land fuel replacement||Scenario||Land fuel replacement||Scenario||Land fuel replacement|
|S1: ICE-gas (ethanol)||9.6%||BTW = 7%||S4: HEV-gas (ethanol)||156%||BTW = 15%||S7: ICE/HEV-gas (ethanol)||54.6%||BTW = 11%|
|S2: HEV-gas (ethanol)||20.7%||BTW = 15%||S5: FCV||229%||BTW = 22%||S8: HEV-gas (ethanol)||74.4%||BTW = 15%|
|S3: HEV-diesel (ethanol)||24.8%||BTW = 18%||S6: SFCV||280%||BTW = 27%||S9: SFCV/SFC/HEV||99.2%||BTW = 20%|
Could we surpass natural photosynthesis?
This study suggests developing next generation biorefineries by integrating high-efficiency solar cells or other electricity-generating systems, water electrolysis, with biological CO2 fixation mediated by cell-free synthetic cascade enzymes (Fig. 3). This cell-free biosystem is believed to work based on the design principles of synthetic biology, knowledge in the literature, and thermodynamics analysis [40, 90]. This hypothetical system could have numerous advantages. First, solar cells have much broader light adsorption spectrum and higher efficiencies than plant pigments. Also, the efficiency of solar cells, unlike plants, does not change in response to insolation variation. Also, it is easy to concentrate nonpoint insolation to a point energy – electricity. Second, hydrogen generated by water electrolysis at daytime can be stored for a few hours so that it can be consumed at a constant synthesis rate for the biological CO2 fixation process at night. Therefore, it is easy to regulate and match changed-rate electricity generation and constant-rate biosynthesis process. Third, the products of artificial photosynthesis are carefully chosen: water-insoluble amylose, volatile alcohols, or water-insoluble fatty alcohols. So the product separation costs could be minimal. Fourth, ultra-high energy efficiency from hydrogen or electricity and CO2 to chemical energy could be achieved, much better than natural processes mediated by living organisms that dissipate energy by respiration [91-93]. Table 5 presents the comparison between natural photosynthesis and artificial photosynthesis. Validation experiments and practical application of these systems will require worldwide collaborative efforts from biologists, chemists, electrochemists, and engineers  (Note: It is important to fix high concentration CO2 generated from power stations rather than to capture atmospheric CO2 because the latter requires extremely high energy inputs, resulting in economical infeasibility ).
Table 5. Comparison matrix between photoautotrophic organisms and artificial photosynthesis based on cell-free cascade enzyme factories plus photovoltaic. Modified from Ref. 
| ||Natural photosynthesis||Artificial photosynthesis|
|Solar to chemical efficiency||Theoretical ~4–10%||Theoretical, 51%|
|Practical: ~0.2–2%||Practical: >10%|
|Sunlight spectrum (e.g., 350–2350 nm)||Only 48% (only 400–700 nm)||Whole spectrum adsorption by solar cells (63%, theoretical; 42% highest; 18%, commercial)|
|Light-harvesting efficiency||Low under nonoptimal conditions||Nearly constant independent of insolation|
|Chemical synthesis pathway||Unmatched reaction rates between light reactions and dark reactions||Constant synthesis rate from stored hydrogen|
|Chemical synthesis efficiency||12–18 ATP + 6NAD(P)H equivalent consumed per hexose synthesis for utilizing low level CO2||ATP-neutral synthesis pathway by using high levels CO2 from power stations|
|Respiration||~50% loss||Not applicable|
|Complicated regulation between primary and secondary metabolisms||A (small) fraction of chemical energy could flux to product||99+% of energy could flux to product because of insulation of biocatalyst synthesis (e.g., cell growth) from product formation|
|Product separation costs||Separate intracellular product from aqueous cells||Generate water-insoluble product (e.g., amylose or fatty alcohols) or volatile products|
|Large water consumption||500 + kg of water needed for 1 kg of carbohydrate produced||0.6 kg water needed for 1 kg of carbohydrate synthesized|
|Contamination||Use of weedicides||Not applicable|
|Operation time||Daytime only||24/7|
|Land resource||Limited due to the combined requirements of temperature, water and insolation||Nearly everywhere by separating solar harvesting systems from product synthesis systems|
|Waste generated||Nonpoint pollutants||Point pollutants from fermenters|
Figure 3. Next generation biorefineries based on artificial photosynthesis that can fix CO2 and hydrogen to starch or other compounds. The enzymes involved in the synthetic enzymatic pathway responsible for CO2 fixation are suggested in the reference , which are different from all natural CO2 fixation pathways . Also, the enzymes responsible for product formation are subject to change.
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In a word, next generation biorefineries based on artificial photosynthesis would not only bridge the current and future primary energy utilization systems aimed at facilitating electricity and hydrogen storage but also address such sustainability challenges such as renewable biofuel and chemical production, CO2 utilization, and fresh water conservation . Its large-scale implementation would foster the switch from fossil fuel-based resources to renewable bioresources.