In our everyday lives, we use a wide range of materials such as plastics, detergents and cleaning agents, pharmaceuticals, and biofuels. A future sustainable economy can use either biomass or carbon dioxide from the atmosphere to produce these materials. The overall demand for the raw materials can be reduced by increasing the recycling rate.
An analysis of the demand and potential of biomass shows that third-generation biomass alone, i.e. the exclusive use of residual components from food production, is not sufficient to supply enough raw materials for a future chemical industry and for the production of biofuels. If development continues as in the past, a bio-based economy will also have to rely on first or second generation biomass. These are food components such as starch, sugar, and vegetable oil, or plants grown on land where also food could be produced, such as grasses and wood. It is therefore inevitable that bio-based compounds will compete for the same land area as food production.
For a bio-economy, all the necessary technologies have been demonstrated at least on a pilot-plant scale. A bio-economy would therefore be technically and economically feasible with existing technology. Already today, various products are obtained from biomass in an economically viable way.
The direct capture of carbon dioxide from the atmosphere requires new technologies for which several proposals are currently being further developed. Due to the low concentration of carbon dioxide in the atmosphere significant technical challenges have to be overcome. As a consequence, the economic feasibility on a larger scale still has to be proven. The subsequent conversion of the separated carbon dioxide into chemicals requires a considerable amount of energy, but already works with production volumes of 5000 tonnes per year.
For a carbon-dioxide economy, no fertile land area would have to be used, since carbon-dioxide capture can be implemented anywhere, including for exampe deserts or permafrost regions. The required energy can also be generated sustainably, for example through photovoltaics in deserts.
Establishing a carbon-dioxide economy, however, will currently not reduce greenhouse-gas emissions as long as we burn fossil resources for energy use. Only after the energy transition could a carbon-dioxide economy make a sustainable contribution to reducing carbon dioxide in the atmosphere.
The repeatedly proposed use of algae either to capture carbon dioxide from the atmosphere or to produce chemicals from carbon dioxide turns out to be less efficient than comparable chemical routes. Algae are therefore not a suitable solution.
Both a bio-economy and a carbon-dioxide economy are in principle technically feasible and allow the material cycles of a circular economy to be closed.
However, as long as there are undernourished people, bio-economy is ethically questionable because of its competition with food production. Without a change in our behavior with respect to animal-based nutrition and number of children the problem of world hunger cannot be overcome. So we decide with our behavior whether we can realize an easily accessible bio-economy or whether we are forced for ethical
In addition to food and energy, we need numerous materials and substances that we use in everyday life. Materials such as plastics are used to create things that enable us to lead a pleasant life. We use substances to produce detergents, cleaning agents, care products, pharmaceuticals, and biofuels. A major constituent of this material base is carbon. If we want to build a sustainable economy, this carbon can ultimately only be obtained from the carbon dioxide in the air. Either technical methods are used to separate the carbon dioxide from the air, for example by absorption, which is then converted into the desired substances in appropriate chemical processes. Large amounts of energy are required for processing the carbon dioxide in this way. Alternatively, sunlight can be used as an energy source to extract carbon dioxide from the atmosphere by cultivating plants that do just that. The energy required to operate the processes that convert biomass into the desired products is much less than for the routes that are based directly on carbon dioxide. With both alternatives, the consumption of land area or energy is so high that the aim should be to keep the demand for new materials as low as possible by achieving the highest possible recycling rate. Obviously, however, not all materials can be recycled, such as biofuels, detergents and care products, because the 'end products' of the respective application are not sufficiently concentrated to be properly processed. For an overall sustainable economy, it is to be expected that the two main routes - products produced using carbon dioxide from the atmosphere or from biomass - will be used together in an interconnected material network, depending on what is optimal for a product and an application. It can also be expected that recycling will be significantly increased, so that the supply of fresh materials into this cycle will be minimized. One form of recycling that is technically feasible but has not yet been implemented is the general incineration of waste and the conversion of the resulting carbon dioxide into the desired products using sustainable energy.
It has already been demonstrated at the "George Olah Renewable Methanol Plant" of Carbon Recycling International on Iceland that the use of carbon dioxide for the production of chemical compounds is technically feasible and economically not completely inefficient. With the help of sustainable electricity and carbon dioxide from underground sources, which is released when geothermal energy is used, methanol is produced which can be used as a raw material in the chemical industry to manufacture a variety of products. In principle, any source of carbon dioxide, including combustion, would allow comparable methanol production.
However, the extraction of carbon dioxide from the atmosphere is critical. Until now various processes based on the principles of absorption and adsorption have been proposed and tested on a smaller scale. The carbon dioxide is washed out of the air either with the aid of a liquid or a solid. Its concentration in the air is comparatively low, around 0.04%, and around 100 tons of the washing liquid or solid are needed to obtain one ton of carbon dioxide. Although these are then purified again and reused, it is still relatively large quantities that have to be treated per ton of carbon dioxide, with the risk that correspondingly high costs will result. On a larger, at least semi-technical scale, however, the cost-effectiveness of these processes, for which many theoretical studies with very different technologies have already been published, has yet to be demonstrated.
If, on the other hand, the carbon dioxide in the atmosphere is converted with the help of sunlight and plants into compounds that can then be used as starting materials for chemical processes, a multitude of options arise. The biomass used can be subdivided according to the generation of the technology used. Biomass of the first generation is defined as raw materials such as grains, sugar beet and sugar cane, which could also be used for food production. Second generation biomass are plants that cannot be used directly as food, but that grow on land that could also be used for food production. Examples are fast-growing grasses such as miscanthus or trees and shrubs such as poplars and willows, which also grow rapidly. Trees are usually grown in short-rotation plantations, where the trees grow very densely and are deforested after only 3 to 10 years, as this provides the highest annual yield. Finally, third generation biomass is biomass that does not compete with food production, such as straw and other waste materials from food production.
The components that can be produced from the biomass are initially sugar and vegetable oil as intermediate substances. These can then serve as raw materials for further chemical steps to produce the substances and materials that are finally used, such as plastics. The diversity of the starting materials and the processes that result gives rise to a multitude of options, for which Figure 1 shows the land area demand that would be required if the respective route alone were to provide all raw materials for the chemical industry. As already mentioned, it is to be expected that a mixture of different routes will ultimately be realized, because different processes will be optimal for different products. Since the shares of the different routes in a future bioeconomy are not foreseeable today, only the individual routes can be compared here. In addition, different end products are indicated for some raw materials. On the one hand, the substances can be used completely, for example as sugar, from which new substances can then be built up. Alternatively, the sugar can be fermented to ethanol, releasing the resulting carbon dioxide into the atmosphere. Accordingly, less ethanol is obtained than sugar has been consumed. This leads to a larger required land area, since part of the original biomass is lost towards the final product. In a further step, ethanol can be converted to ethylene, where water is eliminated, i.e. a smaller mass of product is obtained than the raw material. On the other hand, ethylene is a so-called direct drop-in, i.e. ethylene can be fed directly into existing chemical processes, since ethylene is already the starting material for many processes today, but is produced from crude oil. This means that the starting materials sugar, starch, which can easily be converted into sugar, and cellulose, from which sugar can only be produced with great effort and losses, each have three possible routes, which differ in how much of the biomass ultimately ends up in the product. The route via ethylene would mean the smallest changes in the existing subsequent chemical steps, but is associated with the greatest mass losses.
Figure 1: Per capita area that is required in a bioeconomy to generate different intermediate products from various feedstocks, if the total chemicals demand is to be covered by the respective route. The production routes are sorted by the generation of biomass used. Lighter green bars refer to routes where individual steps have not yet been technically implemented on a larger scale.
Figure 1 shows the different routes sorted by generation of biomass. The total land area shown, which is represented as area per capita, is the area of arable land that will be available to everyone in 2050 if the world population develops according to the UN's high population projection. In addition, the land area used in the land-use balances for the production of bio-based substances and materials is shown as a red dashed line. It defines the goal below which a future bioeconomy would have to remain, so that the overall balances work out as shown. The bars for each of the routes presented result from the fact that two limiting cases were considered with respect to agricultural productivity. The smallest land area requirement results from taking into account a country that has a very high area-specific productivity and is at the same time a large producer of the respective initial biomass. The highest land area requirement is based on average global productivity. The width of each bar thus characterizes the area of land required in countries where cultivation is worthwhile. For second-generation biomass, national or global average productivity values are unfortunately not available, so that the bar width was derived from publications on typical productivity.
It becomes obvious that the use of third-generation biomass, which does not compete with food production, requires by far the largest land area. In addition, about 2/3 of straw, one of the main contributions to third-generation biomass, is best ploughed back into the field to form humus and thus ensure long-term sustainability. If this is taken into account, it becomes clear that more than the sustainably available quantity of by-products of food production will be required to meet all human needs for bio-based products. So solely third-generation biomass is not sufficient to satisfy people's needs. This means that competition with food production for arable land with the production of bio-based products is inevitable, at least if mankind does not change its behavior with regard to the number of children and nutritional habits.
Comparing first and second generation biomass, it is striking that there is a multitude of options available that remain below the target of the red dashed line. In addition, it becomes clear that first and second generation biomass differ only slightly, if at all, in their land area requirements. Since the processes based on first-generation biomass are typically simpler and already established on a large scale, it is probably better to use first-generation biomass. This also has the advantage that proteins, which in first-generation biomass are relatively readily accessible components, can be used for human nutrition, as they are usually not used to produce the main products and materials in the chemical industry. This would therefore create a positive synergy between bio-economy and food production.
If now the options for a sustainable economy are compared, a carbon-dioxide economy does not require any arable land, but large amounts of energy and the separation of carbon dioxide from the atmosphere still involves significant technological and economic risks. A suitable carbon-dioxide capture process has not yet been operated on a large scale under realistic conditions. The bioeconomy, on the other hand, has already been technically proven on a large scale, but requires arable land that competes with food production. Considering that not only a bio-based chemical industry as shown in Figure 1 has to be fed with biomass, but also production of bio-combustibles required for various processes, it is clear that the total area required is 2 to 3 times larger than shown in Figure 1. Bioeconomy is therefore ethically questionable as long as people are starving. As shown, the nutritional situation depends to a large extent on human behavior with regard to the number of children and eating habits. Therefore we decide with our behavior whether we can realize an easily accessible bio-economy or whether we are forced for ethical reasons to aim for a carbon-dioxide economy with all the still unsolved economic and technical challenges. Ethics and feasible technology are therefore directly linked.
Both options, the bio-economy and the carbon-dioxide economy, are feasible in principle and offer a great opportunity to establish a circular economy, in which the material cycles are really closed.
Finally, two further aspects are to be addressed here which are frequently meintioned in this context. Establishing a carbon-dioxide economy will not currently reduce greenhouse-gas emissions as long as we are still burning fossil resources for energy use. This is because the fossil raw materials - gas, oil, and coal - are converted into electricity with an efficiency of only 40%. In order to convert carbon dioxide back into comparable liquid or solid substances, however, due to the physical principle of energy conservation, 100% of sustainably produced electricity is needed to produce the hydrogen required for this, and even more, taking into account technical inefficiencies. It is therefore at least 2.5 times more efficient to feed sustainably produced electricity into the grid and to use the fossil raw materials saved than to extract carbon dioxide from the atmosphere and convert it into products. Only after the sustainable energy transition, i.e. when fossil fuels are no longer used, could a carbon-dioxide economy make a sustainable contribution to reducing carbon dioxide in the atmosphere.
Then, the repeatedly proposed use of algae either to capture carbon dioxide from the atmosphere or to produce chemicals from carbon dioxide shall be examined. In the literature and the media it is often stated that algae are much more efficient in converting carbon dioxide, for example into vegetable oils, than common land-based plants. However, it is often not mentioned that this is only the case if concentrated carbon dioxide is made available to the algae. If the algae are correctly compared with chemical processes for the conversion of carbon dioxide, such as the above-mentioned production of methanol, algae are much less efficient because they use at least 30% of the carbon dioxide to build their cell structure and typically at least 10% of the supplied carbon dioxide escapes into the atmosphere during the process. Thus, in the overall process, the carbon dioxide is only converted into products with an efficiency of less than 70%, while the chemical process has an efficiency of more than 90%. Thus, algae cannot be used sensibly to convert carbon dioxide. Alternatively, algae are proposed to bind carbon dioxide from the atmosphere. This process also has a lower efficiency than land-based plants, taking into account the energy losses during operation of the process. Growing algae instead of normal plants is again a loss of efficiency. Only if algae are cultivated in regions where land-based plants do not flourish, such as savannas and deserts, there would be no competition for land area. However, it is precisely under such conditions that the process losses are particularly considerable because the algae cannot tolerate too much light intensity and high temperatures and, depending on the process, water evaporates to a considerable extent. In all these options, algae are therefore less efficient than the alternatives.