Use these tools to design and evaluate industrial processes and supply chains that rely on renewable bio- and non-bioresources.
Interest in sustainability and sustainable development is driven by three factors: an increasing global population; the increasing per-capita prosperity of the human population, accompanied by increasing consumption; and the potential impacts on the environment that those two trends imply. According to the U.S. Census Bureau, the world’s population has increased from 3 billion in 1960 to over 7 billion in 2016. Worldwide per-capita gross domestic product (GDP) has risen from about $8,200 in 2003 to almost $14,000 in 2011. While increasing prosperity is desirable, the potential impact on the environment is not. The challenge to humanity — how to maintain and increase human prosperity without overtaxing the environment — is a challenge that engineers, in particular chemical engineers, are uniquely qualified to tackle.
There is a subtle difference between sustainability and sustainable development. Rather than add to the substantial body of literature on the subject that already exists, we have limited this discussion to proposing working concepts that we use throughout this article.
Sustainability can be defined as the ability to use resources without completely using them up or destroying them. In this context, sustainability is essentially about maintaining biophysical environmental conditions that can support civilized human existence and social development along whatever ideological or political lines society deems appropriate.
Sustainable development was defined by the World Commission on Environment and Development (in what is known as the Brundtland Report) as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs” (1). Sustainable development adds an element of social equity, i.e., development that benefits all. While this is laudable, it involves discussions of policy and politics that go beyond engineering. Thus, here we focus on what engineers can contribute to the sustainable and efficient use of regional resources.
Renewable regional resources
Renewable resources are resources replenished by natural “income.” Solar radiation is the main source of this natural income. Resources based on solar radiation are direct solar irradiation that may be harnessed by solar thermal, shallow geothermal, or photovoltaic (PV) technologies, as well as resources that are induced by solar radiation, such as wind, hydropower, and biomass.
A common feature of these resources is that they require area, either for their harvest (biomass) or for their conversion (solar irradiation, wind, and hydropower). Because the surface area of the Earth that collects the natural income of solar radiation is finite, these resources — although available for a (practically) infinite time — inherently compete for this limited basic resource of planetary surface area. The per-area yield of these renewable resources varies widely, ranging from 1.7–7.5 kWh/m²-yr for biomass to 250–600 kWh/m²-yr for solar heat. In addition to solar irradiation, wave and tidal forces can also be harnessed to provide a renewable (energy) resource, although they play a minor role. A characteristic feature of all renewable resources is their dependence on spatial context.
Non-bioresources. Renewable non-bioresources are essentially carbon-free energy sources. With the exception of deep geothermal energy, all other non-bioresources provide energy dependent on time. This time dependence may either be seasonal (e.g., for hydropower), cyclic (solar heat, tidal energy) or intermittent (wind). Aligning demand and supply of energy, therefore, becomes more challenging as these renewable non-bioresources become more prevalent. Incorporating them into existing energy systems and industrial processes requires systemic solutions, including storage, smart integration of different sources, and demand management.
Deep geothermal energy is, strictly speaking, not a renewable resource, because it utilizes a natural stock. However, it is abundant and carbon-free, so it is often put in the same category as truly renewable resources. Unlike solar-based resources, geothermal energy does not compete for area but rather is a classical point resource.
Bioresources. Photosynthesis, which harnesses solar energy to generate bioresources, is the only natural pathway to transform solar radiation to material containing carbon and hydrogen. The energy-transformation efficiency of this pathway is relatively unattractive. Maximum conversion rates of solar radiation to energy contained in bioresources are between 4.6% for C3 plants (e.g., rice and barley) and 6% for C4 plants (e.g., maize and sugar cane) (2). In comparison, transformation efficiencies for producing heat by concentrated solar energy are 60% or more.
Because they have low transformation efficiencies, bioresources require considerably more area to provide the same amount of energy than other renewable sources. Their advantage, however, is their versatility. As the only material manifestation of natural income, they can serve as the basis for a wide variety of products and services across the value chain, from polymers to chemicals to materials of construction and bulk products like paper and textiles.
Although non-bioresources have higher transformation efficiencies and much lower area requirements, an important advantage of bioresources is that they can be stored. This makes them an important element of any future energy system by helping to balance supply and demand, as well as providing products and services such as carbon-neutral fuel for long-distance transport (e.g., by airplanes and ships).
Because of this versatility, however, there is competition for these bioresources. In particular, the food sector is a major consumer of agricultural products, while the pulp and paper industry and the energy sector compete for forestry products. As a result, the impact on natural biomass generation has become significant. The global net primary production (NPP), i.e., the rate at which sunlight is converted into useful chemical energy as measured by the mass of carbon fixed by photosynthesis per year, is about 105 Gt/yr. Almost one-quarter of that is already being used by human society.
This highly competitive market for bioresources requires the chemical process industries (CPI) and the energy sector to focus on improving the efficiency of the processes that turn biomass into useful products and services. In particular, it will be necessary for them to utilize byproducts and wastes from agriculture, forestry, and other industrial value chains (e.g., the food sector).
Spatial context and actors
Renewable resources reflect their spatial context. Yield is a function of climate and geography. Thus, every region has its particular mix of renewable resource potentials. For instance, the Great Plains in the U.S. have abundant cropland and considerable wind and solar radiation; Siberia has great wood-biomass potential; Spain boasts high wind and solar radiation potential but lower biomass yields.
Economic and cultural parameters also influence the potential to convert renewable resources into useful products and services. The lack of electricity distribution grids will hamper the use of solar and wind power, while highly concentrated animal husbandry combined with good gas and electricity grids may allow the utilization of low-grade resources like manure.
The physical characteristics of renewable bioresources affect the logistics of their use. Because bioresources contain a significant amount of moisture and they often have low densities (mass-to-volume ratios), volume rather than weight limits their transport. Thus, economical transport distances vary widely from one type of bioresource to another. Heat, either from solar collectors or from combined heat and power (CHP) generation, must be utilized locally, within a few kilometers of the source. Low-grade bioresources, like manure and straw, may be used locally or regionally, at distances up to 20 km. Wood chips and low-grade forestry products may be transported economically on a regional scale up to 100 km. High-grade bioresources, such as crops and wood logs, compacted materials like pelletized straw and wood, and electricity, can be distributed on a continental or global scale. This differentiated spatial context of renewable resources is the basis for equally differentiated roles of the actors in the value chain, as discussed in the rest of this section.
Agriculture and forestry. Agriculture and forestry provide the most critical renewable bioresources for the food sector, the CPI, and the energy sector. Bioresources demand, by far, the largest share of land area for their generation compared with all other renewable...
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