Human beings continue to deplete raw materials at an alarming rate, and we are expending more time and energy to obtain many of the key materials and energy that we require as building blocks in the chemical processes that lead to products. The acquisition of these key dwindling resources also results in an increasing amount of environmental degradation. A large amount of the energy and materials that we use today come from nonrenewable
Green Chemistry and Engineering: A Practical Design Approach, By Concepcio´n Jimenez-Gonzalez and David J. C. Constable
Copyright2011 John Wiley & Sons, Inc.
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sources, which by definition are finite and will be depleted at some point. For example, there is a limited and finite amount of mineral ores on Earth, and they normally require large amounts of energy to be extracted, refined, and converted to the desired form for chemical processing. Once all the copper on Earth has been extracted, one must find ways to keep recycling it or find alternatives to its use.
Renewable resources, on the other hand, are derived from plants, animals, or ecosystems and can be regenerated through sustainable management. These resources, both energy and materials, have the potential to be replenished indefinitely if managed properly. You may recall that in Chapter 4 we explored renewability as one of the metrics for green chemistry and green engineering. We discussed how in the current chemical landscape relatively few chemicals available for routine commercial operations are derived from renewable sources.
Or, if they are from renewable sources, they are often associated with considerable life cycle impacts or there are significant numbers of trade-offs associated with their use.1 One example that we discussed was the current debate over the sustainability of producing and using corn-based ethanol as fuel. We emphasized that the key messages from that debate are that assessments of renewability have to be carried out from a life cycle perspective. We also noted that it is challenging to compare highly developed chemical or petrochemical processes that use nonrenewable feedstocks with processes that use potentially renewable feedstocks that are obtained using processing approaches that are comparatively immature or not as fully developed. Remember that the petrochemical industry has been in existence for over 150 years and has been driven toward ever greater processing efficiencies.
Therefore, if this is the current situation, why do we even bother to look into renewable materials and energy if the answer is not that simple and direct? At least part of the answer is that renewable materials and energy do offer one distinct possibility of breaking the linear production cycle that constrains our innovation and drives us away from achieving greater sustainability. Renewable resources offer the opportunity to operate production systems closer to the way that nature works, in a cyclical manner, where materials and energy can be utilized while minimizing waste (see Figure 22.1).
FIGURE 22.1 Bioprocesses (a) might present an opportunity to close the cycle in the way that goods are produced, in some cases utilizing the waste as feedstock. This is in contrast to the linear production systems (b), where waste invariably has to be treated and disposed of without utilization.
there are many additional advantages in using renewable resources beyond conserving materials and energy, such as the minimization of waste through the use of closed-loop systems and biodegradable by-products that are easier to treat. In an ideal system, we will be able to design products that are derived from renewable sources, and those products, at the end of their useful life, will be reassimilated into nature or into closed-loop industrial systems that utilize product waste as inputs, and when waste is generated, as indeed it will be, it is easily biodegradable. Because the use of renewable resources could enable this transfor- mation in the way we make and use the items we need and want, we must strive to utilize them as much as possible. To underscore their importance, we note that the U.S. National Research Council issued a report in 2000 on bio-based industrial products2 citing the following potential benefits:
. Use of unexploited agricultural productivity
. Use of some agriculture, forestry, or municipal waste
. Potential for more environmentally sustainable products and processes
. Development of better performing or currently unavailable products
. Use of molecular biology to enhance raw materials for easier processing and to reduce the economic and environmental costs of processing
Given these potential benefits and the pivotal role of renewable resources in achieving greater sustainability, the United States and the European Union have set targets to increase the use of renewable resources for products and energy, as shown in Table 22.1. However, to realize the full potential of renewable resources, one must strive to think holistically. For example, the use of renewable resources needs to be managed in a way that ensures long-term availability; that is, they need to be used within their rate of replenishment (e.g., trees grow only so fast) and within the limits of the planet to regenerate itself from the impacts while maintaining the complex equilibrium of living systems. In addition, as we strive to use more renewable materials, we must be aware of and make provisions to counteract the impacts that arise through activities such as agriculture, which is generally material and energy intensive;
has significant impacts on the carbon, nitrogen, and phosphorus cycles; and creates competition between arable land for food and for other items of commerce.
Some people might have the misperception that when utilizing renewable materials or energy, one is therefore free to use as much as possible because it is renewable. This is a TABLE 22.1 U.S. and European Union Goals for the Use of Renewable Resources
in Energy Generation, Transportation Fuels, and Products (%)
United States European Union
Renewable Resource 2001 2010 2020 2030 2001 2005 2010 2020–2050 Bioenergy: share of electricity
and heat demands
2.8 4 5 5 7.5 — 12.5 26 (2030)
Biofuels: share of demand for transportation
0.5 4 10 20 1.4 2.8 5.8 20 (2020)
Bioproducts: share of biobased chemicals
5 12 18 25 8–10
Source:ref. 3.
flawed perception on two accounts. First, renewable resources are renewable only if we use them in a sustainable manner, that is, such that the rate of use is less than the rate at which they can be replenished without creating additional stressors on the environment.