Negative carbon intensity of renewable energy technologies involving biomass or carbon dioxide as inputs
Introduction
Conventional fossil fuel-based energy technologies can achieve efficiency in energy conversion but they are usually completely inefficient in carbon (C) conversion, because they generate significant carbon dioxide (CO2) emissions to the atmosphere per unit energy converted. The emitted CO2 adversely affects natural C cycles by highly irreversible accumulation in the atmosphere. Namely, the content of CO2 in the atmosphere is increasing at a rate exceeding 2 ppmv yr−1. It currently reaches the level of around 391 ppmv [1], about 110 ppmv above the pre-industrial average. The expected further rise in atmospheric CO2 content can pose a significant danger for ocean ecosystem health, i.e. the excess of atmospheric CO2 can acidify oceans leading to damaging the oceanic biosphere, and consequently, it can reduce the present natural ability of oceans to absorb large amounts of CO2 from the atmosphere. Besides, a CO2-induced global warming effect is characterized by significant time delay, i.e. Earth's thermal response lags behind the rise in atmospheric CO2 by more than five decades. Hence, even when future CO2 emissions are reduced and atmospheric CO2 content is stabilized at a certain level, a significant risk exists that Earth's global mean temperature will further rise during forthcoming decades [2]. These effects imply a need for deployment of technologies able to remove CO2 from the atmosphere. Overall, energy systems must be less CO2-intensive because otherwise, unwanted irreversible climate changes might occur [3].
Existing energy technologies are characterized by various C intensities. Only limited number of energy technologies can achieve negative C intensity [4]. There are two large groups of such C negative renewable energy technologies. The first group includes biofuels. These technologies make use of the fact that biomass growth is associated with photosynthesis-based uptake of atmospheric CO2. C negative intensity is achieved when biomass energy conversion is followed by the conversion of C to stable C-rich products such as fuels, fertilizers or food. The negative C intensity can be particularly pronounced when at the end-of-life of these C-rich products the release of C back to the atmosphere is minimized. The second group of technologies utilizes renewable energy to drive the production processes of C-rich fuels from CO2 captured either in fossil fuel-fired power plants or directly from the atmosphere.
The present review seeks approaches which are suitable for renewable energy systems and can significantly negate C intensities of energy. The main idea behind current approaches is to avoid or minimize the use of cost-intensive technologies involving C storage in geological formations. Instead, advanced and more cost-effective methods for achieving negative C emissions are addressed. The main focus is thus on two C management technologies: (i) soil C sequestration and (ii) CO2 conversions to fuels and other useful C-rich products.
The review also highlights how some of these routes can offer a valuable opportunity to introduce renewable energy into the existing energy infrastructures. The focus is thus on techniques allowing ‘drop-in’ of renewable energy into power systems mainly by synthesizing fuels and fertilizers from biomass and CO2.
National energy systems in many countries involve a significant share of C intensive energy technologies such as fossil fuel-firing, in particular coal [4]. Therefore, expanded deployment of renewable energy technologies designed to achieve C negative intensity looks particularly promising for these energy systems [5]. C negative renewable energy can offset positive C intensity of fossil fuel-based energies and hence the overall C intensity of energy can be significantly reduced. The potential for decarbonization of energy systems through adoption of C negative renewable energy technologies is large because biomass production capacity for energy use remains significant in many regions of the world. Besides, some countries have or can have the excess of renewable energy associated with: (i) intermittent operation of wind and solar farms, (ii) locally excessive renewable energy sources (solar energy in hot climates, high-altitude wind and many others). This excess of renewable energy can be used to convert CO2 into useful stable C-rich products. The recycling of C to fuels beneficially offers storage capability for renewable energy. The stored energy can serve a transportation sector as well as it can stabilize energy systems with significant share of renewable energy. Besides, carbon negative renewable energy carries a modest price if we consider total costs to society, i.e. including both subsidies to coal and the negative external economies of coal [6].
The present study analyzes two large groups of renewable energy technologies characterized by C negative intensity of energy: (i) biofuels and (ii) fuels derived from CO2 and renewable energy. The emphasis is put on technological refinements that increase C negativity of energy, methodologies for 'drop-in' of renewable energy, technological reliability of carbon negative renewable energy systems and their cost-effectiveness.
Section snippets
C intensity of energy
Energy-related CO2 emissions to the atmosphere (ECE, in g C) in relation to energy (E, in J) are known as the C intensity of energy (ECE/E, in g C J−1). The C intensity of energy is often related to primary energy or to useful energy obtained from power cycles.
Strategies for C sequestration
The key objective of C sequestration is stabilizing the content of C in the atmosphere. Current C emissions to the atmosphere are around 8 Pg C yr−1. To sequester such huge amounts of C, C sequestration technologies must be massive.
There are two main ideas behind massive C sequestration. The first idea is associated with capturing of C derived from fossil fuel combustion and the second idea is associated with capturing directly atmospheric C. The captured C must be further permanently stored in
‘Drop-in’ of renewable energy
C negative renewable energy technologies must be suitable for 'drop-in' of renewable energy. This 'drop-in' of renewable energy can be achieved differently in bioenergy technologies involving C sequestration in vegetation and soil and differently in non-biomass-based renewable energy technologies involving CO2 capture and recycling. As indicated in Section 3.3 conventional CCS technologies are usually less suitable for 'drop-in' of renewable energy.
The complimentary role of biofuels and C-derived fuels to serve a transportation sector
The total sustainable technical potential of bioenergy in 2050 is projected to be between 80 and 170 EJ yr−1. The midpoint value of this range, 125 EJ, is around one quarter of the current global energy use (about 500 EJ yr−1) and less than one-tenth of the projected global energy use in 2050 (about 1300 EJ yr−1). The global transport energy demand is about 100 EJ and is projected to grow to about 170 EJ in 2050. Assuming that half the available sustainable biomass energy was available for biofuel
C negative biofuels
Plants have evolved highly sophisticated light-harvesting mechanisms that allow for increased environmental tolerances and robustness, enhanced photo-efficiencies and prolonged lifetimes. Existing solar technologies such as PV or CSP still require expensive materials and achieve relatively low efficiencies of solar energy conversion. Therefore, green plants still have great potential for storing solar energy in biomass. Besides, bioenergy offers the provision of useful energy and one additional
C negative fuels derived from CO2 and renewable energy
Renewable energy resources often are relatively unstable and therefore renewable energy systems require energy storage. CO2 capture and recycling (CCR) technology is an ideal candidate for storing renewable energy in the form of useful fuels and, at the same time, the CCR can lead to the net removal of CO2 from the atmosphere. The CCR allows for storing renewable energy in the form of useful C-rich energy carriers which can compete with fossil fuels, at least in transportation energy
Conclusions
According to International Energy Agency global carbon intensity of total primary energy supply in 2009 was 1.54 × 10−5 g C J−1 which corresponds to the increase of atmospheric CO2 content by 2 ppmv yr−1 or by 0.5% v yr−1. Therefore, carbon intensity of energy must be urgently significantly decreased in order to stabilize the content of CO2 in the atmosphere [3] and prohibit unwanted climate change.
This review paper analyzed technological options which could negate carbon intensity of some renewable
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