Analysing existing structures: a brief introduction

BackgroundThis study evaluates the carbon dioxide and other greenhouse gas fluxes to the atmosphere resulting from charcoal production in Zambia. It combines new biomass and flux data from a study, that was conducted in a miombo woodland within the Kataba Forest Reserve in the Western Province of Zambia, with data from other studies.ResultsThe measurements at Kataba compared protected area (3 plots) with a highly disturbed plot outside the forest reserve and showed considerably reduced biomass after logging for charcoal production. The average aboveground biomass content of the reserve (Plots 2-4) was around 150 t ha-1, while the disturbed plot only contained 24 t ha-1. Soil carbon was not reduced significantly in the disturbed plot. Two years of eddy covariance measurements resulted in net ecosystem exchange values of -17 ± 31 g C m-2 y-1, in the first and 90 ± 16 g C m-2 in the second year. Thus, on the basis of these two years of measurement, there is no evidence that the miombo woodland at Kataba represents a present-day carbon sink. At the country level, it is likely that deforestation for charcoal production currently leads to a per capita emission rate of 2 - 3 t CO2 y-1. This is due to poor forest regeneration, although the resilience of miombo woodlands is high. Better post-harvest management could change this situation.ConclusionsWe argue that protection of miombo woodlands has to account for the energy demands of the population. The production at national scale that we estimated converts into 10,000 - 15,000 GWh y-1 of energy in the charcoal. The term "Charcoal Trap" we introduce, describes the fact that this energy supply has to be substituted when woodlands are protected. One possible solution, a shift in energy supply from charcoal to electricity, would reduce the pressure of forests but requires high investments into grid and power generation. Since Zambia currently cannot generate this money by itself, the country will remain locked in the charcoal trap such as many other of its African neighbours. The question arises whether and how money and technology transfer to increase regenerative electrical power generation should become part of a post-Kyoto process. Furthermore, better inventory data are urgently required to improve knowledge about the current state of the woodland usage and recovery. Net greenhouse gas emissions could be reduced substantially by improving the post-harvest management, charcoal production technology and/or providing alternative energy supply.

Deep society-wide decarbonization is a grand challenge of the 21st century requiring the development, manufacture, and deployment of transformative carbon-neutral and carbon-negative technologies on a global scale. Carbon dioxide (CO2) capture coupled with storage or conversion is projected to play a key role in mitigating and even reversing carbon emissions [1]. Present-day carbon capture processes rely on thermochemical cycles where solvents/sorbents absorb and release CO2 at lower and higher temperatures respectively. While functional, these embodiments are energetically-intensive and typically rely on fossil fuel derived heat for CO2 desorption, ultimately limiting effectiveness [2]. Electrochemical approaches enable lower energy CO2 separations, as electrode potential can be modulated to selectively activate sorbents rather than temperature and pressure swings which impact the entire capture media. Moreover, electrochemical systems enable direct integration of renewables, modular deployment, and safe operation at ambient conditions [3]. Current research efforts for electrochemical CO2 separation are largely exploratory, focusing on molecular discovery and proof-of-concept demonstrations [4]. As such, modeling work to identify favorable combinations of molecular properties and device characteristics could benefit the field by providing design criteria.Here, expanding upon a prior thermodynamic modeling framework, we assess the energetics of CO2 separation processes that use soluble, redox-active capture species, and how this is impacted by certain tunable molecular properties [5]. To guide the analysis, we contemplate four distinct system configurations representative of different practical embodiments of the envisioned technology. One example is a 4-stage configuration, where the capture species is activated and deactivated in the electrochemical cell, and absorption and desorption steps are carried out in separate process units, as demonstrated in Figure 1(a). We evaluate thermodynamic and electron efficiencies, as measures of energetic and materials utilization efficiencies, respectively, to investigate how molecular properties and system configuration affect performance. This modeling work reveals a tradeoff between these two efficiencies, as shown in Figure 1(b) for a 4-stage configuration. To further explore this tradeoff, we introduce the combined efficiency, defined as the product of thermodynamic and electron efficiencies. More specifically, an electrochemical CO2 separation system that maximizes combined efficiency is anticipated to be successful due to a suitable balance of energy and materials costs. We find the combined efficiency metric is maximized when certain thermodynamic properties are optimized, such as CO2 binding affinity and CO2 solubility. Accordingly, optimal properties that lead to the maximization of the combined efficiency, will also be dependent upon system configuration and intended application (e.g., post-combustion or direct air capture). Overall, the introduction of combined efficiency as a thermodynamic metric enables assessment of candidate capture molecule and electrolyte pairs as well as the identification of the application and process configuration most suitable for the pairing. These results offer a framework of thinking and design guidelines for capture molecule synthesis as well as the selection of supporting salt and solvent. Acknowledgement We gratefully acknowledge funding support from the Alfred P. Sloan Foundation.