Negative CO2 Emissions Research Articles

High-Efficiency Bioenergy Carbon Capture Integrating Chemical Looping Combustion with Oxygen Uncoupling and a Large Cogeneration Plant

Bioenergy with CO2 capture and storage (BECCS) is a promising negative emission technology (NET). When using sustainably produced biomass as fuel, BECCS allows the production of power and heat with negative CO2 emissions. The main technical challenges hindering the deployment of BECCS technologies include energy penalties associated with the capture process. This work evaluates the performance of an advanced CO2 capture technology, chemical looping with oxygen uncoupling (CLOU), in conjunction with biomass-fired combined heat and power (CHP) generation. Results from a MATLAB/Simulink reactor model were incorporated in a plant and integration model developed in a commercial process simulation software to quantify the key performance indicators of the CLOU-integrated CHP plant. Both energy and exergy analysis were conducted. The results show a remarkably low efficiency penalty of 0.7% compared to a conventional reference plant, and a high carbon capture efficiency of 97%. The low efficiency penalty is due to the high moisture and hydrogen contents of the biomass, and the separation of combustion products and excess air streams in the CLOU process; these together provide an opportunity to recover a significant amount of heat by flue gas condensation at a higher temperature level than what is possible in a conventional boiler. The condensing heat recovery yields an 18 MW generator power increase (3 MW loss in net power output) for the CLOU plant; in the reference plant with conventional boiler, the same scheme could achieve an increase of 9 MW (generator) and a decrease of 8 MW (net).

Techno-Economic Assessment of a Combined Heat and Power Plant Integrated with Carbon Dioxide Removal Technology: A Case Study for Central Poland

The objective of this study is to assess the techno-economic potential of the proposed novel energy system, which allows for negative emissions of carbon dioxide (CO2). The analyzed system comprises four main subsystems: a biomass-fired combined heat and power plant integrated with a CO2 capture and compression unit, a CO2 transport pipeline, a CO2-enhanced geothermal system, and a supercritical CO2 Brayton power cycle. For the purpose of the comprehensive techno-economic assessment, the results for the reference biomass-fired combined heat and power plant without CO2 capture are also presented. Based on the proposed framework for energy and economic assessment, the energy efficiencies, the specific primary energy consumption of CO2 avoidance, the cost of CO2 avoidance, and negative CO2 emissions are evaluated based on the results of process simulations. In addition, an overview of the relevant elements of the whole system is provided, taking into account technological progress and technology readiness levels. The specific primary energy consumption per unit of CO2 avoided in the analyzed system is equal to 2.17 MJLHV/kg CO2 for biomass only (and 6.22 MJLHV/kg CO2 when geothermal energy is included) and 3.41 MJLHV/kg CO2 excluding the CO2 utilization in the enhanced geothermal system. Regarding the economic performance of the analyzed system, the levelized cost of electricity and heat are almost two times higher than those of the reference system (239.0 to 127.5 EUR/MWh and 9.4 to 5.0 EUR/GJ), which leads to negative values of the Net Present Value in all analyzed scenarios. The CO2 avoided cost and CO2 negative cost in the business as usual economic scenario are equal to 63.0 and 48.2 EUR/t CO2, respectively, and drop to 27.3 and 20 EUR/t CO2 in the technological development scenario. The analysis proves the economic feasibility of the proposed CO2 utilization and storage option in the enhanced geothermal system integrated with the sCO2 cycle when the cost of CO2 transport and storage is above 10 EUR/t CO2 (at a transport distance of 50 km). The technology readiness level of the proposed technology was assessed as TRL4 (technological development), mainly due to the early stage of the CO2-enhanced geothermal systems development.

The pyrolysis and combustion of polyoxymethylene in a fluidised bed with the possibility of incorporating CO2

The recycling of polyoxymethylene (POM) was performed in a fluidised bed which was formed from cenospheres (hollow, spherical particles). Air, N2 and CO2 were used as the fluidising agents. The composition of the process products in the flue gases as a function of fluidised bed temperature is shown and discussed for each fluidising gas. A flue gases FTIR analysis together with the spectra deconvolution of multicomponent gaseous samples were used to conduct instant quantitative monitoring of the products of POM thermal degradation. At 400 °C, in atmospheres of either air or CO2, the process of POM thermal recycling was performed for the purpose of monomer recovery and obtained a near 90% efficiency of formaldehyde formation. At temperatures above 600 °C, it was found that the use of air or N2 leads to the recovery of energy or gas rich in CO, respectively. The CO-rich gas was obtained during the pyrolysis of POM when CO2 constituted the fluidising agent. The amount of CO in the flue gas was almost twice as large as the amount of carbon introduced into the process in the polymer material. This means that carbon was converted to CO from the fluidising medium, thus recycling of POM to the CO-rich feedstock was obtained with a negative CO2 emission. The main advantage of the POM pyrolysis in the nitrogen atmosphere was monomer recovery.

Waste-to-energy technology integrated with carbon capture – Challenges and opportunities

Carbon dioxide emission is a serious environmental issue that humankind must face soon. One of the promising technologies for reducing global CO2 emissions is oxy-fuel combustion (OFC) technology, which belongs to the carbon capture methods. OFC involves the use of oxygen and recirculated flue gas as an oxidizer in the combustion process. Application of oxy-fuel combustion in waste incineration can result in negative CO2 emission since some part of the carbon in municipal solid waste is biogenic. Such technology is often described as BECCS or Bio-CCS and it has attracted the attention of scientists recently. In addition to easier CO2 capture, oxy-fuel combustion of municipal solid waste offers other advantages, such as reduced flue gas volume, increased combustion temperature and the possibility of retrofit existing incineration plants. In the present paper, studies of oxy-fuel combustion of waste materials, in particular, municipal solid waste and sewage sludge are presented and summarized. The study shows the opportunities and challenges that have to be addressed to fully exploit the potential of the oxy-fired incineration plant.

Hot corrosion behaviors of TP347H and HR3C stainless steel with KCl deposit in oxy-biomass combustion

Oxy-combustion is one of the most promising technologies for carbon capture and sequestration. When CO2-neutral biomass is burned under oxy-combustion conditions, named “oxy-biomass combustion” a negative CO2 emission can be achieved. However, the high content of potassium and chlorine in biomass results in sever ash deposition and corrosion in air fired furnaces, which are further aggravated in oxy-combustion mode due to the enrichment of corrosive species by flue gas recycle. In this paper, the hot corrosion behaviors and mechanism of two representative materials (TP347H, HR3C) used for superheaters in furnaces are studied. The effects of oxy-combustion atmosphere, KCl deposition, effect of SO2, effect of water vapor, and temperature on the corrosion kinetics at the starting stage are investigated. The corrosion severity of the materials was determined using the weight gain method, and the microstructures and chemical compositions of corrosion layers were characterized by the scanning electron microscopy with energy dispersive spectroscopy, and X-ray diffraction. The results show that the hot corrosion rate is significantly sped up by KCl deposition, more than five times the gas corrosion rate under the same gas composition and temperature. HR3C with higher Cr and Ni contents is more likely to form Cr enrichment on the interface between the corrosion layer and the substrate than TP347H, resulting in stronger resistance to the hot corrosion than TP347H. When the corrosion atmosphere is changed from air-combustion to oxy-combustion, the hot corrosion rate is reduced with a denser Cr oxide film and less metal sulfides. The increase of temperature in the presence of KCl deposition significantly affects the hot corrosion rate, e.g. the corrosion rate at 650 °C is 16 times higher than that at 450 °C. Water vapor and SO2 concentrations have opposite influences on the hot corrosion, respectively. Compared to the dry environment, a high-humidity environment decreases the hot corrosion rate; however, a higher SO2 concentration facilitates the sulfation of KCl deposits, leading to stronger damage to the chromium oxide film and thereby an increased hot corrosion rate.

Impact of Uncertainties on the Design and Cost of CCS From a Waste-to-Energy Plant

Uncertainties are an inherent and important element of novel systems with limited large-scale industrial experience and must be taken into account in order to enable the design of cost-efficient energy systems. This paper investigates the optimal design of carbon capture and storage from a waste-to-energy plant under uncertainties. With the aim of providing a better understanding of the impact of uncertainties on the design and cost of CCS chains, as well as the capture technology selection, the case of a hypothetical 40 MW waste-to-energy plant located in Norway is considered. The impact of key technical and cost uncertainties on the cost of different CO2 capture and CCS chain options are investigated using an in-house techno-economic CCS assessment tool combined with an uncertainty quantification framework. When the different capture options are compared on a deterministic basis, the advanced amine yields the best performances (CO2 avoidance cost of 153 €/tCO2,avoided), followed by the membrane process based on partial capture (200 €/tCO2,avoided) and MEA-based capture (217 €/tCO2,avoided). However, in contrast with the advanced amine, the partial capture considered in the membrane process does not enable net negative CO2 emissions. Once technical and cost uncertainties are taken into account, the advanced amine-based capture remains the best option, however the MEA-based capture outperform the membrane process. Finally, the stochastic optimisation showed that the uncertainties considered do not impact the optimal capture capacity in this case. The full CCS chain perspective is then included through two chain options: a nearby offshore saline aquifer or an offshore CO2 EOR storage located further away. The EOR-based chain leads to the best performances (187 vs. 202 €/tCO2,avoided) both on a deterministic basis and when different uncertainty scenarios are considered. However, as a shared transport and storage infrastructure is considered, uncertainty regarding the amount of CO2 coming from nearby industries leads to a different optimal design of the chain (pipeline diameter and ship capacity). Finally, uncertainties on the EOR response to CO2 injection can significantly reduce the potential of the CO2 EOR-based chain and lead to cases in which the saline aquifer-based chain would be optimal.

Achieving Zero/Negative-Emissions Coal-Fired Power Plants Using Amine-Based Postcombustion CO2 Capture Technology and Biomass Cocombustion.

The strengthening carbon mitigation efforts to meet the 1.5 °C target requires the development of zero/negative CO2 emission technologies to eliminate large direct CO2 emissions from fossil-fuel fired power stations. Amine scrubbing is a dominant technology to capture CO2 from fossil-fuel power stations, but its application in achieving zero/negative emission in power stations is rarely reported. The present study investigates the MEA-based technologies to achieve zero and negative CO2 emission in coal-fired power stations, and their techno-economic performance was evaluated in detail. These zero/negative-emission technologies include 99.7% CO2 capture from flue gas (zero emission), biomass cocombustion with coal integrated with CO2 capture at ratios of 10% biomass/90% CO2 capture and 5% biomass/95% CO2 capture (zero-emission), and 10% biomass/95% CO2 capture for negative-emission power station. Our investigation revealed that these zero/negative-emission technologies are technically and economically viable, and their CO2 avoided costs did not significantly increase compared to the standard 90% CO2 capture. The CO2 avoided cost for 99.7%-capture is estimated at $66.5/tonne CO2, which is $2.6/tonne CO2 higher than that of 90%-capture. The biomass cocombustion zero/negative-emission technologies show better economic performance with CO2 avoided cost of $64.1-64.8/tonne CO2, which is only $0.2-0.7/tonne CO2 higher than the standard 90%-capture. These results indicate that the amine-based CO2 capture integrated with biomass cocombustion technology would be economically competitive to achieve zero or even negative CO2 emissions in coal-fired power stations.

Oxy-combustion characteristics as a function of oxygen concentration and biomass co-firing ratio in a 0.1 MWth circulating fluidized bed combustion test-rig

Oxy-combustion with a circulating fluidized bed (Oxy-CFBC) can facilitate the separation of high CO2 concentration and reduce emissions by biomass co-firing. This study investigated Oxy-CFBC characteristics such as temperature, solid hold-up, flue gas concentrations including CO2, pollutant emissions (SO2, NO, and CO), combustion efficiency and ash properties (slagging, fouling index) with increasing input oxygen levels (21–29 vol%), and biomass co-firing ratios (50, 70, and 100 wt% with domestic wood pellet). The possibility of bio-energy carbon capture and storage for negative CO2 emission was also evaluated using a 0.1 MWth Oxy-CFBC test-rig. The results show that combustion stably achieved with at least 90 vol% CO2 in the flue gas. Compared to air-firing, oxy-firing (with 24 vol% oxygen) reduced pollutant emissions to 29.4% NO, 31.9% SO2 and 18.5% CO. Increasing the biomass co-firing from 50 to 100 wt% decreased the NO, SO2 and CO content from 19.2 mg/MJ to 16.1 mg/MJ, 92.8 mg/MJ to 25.0 mg/MJ, and 7.5 mg/MJ to 5.5 mg/MJ, respectively. In contrast to blends of sub-bituminous coal and lignite, negative CO2 emission (approximately −647 g/kWth) was predicted for oxy-combustion only biomass.

Potassium Ash Interactions with Oxygen Carriers Steel Converter Slag and Iron Mill Scale in Chemical-Looping Combustion of Biomass—Experimental Evaluation Using Model Compounds

Chemical-looping combustion (CLC) is a combustion technology in which a solid oxygen carrier is used to convert fuel. The oxygen carrier is oxidized in air and subsequently transferred to a separate reactor in which it reacts with the fuel. The produced CO2 is inherently separated from the air components, making CLC a promising technology for carbon capture and storage (CCS). CLC of biomass combined with CCS (bioenergy CCS; BECCS) is a way to generate negative CO2 emissions and thus interesting for climate change mitigation. Undesirable chemical reactions between ash and oxygen carriers are a challenge in BECCS because of the reactive nature of biomass ash. This article examines two low-cost steel industry byproducts that have shown desirable fuel conversion properties in CLC: iron mill scale (Glodskal B) and steel converter slag (LD-slag). Their interactions with potassium ash model compounds (KCl, K2CO3,K2SO4, and KH2PO4) in a reducing atmosphere have been investigated. Mixtures of oxygen carriers and potassium salt have been reduced for 6 h in CO and steam in a laboratory-scale fixed-bed reactor at 850 °C. The reduced samples have been analyzed with SEM/EDS and XRD. The reactivity of the mixtures during reduction and oxidation has also been examined by thermogravimetric analysis (TGA). K2CO3 increased the reaction rate for the reduction of Glodskal and inhibited the reactivity of LD-slag. KH2PO4 formed a K−P−Fe component with apparent low melting temperature with Glodskal, causing agglomeration, and decreased the reduction/oxidation rate in TGA. KH2PO4 formed a K−P−Ca component with apparent high melting temperature with LD-slag causing agglomeration but the reduction rate was not affected. The study suggests that the iron mill scale and LD-slag should not be rejected as oxygen carriers for CLC based on potassium ash interaction.

Combustion of solid recovered fuels within the calcium looping process – Experimental demonstration at 1 MWth scale

The calcium looping (CaL) process is an efficient post-combustion CO2 capture technology based on the reversible carbonation-calcination reaction of natural lime. This work presents the results obtained during long-term operation at the 1 MWth CaL pilot plant at Technische Universität Darmstadt. During more than 230 h of representative CaL operation, the calciner was heated by oxy-fuel combustion of waste derived fuels. During the experimental investigation, two types of solid recovered fuels (SRF) were utilized. Both types of SRF were fed to the process in the form of raw fluff, similar to typical industrial applications. The flue gas to be decarbonized in the carbonator, was supplied by an on-site combustion chamber. The CO2 concentration was kept between 9.5 and 10.5 vol% as typical value for waste-to-energy (WtE) plants fueled by municipal solid waste (MSW). Over a wide range of operation conditions, CO2 absorption rates of 80% and total CO2 capture rates over 90% were achieved. Within this work, exemplary long-term data plots of relevant CaL process parameters are shown for each type of SRF. A chlorine balance for a representative operation period is established based on the relevant effluent streams from the CaL system. Thereby, it was found that the calciner fly ash represents the major chlorine effluent. Furthermore, the retention rate of chlorine was above 82% throughout all test points, respectively. When taking into account the organic waste fractions typically contained in SRF and MSW, this work successfully demonstrates the feasibility of net negative CO2 emissions by means of the CaL process at semi-industrial scale.

Potential contribution of fusion power generation to low-carbon development under the Paris Agreement and associated uncertainties

Fusion energy has abundant resources, does not exhibit nuclear runaway, and it has zero-carbon emissions. Long-term strategies for fusion energy development will become critically important to promote future DEMOnstration Fusion Reactor (DEMO) projects by another largescale investment and gain social acceptance. This paper assessed the potential contribution of fusion power generation to low-carbon development, which is prescribed in the Paris Agreement, under the combination of different uncertainties of future socioeconomic development, probability of the 2 °C target, and development of commercial fusion power plants. Global negative CO2 emission in 2100 by drastic decarbonization of energy systems was required to achieve the 2 °C target, and fusion power plants were expected to be installed in the latter half of the 21st century mainly in countries with limited potentials of zero-emission energy sources, such as Japan, Korea, and Turkey, for cost-efficient climate change mitigation. If inexpensive power plants could be developed by enhanced R&D and advanced design in DEMO projects or if the establishment of fission plants in the future is low, fusion power generation will also be deployed in the EU28, India, and China. Further cost reduction by innovative design and alternative concepts are also essential to diffuse fusion plants in zero-emission resource-rich countries.

Loss of soil organic carbon in Swiss long-term agricultural experiments over a wide range of management practices

Soil carbon sequestration (SCS) is one of the cheapest and technically least demanding carbon dioxide (CO2) removal (CDR) or negative CO2 emission technologies. For a realistic assessment of SCS, it is critical to evaluate how much carbon (C) can be stored in soil organic matter under actual agricultural practices. This includes typical crop rotations and fertilization strategies, depends on resources that are available (e.g. farmyard manure (FYM)) and are affordable for farmers. Furthermore, it is important to assess SCS based on given climatic and soil conditions. Here, we evaluate changes in soil C storage for Switzerland using data from eleven long-term field experiments on cropland and permanent grassland that include common local practices.At all sites, changes in soil organic carbon (SOC) stocks were measured in topsoil (∼0-0.2 m) in response to a total of 80 different treatments including different types of mineral or organic fertilization (e.g. FYM, slurry, peat, compost) or soil management (tillage vs. no-till). The treatments were applied to different, diverse crop rotations or grass mixtures that are representative for Switzerland. We found that topsoils lost C at an average rate of 0.29 Mg C ha−1 yr−1, although many of the investigated treatments were expected to lead to SOC increases. Based on a linear mixed effects model we showed that SOC change rates (ΔSOC) were driven by C inputs to soil (harvest residues and organic fertilizer), soil cover and initial SOC stocks. The type of land use or soil tillage had no significant effect. Our analysis suggests that current efforts to manage soils sustainably need to be intensified and complemented with further techniques if Switzerland wants to achieve the goal of the 4 per 1000 initiative.

Numerical study of biomass Co-firing under Oxy-MILD mode

Oxy-MILD (Oxygen Moderate and Intense Low-Oxygen Dilution) combustion is one of the most promising technologies for the mitigation of CO2 emissions from coal-fired furnaces, benefiting from its good performance in flame-temperature controlling and NOx reduction. Under oxy-MILD mode, the combustion or co-firing of biomass (CO2-neutral) can achieve “negative CO2 emissions”. In this paper, oxy-MILD biomass co-firing is numerically studied by CFD modeling for the IFRF furnace NO.1, where Guasare coal and Olive waste are co-fired under air-MILD and oxy-MILD conditions, respectively. The effects of biomass co-firing ratio (0–30%, energy basis) and atmosphere on the temperature and heat flux distribution, and NOx emissions are discussed. The modeling results show that under MILD combustion mode, both oxy-combustion and biomass co-firing can generate a more moderate temperature distribution and lower NOx emissions than air-combustion and coal combustion, respectively. When biomass co-firing ratio increases from 0% to 30%, under oxy-MILD combustion mode, the peak temperature linearly decreases by 28 K and the NOx emissions decrease by 141 ppm; while under air-MILD combustion mode, the peak temperature increases by 15 K and the NOx emissions decrease by only 73 ppm. This modeling work suggests that oxy-MILD biomass co-firing is a more promising technology to achieve “negative CO2 emissions” in coal combustion, with lower furnace temperatures as well as NOx emissions.

Transitioning towards negative CO2 emissions

In this policy perspective we argue that mankind likely needs to achieve negative CO2 emissions before 2050, describe the transition to CO2-neutrality and CO2-negativity, and illustrate the possibly critical role played herein by CO2 use.

Simulation of a 100-MW solar-powered thermo-chemical air separation system combined with an oxy-fuel power plant for bio-energy with carbon capture and storage (BECCS)

The combination of concentrated solar power–chemical looping air separation (CSP-CLAS) with an oxy-fuel combustion process for carbon dioxide (CO2) capture is a novel system to generate electricity from solar power and biomass while being able to store solar power efficiently. In this study, the computer program Advanced System for Process Engineering Plus (ASPEN Plus) was used to develop models to assess the process performance of such a process with manganese (Mn)-based oxygen carriers on alumina (Al2O3) support for a location in the region of Seville in Spain, using real solar beam irradiance and electricity demand data. It was shown that the utilisation of olive tree prunings (Olea europaea) as the fuel—an agricultural residue produced locally—results in negative CO2 emissions (a net removal of CO2 from the atmosphere). Furthermore, it was found that the process with an annual average electricity output of 18 MW would utilise 2.43% of Andalusia’s olive tree prunings, thereby capturing 260.5 k-tonnes of CO2, annually. Drawbacks of the system are its relatively high complexity, a significant energy penalty in the CLAS process associated with the steam requirements for the loop-seal fluidisation, and the gas storage requirements. Nevertheless, the utilisation of agricultural residues is highly promising, and given the large quantities produced globally (~ 4 billion tonnes/year), it is suggested that other novel processes tailored to these fuels should be investigated, under consideration of a future price on CO2 emissions, integration potential with a likely electricity grid system, and based on the local conditions and real data.

Formic acid synthesis – a case study of CO2 utilization from coal direct chemical looping combustion power plant

ABSTRACT Chemical looping combustion (CLC) is an encouraging technology for hydrogen and power co-generation with efficient CO2 capture. Utilization of the captured CO2 is a much more important task to realize negative CO2 emissions. The objective of the work is to explore a case study of carbon dioxide utilization (CDU) in the form of formic acid synthesis using the captured CO2 and co-generated H2 from coal direct chemical looping (CDCL) power plant. Steady-state simulations are performed on the power and H2 co-generation CDCL plant integrated with formic acid synthesis and presented a comparison of overall performance of the CDCL plants with and without CDU integration. The simulation results revealed that the formic acid synthesis integrated CDCL plant avoids release of 62.49 kg.CO2/GJ of fuel input into the environment and utilizes the captured CO2 completely with the net electrical efficiency penalty of 5.8%.

A Downdraft Fixed-Bed Biomass Gasification System with Integrated Products of Electricity, Heat, and Biochar: The Key Features and Initial Commercial Performance

Biomass, as a renewable and clean energy resource, plays a vital role in energy security and greenhouse gas reduction across the world. This paper reports on our newly established technology: a downdraft fixed-bed biomass gasification system using nut shells (mainly apricot kernel shells) for electricity generation, heating and partially activated carbon production at the same time. Particularly, the key features of the gasification reactor will be presented in detail. In the commercial plant (3 MW scale) located in Hebei province, China, the typical energy conversion from apricot kernel shell gasification is as follows: 47% syngas, 44% char (partially activated carbon), 5% hot water, and 4% energy loss. The main gasification temperature is 600–800 °C, while the activation zone is 850–900 °C. The commercial system has currently been in operation for 4 years. Considering the partially activated carbon as a stable carbon carrier, the whole system features negative CO2 emissions.

Life-cycle energy, cost, and CO2 emission of CO2-enhanced coalbed methane (ECBM) recovery framework

This study aims to analyze the technical, economic, and environmental capabilities of CO2-enhanced coalbed methane (ECBM) recovery framework. By incorporating various analysis techniques such as process simulation, techno-economic evaluation, and life cycle assessment, we developed a systematic approach to evaluate the whole process chain of the framework consisting of five sub-processes: CO2 capture, CO2 transport, CO2 injection, raw coalbed methane extraction, and methane purification. As a case study, the CO2-ECBM recovery in the Tavan Tolgoi basin, the largest coal seam in Mongolia, was investigated. The life-cycle energy and cost, and the net CO2 emission were analyzed using twelve realistic scenarios. As a result, the CO2 capture is analyzed as the main cost driver due to its high energy consumption, followed by the CO2 transportation. In addition, the proposed framework is identified as the only negative CO2 emission option compared to typical CBM and flue gas-ECBM frameworks.

Seasonal variations and intricate diel differences in the physio-chemical parameters and CO2 emissions from a typical karst groundwater-fed reservoir in southern China

The quantification of CO2 flux of water reservoirs is still largely uncertain due to complex temporal variations in physio-chemical parameters and CO2 emissions. To address this issue, the diel-scale characteristics of physio-chemical parameters, δ13CDIC, and CO2 emission flux from Dalongdong reservoir (DLD), a karst groundwater-fed reservoir, in southern China were measured covering July, August, and November of 2014 and March and June of 2015. The results showed that the physio-chemical parameters, δ13CDIC, and CO2 emission flux displayed significant seasonal variations and intricate diel differences in the DLD reservoir. At the seasonal scale, the DLD reservoir is a CO2 source because the mean value of the CO2 emission flux is higher than zero. The highest CO2 emission flux was in November of 2014 due to reservoir overturn and the lowest flux was in August of 2014 due to reservoir thermal stratification. Diel differences in pH, DO, DIC, and pCO2 were inconspicuous during the study periods, while the CO2 emission and δ13CDIC showed higher values during the daytime than the nighttime during July of 2014 and March and June of 2015. In August of 2014, mean differences between the daytime and nighttime CO2 emission were inconspicuous due to a negative CO2 emission flux occurrence. Reservoir overturn in November of 2014 resulted in a higher mean CO2 emission flux and larger data range during the daytime than at nighttime. Future studies that calculate CO2 emissions in water reservoirs should consider monitoring temporal frequencies to obtain more accurate estimations of CO2 emission fluxes.

Assessing negative carbon dioxide emissions from the perspective of a national “fair share” of the remaining global carbon budget

We present an assessment of the plausible Paris-aligned fair share nett cumulative carbon dioxide (CO2) quota for an example nation state, the Republic of Ireland. By Paris-aligned, we mean consistent with the Paris Agreement adopted at the 21st Conference of the Parties to the United Nations Framework Convention on Climate Change, at Paris, France, in December 2015 (UNFCCC 2015). We compare and contrast this quota with both the aspirations expressed in the current Irish National Policy Position and current national emission projections. The fair share quota is assessed as a maximum of c. 391 million tonnes of carbon dioxide (MtCO2), equal to 83 tonnes of carbon dioxide (tCO2) per capita, from 2015, based on a precautionary estimate of the global carbon budget (GCB) and specific interpretation of global equity. Given Ireland’s high current CO2 per capita emission rate, this would correspond to sustained year-on-year reductions in nett annual CO2 emissions of over − 11% per year (beginning as of 2016). By contrast, the CO2 mitigation target indicated in the National Policy Position corresponds to nett annual reduction rates in the range of only −4.7% per year (low ambition) up to a maximum of − 8.3% per year (high ambition), and projections based on current and immediately planned mitigation measures indicate the possibility, instead, of sustained increases in emissions at a rate of the order of + 0.7% per year. Accordingly, there is a large gap between Paris-aligned ambition and current political and policy reality on the ground, with a significant risk of early emergence of “CO2 debt” and tacit reliance on rapid deployment of currently speculative (at a relevant scale and feasible cost) negative CO2 emission technologies to actively remove CO2 from the atmosphere. While the detailed policy situation will clearly differ from country to country, we suggest that this methodology, and its CO2debt framing, may be usefully applied in other individual countries or regions. We recommend that such framing be incorporated explicitly into a global mitigation strategy via the statements of nationally determined contributions required to be submitted and updated by all parties under the Paris Agreement processes.