CO₂ mitigation options have been overviewed from an engineering point of view. There have been proposed a number of mitigation options, which can be divided into three categories; 1. reduction of energy intensity; 2. reduction of carbon intensity; 3. carbon sequestration. In this review paper, various mitigation options are reviewed focusing on the carbon sequestration options.
A reduction in energy intensity is essentially an energy saving. A reduction in carbon intensity could be achieved by switching to energy resources with lower carbon contents. Based on the 2001 IPCC report, the mitigation potential related to energy intensity is estimated at 1, 900–2, 600 Mt-C/year in 2010, and 3, 600–5, 050 Mt-C/year in 2020, including other greenhouse gas equivalents. There are additional benefits in implementing these options; they are economically beneficial, and have no associated harmful effects. The carbon sequestration options can be divided into two categories; the enhancement of the natural sinking rates of CO₂, and a direct discharge of anthropogenic CO₂. The relevant sequestration options in the first category include terrestrial sequestration by vegetation, ocean sequestration by fertilization, and an enhancement of the rock weathering process. In the direct discharge options, the CO₂ produced from large point sources, such as thermal power stations, would be captured and separated, then transported and injected either into the ocean or underground. Although the sequestration options are less beneficial in terms of cost per unit CO₂ reduction compared to other options, technical developments in sequestration options are necessary for the following reasons; 1. A huge potential capacity for carbon sequestration, 2. carbon sequestration enables a continuous use of fossil fuels, which is unavoidable at the moment, before switching to renewable energy sources. Each sequestration option has advantages and disadvantages in terms of capacity, cost, the time scale of the sequestration, the stability of sequestered CO₂, and additional environmental impacts, which depend on the location, time, and amount of sequestration. Thus, reliable evaluations of the mitigation efficiency are essential for each sequestration option upon implementation.

View full abstract

Global warming is a serious problem requiring immediate countermeasures. Large-scale afforestation in arid areas for the aim of absorbing CO₂ is considered to be one of these measures and the research activities for it have been carried out in Leonora, Western Australia. In this study, the relationship between the biomass distribution and topographic features was examined in order to collect basic information for the selection of suitable afforestation regions in the research area using a biomass distribution map and some topographic elements. The results were as follows.
(1) Plant growth is related to the movement of and the volume of runoff water derived from inequality of the ground, which leads to a biased distribution of nutrient accumulation and of surface soil thickness.
(2) The total potential biomass estimated from the present topography is greater than the present biomass in the 30 km × 50 km research area. A mass of 319 kt-C could be absorbed in the research area without any improvements in the landform.
(3) A substantial improvement on the environment utilizing vast arid areas by afforestation is necessary to fix enough carbon deterring global warming.

View full abstract

Carbon sequestration by afforestation of arid land is one of a few effective countermeasures against the carbon dioxide problem. In order to attain net fixation of carbon dioxide, the quantity of carbon dioxide generated by the implementation of afforestation should be less than that fixed by the tree growth by afforestation. As an evaluation tool for afforestation technologies for arid land, we propose an integrated simulator consisting of soil-water transport, transpiration, photosynthesis, and plant growth models. The aim of this approach is to estimate the quantity of the carbon fixed by afforestation. The simulation results on the irrigation and soil-water transport indicated the importance of soil depth for water maintenance. Also, the growth efficiency of photosynthesized carbon for planted trees was estimated by comparing simulation results and experimental data. Although the simulator constructed herein was based on the climatic and land conditions at our experimental site at Sturt Meadows in Western Australia, the structure of this simulator can be used not only for afforestation in our experimental site, but also for other areas with similar conditions.

View full abstract

A carbon fixation system by large-scale afforestation in arid regions is being researched near Leonora in Western Australia. A tree species that naturally grows in the area, E. camaldulensis, has been planted at the afforestation trial sites. In arid regions, water use efficiency by a tree, defined as the ratio of its organic dry matter production to its water consumption, is an important factor in tree growth for establishing a strategy for afforestation. In this study, the soil water content conditions under which E. camaldulensis could grow in arid regions were investigated and the water use efficiency, daily sap density flux, transpiration rate, and photosynthetic rates were measured. The results show that a thick soil layer (>2 m) effectively collected runoff water and could maintain the growth of E. camaldulensis. The daily sap flux density of the tree was closely related to the water content of the soil between 0.5 m and 1.0 m in depth. The water use efficiency of productivity, WUE_P [g-dry mass increase/kg-water consumption] depended on the season, and the average WUE_P over a 1-year period was about 5. The maximum WUE_P occurred during the period of minimum soil water content, and decreased with its increase. These results were confirmed by measurements of transpiration and photosynthetic rates.

View full abstract

Rate processes involved in the changes in the ratio of particulate organic carbon and particulate organic nitrogen (POC/PON ratio) in the decomposition of phytoplankton were clarified. First, we did two decomposition experiments using substrates whose high C/N ratio was more than two times higher than the Redfield ratio: (a) decomposition of Porphyridium purpureum (POC/PON = 17) and (b) decomposition of Skeletonema costatum (POC/PON = 6.5) in the presence of three times as much organic matter of glucose, which consequently increased the initial C/N ratio by more than a factor of four. In both experiments, the same mechanism was observed: POC decreased linearly while PON remained constant when the C/N ratio was high (>6). Then, we did decomposition experiments of extracted DOM of Skeletonema costatum in the presence and absence of a predator (heterotrophic nanoflagellates (HNF) and zooplankton). Finally, we developed a simple kinetic model of the phytoplankton decomposition process (PDP model). This model with fixed decomposition rate constants fits well with the results of the decomposition experiments where the initial C/N ratio and the volume ratio of POM and DOM differed by a factor of more than three. Our simple PDP model can explain well both the formation processes of semi-refractory POM (SR-POM) and the processes involved in the changes in the POC/PON ratio.

View full abstract

We conducted experiments at Fukido mangrove, northeastern Ishigaki Island, Okinawa, Japan in March 1999, to quantify the nitrogen cycle for simulations of nitrogen cycling and export. We measured concentrations of TON (total organic nitrogen), nitrate (NO₃^– + NO₂^–), ammonia (NH₄⁺) in the creek water, and as physical parameter, measured flow rate and water level. Concentrations of TON and NO₃^– + NO₂^– were higher at high tide and lower at low tide, whereas NH₄⁺ concentration had an opposite trend. Export flux of NO₃^– + NO₂^– was positive (62 mol·d^–1) but export of TN (total nitrogen) was estimated –21 mol·d^–1, so that Fukido mangrove is a source of nitrogen to its coastal community. And we constructed a model of nitrogen cycle and simulated the export fluxes. Simulated TON export was only 6.6%. In the simulations, the vegetation was a net sink for nitrogen, meaning that it is string nitrogen as biomass. There is no mechanism to input into the ecosystem in this model. Nitrogen fixation may be a significant source of nitrogen to the mangrove ecosystem, and we will need to conduct more studies to clarify here its role in the nitrogen cycle.

View full abstract

The purpose of our current study was to clear the fate of organic carbon and nitrogen in seawater and sediment, especially the difference between organic carbon and nitrogen in dissolved and particulate forms and to determine carbon sequestration by seagrass ecosystem. We had carried out four types experiments: the diel variation for 24 hour, and seasonal changes of DOC (Dissolved Organic Carbon)/DON (Dissolved Organic Nitrogen) and POC (Particulate Organic Carbon)/PON (Particulate Organic Nitrogen) in outside and inside of seagrass communities, and the enclosed chamber experiment for evaluation of release of organic matters from seagrass exuadation and the sediment, decomposition experiments in laboratory and in situ. POM (Particulate Organic Matter) of seagrass leaf litter is resistance to the biological degradation under 25°C. (The rate constant of decomposition is 0.0083 d^–1, namely remineralization time is about 120 days.) While POM in water column shows POC is more rapidly consumed to PON, and POC/PON ratio decresed with time of decomposition. DOM in water column shows DON is more rapidly consumed to DOC and DOC/DON ratio increased with time of decomposition. This suggests that rate of solubilization of DON from PON may dominate a rate of remineralization of nutrients in seagrass communities. We developed a kinetic model of the seagrass decomposition process (SDP) changes in POC/PON ratio with time as well as phytoplankton decomposition model (Fujii et al., 2002). The SDP model was adapted to the results of three kinds of decomposition experiments where the initial C/N ratio and the volume ratio of POM and DOM differed by more than a factor of three. The apparent zeroth-order decrease in POC and the constant PON in the first labile decomposition process are well expressed by the SD model, and consequently, the calculated POC/PON ratio shows good agreement with the experimental result. A realistic model for CO₂ sequestration was also developted using net organic production (net organic production = gross organic production – respiration), sedimentation inside seagrass bed and sedimentation at the open ocean (=export flux). We calcultaed net sequestration of carbon into deep water after 100 years under two different sinking velocities which are for 6 m·d^–1 and 24 m·d^–1. These results showed that 120 t-C for 6 m·d^–1 and 150 t-C for 24 m·d^–1 during 100 years were accumulated into deepwater, suggesting that 1.2 to 1.5 t-C·year^–1 for 31 t-C of gross production of seagrass km^–2 are accumulating into deep water. Seagrass ecosystem is very effective as one of the CO₂ sequestration.

View full abstract

A simple model interpreting vertical distribution of soil carbon was constructed and annual degradation rate constants of soil carbon for various forest soils were estimated. The model is based on the material balance at the soil surface, and assumes that the relationship between soil depth and soil carbon is linear. The estimated soil degradation rates were of the similar order of magnitude, and we concluded that this model might be used for rough estimation of soil carbon degradation rates of forest soils where vertical distributions of soil carbon are available.

View full abstract

For reducing amount of CO₂ emitted by acetylene production, a research on energy efficient plasma technologies for converting methane to acetylene was conducted, resulting in development of a tubular circle-to-plate (CTP) type of reactor. The CTP reactor driven by a pulsed plasma converted mixture of methane and CO₂ to acetylene, ethylene, ethane, carbon monoxide, higher hydrocarbons such as C₃ and C₄, and hydrogen. CO₂ was used mainly for stabilizing the plasma discharge, though it also reacted with methane yielding CO and hydrogen. A maximum energy efficiency of 4 mmol/kJ, which was obtained at a pulse frequency of 625 Hz, indicated a potential to reduce CO₂ emission from the acetylene production. The energy efficiency decreased at a pulse frequency above 625 Hz with the CTP reactor, which differed from results with co-axial cylindrical (CAC) and point-to-point (PTP) type reactors. Acetylene selectivity was 62% at maximum at 8 kHz.
The plasma technology would also be useful for relocating conventional fossil fuel dependent acetylene production to the sun-belt where solar thermal power, which is relatively inexpensive among renewable energies, can be utilized to generate plasma. Such relocation accompanied by replacement of fossil energy with renewable energy would result in reduction of global CO₂ emission.

View full abstract

Oil, proteins, organic acids and amino acids were produced from fish meat by sub-critical water hydrolysis. A reaction model for kinetics of sub-critical water hydrolysis was proposed to explain the generation of major products such as oil, cystine, alanine, glycine, and pyroglutamic acid in sub-critical water hydrolysis of fish meat. Since the reaction kinetics in sub-critical water hydrolysis of fish waste is complicated, a simplified model was developed. It shows relatively good agreement with the experimental results for a wide range of reaction temperatures.

View full abstract

Recently, the control of non-CO₂ greenhouse gases has attracted interest as a way to prevent global warming. Several research studies on the development and assessment of technologies to control CH₄ and N₂O emissions by human activities are under way. In the wastewater treatment field, the development of anoxic/oxic processes and the introduction of bacteria that effectively prevent N₂O emission have been studied. Moreover, it has been clarified that eco engineering technologies such as artificial wetlands and soil trench systems are very effective, especially in developing countries. As for landfill disposal, CH₄ oxidation by cover soil and the use of CH₄ gas as a source of energy have very effectively reduced the emission of CH₄. In the combustion field, it was clarified that N₂O emissions vary according to the structure of each combustor and its operational conditions. And research and development concerning combustors and their optimum operational conditions are under way. Concerning automobile sources, characteristics of N₂O emission from gasoline fueled vehicles installed with a catalyst were examined. Three way catalysts and so on are now being developed. Chemical industries, especially adipic acid production are another non-negligible N₂O source, and cracking process related countermeasures have been applied. CH₄ and N₂O emissions from agricultural land are strongly dependent on the oxidation-reduction environment of soil and on fertilization, so they must be suitably managed. The development and management of ruminants feeding and suitable treatment of animal waste are very important ways to control CH₄ and N₂O from livestock. Research projects to improve the CH₄ and N₂O emission-absorption inventory, and to estimate the suitable technologies to control CH₄ and N₂O emission are now in progress. Combining these projects appropriately will develop technologies and systems and will make great contributions to the control of the emission of GHGs and to global warming.

View full abstract

This paper describes the results of field measurements of nitrous oxide (N₂O) emissions from fluidized bed sewage sludge incinerators. N₂O was monitored continuously at six incinerators for 7–14 days. To study possible seasonal differences in N₂O emissions, measurements were done at two incinerators in both summer and winter. N₂O emissions varied with time, with large fluctuations in all incinerators. This result suggests that N₂O emission factors determined by a grab sampling or short-term monitoring are not reliable. The average emission factors obtained were 1, 520–6, 400 g-N₂O/t-dry sludge, which is much higher than the values of 800–1, 500 g-N₂O/t-dry sludge published in “Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories” by the IPCC.
N₂O emissions were mostly determined by the highest freeboard temperature of the incinerator. N₂O emissions were drastically lower at higher values of highest freeboard temperature. The relation of N₂O emissions to the highest freeboard temperatures was nearly the same in all incinerators. This suggests that one can estimate the N₂O emission factor of an incinerator by analyzing the freeboard temperatures for several weeks.
Seasonal differences in N₂O emissions from a small-capacity incinerator seemed to be due to the change in freeboard temperature. A large-capacity incinerator had the same N₂O emissions in the summer and winter, because the change in freeboard temperature was small. Incinerators having larger incineration capacities tended to have higher N₂O emission factors. This means that large capacity incinerators are operated with relatively low freeboard temperature.

View full abstract

Acidic precipitation is a worldwide problem. Acids result from human activities, such as coal burning and the use of internal combustion engines. Though surface soils work to neutralize acidic precipitation and maintain land water at neutral levels, the environmental impact of acidic precipitation on the neutralization ability of soil has not been clarified. In this paper, the effects of acidic precipitation on the neutralization ability of the soil were experimentally examined in batch and column reactors with acidic solutions as the artificial acidic precipitation. Soil was collected in the Hokuriku area, located in the central part on the Japan-Sea side of Japan.
On that coastal area, winter precipitation contains not only high amounts of acidic components, but also high concentrations of calcium and sodium ions derived from the sea. It was found from experimental results that calcium ions disturbed the neutralization ability of soil, but sodium ions did not. In this paper, the neutralization ability of soil was also assessed by a method based on the chemical equilibrium analysis of the relationship between the concentration of metal ions and pH values in the aqueous phase. The assessment suggested that calcium ions in precipitation prolonged the neutralization ability of soil.
Carbon dioxide residing in small spaces in the soil was confirmed to enhance an increase in pH during the neutralization reaction. The existence of carbon dioxide was also found to promote the neutralization ability of soil. To elucidate the life-time neutralization ability of soil in respect of acidic precipitation, the effect of carbon dioxide residing in small spaces in soil cannot be neglected. This indicated that to achieve the aims of the present study, analyses of the solid, liquid and gas phases were required.

View full abstract

In the field of freeze separation, there have been few studies regarding the separation of solutes by forming ice spheres from the solution and melting them. In order to enhance the elution of sulfate ions from ice, spheres formed of sulfate-containing water were examined, and melting experiments were performed.
First, ice spheres containing sulfate ions were packed in a column and melted, the concentration of sulfate ions in the drained effluent was then measured. The ice spheres were formed using two different methods. In the first method, a plastic mold having spherical holes of 18.0 mm diameter was used. The mold was refrigerated for 12 h in order to form spherical ice crystals. The other method involved the formation of falling droplets of the solution through a refrigerant controlled at about –30°C and rapidly freezing them. The ice spheres formed in this way had an average diameter of 3.5 mm. Second, the ice spheres including sulfate were melted from the surface to the center in a glass vessel filled with an organic solvent and the change in the concentration of sulfate due to the melting was measured and the sulfate distribution in the sphere was estimated. In this experiment, the ice spheres having a diameter of 18.0 mm were melted.
The results of the melting experiment using the column showed higher concentrations of sulfate ions in the effluent drained first and lower concentrations in the later effluent. When using solvent for the melting experiment, the distribution of sulfate in an ice sphere was found to be biased; the amount of sulfate was apparently larger near the surface than that around the center of the sphere. It was also found that the solution containing a relatively high amount of sulfate ions for forming the ice sphere tended to establish a uniform sulfate distribution. The maximum concentration near the surface was lower than that observed in the first effluent using the column. Consequently, it was suggested that when the ice spheres containing sulfate ions were packed in a column and melted, a higher concentration of sulfate ions preferentially appeared in the first drained effluent according to the repeated melting and refreezing cycle.

View full abstract

Iron-conditioned zeolite was prepared and used in arsenic removal from groundwater at pH 7.8 ± 0.2 and temperature 293 ± 1 K. The effects of initial arsenic concentration and intermittent agitation were investigated in a batch reaction vessel. Kinetics analysis showed that the adsorption reaction between arsenic and the binding surfaces (probably hydrous ferric oxide) can be approximated by a pseudo-second order rate equation and that chemical reaction is the essential rate-controlling step. The rate constant, the equilibrium sorption capacity and the initial arsenic sorption rate were calculated. The initial arsenic uptake and equilibrium uptake increased with an increase in initial concentration. The intermittent agitation was superior in arsenic uptake, compared to continuous agitation. The external and intra-particle diffusion resistances to mass transfer slightly varied with an increase in initial arsenic concentration.

View full abstract

As a second step in our work of metal recovery from heavy oil fly ash, mutual separation of V/Fe was studied using a chelating resin, DIAION CR11 or a strong acid cation resin, DIAION SK1B. A prototype of a moving bed has been developed and applied for the separation of V/Fe. Sulfuric acid solution of V and Fe was one stream in the leaching process proposed by the authors. Metals should be separately removed from the solution so as to reuse the sulfuric acid solution. The resin CR11 selectively adsorbs Fe which is a major component in the stream, while SK1B removes a large qiantity of both metals. Vanadium can be recovered from the effluent with a bed of the resin SK1B. A moving bed applied with liquid pulse has been developed. Giving pulse to the liquid is effective to obtain a stable plug flow of resin in the bed and the resin flow rate is successfully controlled by amplitude and frequency of the pulse. Characteristics of V/Fe separation are studied under various operating conditions. Degrees of separation of V/Fe are enhanced by the operation applying a large bed height at a small resin flow rate.

View full abstract

In order to recover nickel from waste plating solution, the precipitation of nickel by sulfuration was applied in the present study. Sodium sulfide (Na₂S), sodium disulfide (Na₂S₂) and sodium tetrasulfide (Na₂S₄) were employed as sulfurating agents and were compared in terms of their ability to form Ni precipitates. From the results, it was found that the initial nickel concentration of 100 mg/dm³ was reduced to 0.594–3.37 mg-Ni/dm³ in the filtrate by addition of sulfurating agents. The average specific filtration resistance of the slurry produced by addition of a sulfurating agent was smaller than that of nickel hydroxide slurry; in particular, the lowest specific resistance of the slurry was found to be produced by addition of sodium tetrasulfide at a controlled pH 7 ± 1. The low filtration characteristic of nickel sulfide precipitates obtained by addition of sodium tetrasulfide was attributed to their particle size.

View full abstract

Human and industrial activities have recently lead to problems in regard to the eutrofication of isolated bodies of water due to influx of phosphate ions from industrial and domestic wastewater. Various methods have been developed for the removal of phosphate ions. And in particular, adsorption allows efficient and inexpensive recovery of phosphate ions in which the adsorbant and adsorbate are regenerable. In this study, investigations were carried out regarding the production of aluminum oxide hydroxide and granular aluminum oxide hydroxide, the adsorption and desorption capacities of these substances with respect to phosphate ion, and desorption efficiency and phosphate ion recovery levels obtained using sodium hydroxide and sodium carbonate. Adsorption and desorption were repeated 50 times in order to evaluate the effectiveness of aluminum oxide hydroxide, and it was determined that the adsorption performance of aluminum oxide hydroxide and granular aluminum oxide hydroxide with respect to phosphate ions is highly relative to aluminum oxide. The quantity of phosphate ions adsorbed was seen to decrease with repeated use of aluminum oxide hydroxide. The adsorption mechanism for phosphate ions onto aluminum oxide is conjectured as involving the presence of active hydroxyl groups contributing to ion exchange at the surface of the aluminum oxide hydroxide, ion exchange with phosphate ions, and the chemical bonding of hydroxyl groups with phosphate ions. It was thus clarified that aluminum oxide hydroxide is suitable for use as an adsorbant for the recovery of phosphate ions. Moreover, the investigations suggested that this material would be useful for preserving phosphate ion resources by recovering phosphate, a cause of eutrophication, as a value-added sodium phosphate product.

View full abstract

The removal of harmful organic materials in water has been carried out using the adsorption and desorption characteristics of a temperature-sensitive polymer gel which is synthesized from polyvinylalcohol (PVA). Two types of polymerization degrees (1700 and 2500) of PVA were used in this experiment. The gel volume in water was induced at around 310 K for 1700 and 320 K for 2500. At higher temperatures, the temperature sensitive polymer gel shrinks because of discharging water, whereas, in contrast, at lower temperatures, the gel swells as a result of absorbing water. The reversibility of the volume change of the synthesized polymer gel is confirmed by changing temperature.
The adsorption behavior of organic compounds onto PVA polymer gels in water was investigated at various temperatures. The amount of adsorption of organic compounds increases remarkably at temperatures higher than about 310 K for 1700 and 320 K for 2500. The organic compound in water could be adsorbed and desorbed reversibly onto PVA polymer gel by the temperature swing. The mechanism of adsorption and desorption of organic compounds onto gel can be explained by the hydration and dehydration of the polymer gel. The driving force of the adsorption is thought to be the hydrophobic interaction between PVA polymer gel and organic compounds.

View full abstract

The adsorption behavior of nitrate anion was investigated from aqueous solution using activated carbon (AC) prepared from coconut shells and charcoal (CB) prepared from bamboo. The adsorption of nitrate anions on these adsorbents exhibited maximum values in the region of equilibrium pH 2–4, and was explained by the adsorption of the Langmuir type. The adsorption capacity and the adsorption equilibrium constant for AC were 2.66 × 10^–1 mmol·g^–1 and 2.72 dm³·mmol^–1, respectively, and those for CB were 1.04 × 10^–1 mmol·g^–1 and 3.53 dm³·mmol^–1, respectively, at 303 K. The order of the adsorption capacity was the same as the order of their specific surface areas. This suggests that the specific surface area is one of the factors that determine the adsorption ability for nitrate anions. The theoretical curves calculated using these values were in good agreement with the experimental data. From the obtained thermodynamic parameters, it was found that the adsorption of nitrate anions on AC and CB contributes to the hydrophobic interaction.

View full abstract

An environmentally benign formation method of polymeric microspheres is reported using supercritical carbon dioxide (SC-CO₂). The polymer microspheres were synthesized by the precipitation copolymerization of glycidyl methacrylate (GMA) with methyl methacrylate, n-butyl methacrylate or diethylaminoethyl methacrylate, methacrylic acid (MAA) or 2-hydoxyethyl methacrylate (2-HEMA) in SC-CO₂. Scanning electron microscopy (SEM) showed that the products were spherical microparticles with the addition of MAA and/or 2-HEMA as the monomer. The mean diameter of the particles is 0.5–5 μm. During the copolymerization of GMA with MAA and/or 2-HEMA in SC-CO₂, it was found that MAA and 2-HEMA served as a stabilizer for GMA. The obtained polymer particles were applied as powder coatings, which have been attracting much attention as environmentally benign coating systems. The resulting coatings have a smooth and coherent film. The particle size and morphology were controlled by changing the pressure, temperature, and initial GMA/MAA and/or GMA/2-HEMA concentration ratio. The initial concentration ratio was more effective than the other factors in controlling the particle size.

View full abstract