The objective of the study was actually the investigation of the effect of various organic wastes on the ability of urine in absorbing CO 2. Urine alone or mixed with olive-oil-mill waste waters (O), poultry litter (P) or meat bone meal (M) was used on the absorption of CO 2 from a gas bottle. The absorption capacity (1.35 - 2.85 gCO 2/gNH 4) was bigger than other solvents such as ammonia and amines. The range of CO2 absorption was significantly bigger for the organic mixtures P and PM with urine (9.1 - 11.8) g/L than urine alone 6.5 g/L. These organic wastes could be used to increase CO 2 absorption in urine and reduce gas emissions.
Global warming resulted from emission of greenhouse gases has received widespread attention. Among the greenhouse gases CO2 contributes more than 60% to global warming because of its huge emission amount. Though various CO2 capture technologies including physical absorption, chemical absorption, adsorption and membrane exist, they are not matured yet for post-combustion power plants. Among these technologies chemical absorption using aqueous alkanolamine solution is proposed to be the most applicable technology for CO2 capture. Alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), and N-methyldiethanolamine (MDEA) are widely used as absorbents for CO2 capture [
However, the MEA process suffers the following disadvantages for CO2 separation from flue gases: Low carbon dioxide loading capacity (Kg CO2 absorbed per Kg absorbent); high equipment corrosion rate; amine degradation by SO2, NO2, HCL and HF, and Oxygen in flue gas; high energy consumption during high tem- perature absorbent regeneration.
An ideal absorbent should have a CO2 loading capacity of at least 1 Kg of CO2 per Kg of solution and that the regeneration energy requirement must be much lower than the MEA process. The aqua ammonia process seems to have avoided the shortcomings of the MEA process: Aqueous ammonia has high loading capacity; aqueous ammonia does not pose a corrosion problem; there is no absorbent degradation problem, thus reducing absorbent makeup rate; the energy requirements for absorbent regeneration is predicted to be at least 75% less than in the MEA process [
CO2 removal efficiency and absorption capacity of NH3 solvent are better than those of MEA. The maximum CO2 removal efficiency by NH3 solvent can achieve 99% and the CO2 absorption capacity can approach 1.2 KgCO2/Kg NH3 while for MEA it is only 0.4 KgCO2/Kg MEA [
Aqueous ammonia can be used to capture CO2 from flue gas of coal-fired power plant with quick reaction rate, high removal efficiency, and high loading capacity of CO2 [
However, the absorbents should not be limited to alkanolamines but ionic liquid and other alkaline absorbents as well as the mixtures are also needed to test their potential. The development of suitable absorbents with high CO2 adsorption capacity is still demanded [
In animals ammonia removal is performed through the CO2 by the urine. On the other hand to increase CO2 absorption we could think of using any organic residue as a source of ammonia. So, Ammonia in solution from urine has been used to capture CO2 gas and produce ammonium bicarbonate [
During the hydrolysis of urea of urine or waste-water solutions pH is increased and NH4 produced.
NH4 is in equilibrium with dissolved and gaseous ammonia. The pKa value for this equilibrium is 9.3 at 25˚C.
For other wastes, mineralization is the process by which microbes decompose organic N from manure, or- ganic matter and crop residues to ammonium.
Cumulative net N mineralized in P was fitted to a two-pool, fast and slow first-order models. The fast N pool varied from 11.6% to 56.9% of organic N and could be predicted from uric acid-N-concentration in the litter. Total mineralizable N (fast + slow N pools) range from 46.5% to 86.8% of organic N and could be predicted from uric-acid N and total N concentration. Differences that affect N mineralization rates of P include uric acid concentration (which depends on diet) and moisture content of the litter [
M a potential organic fertiliser for agricultural crop contains considerable amounts of nutrients (on average) 8%N, 5%P, 1%K and 10%Ca [
If the aqueous ammonia is the agent that can remove CO2, that may exist in the flue gas [
The urine used in the experiments was my own family urine, pH 5.9 ± 0.3; EC 13.8 ± 3.5; NH4-N 6 ± 2.3. The O, collected from St. Anthony oil mill of Viznar (Granada, Spain) in January 2013, had a pH of 4.2, EC of 12 dSm−1. The CO2 came from an industrial gas cylinder supplied by the company Air Products Ltd.
Two samples of 200 mL of hydrolysed urine were used as controls. The different treatments for Sample 1 were: (A-urine alone; AP1-urine mixed with 0.25%P in weight; AP2-urine mixed with 0.5%P in weight; AP1O-urine mixed with 0.25%P in weight and 2.5%O in volume; AP2O-urine mixed with 0.5%P in weight and 2.5%O in volume. For the sample 2 were: (B-urine alone; BM-urine mixed with 0.25%M in weight; BMO- urine mixed with 0.25%M in weight and 1%O in volume; BMP-urine mixed with 0.125%M and 0.125%P in weight; BMPO-urine mixed with 0.125%M and 0.125%P in weight and 1%O in volume. When pH increased above 8.5, the BC half samples were kept 45 minutes in a reactor at a pressure of 6 bar CO2 (Days 15-22). In the end the pH of pressured samples decreased to around 7.5. Similarly, the AC half samples were kept 40 minutes in the reactor (Days 11, 13 and 19). All the samples were stored in stove to 25˚C for 3 months with two replicates for each treatment.
Every week, the pH, CO2, and NH4 values as well as the EC of each sample were measured. The pH was monitored using a pH/ion meter, and EC using a conductivity meter (both Crison 2002). The CO2 was analysed following the procedure reported by Lin and Chan [
The results were subjected to an analysis of variance and comparison of means using the PC computer pro- gram Statistic 8.0 (Analytical Software, FL, USA). Also, the figures were plotted with this program.
The average values of pH, EC, NH4, CO2, CO2-Absorption and Absorption capacity (CO2/NH4) of each treat- ment are shown in
In Sample 1, the carbonated treatments increased the CO2 absorption and EC in all samples, indicating that part of CO2 (6.6 - 13.9 g/L) was absorbed by the samples (
The treatments with the highest CO2 absorption were AP1C and AP1OC (11.8 - 11.7 g/L) (see
Treatment | pH | EC (dS/m) | NH4 (g/L) | CO2 (g/L) | CO2-Absorption (g/L) | Absorption Capacity |
---|---|---|---|---|---|---|
A | 9.0c | 44.6b | 6.0ab | 8.2a | 0 | 1.3a |
AC | 8.2ab | 48.7bc | 7.0bc | 14.8b | 6.6a | 2.1b |
AP1 | 9.0c | 35.6a | 5.1a | 8.4a | 2.0a | 1.6ab |
AP1C | 7.8a | 57.3d | 7.6c | 20.6c | 13.8c | 2.7c |
AP1OC | 7.8a | 56.8d | 7.6c | 20.6c | 13.7c | 2.7c |
AP2 | 8.8bc | 35.7a | 5.1a | 10.4a | 3.7a | 2.0b |
AP2C | 8.0a | 54.5cd | 7.6c | 19.9c | 13.0bc | 2.6c |
AP2OC | 7.7a | 55.0d | 7.6c | 20.8c | 13.9c | 2.8c |
B | 9.2c | 30.7a | 5.5ab | 4.2a | 0a | 0.8a |
BC | 8.4abc | 33.2a | 5.3a | 10.1b | 5.9b | 2.0c |
BM | 9.1c | 30.9a | 5.4ab | 7.4ab | 3.2ab | 1.4b |
BMC | 8.3ab | 39.7b | 6.5bc | 17.3c | 13.1c | 2.7d |
BMOC | 7.9a | 41.8b | 7.0c | 19.1c | 14.9c | 2.7d |
BMP | 9.0bc | 30.7a | 5.4ab | 9.5b | 5.2b | 1.8bc |
BMPC | 8.2a | 40.8b | 6.9c | 19.1c | 14.9c | 2.8d |
BMPOC | 7.9a | 39.2b | 6.5bc | 18.6c | 14.4c | 2.9d |
Different letters (a, b, c, d) within a column indicate significative differences between treatments at level of significance (P < 0.05) according to Tukey’s test.
The carbonated treatments increased the mineralised NH4 in all samples (
In Sample 2, the carbonated treatments increased the CO2 absorption and EC in all samples, indicating that part of CO2 (5.9 - 14.9 g/L) was absorbed by the samples.
The treatments with the highest CO2 absorption were BMOC and BMC (11.7 - 10.0 g/L) (see
The carbonated treatments increased the mineralised NH4 in all samples (
Liu et al. [
In our samples with a concentration of aqueous ammonia smaller than 1%, we should think in an absorption time of about 2 hours to get an efficient removal of 100% CO2. The absorption time was 120 minutes for sample 1 and 90 minutes for Sample 2. The shorter absorption time (Sample 2 compared to 1), could explain a smaller increase NH4 and insufficient removal of CO2.
Theoretically according Equation (1), absorption capacity of ammonia was 2.5 (44gCO2/17gNH3). Previous research shows that aqueous ammonia has a higher absorption capacity than that of monoethanolamina (MEA) at same temperature and pressures. Absorption capacity of aqueous ammonia can approach 1.2 KgCO2/KgNH3 while for MEA it is only 0.4 KgCO2/Kg MEA [
Our research shows that the absorption capacity of urine was similar to aqueous ammonia (between 0.8 - 1.3 gCO2/gNH4). However, the absorption capacity of urine-organic samples mixtures (
The absorbed CO2 lowered the pH of the urine mixtures due to the formation of carbonic acid. For all the samples, the pH variations could be explained in function of the CO2 and NH4 increases. So, the pH, CO2, and NH4 showed a statistically significant relationship (P < 0.0001) (correlation coefficient 0.624) at the 95% confi- dence level (see
The addition of a low percentage of O (acid pH) could reduce urine pH and increase buffering capacity due to fatty acids. To more buffering capacity there is a smaller variation in pH for the addition of CO2 with a better stabilization of the CO2 absorbed [
NH4 volatilization increased with the pH, so that all the factors that tended to lower the pH reduced NH4 losses [
The fact that pH of the carbonated samples reaches a next value to 8.0 could be explain because the minerali- zation is still producing new NH4 which is not neutralised by new contributions from CO2.
Ammonia availability rate has a fast decreasing after 30 days [
With the use of organic waste power consumption is minimized because it is not necessary to recover the ab- sorbent again. The CO2 absorbed by the urine mixtures is fixed to the soil as carbonate or organic carbon through fertilization [
The advantages of this technology are undeniable: A process for removal of CO2 at room temperature, with- out catalyst, with relative low-cost and low-energy requirements. The urine and other wastes produced by a family could be stored and be useful to reduce emissions into towns.
Fertilization with urine and O has previously been demonstrated to be a feasible method for reducing the en- vironmental impact of O and CO2 [
P and M could provide N and other plant nutrients when used as fertilizers. The effects of M as N fertiliser were evaluated by Salomonsson (1994) [
However, new wastes, manures, and compost need to be tested with added urine as a means for increasing NH4 contents and CO2 adsorption. The proposed strategy requires further research to reduce the risks associated with the waste-water reuse [
In conclusion, hydrolysed urine mixed with a small percentage (0.25% in weight of P, M, and 1% - 2% in vol- ume of O could be considered a stable long-term system for greenhouse-gas absorption. The absorption coeffi- cient (1.35 - 2.85 gCO2/gNH4) was bigger than other solvents such as ammonia and amines. The range of CO2 absorption was significantly bigger for the organic mixtures P and PM with urine (9.1 - 11.8) g/L than urine alone 6.5 g/L.
Some organic wastes similar to urine should be tested as CO2 sinks. In addition, the reduction of CO2 emis- sions requires further research to increase the NH4 contents and CO2 absorption.
This study was funded by the Institute of Agricultural Research and Training, Andalusian Government. The au- thor is grateful to the anonymous reviewers for insightful comments which greatly improved the quality of the manuscript.