Biogas upgrading by chemical absorption using ammonia rich absorbents derived from wastewater.

TLDR: This demonstrates that ammonia rich wastewaters can facilitate chemically enhanced CO2 separation which eliminates the need for costly exogenic chemicals or complex chemical handling which are critical barriers to implementation of chemical absorption.

About: This article is published in Water Research.The article was published on 2014-12-15 and is currently open access. It has received 42 citations till now. The article focuses on the topics: Ammonium bicarbonate & Nitrification.

Figures

Fig. 4. Effect of increasing NH3 concentration on NH3 slip into the gas phase. Fixed liquid and initial gas flow rates (QL , 1.67 x 10 -6 m3 s-1; QG , 1.25 x 10 -5 m3 s-1). The dotted line represents the concentration limit for NH3 in biomethane for gas-to-grid and vehicular use (0.02 g m -3).

Fig. 5. Effect of variable gas-to-liquid ratio upon enhancement factor (E, dimensionless), determined by ratio of CO2 flux for return liquor or IEX regenerant (2325 g m -3 and 447 g m-3 respectively) against CO2 flux in DI water.

Fig. 6. Effect of variable liquid flow rate (QL, 0.17 x 10 -5-1.7 x 10-5 m3 s-1) upon enhancement factor (E, dimensionless) determined by ratio of CO2 flux for (a) return liquor or (b) IEX regenerant against CO2 flux in DI water. Parity line plotted for reference (dotted line).

Table 3. Characterisation of raw wastewater matrices and subsequently derived absorbents

Fig. 1. (a) Set-up used for gas absorption in a polypropylene micro-porous hollow fibre membrane contactor; (b) ion exchange (IEX) column for removal of ammoniacal nitrogen (NH4-N) from raw sewage by clinoptilolite and subsequent production of NH4-N rich IEX regenerant. SP indicates sampling point.

Fig. 2. Influence of pH and temperature upon CO2 flux (JCO2) in analogue NH3 absorbents with concentrations of: (a) 10000 gNH3 m -3; (b) 1000 gNH3 m -3; (c) 100 gNH3 m -3; and (d) 10 gNH3 m -3 under fixed liquid and gas flow rates (1.67x10-6 m3 s-1 and 1.25x10-5 m3 s-1 respectively).

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Biogas upgrading by chemical absorption using ammonia rich absorbents derived from wastewater" ?

The use of ammonia ( NH3 ) rich wastewaters as an ecological chemical absorption solvent for the selective extraction of carbon dioxide ( CO2 ) during biogas upgrading to ‘ biomethane ’ has been studied. Aqueous ammonia absorbents of up to 10000 gNH3 m -3 demonstrated CO2 absorption rates higher than recorded in the literature for packed columns using 20000-80000 gNH3 m -3 which can be ascribed to the process intensification provided by the hollow fibre membrane contactor used in this study to support absorption. When testing NH3 analogues, the potential to recover the reaction product ammonium bicarbonate ( NH4HCO3 ) in crystalline form was also illustrated. This is significant as it suggests a new pathway for ammonia separation which avoids biological nitrification and produces ammonia stabilised into a commercially viable fertiliser ( NH4HCO3 ). 

Q2. What is the disadvantage of aqueous ammonia solvents?

The key disadvantage of aqueous ammonia solvents is that the high saturated vapour pressure introduces NH3 slip into the gas phase, where outlet flue gas concentrations of up to 2000 ppmv NH3 have been reported (Kozak, 2009). 

Q3. What is the effect of the IEX regenerant on the absorption rate of CO2?

the IEX regenerant provided lower enhancement than an analogue of equivalent concentration which can be ascribed to the salt concentration (50 g L-1) which is known to reduce the physical solubility of CO2 into the absorbent (Mcleod et al., 2013). 

Q4. What is the effect of temperature on the NH3-CO2 flux?

stagnant NH3 within the gas-filled membrane pores, absent in column-based systems, will likely resist NH3 mass transfer from the liquid phase, which could conceivably also play a role. 

Q5. What is the NH3 concentration in the treated gas phase?

At an aqueous NH3 absorbent concentration of 10000 g m-3, which has been achieved in ecological absorbents through concentrating ammonia using ion exchange (Mackinnon et al., 2003), an NH3 concentration of only 0.002 gNH3 m -3 was measured in the treated gas phase. 

Q6. What is the common method of removing biogas impurities?

Whilst trace biogas impurities, such as hydrogen sulfide H2S and particulates, are routinely removed (e.g. by activated carbon) prior to further biogas utilisation (Rautenbach and Welsch, 1994); upgrading to produce biomethane requires additional removal of the bulk CO2 fraction to increase methane content to the equivalent of natural gas and is most commonly undertaken by absorption (Persson et al., 2007). 

Q7. What is the role of the membrane in the formation of ammonia bicarbonate?

the use of the membrane for ammonia CO2 absorption can enable ammonium bicarbonate crystallisation at lower CO2 loadings than previously proposed in packed column investigation. 

Q8. What is the reason for the high availability of Na+ ions?

The high availability of [Na+] ions can be ascribed to the considerable sodium hydroxide addition needed to overcome the liquors buffering capacity to reach pH11. 

Q9. What is the common method of converting sewage sludge to natural gas?

Biogas produced through the anaerobic digestion of sewage sludge can be exploited either through co-generation for electricity and heat production (CHP) or it can be upgraded to natural gas standards (biomethane). 

Q10. How can the NH3-CO2 flux be controlled by a low temperature?

Therefore within environmental absorbents, the reduced reactivity imposed by lower absolute NH4-N concentration can be offset by shifting the equilibrium toward free ammonia using mild pH correction. 

Q11. What is the advantage of a hollow fibre membrane contactor for biogas upgrading?

This is particularly attractive for biogas upgrading using ammonia rich wastewater as the chemical solvent since stabilisation of ammonium into the reaction product circumvents the need for direct biological treatment of ammonium in the wastewater. 

Q12. What did Niu et al. (2012) note about NH3 slip?

Niu et al. (2012) also noted NH3 slip was dependent on initial NH3 concentration when testing ammonia absorbents within the same concentration range for CO2 separation from a packed column. 

Q13. What was the effect of the IEX regenerant on the NH3 absorbent?

Whilst no solid was visible within the absorbent reservoir, SEM analysis of the fibre revealed that NH4HCO3 crystals had grown in situ on the membrane surface. 

Q14. What is the effect of the return liquor on the enhancement of CO2?

Chemical enhancement provided by the return liquor was equivalent to that of the analogue indicating that enhancement was ostensibly a function of initial ammonia concentration. 

Q15. What was the maximum E of the return liquor and IEX regenerant?

A maximum E of 14.9 and 2.9 were determined for the return liquor (2325 gNH3 m -3) and IEX regenerant (447 gNH3 m -3) respectively at a G/L of 37.5. 

Q16. How is the revenue generated from biomethane compared to a conventional WWTW?

To establish key financial benefits from this study, revenue generation from biomethane iscompared to a conventional WWTW where biogas is used in CHP to produce electricity and NH4-N is treated by nitrification.