Clean Conversion of Aqueous Ammonia Using a Solid Oxide Cell
Authors: Ragu Marta , Laycock Christian James , Owen Gareth , Lloyd Gareth , Guwy Alan 
Published: July 2021

Ammonia (NH3) is present as an impurity in coke oven gas (COG), which is a by-product of coke-making processes required for the manufacture of steel. Due to its highly corrosive and toxic nature, ammonia is removed from COG by water scrubbing processes generating a highly problematic aqueous waste stream containing ammonia (19 vol%), hydrogen sulphide (4 vol%), carbon dioxide (14 vol%) and tars. At Tata Steel Port Talbot (South Wales, UK) alone, approximately 44,000 m3 of COG and 264 kg of ammonia are produced every hour. This is currently disposed of via incineration, contributing to air pollutant emissions and wasting a valuable and useful resource.

It is well known that solid oxide fuel cells (SOFCs) can convert ammonia gas to electrical power and heat when operating in fuel cell mode [1-3]. In this work, conversion of a liquid aqueous ammonium hydroxide solution (35%) using a commercially available anode-supported button cell was investigated in fuel cell and electrolysis mode. This mixture served as a basic initial simulation of the waste ammonia streams produced from coke-making processes. The electrical performance of the cell was characterised using I-V curves and electrochemical impedance spectroscopy, and the output gases of the cell were measured in real-time using quadrupole mass spectrometry (QMS).

The QMS data show the ammonia decomposed to nitrogen and hydrogen at the open circuit voltage (OCV). In fuel cell mode, the cell utilised hydrogen to produce electrical power. I-V curves were measured showing the effect of the ammonium hydroxide solution on cell efficiency compared with deionised water (see figure). Increasing the deionised water flow rate decreased the OCV considerably and substantially increased the concentration losses; under a 20/80 vol% H2/H2O mixture, the cell produced 50-80% of the current yielded under pure H2. With the ammonium hydroxide solution, OCV and concentration losses were affected much less as the flow rate of the mixture was increased, indicating conversion of ammonia into electrical power with high efficiency. The I-V curves and electrochemical impedance spectra indicated very low activation losses.

In electrolysis mode, the presence of ammonium hydroxide decreased performance and this was attributed to the decreased presence of H2O, which the I-V curves (see figure b) and impedance spectra show increased the activation and concentration losses. However, the QMS data have demonstrated that electrolysis of H2/NH4OH mixtures increases the H2 production by a factor of approx. 2.3 and yields a gas mixture composed of approx. 87 vol% H2 balanced in N2, with no NO
x
production observed.

References:

Fuerte, A., R.X. Valenzuela, M.J. Escudero, and L. Daza, Ammonia as efficient fuel for SOFC. Journal of Power Sources, 2009. 192(1): p. 170-174.

Wojcik, A., H. Middleton, I. Damopoulos, and J. Van herle, Ammonia as a fuel in solid oxide fuel cells. Journal of Power Sources, 2003. 118(1-2): p. 342-348.

Stoeckl, B., V. Subotić, M. Preininger, M. Schwaiger, N. Evic, H. Schroettner, and C. Hochenauer, Characterization and performance evaluation of ammonia as fuel for solid oxide fuel cells with Ni/YSZ anodes. Electrochimica Acta, 2019. 298: p. 874-883.

Figure. I-V and power curves of anode supported cell operating on: (a) H2/H2O and (b) H2/NH4OH. H2O and NH4OH were fed into the test setup in the liquid state. NH4OH was added as a 35% aqueous solution. The effects of H2/H2O ratio and H2/NH4OH are shown.

Figure 1