Proton exchange membranes (PEMs) are often used in microbial fuel cells (MFCs) to separate the liquid in the anode and cathode chambers while allowing protons to pass between the chambers. However, negatively or positively charged species present at high concentrations in the medium can also be used to maintain charge balance during power generation. An anion exchange membrane (AEM) produced the largest power density (up to 610 mW/m2) and Coulombic efficiency (72%) in MFCs relative to values achieved with a commonly used PEM (Nafion), a cation exchange membrane (CEM), or three different ultrafiltration (UF) membranes with molecular weight cut offs of 0.5K, 1K, and 3K Daltons in different types of MFCs. The increased performance of the AEM was due to proton charge-transfer facilitated by phosphate anions and low internal resistance. The type of membrane affected maximum power densities in two-chamber, air-cathode cube MFCs (C-MFCs) with low internal resistance (84-91 omega for all membranes except UF-0.5K) but not in two-chamber aqueous-cathode bottle MFCs (B-MFCs) due to their higher internal resistances (1230-1272 omega except UF-0.5K). The UF-0.5K membrane produced very high internal resistances (6009 omega, B-MFC; 1814omega, C-MFC) and was the least permeable to both oxygen (mass transfer coefficient of k(O) = 0.19 x 10(-4) cm/s) and acetate (k(A) = 0.89 x 10(-8) cm/s). Nafion was the most permeable membrane to oxygen (k(O) = 1.3 x 10(-4) cm/s), and the UF-3K membrane was the most permeable to acetate (k(A) = 7.2 x 10(-8) cm/s). Only a small percent of substrate was unaccounted for based on measured Coulombic efficiencies and estimates of biomass production and substrate losses using Nafion, CEM, and AEM membranes (4-8%), while a substantial portion of substrate was lost to unidentified processes for the UF membranes (40-89%). These results show that many types of membranes can be used in two-chambered MFCs, even membranes that transfer negatively charged species.

This work is devoted to the experimental study of biomass gasification in a pilot-scale circulating fluidized bed, and development of an equilibrium model of the process based on Gibbs free-energy minimization. Biomass gasification has considerable potential for reducing greenhouse gas emissions. In the present study, six types of sawdust were gasified in a pilotscale air-blown circulating fluidized bed gasifier to produce low-calorific-value gases. The pilot gasifier employs a riser 6.5 m high and 0.1 m in diameter, a high-temperature cyclone for solids recycle and a ceramic fibre filter unit for gas cleaning. The riser temperature was maintained at 970-1120 K (700-850°C), while the sawdust feed rate varied from 16-45 kg/h, corresponding to a superficial gas velocity of 4-10 m/s. It was found that gas composition and heating value depended heavily on the air or O/C ratio, and to a lesser extent on operating temperature. The higher heating value of the product gas decreased from 5.6 to 2.1 MJ/Nm 3 as the stoichiometric air ratio increased from 0.22 to 0.54. The gas heating value was increased by increasing the overall suspension density in the riser. Fly ash re-injection and steam injection led to increases in gas heating value for the same Q/C molar ratio. Tar yield from biomass gasification was found to decrease drastically from 15 to 0.54 g/Nin 3 as the average suspension temperature increased from 970 to 1090 K. Elevating the operating temperature provides the simplest solution for tar removal in the absence of any catalyst. Secondary air had only a very limited effect on tar removal with the total air ratio maintained constant. A nickel-based, catalyst proved to be effective in reducing the tar yield and in adjusting the gas composition. Ill The cold gas efficiency decreased with increasing air ratio (or O/C molar ratio), though the carbon conversion increased. The cold gas efficiency provides a better criterion for evaluating the gasification process than the carbon conversion. Experimental data showed that the gasification efficiency can be maximized within an optimum range of air ratio (a = 0.30-0.35, or O/C = 1.5-1.7), while keeping the tar yield acceptably low. A non-stoichiometric equilibrium model based on Gibbs free energy minimization was developed for biomass gasification. Five elements (C, H, O, N and S) and 44 species were considered in the model. Both pure equilibrium and situations where kinetic factors cause a partial approach to equilibrium are considered. The equilibrium model predicts that the product gas composition from gasification of woody biomass (e.g. sawdust) depends primarily on the air ratio. An air ratio of 0.2-0.3 is predicted to be most favourable for producing CO-rich gas, while temperatures of 1200-1400 K and an air ratio of 0.15-0.25 are predicted to be optimum for H 2 production. The predicted cold gas efficiency reached a maximum at an air ratio of about 0.25. The model successfully predicts the onset of carbon formation in a C-H-O-dominated system when the relative abundan...

No abstract

Ammonia losses during swine wastewater treatment were examined using single- and two-chambered microbial fuel cells (MFCs). Ammonia removal was 60% over 5 days for a single-chamber MFC with the cathode exposed to air (air-cathode), versus 69% over 13 days from the anode chamber in a two-chamber MFC with a ferricyanide catholyte. In both types of systems, ammonia losses were accelerated with electricity generation. For the air-cathode system, our results suggest that nitrogen losses during electricity generation were increased due to ammonia volatilization with conversion of ammonium ion to the more volatile ammonia species as a result of an elevated pH near the cathode (where protons are consumed). This loss mechanism was supported by abiotic tests (applied voltage of 1.1 V). In a two-chamber MFC, nitrogen losses were primarily due to ammonium ion diffusion through the membrane connecting the anode and cathode chambers. This loss was higher with electricity generation as the rate of ammonium transport was increased by charge transfer across the membrane. Ammonia was not found to be used as a substrate for electricity generation, as intermittent ammonia injections did not produce power. The ammonia-oxidizing bacterium Nitrosomonas europaea was found on the cathode electrode of the single-chamber system, supporting evidence of biological nitrification, but anaerobic ammonia-oxidizing bacteria were not detected by molecular analyses. It is concluded that ammonia losses from the anode chamber were driven primarily by physical-chemical factors that are increased with electricity generation, although some losses may occur through biological nitrification and denitrification.