Бурение грунтовых зондов, установка энергетических колодцев

In biological processes, the growth and product formation rate of microorganism may be reduced or even inhibited by products, contaminants, and by impurities in the syngas. For example, pro­duction of organic acids is known to be associated with hydrogen formation. However, increase of the H2 partial pressure in the gas phase, as well as accumulation of H2 in the fermentation medium, may inhibit the fermentation and acetogenesis due to the altera­tion of carbon flow in the biological pathway of organism [34]. In addition, higher concentrations of CO2 can be a possible source of inhibition as well, as CO2 dissolves in water making carbonic acid or its carbonate derivatives affecting the medium pH.

The impurities in the syngas can also affect the growth of micro­organisms and product yields [35,36]. Once an impurity trans­fers from the syngas into the bioreactor media, the impurity may directly affect the organism by causing cell toxicity by enzyme inhi­bition and this may affect product distribution as well, or indirectly affect the fermentation process by changing process conditions like pH, osmolarity, redox potential, etc.

Depending on the concentration of impurities, syngas can be cleaned up before the fermentation. Selection of commercial tech­nologies suitable for syngas cleanup is mainly based on the cost and the ability to meet the end-user specifications. The impurities in biomass-derived syngas can be categorized into solid impurities, tars and gaseous impurities [37,38]. Filters and cyclones are com­monly utilized for removal of particulate matter or solid impurities. In general, tar removal technologies can be branched into primary methods inside the gasifier treatments and secondary methods after the gasifier treatments. Generally primary inside the gasifier tar cracking methods can effectively convert the heavy and light hydrocarbons to negligible levels. In addition to this, scrubbing with water can be employed for removal of water soluble gaseous impurities such as ammonia, HCl and chlorine and other trace impurities. Zinc oxide beds are also popular for removal of sulfur in the syngas. A more detailed discussion on syngas cleaning is in Section 11.10 in Chapter 11.

The toxicity of impurities on bacterium are due to inhibition of various enzymes, and these enzyme inhibitory effects of common gaseous impurities NH3, NO, NO2, H2S, COS, and SO2 are summa­rized in Table 12.3.

Effect of NH3

Ammonia in the syngas can affect enzymes alcohol dehydrogenase and amidase. Xu and Lewis have studied the effects of ammonia impurity in raw syngas on dehydrogenase activity [45]. In this work, it was shown that NH3 rapidly converts to ammonium ion (NH4+) following exposure of fermentation broth to NH3, and they found that accumulated NH4+ also inhibited dehydrogenase activity

and cell growth. A kinetic model for dehydrogenase activity that included inhibition effects from NH4+ was developed in this work, and K (Michaelis-Menten constant) and KNH+ (the inhibition con­stant for NH4+) were included as model parameters. Experimental results showed that NH4+ behaves as a non-competitive inhibitor for dehydrogenase enzyme with KNH+ of 649 ± 35 mol m-3. As part of the work, Xu and Lewis have been able to distinguish the unique aspect of NH4+ inhibition by comparison with other species such as K+ and phosphate ions, by proving that potassium and phosphate ions had no effect on hydrogenase activity. Since NH4+ can easily be accumulated in fermentation media and transport across the cell membrane, they concluded that it is crucial to remove NH3 impu­rity from raw syngas to minimize the reduction in alcohol dehydro­genase activity.

Effect of Nitric Oxide

Nitric oxide (NO) present in the syngas at concentrations greater than 0.004 mol% can inhibit the enzyme hydrogenase, and this is a reversible, non-competitive inhibitor activity. In addition, NO also had an adverse effect on cell growth and may contribute to increased production of acetic acid. Syngas fermentation using C. carboxidivorans has shown that NO concentrations less than 0.004 mol% had no effect on the efficiency of the process. Therefore, gas cleanup up to 0.004 mol% NO is sufficient in most of the syngas fermentation operations.

Комментарии запрещены.