31.4 Technological Trends and Challenges in the Anaerobic Biorefinery

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technique for lignocellulosic waste is not available yet, so various methods are often

combined to optimize biological conversion into biogas.

The advances in biochemical reactor engineering mainly focus on process integra-

tion and intensification to increase overall energy production and substrate decom-

position, reduce the number of required process steps, and decrease the required

reactor volume. Up to now, biogas production by anaerobic digestion, upgrading

to the quality of natural gas, and its necessary compression to be injected into the

national gas grid are three separate procedures. Nowadays, a technique based on

high pressure favors the upgraded biogas production reaching 95% of methane in

biogas [25].

The Role of High Pressure in Anaerobic Digestion

Pressure changes affect the performance of anaerobic digestion and the solubility

and release of the gaseous end products. Several studies examined the impact of

elevated pressure on biogas quality. According to Henry’s Law, at a given temper-

ature, an increment of the total pressure increases the partial pressure and conse-

quently the solubility of CO2. The equilibrium of CO2 and HCO3−in the liquid form

is affected, and thereupon the pH and buffering capacity of the digester influence the

biogas composition [26]. CO2 is sparingly soluble in water, and its solubility depends

on the partial pressure of the individual species according to Henry’s law:

CCO2 = yCO2 ∗PT∕HCO2

(31.1)

where CCO2 is the liquid-phase concentration of CO2, yCO2 is the gas-phase mole

fraction of CO2, PT is the total pressure, and HCO2 is Henry’s law constant for CO2.

Recently, a novel process condition based on elevated pressure (up to 100 bar)

within the digester reached a methane composition of up to 95%. The goal of

high-pressure digestion is to combine biogas production and purification into a

single process in such a way that the natural gas network accepts this produced

and purified biomethane. At the gas–liquid interface, the concentration of each

gas is in equilibrium, and its diffusivity is affected by any total pressure change.

The gas-to-liquid transfer rate is related to the diffusion, which is driven by the

concentration difference.

As mentioned, biogas mainly consists of CH4 and CO2. The solubility of the two

gases significantly differs under pressure. The CO2 is dissolved much more read-

ily in water, therefore increasing the methane content in the biogas [27]. During

the degradation of organic matter, microorganism produces gases in the liquid. The

gases escape when the liquid is fully saturated and enter the gas phase. Lindeboom

et al. showed that pressure up to 20 bar increased the methane yield suggesting that

the high-pressure auto-generative AD more efficiently degrades the substrate [28].

If more CO2 dissolves in the water under high pressure, the biogas contains less

than 5% CO2. Merkle et al. (2017) studied the anaerobic digestion up to 100 bar

using grass and maize silage hydrolysate as substrate [29]. The results showed sig-

nificantly higher methane yield; however, more research is required to determine

the pressure dependence of the microbial processes. However, the use of multistage