case, the simulation considered an inert fraction of the organic matter of 60%, given
the amount of methane produced in the BMP test compared to the assay carried out
with raw microalgae. The values of the hydrolysis coefficient are shown in Table 5.
The AD of the residual microalgae outperformed the AD of raw microalgae in
continuous mode as OLR increased, which may be explained by the low
biodegradability of the raw microalgae (Fig. 1). However, methane production in
the digester fed with residual microalgae dropped to zero at high OLR values, likely
due to the low values of the hydrolytic constant. In contrast, the digester operated
with raw microalgae supported a low but stable methane productivity, likely due to
the retention of the hydrogenotrophic methane population inside the reactor.
7 Biogas Upgrading to Biomethane
Biogas from the anaerobic digestion of microalgae is typically composed of CH 4
(60–75%), CO 2 (25–30%), H 2 S (0–1%), O 2 (0–1%), N 2 (0–4%) and trace levels of
NH 3 , volatile fatty acids (VFAs) and siloxanes (the latter present in microalgae
grown in domestic wastewater) (Alzate et al. 2012). Biogas composition determines
the final energy use of this renewable energy feedstock, which ranges from on-site
combustion for heat (boilers) or heat/electricity generation (internal combustion
engines, turbines, fuel cells), use as a vehicle fuel, and injection into natural grad
grids (Bailón and Hinge 2012). In this context, while boilers and internal combustion engines require a removal of H 2 S below 0.02–0.1% levels (depending on
the manufacturer), micro-turbines and turbines can stand H 2 S concentrations in the
range of 1–7%. However, the latter require an efficient removal of siloxanes
(<0.03–0.1 ppm v ), while internal combustion engines and boilers can cope with
concentrations of 5–28 mg Si m
−3 . Nowadays, the technical requirements for
biogas injection into natural gas grids or biogas used as a vehicle fuel are
country-specific, although a European draft for biogas quality is currently under
approval (Table 6). This entails the need for a biogas-upgrading step prior biogas
valorization, which will be stricter when biogas is to be injected into natural gas
networks (in the form of biomethane).
Biogas-upgrading technologies can be classified into physical/chemical and
biological as a function of the mechanisms governing pollutants removal from
biogas. Nowadays, O 2 and N 2 can be only removed by physical/chemical methods
(such as membrane separation or low-pressure PSA) (Muñoz et al. 2015), while the
removal of CO 2 , H 2 S, NH 3 , VFAs and even siloxanes can be carried using both
platform technologies.
Today, the market of CO 2 removal is mainly dominated by water scrubbing
(with a 41% of the market share), followed by chemical scrubbing (22%), pressure
swing adsorption (21%), membrane separation (10%) and organic solvent scrubbing (6%) (Thrän et al. 2014). Physical/chemical technologies for CO 2 removal
from biogas exhibit a high efficiency and robustness at the expenses of high
investment and operating costs. Typical CH 4 concentrations in the biomethane
12 Biofuels from Microalgae: Biomethane
261
the amount of methane produced in the BMP test compared to the assay carried out
with raw microalgae. The values of the hydrolysis coefficient are shown in Table 5.
The AD of the residual microalgae outperformed the AD of raw microalgae in
continuous mode as OLR increased, which may be explained by the low
biodegradability of the raw microalgae (Fig. 1). However, methane production in
the digester fed with residual microalgae dropped to zero at high OLR values, likely
due to the low values of the hydrolytic constant. In contrast, the digester operated
with raw microalgae supported a low but stable methane productivity, likely due to
the retention of the hydrogenotrophic methane population inside the reactor.
7 Biogas Upgrading to Biomethane
Biogas from the anaerobic digestion of microalgae is typically composed of CH 4
(60–75%), CO 2 (25–30%), H 2 S (0–1%), O 2 (0–1%), N 2 (0–4%) and trace levels of
NH 3 , volatile fatty acids (VFAs) and siloxanes (the latter present in microalgae
grown in domestic wastewater) (Alzate et al. 2012). Biogas composition determines
the final energy use of this renewable energy feedstock, which ranges from on-site
combustion for heat (boilers) or heat/electricity generation (internal combustion
engines, turbines, fuel cells), use as a vehicle fuel, and injection into natural grad
grids (Bailón and Hinge 2012). In this context, while boilers and internal combustion engines require a removal of H 2 S below 0.02–0.1% levels (depending on
the manufacturer), micro-turbines and turbines can stand H 2 S concentrations in the
range of 1–7%. However, the latter require an efficient removal of siloxanes
(<0.03–0.1 ppm v ), while internal combustion engines and boilers can cope with
concentrations of 5–28 mg Si m
−3 . Nowadays, the technical requirements for
biogas injection into natural gas grids or biogas used as a vehicle fuel are
country-specific, although a European draft for biogas quality is currently under
approval (Table 6). This entails the need for a biogas-upgrading step prior biogas
valorization, which will be stricter when biogas is to be injected into natural gas
networks (in the form of biomethane).
Biogas-upgrading technologies can be classified into physical/chemical and
biological as a function of the mechanisms governing pollutants removal from
biogas. Nowadays, O 2 and N 2 can be only removed by physical/chemical methods
(such as membrane separation or low-pressure PSA) (Muñoz et al. 2015), while the
removal of CO 2 , H 2 S, NH 3 , VFAs and even siloxanes can be carried using both
platform technologies.
Today, the market of CO 2 removal is mainly dominated by water scrubbing
(with a 41% of the market share), followed by chemical scrubbing (22%), pressure
swing adsorption (21%), membrane separation (10%) and organic solvent scrubbing (6%) (Thrän et al. 2014). Physical/chemical technologies for CO 2 removal
from biogas exhibit a high efficiency and robustness at the expenses of high
investment and operating costs. Typical CH 4 concentrations in the biomethane
12 Biofuels from Microalgae: Biomethane
261