Biohydrogen Generation and Other Methods - Hydrogen Generation, Storage, and Utilization - page 51

“photobiohydrogen” with microorganisms capable of using solar photons to
separate oxygen from water. The second is production using biotechnologies-
based microorganisms that produce hydrogen naturally.
However, obtaining hydrogen from biomass has major challenges in prac-
tice. Currently, there are no completed technology platforms for large-scale
biohydrogen generation even though small or laboratory scale demonstra-
tions have been done. The yield of hydrogen is low from biomass since the
hydrogen content in biomass is low to begin with (approximately 6% versus
25% for methane) and the energy content is low due to the 40% oxygen
content of biomass. Due to the potential advantages mentioned earlier, sub-
stantial research efforts have been made to explore the potential of biohy-
drogen generation.
4.2 PATHWAYS OF BIOHYDROGEN PRODUCTION FROM BIOMASS
The methods available for hydrogen production from biomass can be divided
into two main categories: thermochemical and biological routes. The major
biomass-to-hydrogen pathways are shown in Figure 4.1 [3]. Hydrogen can
be produced from biorenewable feedstock via thermo chemical conversion
processes such as pyrolysis, gasification, steam gasification, steam reforming
of bio-oils, and supercritical-water gasification. Biological production of
hydrogen can be classified into the following groups: (i) biophotolysis of
water using green algae and blue-green algae (cyanobacteria), (ii) photofer-
mentation, (iii) dark fermentation, and (iv) hybrid reactor system.
The advantage of the thermochemical process is that its overall efficiency
(thermal to hydrogen) is higher (η∼52%) and production cost is lower [4].
The yield of hydrogen that can be produced from biomass is relatively low,
16-18% based on dry biomass weight [5]. Hydrogen yields and energy con-
tents, compared with biomass energy contents obtained from processes with
biomass, are shown in Table 4.1 [6]. In the pyrolysis and gasification pro-
cesses, water gas shift is used to convert the reformed gas into hydrogen,
and pressure swing adsorption is used to purify the product. Compared with
other biomass thermo chemical gasification such as air gasification or steam
gasification, the supercritical water gasification can directly deal with the wet
biomass without drying, and has high gasification efficiency in lower tem-
perature [7]. The major disadvantage of these processes is that the decom-
position of the biomass feedstock leads to char and tar formation [8]. In order
to optimize the process for hydrogen production, efforts have been made by
researchers to test hydrogen production from biomass gasification/pyrolysis
with various biomass types and under different operating conditions.