One-stage H₂ and CH₄ and two-stage H₂ + CH₄ production from grass silage and from solid and liquid fractions of NaOH pre-treated grass silage

Abstract

In the present study, mesophilic CH₄ production from grass silage in a one-stage process was compared with the combined thermophilic H₂ and mesophilic CH₄ production in a two-stage process. In addition, solid and liquid fractions separated from NaOH pre-treated grass silage were also used as substrates. Results showed that higher CH₄ yield was obtained from grass silage in a two-stage process (467 ml g⁻¹ volatile solids (VS)_original) compared with a one-stage process (431 ml g⁻¹ VS_original). Similarly, CH₄ yield from solid fraction increased from 252 to 413 ml g⁻¹ VS_original whereas CH₄ yield from liquid fraction decreased from 82 to 60 ml g⁻¹ VS_original in a two-stage compared to a one-stage process. NaOH pre-treatment increased combined H₂ yield by 15% (from 5.54 to 6.46 ml g⁻¹ VS_original). In contrast, NaOH pre-treatment decreased the combined CH₄ yield by 23%. Compared to the energy value of CH₄ yield obtained, the energy value of H₂ yield remained low. According to this study, highest CH₄ yield (495 ml g⁻¹ VS_original) could be obtained, if grass silage was first pre-treated with NaOH, and the separated solid fraction was digested in a two-stage (thermophilic H₂ and mesophilic CH₄) process while the liquid fraction could be treated directly in a one-stage CH₄ process.

Introduction

Hydrogen (H₂) and methane (CH₄) are both valuable fuels and can be used for heat and electricity production or used as traffic fuels – either separately or as a mixture of H₂ and CH₄ known as hythane [1]. H₂ is the preferred fuel for fuel cells, while CH₄ can also be used in solid oxide fuel cells (e.g. see Ref. [2]). On mass basis, H₂ has a lower heating value (LHV) of 33.33 kWh kg⁻¹ which is over two times higher than LHV of CH₄ (13.90 kWh kg⁻¹). On volume basis, the energy content of H₂ (LHV of 2.995 kWh Nm⁻³ (N meaning here the normal conditions of temperature and pressure)) is, however, three times less than that of CH₄ (LHV of 9.968 kWh Nm⁻³) (e.g. see Refs. [3], [4]).

Anaerobic degradation of organic matter is a complex series of metabolic interactions among different anaerobic microorganisms and is classified into four main stages, namely, hydrolysis, acidogenesis, acetogenesis and methanogenesis. During hydrolysis, organic polymers, such as cellulose, are degraded and solubilized into monomers, e.g. glucose. Acidogenic bacteria then convert these solubilized monomers to, e.g. volatile fatty acids (VFA) and H₂. In the traditional anaerobic digestion process H₂ is usually not detected as H₂ is consumed during, e.g. homoacetogenesis and methanogenesis to produce CH₄ and CO₂ as final end products. However, the process can be moved towards H₂ production instead of CH₄ by controlling the operational parameters, e.g. pH and temperature (e.g. see Refs. [5], [6]).

Owing to the advantageous properties of H₂ as a fuel, there is an increasing interest in H₂ production from organic substrates through anaerobic digestion, i.e. dark fermentative H₂ production (e.g. reviews of Refs. [5], [6]). Dark fermentative H₂ production has been intensively studied with model organic compounds, e.g. glucose and sucrose, whereas less work has been done with solid substrates (e.g. see Refs. [7], [8]) and real wastewaters. H₂ yields from solid substrates were shown to be dependent on the chemical nature of the substrate and operational conditions. Previous researchers reported H₂ yields of 36 ml g⁻¹ total solids (TS, converted by the authors from 1.6 mmol g⁻¹ TS) from olive pulp [9], 170 ml g⁻¹ VS from sugarcane [10], and 360 Nml g⁻¹ VS_removed from organic fraction of municipal solid waste (OFMSW) [11].

However, the H₂ yields obtained through dark fermentation are typically only about 10–20% of the energy content of the substrate [12]. Digestates or effluents from dark fermentation can further be used to recover the residual energy content as they usually contain VFAs and other degradation products which are not degraded further to H₂ due to thermodynamic restrictions (e.g. see Ref. [13]). According to Hawkes et al. [14] there are three different possibilities for a second stage in digestate treatment. These are photofermentation of acetate and butyrate to H₂, microbial fuel cells converting acetate and butyrate to electricity, and an anaerobic digestion process converting VFAs to CH₄. A combined two-stage H₂ and CH₄ process has been proposed as a promising technology as it has shown enhanced hydrolysis and higher energy yields than a one-stage methanogenic process (e.g. see Refs. [14], [15], [16]). Previous studies have demonstrated that CH₄ yields from household solid waste [16], artificial organic solid waste [15] and wastewater sludge [17] can be increased in two-stage processes compared to one-stage processes. Moreover, two-stage H₂ and CH₄ production in a pilot scale (first stage 50 l, second stage 220 l) has also been demonstrated successfully. In the above study, H₂ and CH₄ yields of 0.29 and 0.24 m³ kg⁻¹ VS added were obtained from food waste at an organic loading rate of 12.5 kg VS m³⁻¹ d⁻¹ [18]. In a two-stage system, the growth of acidogenic and methanogenic bacteria is optimized separately. In the first stage, low pH (e.g. see Refs. [5], [6]) and short hydraulic residence times (HRT) (1–2 days) are maintained thus favoring acidogenic, H₂-forming bacteria, while the conditions for slower growing methanogens with neutral pH and longer HRT (typically 10–20 days) are maintained in the second stage (e.g. see Refs. [12], [16]). In addition, a thermophilic or hyperthermophilic first stage, which improves hydrolysis, is also an efficient method of pathogenic destruction (e.g. see Ref. [16]) and thermophilic conditions have so far shown to favour H₂ formation (e.g. see Ref. [11]) by depressing H₂ consuming reactions [19].

Energy crops, i.e. crops grown specifically for the purpose of energy production, are abundant producers of biomass. The produced biomass can be used for CH₄ and/or H₂ production. Crops are mainly composed of lignocellulose, that is, cellulose, hemicelluloses and lignin, which are tightly linked to each other [20]. For successful utilisation of lignocellulosic biomass for bioenergy production, pre-treatment such as thermo-chemical [21] or steam-explosion [22] might be essential. These pre-treatments have been shown to increase carbohydrate availability and thus CH₄ and H₂ yields, e.g. see Refs. [21], [22]. In addition, alkaline treatments have also shown to increase CH₄ production of different lignocellulosic materials, e.g. see Refs. [23], [24], [25], [26]. For example 9 and 15% more CH₄ was obtained from alkaline-treated grass and sugar beet tops, respectively, compared to untreated crops [26]. The increase in CH₄ yield was attributed mainly to the improved hydrolysis as alkalis are known to break the bonds between hemicelluloses and lignin as well as swell the fibres and increase the pore size, e.g. see Refs. [23], [24], [25], [26]. However, Na and K ions present in the alkali can inhibit H₂ and CH₄ production [23], [27] and alkaline treatment can cause the degradation of lignocellulose to refractory and/or inhibitory aromatic compounds, e.g. see Ref. [23].

The objective of this study was to evaluate the CH₄ (mesophilic) production from grass silage in one-stage process and to compare that with the combined H₂ (thermophilic) and CH₄ (mesophilic) production in two-stage process. In addition, solid and liquid fractions separated from NaOH pre-treated grass silage were also used as substrates. Finally, the total energy production from one- and two-stage processes was estimated. A potential energy crop, namely, grass silage, was chosen as a substrate, as it is rather abundant in agricultural sector and also showed potential in CH₄ and H₂ production in our previous studies [28], [29].