Hydrothermal pretreatment of rice straw biomass: A potential and promising method for enhanced methane production

Abstract

This paper presents the results of an experimental batch methane fermentation (at 37 °C mesophilic temperature) study carried out on untreated and pretreated substrates of rice straw using NaOH and hydrothermal pretreatments. 3% NaOH pretreatment was given to ground rice straw biomass for 120 h at 37 °C and hydrothermal pretreatment was given for 10 min at 200 °C. It was observed that NaOH addition is a mandatory requirement for maintaining a suitable range of pH and starting the biogas production from hydrothermal pretreated biomass slurry of rice straw. The fed substrate concentrations were maintained at 5% TS (50 g TS/L). The study revealed into 140.0 L/kg VS_a biogas and 59.8 L/kg VS_a methane from untreated rice straw substrate. However, NaOH pretreated substrate resulted into 184.8 L/kg VS_a biogas and 74.1 L/kg VS_a methane. Hydrothermal pretreated followed by 5% NaOH added substrate resulted into highest biogas and methane production yields as 315.9 L/kg VS_a and 132.7 L/kg VS_a, respectively. NaOH pretreated substrate showed an increase of 132.0% in biogas production and 123.9% in methane production relative to the untreated substrate. However, the hydrothermal pretreated substrate had resulted into an increase of 225.6% in biogas production and 222.0% in methane production relative to untreated rice straw substrate. Hydrothermal pretreatment provided an accelerated pre-hydrolysis of biomass contents during the treatment process and thereby resulted into enhanced biogas and methane production yields.

Introduction

The combustion of fossil originated fuels is a big contributor for increasing the level of CO₂ in the atmosphere, which in turn is directly associated with global warming [1], [2]. Further, the future energy security concerns along with the increasing concentration of the carbon dioxide and methane greenhouse gases emission problems has strengthened the interest in development and utilization of alternative, non-petroleum based renewable sources of energy [3], [4]. Bio-fuels and bio-products produced from plant biomass would have potential of mitigating the problem of global warming. Biomass absorbs CO₂ during growth, and emits it during combustion. Therefore, biomass helps the atmospheric CO₂ recycling and does not contribute to the greenhouse gases effect. Biomass production consumes the same amount of CO₂ from the atmosphere during its growth as is released during combustion. In addition, overall CO₂ emissions can be reduced because biomass is a CO₂ neutral fuel. Simultaneously, bio-fuel production along with bio-products can suitably provide new income and employment opportunities in rural areas [5], [6].

The second generation of bio-fuel production from renewable resources ‘plant biomass’ refers particularly to the lignocellulosic biomass materials, as this makes up the majority of the cheap and abundant non-food materials available from plants. Therefore, lignocellulosic feedstock can offer the potential to provide novel bio-fuels of the “second generation of bio-fuels production” [7]. Biogas is an important renewable fuel among the various biomass derived renewable fuel, particularly for rural areas. It is an environment friendly, clean, cheap and versatile fuel. In fact, all over the world biogas has been extensively used for heating purposes and/or electricity generation. Furthermore, methane fermentation technology is a most efficient way of handling and energy generation from biomass in term of energy output/input ratio (28.8 MJ/MJ) in comparison to all other technology of energy production through biological and thermo-chemical routes of energy conversion processes [8], [9]. Methane production from any biomass is always a better option than bio-ethanol production, as most part of the biomass contents (carbohydrates, fats and proteins) in anaerobic digestion process, is converted into simple derivatives and finally into methane and carbon dioxide with help of the different types of anaerobic and methanogenic bacteria. However, in case of alcoholic fermentation process only carbohydrates is converted into simple sugars and finally into ethanol. Thus, the ethanol production yield is lower than the methane production yield [10]. Lignocellulosic agricultural biomass is primarily composed of cellulose, hemicellulose and lignin. However, it also contains about 1–2% fat and 3–4% crude protein. The fat and protein contents part of biomass remains unutilized in alcoholic fermentation process. According to the IPCC 2001, the global warming potential of methane over 100 years relative to the carbon dioxide is 23 times higher [11]. Therefore, it is essential to reduce natural emission of methane that naturally occurs due to self-decomposition of biomass left outside (particularly wet and highly perishable biomass containing high moisture).

Further, the Asia’s energy security is considered one of the most fragile in the world because the region is heavily dependent on imported fossil oil particularly petroleum based fuels such as gasoline, diesel fuel, liquefied petroleum gas, and compressed natural gas to satisfy the demand of its transport sector. Therefore, a widespread recent interest in the development of renewable energy technologies and policies, particularly those that promote the expansion of bio-fuel production, is believed to be one of the paths to achieve energy security vis–a–vis an overall sustainable development [12], [13], [14]. Huge amount of agricultural biomass is burnt annually in field/open environment that releases harmful gases and is not a way of sustainable development of society. As the prime concern of “second generation of bio-fuels production” is to produce renewable fuels/chemicals from agricultural waste biomass in order to meet the goals of sustainable and ecological development of earth planet. Lignocellulosic agricultural crops waste/biomass has huge unutilized potential in order to meet energy/fuel demand. Furthermore, in order to generate renewable fuel/energy in a sustainable way, the energy efficient fermentation of lignocellulosic agricultural biomass seems to be a best idea for developed as well as developing countries. Rice ranked at third as major agricultural crop grown in the world, in term of total cultivated area of 161.42 million hectare with a gross grain yield production of 678.69 million tonnes in year 2009. The total estimated dry lignocellulosic biomass production from rice cultivation stood at about 905 million tonnes per annum [15]. Further, rice straw is one of the most abundant lignocellulosic agricultural crop wastes in Japan, with an annual production of about 13.0 million tonnes. According to the data of the Food and Agriculture Organization of the United Nations, 1.9% of total global rice production was from Japan in 2009–2010, which amounted to 9.74 million tonnes [16].

The pretreatment of lignocellulosic biomass offers higher biodegradation rate and overall main product yield in any biological energy conversion processes. He et al. [17] reported that the solid state NaOH pretreatment of rice straw (Chinese rice) at 4%, 6%, 8% and 10% (dry weight basis NaOH) had increased the biogas production yield by 3.2–28.6%, 27.3–64.5%, 30.6–57.1% and 15.2–58.1%, respectively, than that of untreated rice straw for organic loading rates of 35, 50, 65 and 80 g/L in the fermentation digester. They also reported that the highest biogas production occurs at organic loading rate of 50 g/L. Pang et al. [18] reported that 6% NaOH pretreated corn stover at 65 g/L organic loading rate had increased 48.5% more biogas yield and 71.0% more bio-energy as compared to the untreated corn stover substrate. Alkali pretreatment of lignocellulosic biomass has been found to cause swelling, leading to increase in internal surface area, decrease in degree of polymerization and crystallinity, separation of structural linkages between lignin and carbohydrates (cellulose and hemicellulose), and disruption of lignin structure. Further, alkaline pretreatment reduces the degree of inhibition during fermentation and provides a lower production cost compared to other pretreatment methods [19], [20], [21]. Reaction temperature and residence time are two very important parameter for disruption of lignocellulosic structure. Reaction temperature of 190 °C and residence time of 5–10 min is high enough to destroy crystalline structure of cellulose and hemicellulose. Lesser reaction residence time can save energy in contrast to higher residence time [22].

Hydrothermal pretreatment of lignocellulosic biomass for enhanced bio-ethanol and biogas production is gaining high importance in the 21st century. Water under high pressure and temperature can penetrate into the biomass, hydrate cellulose, and removes hemicellulose and part of lignin. The major advantages are no addition of chemicals and no requirement of corrosion-resistant materials for hydrolysis reactors in the hydrothermal pretreatment process. The feedstock size reduction is a highly energy demanding operation on a commercial scale. However, there could be no need for size reduction of biomass in hydrothermal pretreatment process and requires much lower need of chemicals for neutralization of the produced hydrolyzate and produces lower amounts of neutralization residues compared to many processes. Hydrothermal pretreatment enhances the accessible and susceptible surface area of the cellulose and makes it more accessible to hydrolytic enzymes [23], [24].

The novel concern of the present study was to understand the hydrolysis behavior of untreated, NaOH and hydrothermal pretreated substrates of rice straw biomass and to present the suitability and comparative effectiveness of the pretreated substrates for improved methane fermentation, in order to produce good quality and quantity of methane.