Steam Explosion for Biomass Pretreatment
Steam explosion has become a promising hydrothermal pre-treatment technology that is fundamental for future bio-refineries. Steam explosion is the most frequently used physico-chemical pretreatment for lignocellulosic biomass (Muzamal, et al. 61). Lignocellulosic biomass is the raw material that is normally used in making bio-fuels. The pre-treatment process is considered as an essential part in activating the bioethanol production out of lignocelluloses. Wood and wheat straw are some of the examples of biomass materials, but wood is a compound material that has a high mechanical strength. Nowadays, steam explosion is very crucial in resolving problems of energy, environment, as well as poverty, due to its effective pre-treatment technology. Steam explosion is very critical in the pre-treatment process that involves opening up of the fibers, to make the biomass polymers more exposed to subsequent processes.
Steam explosion has been termed as a pre-treatment process that is normally utilized on wood to make bio-ethanol and biogas. According to Stelte, steam explosion involves a process where biomass is treated using hot steam that range from 180 oC to 240 oC, under pressure (in the range of 1 to 3.5 MPa), which is followed by an explosion that compresses the biomass to cause a rupture on the biomass fibers rigid structure (3). The key idea behind steam explosion pre-treatment is conquering the lignin recalcitrance in softwood, in addition to “facilitating effective enzymatic access to released cellulosic mono-sugar from auto-hydrolysis during steam treatment” (Tannous 36).
In steam explosion process, the biomass is exposed to heat through steam, which is maintained for a particular time (Chen 151). The steam explosion pretreatment involves a three-step process (Muzamal, et al. 61). These steps include:
- Treatment of wood using pressurized steam for a specific period
- Explosion of wood chips after rapid release of pressure, and
- The effect of softened wood chips after interacting with other chips and the vessel walls
Steam explosion treatment is normally carried out in a digester that can attain a temperature of about 190 oC (Martin-Sampedro, et al. 4924). The digester incorporates a heat exchanger; electro-valves that allow steam in; and a ball valve for discharging steam. When chips are allowed in the digester, the charging steam has to achieve and maintain the operational conditions. After duration of about 10 minutes, the pressure is minimized and the chips are released into the blowing tank. The second cycle incorporates exploding the chips that are released from the first cycle and washing them using cold water. The chips are then subjected to similar procedure as the first cycle.
The main objective of pre-treatment is to crack the lignin structure, as well as split the crystalline structure of cellulose to enable enzymes to enter and hydrolyze the cellulose. Under high temperature and high atmospheric pressure, the functional composition of hemicelluloses produces organic acids and subsequently engages in depolymerization process of hemicelluloses and lignin (Chen 151). Water is also a vital component in steam explosion pre-treatment, as it possesses certain acid properties when at high temperature, which facilitates hemicelluloses hydrolysis (Zheng, et al. 40). After treatment, the finished samples are then washed with water and dried at room temperature before they are stored in polythene bags. Figure 1 below is a sketch that illustrates how a digester, which is used in the steam explosion experiments, operates.
Figure 1: A sketched digester for steam explosion experiments (Source: Martin-Sampedro, et al. 4924)
Advantages of Steam Explosion Pre-treatment
According to Stelte, some of the advantages of steam explosion process that make it stand above other pre-treatment technologies include:
- It utilizes no other chemical except water
- Pre-treatment conditions can be optimized effortlessly to generate high yields of certain chemicals from a variety of plant biomass
- It generate high yield of hemicelluloses that has low degraded byproducts
- Stages of acid handling, as well as acid recycling are evaded
- The process ensures purity of the conversion products after separating components of hemicelluloses, as well as water soluble components
- Corrosion of equipment is minimized due to the mild pH in the reaction process, as compared to acid hydrolysis processes (3-4)
Steam explosion treatment has become vital process in wood pellet industry for manufacturing stable and durable pellets. The treatment is also utilized in the pulping process in the biogas production. Steam explosion softens the lignin and release it from the cell wall before distributing it evenly onto the raw material (Stelte 3). Pellets made through steam explosion process are usually stiff and dark-brown in color. The discharge of lignin, the softening of material, and the flow during the pelletizing process, contributed in the deposition of lignin around the surface of wood particles, making them stiff and water-resistant. Several studies have also indicated that steam explosion pre-treatment can assist in reducing alkali substances in the biomass.
Modes of Operations in Steam Explosion
Steam explosion for biomass treatment incorporates a physiochemical process where structural elements of lignocellulosic materials crash when exposed to heat from steam. Steam explosion pre-treatment processes can be operated either in a batch system or in a continuous mode. A batch system is a small-scale application process that is usually carried out in a laboratory environment while a continuous system is a large-scale operation, mainly in an industrial process environment.
- Lab-scale Batch System
The batch system is normally used in the experimental laboratory as a small-scale operation. During the steam explosion process, the biomass is placed in a small vessel while the pressure is applied into the vessel. The vessel is then heated using steam, which is later released into another vessel during hydrolysis process.
- Continuous Steam Explosion System
A continuous system is a sophisticated method utilized in industrial production processes where raw materials are pre-steamed before they are supplied into the digester through a coaxial feeder and a dense plug. The dense plug is utilized to squeeze out moisture after pre-steaming (Haixia, James, and Xiao 4469). The exposure time is regulated through varying speed of the refining disk. The refined materials are passed into the cyclone (a region of low pressure) where steam explosion occurs. Continuous systems are preferred to batch systems due to high productivity in the pre-treatment reactor. Such systems are not limited by size, and are appropriate for commercial production in industries. Figure 2 represents a continuous steam explosion device as assembled in a pilot plant for experiment.
Figure 2: A continuous steam explosion apparatus for experiment (Source: Haixia, James and Xiao 4470)
Physical and Chemical Changes in Biomass
The main elements of wood fibers are lignin, cellulose, and hemicelluloses. Cellulose is the most plentiful organic compound that can be found in nature (Uzelac 5). Steam explosion treatment process unlocks the wood fiber and destroys the wood cell wall, leading to the release of individual components. The treatment process allows the release of hemicelluloses from the cell walls in wood. When hemicelluloses are released from the walls, they are exposed to chemical and biochemical degeneration (Stelte 11). The hydrolysis of hemicelluloses that occurs during steam explosion pre-treatment is usually completed by the organic acids, which include acetic acids, formic acids, and levulinic acids (Mood, et al. 82).
Hemicelluloses are essential in conveying viscoelastic properties in wood. Degeneration of hemicelluloses make wood become more fragile and stiff. The brittleness and resistance to moisture are two properties that support the use of exploded biomass in the manufacture of fuel pellets that can act as a substitute to coal in heat, as well as power stations (Stelte 11). Lignin and celluloses are also altered by steam explosion conditions. While lignin form low molecular weight lignin after undergoing condensation reaction, celluloses are degraded to form furfural at high pressures.
The use of steam explosion has become widespread, particularly in the conversion of biomass into other new products, due to its capacity to induce physical, as well as chemical modifications that enhance the accessibility of enzymes. In the alteration of lignocellulosic biomass into fuel, treatment of biomass is necessary to expose the cellulose within the plant fibers. Wood fibers contain hemicelluloses, lignin, and cellulose, which are altered during steam explosion process, to allow other processes to take place. Some of the advantages of steam explosion method are that it does not utilize other chemicals except water, and the pre-treatment conditions can be optimized with ease to generate high yields of certain chemicals from a variety of plant biomass. Steam explosion process for biomass is essential for industrial manufacturing of biogas, as well as in wood pellets industry.
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