Chapter 4. Technical Characteristics of Renewable Energy by Source
4.1 Status of RE Technology by Source
4.1.4. Bioenergy
[Figure 4-10] Overview of bioenergy technology
4.1.4.2.1 Solid Fuel Technology and Applications
Burning biomass to produce heat is the first bioenergy technology that human beings have applied.
Many developing countries currently obtain energy for cooking or heating directly by burning biomass.
However, biomass applications in large scale energy production, such as districted heating, are challenging, as biomass is usually scattered over large geographic areas, leading to high collection and transport costs.
Therefore, pelletizing is used commonly to increase the energy density of wood-based biomass, with the biomass pulverized and compressed at high pressure ([Fig. 4-10]). Compared with forest-thinning by- products that must be collected directly from mountainous regions, pelletized biomass is more suitable as a fuel for large scale energy supply because of its significantly higher energy density ([Fig. 4-11]).
[Figure 4-11] Technological development trend of solid biofuels transition
Source: KIER
In addition, torrefaction technology is currently being developed to enhance biofuel densification. Such solid biofuels are combusted in large boilers to produce thermal energy for household or district heating.
Solid biofuels are also used as dual-fuel in coal-fired plants or as dedicated-fuel in dedicated biomass plants in response to the growing need for deployment of RE in the power generation sector ([Fig. 4-12]).
[Figure 4-12] Applications of solid biofuels
4.1.4.2.2 Gaseous Fuel Production Technology and Applications
Gaseous fuels derived from biomass include biogas, which can be produced by anaerobic digestion with anaerobic organisms, and bio-syngas, which can be created by the pyrolysis of wood-based biomass ([Fig. 4-13]).
[Figure 4-13] Gaseous biofuel technology
Source: KIER
<pellet boiler> <Biomass Power Plant>
Source: KIER
Organic waste with high water content is decomposed by micro-organisms under anaerobic conditions and generates biogas, which is primarily methane. Because of its high efficiency in organic waste reduction, as well as its capability to produce energy, the anaerobic digestion process is particularly useful in areas where landfill sites are limited. The anaerobic digestion of organic waste comprises three stages, namely, pre-treatment for the removal of impurities, pre-conditioning (conditioning), and digesting for conversion to methane. The resulting biogas is used as energy for heat or electricity production. Residues after methane production are used as compost or landfilled and incinerated ([Fig. 4-14]).
[Figure 4-14] Anaerobic digestion process flow chart
Source: KIER
The anaerobic digestion process is classified into several types depending on the operating temperature, solid content in the raw materials, and operating mode ([Fig. 4-15]). To achieve higher process efficiency, the optimal operating mode should be chosen to meet the process requirements and the characteristics of the different types of waste.
[Figure 4-15] Classifications of anaerobic digestion process
Source: KIER
In EU countries, biogas is being processed to a higher purity standard to increase its calorific value and is utilized as an alternative fuel to city gas or fuel for compressed natural gas (CNG) vehicles.
The production and application process of biogas is summarized in [Fig. 4-16].
[Figure 4-16] Biogas production and application
Source: KIER
When wood-based biomass with a low water content is heated to a high temperature above 800 °C under anaerobic condition, bio-syngas is generated, which is composed mainly of CO, H2, and methane.
This gaseous fuel is converted into fuel for power generation or F-T fuel by using catalyst after purification ([Fig. 4-17]).
[Figure 4-17] Conversion of biomass into bio-syngas and F-T fuel production process
Source: KIER
4.1.4.2.3 Liquid Fuel Production Technology and Application
Bioethanol
Ethanol is currently used as an alternative fuel to gasoline in many countries. Bioethanol can be produced from carbohydrates or starches, such as sugar cane or corn. As shown in ([Fig. 4-18]), carbohydrate-derived bioethanol is made by ethanol fermentation with concentrated sugar cane juice. The resulting solution is concentrated to increase the ethanol concentration level up to 99.3% or higher and this concentrated anhydrous ethanol is used as a gasoline additive (up to 10% ethanol mixed with gasoline) for vehicles.
Bioethanol derived from starchy crops, such as corn, requires the additional process of converting the starchy raw materials into sugar. The subsequent process is the same as that of the carbohydrate-derived bioethanol production process.
[Figure 4-18] Flow chart of ethanol production process from carbohydrates
Source: A. Bonomi, Presented at IEA Task 39 Meeting (2012)
The wood-based ethanol production process requires pre-treatment to separate the sugar from the wood- based feedstock. After saccharifying the separated sugar polymers, the same steps are followed as for the carbohydrate-derived ethanol production process.
The production processes of bioethanol for each type of biomass feedstock are compared in ([Fig. 4-19]).
[Figure 4-19] Comparison of ethanol production processes by biomass feedstock
Source: S. Saka, Presented at IEA Task 39 Meeting (2012)
Biodiesel
Biodiesel is used often as an alternative fuel to diesel and can be produced by chemical catalysts. As shown in [Fig. 4-20], which describes biodiesel production using chemical catalysts, 3 moles of biodiesel together with 1 mole of glycerol as by-product can be obtained from a chemical reaction after adding the catalyst to 1 mole of vegetable oil and 3 moles of alcohol.
[Figure 4-20] Biodiesel production reaction from vegetable oil
Catalyst
Vegetable oil + 3⋅alcohol<======> 3⋅alkylester (biodiesel) + glycerol
Source: KIER
Catalysts for biodiesel production include acidic and alkaline catalysts, and their characteristics are summarized in <Table 4-6>.
❙ Table 4-6 ❙ Comparison of operation specifications by catalyst
Acidic catalysts Alkaline catalysts
Operating pressure Max. 80 atm Max. 9 atm
Operating
temperature Max. 250 °C Max. 100 °C (typically 60~80 °C)
Reaction time 2~4 h 10~30 min
Target feedstock High free fatty-acid content
(more than 2%) Low free fatty-acid content (less than 2%) Source: KIER
As regards using vegetable oils (canola oil, soybean oil, palm oil, and others) with a low acid value as feedstock, it is preferable to apply an alkaline catalyst (NaOCH3), which shows high activity in the biodiesel production reaction. However, as regards the base catalysis, of which the allowable free fatty-acid content is less than 0.5% and the water content is less than 0.4%, it is desirable to keep the free fatty-acid and water content as low as possible to increase yield. When the acid value or water content exceeds threshold values, salts are produced, making it difficult to separate the biodiesel. This, in turn, lowers the yield. Currently, the
requires a complex refining process to produce biodiesel that meets specifications. Furthermore, low-purity glycerol (about 85%) is an obstacle to applying the liquid catalyst as a high value-added feedstock ([Fig. 4- 21]). Research on the development of a solid-base catalyst is being conducted to provide a solution to this problem. Employing a solid catalyst would enable the simplification of the post-treatment process. In addition, high-purity glycerol (approximately 98%) will allow the solid catalyst to be employed as a feedstock for high value-added production.
[Figure 4-21] Flow chart of biodiesel production process using solid catalyst
However, owing to inactivation of the alkaline catalyst, feedstock oils with a high acid value, such as waste vegetable oil, require a two-phase reaction, as the acidic catalyst must be added after pre- treatment ([Fig. 4-22]).
[Figure 4-22] Flowchart of two -phase biodiesel production from waste oil conversion
All types of alcohol can be used for reaction but methanol is used frequently as it is the cheapest.
However, in Latin America and the Caribbean, bioethanol is used widely because it is abundant.
To design a process to produce biodiesel efficiently, the first requisite is understanding its features when it is being reacted. As described in [Fig 4-20], biodiesel is produced through a reversible reaction; therefore,
Source: KIER
Acidic catalyst
phase 1: free fatty acid + methanol ---> biodiesel + water base catalyst
phase 2: vegetable oil + methanol ---> biodiesel + glycerol
Source: KIER
to raise its transference rate, it is important to increase the concentration rate of the reactant in the reactor or to maintain the low concentration rate of the product. Second, oil does not mix well with methanol, as the former is non-polar, whereas the latter is polar. Consequently, to achieve a vibrant reaction, the reactants must be mixed evenly. Third, the product biodiesel, as well as glycerin have extremely low solubility because of polarity and are separated easily, as glycerin has high density. If the production process of biodiesel could be designed to accommodate these differences, the efficiency of the process could be enhanced.
In respect of the first feature, alcohol is typically put 100% compared to stoichiometry in the biodiesel production reaction. Also, when 80% of the reaction process is completed, the reaction should be paused. It should be resumed after separating of glycerin and re-adding alcohol and catalyst. As regards the second feature, there are measures to agitate or add sub-solvent to raise mutual solubility.
4.1.4.3 Summary
The major technologies to generate and utilize biomass, mentioned above, are summarized in ([Fig 4- 23]) below.
[Figure 4-23] Type and usage of major biomass energy Biomass Bioenergy Application