Boiler Biomass Gasification

Gasification of Biomass Boilers

Biomass Definition

Biomass is a form of renewable energy derived from recently deceased organisms, in contrast to fossil fuels. Common biomass sources include wood fuel, waste, and alcohol. This energy form is considered renewable because plants can regenerate on the same land. Currently, wood is the predominant biomass source, accounting for almost 90% of biomass fuel in the United States. Biomass is deemed carbon-neutral, absorbing carbon from the atmosphere during plant growth and releasing it when burned. This closed carbon cycle theoretically prevents an increase in atmospheric carbon dioxide (CO2) levels. Biomass encompasses various organic materials such as wood, agricultural waste, alcohol, and refuse.

Sources of Biomass

In Indonesia, firewood, twigs, agricultural and plantation waste, as well as household, municipal, and industrial waste, hold substantial potential as biomass energy sources. These sources can be utilized for electricity generation through direct energy conversion processes and intermediary energy conversion.

Biomass, originating from photosynthesis, includes materials left over from plants or animals, like wood from forests, agricultural waste, and organic human and animal waste. The sun's energy is stored in biomass through photosynthesis, converting carbon dioxide into carbon molecules (such as sugar and cellulose) in plants. Different types of biomass have varying energy content, for instance, green wood has 8 MJ/kg (equivalent to 2.22 kWh), oven-dried material has 20 MJ/kg (equivalent to 5.56 kWh), and methane gas has 55 MJ/kg (equivalent to 15,277 kWh).

Biomass Conversion

Historically, our ancestors used biomass boilers to produce heat, burning biomass directly. In modern applications, combustion heat is converted into electricity through turbines and generators. The combustion of biomass produces steam in the boiler, which, in turn, drives the turbine connected to a generator. To enhance the benefits of biomass as fuel, various technologies are employed.

Generally, biomass conversion technology can be classified into three categories: direct combustion, thermochemical conversion, and biochemical conversion. Direct combustion is the simplest technology, involving the direct burning of biomass to produce heat. The result of biomass combustion is thermal energy that can be converted into electricity.
a.Biobriquettes
Biobriquettes involve compressing biomass into a more orderly form, similar to the well-knowncoal briquettes. Various biomasses such as husks, charcoal, sawdust, and other biomass wastescan be used to create briquettes. The process is not overly complicated, and various machines,ranging from manual to semi-mechanical, are used for production.
b.Pyrolysis
Pyrolysis involves the breakdown of biomass (lysis) due to heat ( pyro) at temperatures exceeding 150°C.
The process includes primary pyrolysis, which occurs in the raw material (feed), and secondarypyrolysis, which occurs in particles and gases/vapors produced during primary pyrolysis. It's crucial to note that pyrolysis is a heat-induced decomposition process, and the presence of O2 is avoided to prevent combustion reactions. This process is part of carbonization,specifically high-temperature carbonization (HTC), exceeding 500°C.
Pyrolysis produces solid fuel products like carbon and liquid products in the form of a tarmixture, along with other substances. Gases released in this process include carbon dioxide(CO2), methane (CH4), and small amounts of other gases.
c.Liquefaction
Liquefaction is the process of transforming gas into liquid through condensation, typicallyvia cooling, or converting solid to liquid through melting, heating, or mixing with other liquids tobreak bonds. In the energy field, liquefaction occurs with coal and gas turning into liquid forefficient transportation and utilization.
d.Transesterification
Transesterification is a chemical process that exchanges alkoxy groups in esters with alcohol.
e.Densification
To maximize the benefits of biomass, it can be densified into briquettes or pellets for easy handling. Densification aims to increase density, easing storage and transportation. Generally, densification provides advantages such as higher calorific value per unit volume, convenient storage and transportation, and uniform size and quality (Bhattacharya et al., 1996).
f.Carbonization
Carbonization is the process of converting organic materials into charcoal. This process releases combustible substances like CO, CH4, H2, formaldehyde, methane, formic, and acetic acid, along with non-combustible substances like CO2, H2O, and liquid tar. The gases released have a high heating value and can be used to meet the heat requirements of the carbonization process.
g.Anaerobic digestion
Anaerobic Digestion is a process involving microorganisms without oxygen in a digester. It produces product gas in the form of methane (CH4) and carbon dioxide (CO2), along with small amounts of gases like H2, N2, and H2S. This process can be dry or wet anaerobic digestion, with dry anaerobic digestion having a biomass content of 25-30%, while wet types have a biomass content of less than 15% (Sing and Misra, 2005).
h.Gasification
In simple terms, biomass gasification is the process of converting cellulose material into fuel in a gasification reactor (gasifier). The resulting gas is utilized as a motor fuel to power a generator. Gasification serves as an alternative for energy conservation and diversification. It addresses the handling and utilization of agricultural, plantation, and forestry waste. The gasification device comprises three main parts: (a) the unit to convert raw material (feed) into gas, known as a gasification reactor or gasifier; (b) gas purification unit; (c) gas utilization unit.
i.Biochemistry
Another application of biomass boiler energy involves biochemical processes such as hydrolysis, fermentation, and anaerobic digestion. Anaerobic digestion decomposes organic matter or cellulose into CH4 and other gases through biochemical processes. The stages of the anaerobic digestion process are shown in the figure. Ethanol production from biomass is also considered biochemical conversion. Biomass rich in carbohydrates or glucose can be fermented to break down into ethanol and CO2. However, carbohydrates must undergo hydrolysis first to convert into glucose. The fermented ethanol typically has a high water content and needs distillation to achieve ethanol levels above 99.5%.

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