Gasification of coal or biomass

Gasification of carbonaceous fuels - Gasification is a process of converting carbonaceous fuel into gaseous product with a useable heating value. Carbonaceous fuel includes solids, liquids and gases such as coals, biomass, residual oils and natural gas. Gasification is not a single step process, but involves multiple sub processes and reactions. The generated Synthesis Gas or ‘Syn Gas’ has wide range of applications ranging from direct house hold applications to power generation. Out of all gasification processes, coal gasification process gained much interest from last three decades. Coal can be gasified by reacting it with mixture of air and steam at various temperature and pressure conditions. Biomass gasification is also gaining attention in recent times as it represents an attractive sustainable energy alternative.

Gasification Process Reactions - Gasification is not a single step process and involves following subprocesses / stages and corresponding reactions - Drying of fuel, Devolatalization, Oxidation of char and Reduction in absence of oxygen. Syn-gas is produced at the end of the reduction subprocess in the gasifier. All these substeps generally take place in different zones of the gasifier, depending on the type of gasifier used.

Gasification Process Types - Gasification processes can be categorized into three groups: entrained flow, fluidised bed and moving bed. These gasification processes differ from each other on account of fuel particle size, residence time in the gasifier and operating temperatures. Due to these differences, composition of the product gases and percent conversion of coal is also different for each process. When selecting a specific gasification process for syn gas generation, different factors such as fuel type (coal or biomass etc.), plant size, reactivity of fuel and composition of air-oxygen oxidant mixture have to be taken into consideration.

Selection of a gasification process - For selecting a specific gasification process following factors are evaluated - characteristics of coal or biomass used (such as particle size, ash content), reactivity of the fuel, type and composition of the oxidant (air/oxygen), plant size. General guidelines for gasification process selection are discussed briefly in this article.

Gasification Fuels - Characteristics of various fuels used for gasification is discussed including fuels such as different types of coal, liquid fuels and variety biomass feedstocks such as wood biomass, agricultural waste, municipal organic waste etc. Suitability of these fuels for syn gas generation by gasifiction process depends on these characteristics such as - ash content, heating value, voidage etc.

Gasification Products - Synthesis gas or syngas is the primary product of gasification. Main constituents of syngas are carbon monoxide and hydrogen. If the syngas contains some non-combustible gases such as carbon dioxide and nitrogen, the mixture is commonly known as producer gas. If the generated synthesis gas contains only carbon monoxide and hydrogen, then the mixture is known as water gas. Water gas is commonly used for producing pure hydrogen for fuel cell applications, hence purity of the carbon monoxide and hydrogen mixture is important for water gas. Applications of syngas, producer gas and water gas are also discussed here.

Coal analysis for coal gasification process - Since coal is a widely used feed for gasification, knowledge of the different ways to analyse coal and interpretation of that analysis becomes important. The main purpose of coal analysis is to determine the rank of the coal along with its intrinsic characteristics. For different types of coal, following properties are important in analysis - moisture, volatile matter content, ash content, fixed carbon content. Different analysis techniques such as proximate coal analysis, ultimate coal analysis, ash analysis etc. are briefly discussed.

Types of gasifier equipments - Gasifier equipments are generally classified as upward draft, downward draft and cross draft gasifiers, based on the direction of air/oxygen flow in the equipment. It should  be noted that there are types of gasifier equipment which are different from types of gasification processes. The choice of the one type of gasifier ober there is mostly determined by the fuel, its final available form, its size , moisture content and ash content.  First three type of gasifiers are mostly used in entrained bed gasification process and moving bed gasification process. While the last one is fluidised bed gasification process.

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