Thermochemical Conversion of Coal and Biomass to Liquid and Gaseous Fuels

Thermochemical Conversion of Coal and Biomass to

Liquid and Gaseous Fuels

Abstract

Venkata Abhijit Bhagavatula¹, Naresh Shah² and Gerald Huffman²

 Consortium for Fossil Fuel Science, Department of Chemical and Materials Engineering,

University of Kentucky, Lexington, KY – 40508

Introduction:

The use of biomass as a renewable source of energy has been increasingly gaining importance over the past few years. The growing concerns over green house gas emissions, along with the need for energy independence have accentuated the need for using alternative sources of fuel and chemical feedstocks. A flowchart illustrating the process of converting solid coal/biomass into gaseous and liquid fuel is shown in Figure 1.

Figure 1. Flowchart illustrating the process of converting solid coal/biomass into gaseous and liquid fuel.

Gasification is the thermochemical process of converting a solid carbonaceous source such as coal or biomass into synthesis gas (CO + H₂) which is used as a feedstock for producing fuels and chemicals. Gasification, generally done in the presence of mixtures of air/pure oxygen and steam at temperatures ranging between 600 ºC and 900 ºC, produces a gaseous mixture of hydrogen, carbon monoxide, steam, methane and light hydrocarbons along with some undesirable effluents such as inorganic particulates and condensable organic vapors or tars, as they are commonly known.

Using biomass alone as the gasification feed greatly reduces the green house gas emissions but the process not only involves higher operating costs but also produces higher amount of tar when compared to that of coal. The high tar content of product gases from biomass gasification is a major and widely recognized problem. These high tar contents arise mainly from the lower temperatures and shorter residence times in gasifiers constructed for biomass processing compared to those designed for coal gasification. Tar yields from ligno-cellulosic biomass materials tend to be considerably higher than tar yields from coals. Co-gasification of blends of coal and biomass to produce syngas (CO + H₂), which in turn can be used as feedstock for processes such as Fischer-Tropsch Synthesis used for producing liquid fuels, is a major area of research.

The main objective of this research is to perform the co-gasification of different blends of coal and biomass in a fixed bed reactor and analyze the products in order to achieve high concentration of synthesis gas. Two different types of coal namely, sub-bituminous and lignite, and different types of biomass such as, pine and poplar, have been used for the co-gasification process. Using thermogravimetric analysis, the pyrolysis characteristics of mixtures of coal and different biomass samples have been analyzed. The effects of various coal and biomass blends, temperature, pressure and oxygen to steam ratio, in addition to the particle size effects, on the gasification products have been studied thoroughly. 

The energy content of the gas produced through gasification depends on numerous factors, such as the oxidizing agent, reactor type, fuel type and form, etc. The oxidizing agent can be chosen as air, oxygen, steam, or a mixture of these. When air is used, the resulting gas has a low calorific value. This can be increased by using oxygen or steam but in the latter case sufficient heat should be provided because steam gasification is an endothermic process.