5.1.2 Similarities and Differences between Biomass and Coal as Feedstock

Even though biomass gasification has long been practiced on a variety of scales with and without modern scientific understanding, the subject itself has greatly benefited from the coal science and technology which has been studied far more in depth [15]. Most of the scientific tools developed for and applicable to coal technology are more or less relevant to biomass utilization technology and they include analytical methods, solids handling technol­ogy, chemical reaction pathways, reactor designs and configurations, pro­cess integration, waste heat recovery and energy integration, gas cleanup, product separation, safety practice and measures, and much more. However, the compositional differences between coal and biomass feedstock as well as their impact on processing technologies must be clearly understood for full and beneficial exploitation of the advances and innovations made in coal processing technology. The differences between biomass and coal feedstock are summarized below.

1. The hydrogen content in biomass is significantly higher than that of coal. Coal is a very mature product of a lengthy and slow coalifi — cation process whose principal chemical reaction is carbonization, whereas biomass is not. The coal rank basically indicates the degree
of carbonization progressed and a higher rank coal is a petrologically older coal. Therefore, the H/C ratio of coal is much lower than that of biomass. The H/C ratio of a higher rank coal is also lower than that of a lower rank coal.

2. A higher H/C ratio of biomass feedstock makes it generally more reactive than coal for conventional transformational treatments.

3. Biomass typically has much higher moisture content than coal. This statement is applicable to both forms of moisture: equilibrium and chemically bound. Among various ranks of coal, lignite, the lowest rank coal which is also the youngest, has the highest mois­ture content. Therefore, of all ranks of coal, lignite may be consid­ered the closest to biomass in terms of both proximate analysis and ultimate analysis.

4. Biomass contains a significantly higher oxygen content than coal due to its oxygenated molecular structures of carbohydrates (or polysac­charides), cellulose, glycerides, fatty acids, and the like. Weathered coals (i. e., coal exposed to atmosphere after mining) show an increased level of oxygen content over that of freshly mined coal. However, the oxygen content of weathered coal is still far lower than that of typical biomass. Due to the high oxygen content of biomass fuel, its heating value is substantially lower than that of coal.

5. Coal contains 0.5-8 wt% sulfur (S), whereas biomass has little or no sulfur content. Coal with lower than 1 wt% sulfur may be classified as low-sulfur coal, whereas coal with higher than 3 wt% sulfur may be considered high-sulfur coal. No such designation or classification is needed for typical biomass. In coal syngas sulfurous compounds, if not removed, affect the downstream processing severely by poi­soning the catalysts and also by starting corrosion on metallic parts and equipment. In this regard, biomass is considered a sulfur-free raw material. Sulfurous compounds in coal syngas typically include H2S, carbonyl sulfide (COS), and mercaptans (R-SH), whose prevail­ing abundance depends largely on the gasifying environment as well as the feed coal composition. Furthermore, coal sulfur is subdi­vided largely into three different forms: pyritic sulfur, organic sul­fur, and sulfatic sulfur [14]. However, such a subcategorization for forms of sulfur is unnecessary for biomass.

6. Alkali metals such as sodium (Na) and potassium (K) as well as low — boiling heavy metals such as lead (Pb) and cadmium (Cd) are typi­cally present in raw biomass syngas [16]. This is not as severe for coal syngas and the trace element problems with coal syngas are more source-specific. Due to the trace mineral components in the biomass syngas, downstream processing of biomass syngas, in particular catalytic processing, requires rather comprehensive purification

pretreatment of feed syngas or use of robust and poison — and foul — ing-resistant catalysts.

7. Fuel analysis of both coal and biomass is represented by proximate analysis and ultimate analysis. Proximate and ultimate analyses of a variety of biomass samples found in the literature are presented in Tables 5.1 and 5.2, respectively.

8. Due to the high abundance of moisture, high oxygen content, and noncombustible impurities in biomass, the heating value of biomass is typically much lower than that of coal. The energy density of bio­mass feedstock on a volume basis is therefore substantially inferior to that of coal.

9. Biomass has substantially higher volatile matter (VM) content than coal, although it has much lower fixed carbon (FC) content than coal. Therefore, a large amount of hydrocarbon species can be extracted/ obtained from biomass simply via devolatilization or pyroly­sis, whereas devolatilization or pyrolysis of coal generates a high amount of char.

10. Biomass is generally composed of softer organic materials and its grindability or pulverizability is poor using common size reduc­tion equipment. Considering the irregular shapes and nonuniform compositions of untreated biomass components, cost-effective size reduction for manageable transportation as well as continu­ous reactor feeding often becomes a technological challenge. Pretreatment of biomass feedstock is usually required for industri­alized utilization.

11. Both biomass gasification and coal gasification encounter varying degrees of tar formation during thermal/chemical transformation, however, the severity of tar formation is typically more significant with biomass gasification. Although tar is collectively a carcinogenic species, it condenses at reduced temperatures, thereby blocking and clogging pipelines and valves as well as fouling process equipment and parts.

5.1.3 Analysis of Biomass

As mentioned in the previous section, the proximate and ultimate analy­ses of specific biomass material provide very valuable information about the biomass feedstock. This compositional information provides the science and engineering information needed to identify or determine the fuel heating value, ash amount projected, maximum achievable gasification and liquefac­tion efficiency, moisture content of feedstock, predicted behavior of the feed­stock in a processing environment, and much more. The proximate analysis is a procedure for determination, by prescribed methods, of moisture (MO), volatile matter (VM), fixed carbon (FC), and ash. The amount of fixed carbon is determined by difference. The term proximate analysis involves neither determination of quantitative amounts of chemical elements nor determina­tion other than those categorically named or prescribed. The group of analy­ses involved in proximate analysis is defined in ASTM D 3172. On the other hand, the ultimate analysis is a procedure of the determination of the ele­mental composition of the organic portion of carbonaceous materials, as well as the total ash and moisture. The ultimate analysis is also called elemental analysis. And this analysis is also determined by prescribed methods.

An extensive tabulation of both proximate and ultimate analysis data on over 200 biomass species was presented in Channiwala’s PhD dissertation (1992) [17]. Some representative values of proximate and ultimate analyses of a variety of biomass species, as found from the literature sources, are pre­sented in Tables 5.1 and 5.2, respectively. For comparison, the classification of coal and typical analysis is also shown in Table 5.3.

Comparing between the analyses of coal and biomass, the following gen­eralized statements can be made.

1. Biomass has a very high oxygen (O) content, which is the second most abundant atomic species present in biomass and is nearly as much as the carbon (C) content. However, the oxygen content in coal is much lower than the carbon content and this trend is even more noticeable with higher rank coals. The higher the rank of a coal, the lower its oxygen content is. It may be said that deoxygenation (i. e., oxygen rejection) was an important part of a petrochemical process of coalification or carbonization.

2. Due to the higher oxygen content in biomass, the heating value of biomass is much lower than that of coal. Bio-oil derived from bio­mass also has a high oxygen content, which makes the oil more corrosive to metallic parts and piping. Therefore, efficient use of bio­mass as fuel or fuel precursor will involve a certain level of oxygen rejection (i. e., deoxygenation) in its process scheme.

3. The H/C ratio of biomass is substantially higher than that of coal. The reactivity of biomass is generally higher than that of coal and its processability is also better than coal’s.

4. Among various ranks of coal, lignite is the closest to biomass in a number of properties including its high moisture content, high oxy­gen content, low carbon content, and low heating value. As such, lignite has often been considered as a co-fed companion fuel with biomass.

The standardized analysis of biomass fuel is conducted following the ASTM standards and Table 5.4 shows the list of these codes for specific analyses.