Devolatilization, of biomass

As a particle is heated, it dries and devolatilizes. Water vapour and combustible gases are produced and transported to the surface of the particle and out into the gas between the particles, where the gases ignite if the conditions are suitable. Devolatilization of biomass starts already at temperatures about lOCC and spontaneous ignition of... 

As already indicated, a physical description in terms of a process with a single time constant is fair but an assessment on the basis of two time-scales gives even improved results. Therefore, more research has to be carried out to determine the characteristics of the most important additional phenomena. As an example a two time-scale model is applied to the previously reported measurements of Fig. 8.3 and displayed in Fig. 8.4. Clearly, the release is governed by two rates, typically a smaller and a larger time scale appear compared with the single rate case. However, the single rate results are still very valuable because they describe the apparent rate very well and this would be the only thing that can be described in coarse scale models of devolatilization, e.g., in CFD of biomass conversion. 

The same trend has already been found in experimental studies [6, 9, I8j. In Figure 3 the conversion of fuel-N to NO and NHj during devolatilization of single fuel particles (ranging from bituminous coal to different biomass fuels) at 800 C and 10 kPa oxygen concentration is shown. It can be seen that these experiments provide the same qualitative trend, though for the data obtained in the Formation Rate Unit (FRU, refer to [6]) the sum of NO and NH] have to be viewed, since the residence time in this lab-scale unit is very short. 

Lai, WC, I. Rodriguez, and B. Krieger-Brockett (1993) Composition Effects on the Devolatilization Behavior of Biomass and Municipal Solid Waste, Advances in Thermochemical Biomass Conversion, Blackte Academic Publishers, London, pp. 818-832. 

The influence of other inorganic materials and moisture content in biomass on organic bonded potassium release is negligible due to slow reactions at low temperature and short devolatilization time of biomass.. ... 

The kinetic model of biomass devolatilization is enqsloyed to describe the potassium release process as given in Eq. 1. 

Another direct measure of reactivity is the determination of maximum volatile yield from pyrolysis or devolatilization of any fuel. Figure 4-5 compares the maximum volatile yield of urban wood waste and sawdust to the maximum volatile yield of Black Thunder and Pittsburgh 8 coal. Note that the maximum volatile yield for both woody biomass samples exceeded 90 percent. This is in direct contrast to the coals, and to the petroleum coke as shown in Chapter 2. 

Slow pyrolysis of biomass operates at relatively low heating rates (0. l-2°C/s) and longer solid and vapor residence time (2-30 min) to favor biochar yield (Nanda et al., 2014b). Slow pyrolysis operates at temperature lower than that of fast pyrolysis, t q)ically 400 10°C and has a gas residence time usually > 5 s. Slow pyrolysis is similar to carbonization (for low temperatures and long residence times). During conventional pyrolysis, biomass is slowly devolatilized facilitating the formation of chars and some tars as the main products. This process yields different range of products with their properties dependent on temperature, inert gas flow rate and residence time. 

A potential source of particulates is char formed when the biomass feedstock is incompletely gasified. These particles undergo devolatilization and subsequent reactions at gasification temperatures that leave them less reactive than fresh biomass feedstock. They can pass through the gasifier before they are completely gasified, particularly in those configurations with turbulent beds. Char entrained in the product gas also represents unconverted biomass that contributes to lower conversion efficiencies. 

Saastamoinen J.J., Model for Drying, Devolatilization, and Combustion of Small Biomass Particles in Entrained Flow, Iso-thermal Furnaces , project 4450063-4/443-3/30, version 5, Domestic Fuel Laboratory, Jyvaskyla, Finland, (1985)... 

Biomass feedstocks contain a high proportion of volatile material, 70 to 90% for wood compared to 30 to 45% for typical coals. A relatively large fraction of most biomass feedstocks can be devolatilized rapidly at low to... 

It was shown, that the N-content in the fuel is a governing parameter determining the emissions of NO. Though rather simple assumptions were applied in this work, reasonable agreement to emissions measured for various fuels in different combustors was obtained. This confirms that devolatilization is dominating NO, emissions for high volatile biomass fuels. 

Volatiles composition is an important factor influencing the conversion of volatile nitrogen to NO and N O. The high CO content in biomass volatiles seems also to be partly responsible for the low levels of NjO emissions in biomass combustion. As the effect of most parameters is contrary a simultaneous reduction of NO and N O by primary measures is difficult, especially as other emission limits (e.g. for CO) have also to be captured. However, affecting devolatilization conditions to favor NH3 to HCN release and minimizing air-to-fuel ratio seems promising. 

Our methods and experiments (UW) previously addressed composition effects in pyrolysis of RDF (Lai, et al 1993) and wood (Krieger-Brockett, et al 1997). In those papers and this one, even minor components are shown to alter pyrolysis slate when appropriate statistical methods are used. This paper briefly summarizes our work on pyrolysis product slates resulting from large- or macro-particle devolatilization (in which heat transfer is a slow process) of native biomass compositions in under-utilized species. The method has general applicability and owing to the limited scope of this article, the reader is referred to Somasundaram (1990), Lai (1991) and Rodriguez (1996) for details and extensive literature reviews with only a few relevant articles mentioned here,... 

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