Biomass char

Kannan, M.P. Richards, G,N.(1990). Gasification of biomass chars in carbon dioxide dependence of gasification rate on the indigenous metal content. Fuel,... 

Kinetic parameters from selected publication on CO2 reactivity are shown in Table 1. A wide span of apparent activation energies is published for biomass chars, 80.3 kJ/mol -318 kJ/mol and for coal chars, 79 kJ/mol -359.5 kJ/mol Some of the discrepancy can be explained as due to different experimental procedures (such as sample load, particle size and sample preparation) and the application of different analysing equipment. Concerning the latter one, the potential role of systematic errors in temperature measurements among various thermobalances is evident. Variations might also be explained by different extraction procedures, lack of accuracy caused by the approximations used in the different computational methods and the kinetic compensation effect. 

In the present paper investigation are made in order to produce operational data for the gasification reactivity for biomass chars and coal chars. Furthermore the differences between chars derived from biomass and coal is illustrated. Thermogravimetric analyses have been used to obtain information about the kinetic values of the C-CO2 reaction. 

The structural profiles of both biomass chars show consistency within the studied temperature and pressure range. This result is in agreement with earlier findings. Our reactivity profiles obtained under different reactants (e.g. CO2 and O2) gives reactant specific reactivity profiles. Which is consistent with Salatino and Floess. ... 

A comparison of the reactivity of wheat, spruce and Longyear coke is shown in Fig. 4. The two biomass chars exhibit almost identical reactivity while the reactivity for the much more dense metallurgical coke is clearly lower. This is in general agreement with the reviewed literature. 

Heterogeneous tar reactions on the surfaces of (partially) reacted biomass, char particles etc. The same types of reactions as in the homogeneous case (cracking, oxidation, repolymerization) may occur but the surfaces may have a catalytic effect on the reactions. 

These authors consider that a system of good kinetic equations for the biomass-char gasification reactions under the true gasifier atmosphere do not exist yet. This conclusion can be discussed and even criticised, of course. The authors are open to a discussion and experimental checking with the person(s) who considers that holds a system of kinetic equations to correctly describe the network of reactions 4a-4d. These authors do not know such system of kinetic equations and have again to look for an empirical and easy-to-use approach. 

All the above problems can be applied to the upper part of the CFB gasifier. With an accurate fluid dynamic model the axial profiles of biomass, char, calcined dolomite and silica sand could be calculated, but the lack of accurate kinetic information on the effect of the concentration of these solids on the rate of all reactions involved avoids to calculate the axial profiles of the gaseous confounds (H2, CO, CO2... and tars). So, it... 

The ratio biomass char to bed material has to be a certain range. 

According to the bench-scale reactor at Royal Institute of Technology in Sweden a ratio of height to diameter of the fluidised bed of 1 is chosen. The ratio of biomass char to bed material is between 10-15 % taking into account the experiments. Higher ratios could lead to bed-disturbances. These boundary conditions lead to the design described above and to a calculated fluidised bed diameter of approx. 1.5 m. 

Biomass Char Liquids Gases Adsorbent Remarks... 

There was, within the uncertainty of the heal capacity data, no variation with preparation conditions (neither initial pellet density nor pyrolysis temperature made much difference). The values in Figure 2 are at considerable variance with some values used in modeling of wood and biomass pyrolysis, depending upon temperature. For example, values for wood and biomass chars have been taken to be 1.38 [25], 2.5 [26], 1.0 [27-29]. 0.71 [30], 0.67... 

As the ash content of different biomass could influence the gasification reactivity of biomass chars, a study has been conducted to determine the influence of heavy metals on the gasification process [6]-[7]. They show that the alkali metals increase the reactivity of wood char and that lead, copper and zinc, especially as chlorides, inhibited the gasification of the char (about 2.5 times slower that the char fromtmtreated wood). 

The equations for transient heat transfer given in Section 3.2.1.5 can be used to derive the heating rate of a biomass/char particle. The particles are heated in the gasifier by heat transfer from the hot surrounding gas and by radiation from the reactor wall. To estimate the thermal resistance by convective heat transfer to the particle compared to the resistance by heat conduction in the particle, we use the Biot number [Eq. (3.2.47)] for a spherical particle ... 

The reaction kinetics found in the literature is summarized in Table 1. Because of the different types of solids involved (biomass, char), all reaction rates are expressed per mj. Values for the pre-exponential factors and the activation energies are given in Table 2. The combustion of hydrogen is considered infinitely fast. 

This study was carried out as a part of the research project named Fundamental Research on the Reduction and Preparation of Biomass Char-Iron Concentrate Briquette ofNatural Science Foundation of China (No. 51104014). Financial support from NSFC (Natural Science Foundation of China) is gratefully acknowledged. 

Cetin, E., Moghtaderi, B., Gupta, R., and Wall, T.F. (2004) Influence of pyrolysis conditions on the structure and gasification reactivity of biomass chars. Fuel, 83, 2139-2150. 

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