Chapter 10

What are advantages and disadvantages of the use of air as the oxidizing agent as compared to steam for biomass gasification for combined heat and power production?

Consider a perfectly stirred reactor to be used for dry wood gasification. The composition of wood is given as overall molecular formula: \(\mathrm{CH}_{1.4} \mathrm{O}_{0.6}\). The reactor volume \((\mathrm{V})\) is \(4 \mathrm{~m}^{3}\), and the mass flow rate of wood is \(2 \mathrm{~kg} \cdot \mathrm{h}^{-1}\). Primary air consists of \(23 \mathrm{wt} \% \mathrm{O}_{2}\) and is fed to the gasifier with a mass flow rate \(\phi_{\mathrm{m}, \text { air. }}\) Oxygen in the air reacts with biomass in an idealized way so as to form only \(\mathrm{CO}\) and \(\mathrm{H}_{2}\). The reaction is given as \(\mathrm{CH}_{1.4} \mathrm{O}_{0.6}+\left|\mathrm{v}_{1}\right| \mathrm{O}_{2} \rightarrow \mathrm{v}_{2} \mathrm{CO}+\mathrm{v}_{3} \mathrm{H}_{2}\) with \(v_{i}\) being the stoichiometric coefficients. The rate of consumption of \(\mathrm{O}_{2}\), \(R_{\mathrm{O} 2,1}\), in \(\left[\mathrm{kmol} \cdot \mathrm{m}^{-3} \cdot \mathrm{s}^{-1}\right]\) is given by $$ R_{\mathrm{O} 2,1}=\mathrm{k}_{1} \mathrm{Y}_{\mathrm{O} 2} \exp \left(-\mathrm{T}_{\mathrm{a} 1} / \mathrm{T}\right) $$ with \(\mathrm{Y}_{\mathrm{O} 2}\) being the \(\mathrm{O}_{2}\) mass fraction, \(\mathrm{k}_{1}=10^{7} \mathrm{kmol} \cdot \mathrm{m}^{-3} \cdot \mathrm{s}^{-1}\), and \(\mathrm{T}_{\mathrm{a} 1}=2.5 \times\) \(10^{4} \mathrm{~K}\). The reactor is operated at steady state and at isothermal conditions with \(\mathrm{T}=1000 \mathrm{~K}\). a. Calculate \(\lambda\). b. Suppose that just enough air is fed into the reactor for complete wood conversion into \(\mathrm{CO}\) and \(\mathrm{H}_{2}\). Compute \(\phi_{\mathrm{m}, \text { air }}\) c. Write down the conservation equations for total mass and \(\mathrm{O}_{2}\) (mass fraction), respectively. d. Determine \(\mathrm{Y}_{\mathrm{O} 2}\) in the reactor by solving the equations. N. B. \(\mathrm{Y}_{\mathrm{O}_{2}}>0\), though just enough air is introduced in the reactor for complete conversion.

Why are fixed/moving beds not suitable for large-scale synthesis gas generation based on biomass gasification?

Why does a model based on chemical equilibrium calculations predict higher \(\mathrm{CH}_{4}\) formation at higher pressure and typical fluidized bed gasifier temperatures of about \(850^{\circ} \mathrm{C}\) ?

For Example \(10.2\), check whether the criterion for negligible intraparticle heat transfer limitation is fulfilled. Take \(\Delta_{\mathrm{r}} \mathrm{H}=227.5 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}\) and \(\mathrm{E}_{\mathrm{a}}=\) \(220 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}\). Assume the gas properties are those of pure nitrogen at \(900^{\circ} \mathrm{C}\). These are \(\lambda=0.074 \mathrm{~W} \cdot \mathrm{m}^{-1} \cdot \mathrm{K}^{-1}, \rho=0.2875 \mathrm{~kg} \cdot \mathrm{m}^{-3}\), and \(\mathrm{c}_{\mathrm{p}}=1.199 \mathrm{~kJ} \cdot \mathrm{kg} \cdot \mathrm{K}^{-1}\).

When a biochar particle is converted in Regime I, its conversion rate depends on its particle size. Right or wrong?

Supercritical water has a high density and is a highly nonideal gas state. At \(24.0 \mathrm{MPa}\) and \(380^{\circ} \mathrm{C}\), calculate the density according to ideal gas law. Also use the van der Waals equation of state to calculate density and finally the steam tables. Compare the values obtained. What is your conclusion?

A high value of the Thiele modulus during a biochar particle conversion process corresponds to a Regime I conversion. Right or wrong?

Consider an aqueous feedstock with \(10 \mathrm{wt} \%\) organics (take glucose for the calculations). This solution is gasified at \(600^{\circ} \mathrm{C}\) and \(25.0 \mathrm{MPa}\) (abs.), and the reactor effluent is heat exchanged with the feed at an efficiency of \(70 \%\). Estimate the amount of energy that has to be provided to the reactor per \(\mathrm{kg}\) of (wet) feed. You may assume that the reaction reaches equilibrium.

What does it mean when a particle’s gasification process is limited by intraphase heat transfer?

Recap the pros and cons of the use of circulating fluidized bed gasifiers versus entrained flow gasifiers.

What are the main differences between bubbling and circulating fluidized bed gasifiers?

In a BFB gasifier, biomass fuel can be fed from the top or from the bottom part (in-bed feeding); what are the implications of these different ways of feeding for the gas composition?

$$ \text { Explain the role of } \mathrm{NaOH} \text { (aq.) addition in the SCW gasification process. } $$

Which problems do you foresee when applying SCW gasification with seaweed (macroalgae) as feedstock?

Name a few advantages and disadvantages of gas cleaning at high temperature versus low (near ambient) temperature.

Often, a (multi)cyclone is positioned before a filter. Mention one advantage and one disadvantage of such a particle cleaning constellation.