Chapter 5, Problem 5.59P

The current global reliance on fossil fuels for heating, transportation, and electric power generation raises concerns regarding the release of CO₂and CH₄, which are greenhouse gases thought to lead to climate change, and NO, which contributes to smog. One potential solution to these problems is to produce transportation fuels from renewable biomass.

You have been asked to evaluate a proposed process for converting forest residues to alcohols that may be used as transportation fuels. In the ?rst stage of the process, steam and dry wood from hybrid poplar trees (which grow between ?ve and eight feet a year and can be harvested roughly every ?ve years) are fed to a gasi?er in which the biomass is converted to light gases in the following reactions:

   $\begin{array}{l}{\text{C+H}}_{\text{2}}\text{O}\to {\text{CO+H}}_{\text{2}}\\ {\text{CO+H}}_{\text{2}}\text{O}\to {\text{CO}}_{\text{2}}{\text{+H}}_{\text{2}}\\ {\text{C+CO}}_{\text{2}}\to \text{2CO}\\ {\text{C+2H}}_{\text{2}}\to {\text{CH}}_{\text{4}}\\ {\text{CH}}_{\text{4}}{\text{+H}}_{\text{2}}\text{O}\to {\text{CO+3H}}_{\text{2}}\end{array}$

The ef?uents from the reactor are a gas stream containing H₂, CO, CO₂, CH₄, and H₂O, and a solid char stream that contains only carbon and hydrogen. The char is discarded and the gases go through additional steps in which the hydrogen and carbon monoxide are converted to mixed alcohols. This problem only concerns the gasi?er.

Data:

• Elemental composition of biomass: 51.9 mass% C. 6.3% H, and 41.8% O
• Pressure and temperature of entering steam: 155°C. 4.4 atm
• Feed ratio of steam to biomass: 1.4 kg steam/kg biomass
• Yield and dry-basis composition of product gas: 1.35 kg dry gas/kg biomass at 700°C, 1.2 atm; 50.7 mol% H₂, 23.8% CO, 18.0% CO₂, 7.5% CH₄
(a) Taking a basis of 100 kg of biomass fed, draw and completely label a ?owchart for the gasi?er incorporating the given data, labeling the volumes of the steam fed and the gases produced. Perform a degree-of-freedom analysis.

(b) Calculate the mass and mass composition of the char and the volumes of the steam feed and product gas streams.

(c) List advantages and possible drawbacks of using biomass rather than petroleum as a fuel source.