**2.1.1 Absorption**

134 Mass Transfer in Chemical Engineering Processes

placed in direct contact. Sulfur stress cracking (SSC) is the most common corrosive mechanism that appears when the metal makes contact with H2S. Sulfides of iron and atomic hydrogen are formed in this process. This mechanism starts to take place when the H2S concentration is higher than 50 ppm (Gosh, 2007). The admission valves, bronze gears and the exhaust system are also attacked by the presence of H2S. The degree of deterioration of the engines varies considerably. Results obtained experimentally on this regard are contradictory (Gonzalez et al, 2006; Marchaim, 1992). It has been found that H2S in biogas diminishes the life time of the engine by 10 to 15% (Horikawa & Rossi, 2004). Finally, time between oil changes is reduced since lubricant oils contain H2S and corrosion inhibitors to protect the engine. It increases the maintenance cost of the engine. Users consider the high maintenance cost as the main

**waste Landfills Industrial** 

CH4 50-80 50-80 50-70 >70 CO2 30-50 20-50 30-50 <10 H2O Saturation Saturation Saturation N/S H2 0-2 0-5 0-2 N/S H2S 0.70 0.1 0.8 < 0.01 NH3 Traces Traces Traces N/S CO 0-1 0-1 0-1 N/S N2 0-1 0-3 0-1 N/S O2 0-1 0-1 0-1 N/S

Typically, small scale power plants based on biogas range from 0.1 to 1 MW. This implies a volumetric biogas flows between 60 and 600 m3/hr. For this small scale application an additional practical consideration arise. Out of the bio-digester or landfill, the biogas gauge pressure is negligible, and due to economical considerations the use of any device to increase pressure should be avoided. Engine suction is the only driving force available to make the biogas to flow from the bio-digester or landfill to the engine. Therefore, the

To address this need, the present document describes the design, manufacturing and testing of a system to reduce H2S and CO2 content to less than 100 ppm and 10%, respectively, from 60 to 600 m3/hr biogas streams, with minimum pressure drop, for applications in small scale power plants (0.1 to 1 MW) based on internal combustion engines fueled with biogas. Initially, this document describes and compares the existing alternatives to trap H2S and CO2 from gaseous streams. From this analysis it is concluded that amines treatment is the most appropriate for this application. Since there is no reported data for the H2S and CO2 absorbing capacity of these substances, a method is proposed to measure it by means of a bubbler. This information is used in the design process of biogas treatment system. Details of the manufacturing process are also included. Then, results of the experimental work are reported, and finally, an economical analysis on the use of this type of systems is presented.

**Composition (%)** 

**Waste** 

**Desired composition** 

withdraw of these types of systems.

**Agricultural** 

Table 1. Biogas composition. Most of the data from (Carrillo, 2003).

pressure drop across the biogas treatment system should be the least possible.

**Component** 

(N/S Not specified)

It refers to the process by which one substance, such as a solid or liquid, takes up another substance, such as a liquid or gas, through minute pores or spaces between its molecules. The absorption capacity of the absorber depends on the equilibrium concentrations between gaseous phase and the liquid phase. For diluted concentrations in many gases and in a wide interval of concentrations, the equilibrium relation is given by Henry's Law, which quantifies the gas absorption capacity in the fluid (Cengel & Boles, 2008). A gas absorbing unit should ensure complete contact between the gas and the solvent, in such a way that diffusion occurs at the inter-phase.
