**1. Introduction**

132 Mass Transfer in Chemical Engineering Processes

Ying-Yue, W.; Chi-Tzong, H.; Wen-Ta, C. & Jia-You, F. (2001). In vitro and in vivo

*International Journal of Pharmaceutics*, Vol.224, 89-104.

evaluations of topically applied capsaicin and nonivamide from hydrogels.

Due to strategic and environmental reasons, currently, there is an increasing interest in biofuels as alternative energy source. Bio-alcohols and biodiesel are the alternatives been considered for auto-motion while biomass and biogas are the alternatives been considered for electrical power generation.

Biogas is a medium-energy content fuel (~22 MJ/kg) derived from the organic material decomposition under anaerobic conditions (Horikawa et al, 2004). It can be obtained from landfills or from bio-digesters that transform manure and biomass into natural fertilizer in farms after 25-45 days of residence time. Due to its gaseous nature and the impossibility of producing it intensively, it is not attractive for large scale power generation.

However, recently, a new approach for electric power generation has been emerging. It consists of inter-connecting thousands of small and medium scale electrical plants powered by renewable energy sources to the national or regional electrical grids. It is considered to interconnect the hundreds of the existing small aero generators and solar panels (Pointon & Langan, 2002). Even though, there are still several technical issues to be resolved, this alternative of distributed electrical power generation is being considered as the best alternative to bring electricity to the rural communities located far away from the large urban centers.

In this case, the use of the biogas generated in the thousands of existing farms and landfills, as fuel for internal combustion engines connected to an electric generator becomes a very attractive alternative for electric power generation because of its very low cost, high benefitcost ratio and very high positive impact on the environment.

Biogas is made up mainly of methane (CH4) and carbon dioxide (CO2). It also contains traces of hydrogen sulfide (H2S). Its composition varies depending on the type of biomass. Table 1 shows its typical composition.

The biogas calorific power is proportional to the CH4 concentration. To be used as fuel for internal combustion engines, it has been recommended a CH4 concentration greater than 90% (Harasimowicz et al, 2007). However CO2 has a typical concentration of ~ 40%. This high CO2 concentration reduces the engine power output proportionally to its concentration, limiting the use of biogas in electrical power plants driven by internal combustion engines (Marchaim, 1992).

The high content of H2S (~3500 ppm) causes corrosion in the metallic parts at the interior of the engine. The H2S is an inorganic acid that attacks the surface of metals when they are

Removal of H2S and CO2 from Biogas by Amine Absorption 135

CO2 removal from gas streams has been of great interest, especially in large thermal power plants, due to its greenhouse effect (Romeo et al, 2006). Table 2 compares the different

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

It refers to the process by which molecules of a substance, such as a gas or a liquid, collect on the surface of a solid. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid (Tondeur & Teng, 2008). It could be physical or chemical. In physical adsorption processes, gas molecules adhere to the surface of the solid adsorbent as a result of the molecules attraction force (Van der Walls Forces). Chemical adsorption involves a chemical reaction. Usually, adsorbents are 12 µm to 120 µm high porosity solid grains, inert to the treated fluid. The most used adsorbents for CO2 are activated charcoal,

It is the process of converting a gas into a liquid by reducing temperature and/or increasing pressure. Condensation occurs when partial pressure of the substance in the gas is lower

A membrane is a layer of material which serves as a selective barrier between two phases and remains impermeable to specific particles, molecules, or substances when exposed to the action of a driving force. The driving force is the pressure difference between both sides of the membrane. Gas permeability through a membrane is a function of the solubility and diffusivity of the gas into the material of the membrane. Membranes are expensive and their

Table 3 compares the different alternatives reported for H2S removal from gas streams

It refers to processes where the cleaning reagent, once it becomes saturated, regains its

than the vapor pressure of the pure substance at a given temperature.

**2. Biogas treatment methods 2.1 CO2 removal from gas streams** 

diffusion occurs at the inter-phase.

silica gel, zeolites and synthetic resins.

separation efficiencies are low (Ramírez, 2007).

removal capacity through a change in the external conditions.

**2.2 H2S removal from gas streams** 

**2.2.1 Regenerative processes** 

existing technologies.

**2.1.1 Absorption** 

**2.1.2 Adsorption** 

**2.1.3 Condensation** 

**2.1.4 Membranes** 

(Walsh et al, 1988).

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 withdraw of these types of systems.


#### (N/S Not specified)

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

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

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.
