Gasification from waste organic materials

Santiago Ramírez Rubio¹, Fabio Emiro Sierra², Carlos Alberto Guerrero³

¹ Aeronautical Engineer, Universidad San Buenaventura, Masters in Mechanical Engineering, Universidad Nacional de Colombia. Professor, Universidad San Buenaventura. santiagoramirezr@gmail.com

² Doctor of Engineering, Universidad de Kassel. Associate Professor, Universidad Nacional de Colombia. fesierrav@unal.edu.co

³ Chemical Engineering and Mechanical, Master in Environmental Engineering PhD in Chemical Engineering. Associate Professor, Universidad Nacional de Colombia. caguerrerofa@unal.edu.co

ABSTRACT

This article describes the fixed bed biomass gasifier operation designed and built by the Clean Development Mechanisms and Energy Management research group, the gasifier equipment and the measurement system. The experiment involved agro-industrial residues (biomass such wood chips, coconut shell, cocoa and coffee husk); some temperatures along the bed, its pressure, inlet air flow and the percentage of carbon monoxide and carbon dioxide in the syngas composition were measured. The test results showed that a fuel gas was being obtained which was suitable for use with an internal combustion engine for generating electricity because more carbon monoxide than carbon dioxide was being obtained during several parts of the operation. The gasification experimentation revealed that a gasifier having these characteristics should be ideal for bringing energy to areas where it is hard to obtain it (such as many rural sites in Latin-America) or other places where large amounts of agro-industrial wastes are produced. Temperatures of around 1,000°C were obtained in the combustion zone, generating a syngas having more than 20% carbon monoxide in its composition, thereby leading to obtaining combustible gas.

Keywords: gasification, biomass, fixed bed, syngas, renewable energies

Received: August 27th 2010 Accepted: November 4th 2011

Introduction

The Universidad Nacional de Colombia's Clean Development Mechanisms and Energy Management research group has designed and built a system for biomass gasification involving a fixed-bed parallel flow reactor. The system consisted of the reactor where the necessary reactions were produced to obtain the synthesis gas (syngas), a cyclone responsible for collecting par-ticulate matter, a heat exchanger condensing the bulk of the water and tar present in the gas so produced, a ventilator creating the necessary suction to facilitate gas flow through the system and a module for data acquisition and measurement. Figure 1 shows the gasified system (reactor and gas preparation) and the aforementioned external modules.

Biomass gasification is widely used as an energy resource around the world. Several authors have documented its use, such as biomass as energy source in Brazil (Lora and Andrade, 2009), gasification in Japan (Min et al., 2005) and gasification of cane leaves for producing electricity in India (Jorapur and Rajvanshi, 1995).

Several works on fixed-bed biomass gasification (with air) have found similar results to those presented in this article (Dogru et al., 2002; Sharma, 2009; Sheth and Babu, 2009; Zainal et al., 2002). All evaluated the process in terms of temperature and carbon dioxide and monoxide production. Although not the case in this article, some authors have worked on gasification using different agents (Ahmed and Gupta, 2009) such as water vapour in the gasification of paper. Figure 2 gives the biomass gasification flow diagram.

Gasification parameters

This research needed to take different parameters into account; the five major variables affecting gasification are given below.

Temperature

Gasification is carried out at constant temperature in permanent contact with a gasified agent; such temperature remains constant until there is no more loss of mass, meaning until only ashes are left. Gasification involves low medium and high temperatures.

Mass transfer and pore diffusion are faster than the chemical reaction at 800°C to 900°C; chemical kinetics therefore controlling gasification speed. Moreover, temperature defines the pyro-lysis (400°C-700°C), combustion (800°C-1,300°C) and reduction (700°C-900°C) zones involved in gasification (Sierra, 2008).

Residence time

Increasing the time during which the biomass is allowed to react with the oxidising agent and the gasification temperature leads to a decrease in the amount of solid material remaining and increases the percentage of gases.

Oxidising agent

Air, oxygen, carbon dioxide, steam or a mixture of all of them may be used for gasification; however, each one has different reaction characteristics.

Air gasification produces poor quality gas regarding calorific value (4-7 MJ.m^-3) due to its low energy density.

Oxygen gasification produces better quality gas (10-18 MJ.m^-3) but it implies high oxygen production costs and risk is higher during the process.

In steam gasification or carbon dioxide as gasified agent, the gas so obtained has a similar quality to that obtained with oxygen; it also avoids high production costs and operating risks. This is 3 to 5 times faster than carbon dioxide gasification (Sierra et al., 2009).

Feedstock

Gasification feedstock (i.e. any organic matter) leads to a carbonised product being obtained at the end of pyrolysis; when completed at high temperatures, a larger surface area is obtained, indicating great carbon availability for the reaction (Sierra et al., 2009).

Elements which may be formed from the ashes, such as potassium, magnesium, sodium, iron and calcium are called biomass catalysts for steam or carbon dioxide gasification.

Reactivity

The carbonised products' reactivity is influenced by their chemical structure determining the number of active sites, the internal surface area and porosity controlling diffusion speed and the inorganic compounds which might have a catalyst or inhibitor effect (Sierra et al., 2009).

Chemical kinetics

The physicochemical process occurring during gasification consists of transition from feedstock until being carbonised and its subsequent reduction.

Carbonised product and volatile production are important in a fixed-bed gasifier due to the low heat rate (<100°C/min) and the solids' long residence time. The organic matter's thermal behaviour is frequently studied by thermogravimetric analysis (TGA) measuring a sample's weight loss rate regarding time and temperature. The TGA only provides a semi-quantitative analysis of pyrolysis because weight loss rates are related to time, temperature and a particular sample's size and density, thereby leading to adjusting the main equation as follows:

where t is time, K constant rate equal to A exp(-E/RT), A is the pre-exponential factor, E the activation energy, R the universal gas constant, V total volatile material at temperature T, and HR the heating rate equal to dT/dt. The equation may even estimate the decomposition rate of the cellulose present in the biomass, over a range of low rates of heating. The analysis cannot predict the variation of the carbonised formation and the gaseous products in different pyrolysis conditions (Sierra et al., 2009).

Table 1 gives a summary of the basic reactions driving gasification.

Research methodology

The carbon dioxide (CO₂) and monoxide (CO) concentration in the percentage of gas produced, the temperature in the centre of the bed throughout the reactor, the reactor's weight (whose variation per time unit is associated with solid conversion into gas) and the flow of air supplied to the reactor can all be measured during gasification. Figure 2 shows the module where all the digital signals regarding the aforementioned instruments converge ( except for inlet air flow because its signal is analogous); the reading is done by computer. That unit is also where the syngas is cleaned to enter the gas analyser equipment which measures the CO and CO₂ concentration.

The gas analyser's gas preparation unit is located in the same module; it contains a couple of filters which exclude tar remnants to avoid the gas analyser becoming clogged and a flow regulator that allows enough gas to enter the analyser.

The system consisted of 3 load cells (located in a Y structure which can be observed at the bottom of Figure 3) which allowed reviewing reactor weight variation per time unit (Figure 4). The temperature throughout the bed could be checked through a series of thermocouples installed in the reactor (Figure 5). A computer read these instruments digital signals whilst the instrument measuring inlet air flow read analogue signals (Figure 6).

Biomass such as wood, cocoa waste, coconut and coffee was used for the gasification carried out in the laboratory equipment (Figure 7). Each feedstock used had particular characteristics regarding the process, gas and tar production. Although they all generated a good combustible gas there were significant differences leading to conclude that some were better than others: It was found that the coffee husks were good biomass for gasification because their particle size did not require preparation and they involved low tar production.

Wood is one of the most common materials for gasification because it is easily obtained. Figures 8 and 9 show the data obtained for a wood-based gasification. The first shows the temporary changes in temperature measurement, the composition of the gas so produced and the bed weight.

Results and Analysis

Figure 8 shows that the highest temperature was measured by thermocouple 3, which is the area where combustion occurred. It can be seen that temperature decreased as measurement moved away from the aforementioned area and temperature pattern began to become a bit more linear. Temperature decreased as the biomass moved down the reactor due to the presence of endothermic reactions which absorbed heat in these areas. Temperatures in the upper combustion areas had a very low temperature, significantly lower than the lower areas, due to the reactor's operation type. This was because parallel flow equipment means that the hot combustion gases flow in the same way as the biomass, to the bottom part, thereby avoiding good convection heating becoming present at the top. Conduction heating was very poor because wood, being the major part of the biomass, has a very low conduction coefficient, stopping the heat flowing freely through the contact between the solid particles.

Figure 9 shows that the gasification results were good, because there was a good carbon monoxide percentage in the syngas so produced at the end of the gasification (around 20%). However, the carbon dioxide results were higher when they should have become decreased. It is possible that the gasification was done with an air flow high enough to fully oxidise an important part of the carbon present in the biomass. Such hypothesis was reinforced by noting the increase in CO percentage and consequent CO₂ reduction at the end of the process, indicating a decreased air supply (aimed at shutting down the reactor) involving less complete combustion reactions and the carbon reacted with a larger amount of oxygen. Regarding the variation in reactor weight per time unit, it was interesting to see the breaks in the graph where such abrupt variations were attributed to the periodic shaking aimed at avoiding the biomass clogging inside the gasifier.

Conclusions

Gasification in the equipment used here produced good quality gas because it was combustible and achieved a good amount of carbon monoxide and low carbon dioxide at the proper temperature;

Better gasification quality could be achieved when biomass humidity was lower because the water in the biomass decreased the combustion temperature and increased the amount of tar produced;

Temperature was a significant influence on gasification rate, i.e. a high temperature in the combustion area would have a higher combustion rate and vice versa (this can be seen on the weight change graph); and The gasification product was a fuel which could be burned in actual combustion systems. Recent research in this field has been hopeful; it is expected that developments in the near future will allow commercial gasification systems in Colombia to produce low-cost energy, as has been happening around the world where equipment having integrated gasification systems can be found (Treviño, 2008). These integrated systems have the advantage that they can use low-quality fuels like sulphur-rich coal which involves a high environmental cost when being used.

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