ABSTRACT
Most stoves based on the principle of micro gasification have improved thermal efficiencies with low emissions, however, knowledge on the effect of the stove operation at different air flow rates on thermal efficiency, fire power, emissions, specific fuel consumption and burning rate is scarce. The main objective of this research was to evaluate performance a micro gasifier cook stove. An experimental forced draft cook stove was therefore developed using the available materials based on the design equations and household energy requirements. Simulation of air flow was integrated to help in the selection of the fan. The water boiling test was used and carried out at volumetric air flow rates of 0.014 m3s-1, 0.020 m3s-1, 0.027 m3s-1 and 0.034 m3s-1 with three replications. Performance was based on carbon dioxide, carbon monoxide, particulate matter, temperature near the pot and time for boiling water recorded real time. The average thermal efficiency and boiling time were 33±4%, and 13.5±3 minutes, respectively. There was linear proportionality for variation of air flow rate with the fire power of the stove in both cold and hot phases. The resistance to airflow exerted by the fuel and by the char inside the reactor during gasification was an average of 0.125 cm of water which was the minimum resistance needed by the fan. Burning rate increased with increase in volumetric air flow rate in both cold & hot phases. Specific fuel consumption increased linearly up to 0.027 m3s-1 and then dropped drastically in cold Phase. Considering Carbon monoxide & particulate matter emissions, the optimum air flow rate was 0.021 m3s-1 that corresponded to an average thermal efficiency of 33.5% for cold phase high power. During hot phase, the optimum air flow rate was 0.029 m3s-1 which resulted to thermal efficiency of 34%. Therefore, the general performance of the stove represents tier 3 according to International Workshop Agreement. This knowledge is finally useful to the users of gasifier stoves and designers in minimizing emissions at optimum efficiency.
CHAPTER ONE
INTRODUCTION
Background
Micro-gasification is a process of producing gas from solid fuels in gasifiers` small enough in size to fit under a cooking pot at a convenient height. The principle was invented in 1985 and the first commercial micro-gasifier cook stove was available in 2003 (Roth, 2011). Gasifiers are therefore devices that enable converting of solid to gaseous fuel by thermo chemical process. This process involves drying at temperature above 100 0C, Pyrolysis at temperatures beyond 300 0C and wood- gas combustion (Birzer et al., 2013). The principle of gasification became particularly important in the European scene during the Second World War when the fossil fuel availability was scarce. However, research and development reduced drastically when fuel availability became to normal (Shafiee and Topal, 2009).
Most of the improvements on biomass stoves have been based on intuitive approaches to examine heat transfer aspects relegating the combustion issues to a peripheral state (Ruiz- Mercado et al., 2011). Integration of simulation in the design phase provides solution in this case. A simulation based design improves the accuracy and minimizes the cost of producing many prototypes (Kshirsagar and Kalamkar, 2014 ). It is difficult to predict cook stove performance without measurements. Therefore, testing is an important tool for any designer to develop solutions and estimate potential environmental, health, social, and economic impacts (Onuegbu et al., 2011).
The performance of a stove is evaluated using water boiling test (WBT) based on thermal efficiency, emissions, specific fuel consumption, firepower and safety (Jetter and Kariher, 2009). The hazardous indoor air pollution that should be minimized includes Carbon Monoxide and Particulate Matter that have major health concerns (Smith and Mehta, 2003). Thermal efficiency is an estimate of the proportion of total energy produced by fuel that is used to heat the water pot. Most biomass based stoves have utilization efficiency of between 10 and 20% which is very low (Bhattacharya et al., 2002). There is therefore need to develop improved energy conversion devices to reduce heat losses and indoor emission pollutants.
As indicated earlier, Water boiling test was used in the evaluation of the developed experimental cook stove. It however important to note that there exist other tests that include controlled cooking test and kitchen performance test. WBT consist of three phases: a high power phase with a cold start, a high power phase with hot start and low power phase which is the simmering phase. Each phase involves a series of measurements and calculations (Arora et al., 2014).
Different types of cooking like simmering and levels of heat are needed for the wide variety of dishes around the world. Cooks control the heat of the fire by adjusting the fuel or air supply to the fire (Jetter and Kariher, 2009). Design features for easy air adjustment allow the cook to prepare a variety of dishes with one stove. However, changes in air supply can also affect the fuel burn rate, thermal efficiency, and completeness of combustion. Therefore, benefits to the user need to be balanced with performance. Air supply in a cook stove is typically divided into two modes based on location relative to the fire (Raman et al., 2013). Primary air enters directly to the combustion zone and reacts with the fuel. On rocket stoves, primary air enters through the fuel opening (Reed and Larson, 1997). Some stoves have inlet openings on the bottom of the stove underneath the fuel, which can be preheated before entering the combustion zone and supplies oxygen to the bed of burning charcoal residue. Secondary air is routed into the stove downstream of the combustion zone, supplying oxygen to react with producer gas (Panwar and Rathore, 2008).
Kenya is among the developing nations facing limited access to clean energy sources (Brew- Hammond, 2010), however, biomass gasifier stove technology could be part of the solution due to the following advantages not only to users but to the general public as well: It is a good replacement for LPG stove, particularly in terms of fuel savings and quality of flame (McKendry, 2002), it will also help to minimize environmental pollution especially the burning of waste on roadsides and the dumping of the same along river banks (Demirbaş, 2001), in addition, it will help reduce the carbon dioxide, Carbon monoxide and particulate matter emission in the air brought about by the excessive burning of wood & other biomass fuel in the traditional cook stoves, which contributes to the ozone layer depletion & consequently in the ―GHG effect‖ into the atmosphere (Roden et al., 2009), Finally, it will help preserve the forest by reducing the cutting of trees for the production of wood fuel and wood charcoal thus, minimizing problems concerning drought during summer and flood during rainy season.
Gasifier stoves using wood as fuel have been developed in countries like the US, China, India and other developing countries in Asia. These gasifier stoves like the Philips Wood stoves and Teri gasifiers produce a flammable gas by burning the fuel with limited amount of air (Yohannes, 2011). In Kenya the technology is new and few attempts have been made like Kenya Industrial Research and Development Institute Gastove and Sustainable Community Development Service gasifier stoves.
Design parameters like air flow rates, diameter and height of the reactor are paramount to successful forced draft cook stoves. Power output of the stove is highly dependent on the diameter of the reactor hence the bigger the diameter of the reactor, the more energy that can be released by the stove. This also means more fuel is expected to be burned per unit time since gas production is a function of the gasification rate in kg of fuel burned per unit time & area of the reactor (Yohannes, 2011). In addition, the total operating time to produce gas is affected by the height of the reactor. The higher the reactor, the longer is the operating time. However, the height of the reactor is limited by the height at which the stove is to be installed in the kitchen (Kartha and Larson, 2000). Finally, the size of the air in late is dependent on the size of the reactor. The bigger the diameter of the reactor, the more airflow is needed. The higher the reactor, the more pressure is needed in order to overcome the resistance exerted by the fuel (Belonio, 2005).
Almost any carbonaceous or biomass fuel can be gasified under experimental or laboratory conditions. A gasifier stove is fuel specific and it is tailored around a fuel rather than the other way round (Somashekhar et al., 2000). It is therefore necessary to evaluate the fuel to determine its moisture content, carbon content, volatile material, heat energy calorific value and ash content. In this research, saw dust pellets of diameter 6-10 mm and length < 40 mm were used to meet the uniformity requirement.
The objective of this research was to evaluate the performance of an experimental biomass micro gasifier cook stove. The development was based on design formulas with an integration of simulation of air flow in the packed bed reactor.
Statement of the Problem
Exposure to indoor air pollution resulting from inefficient burning of biomass in traditional cook stoves is a major health hazard affecting people in less developed countries Kenya included. This is expected to grow especially with the continual use of biofuels creating need for efficient energy technologies. Most of the stoves based on the principle of micro gasification have improved thermal efficiencies with low emissions; however, knowledge into the effect of the stove operation at different air flow rate on thermal efficiency, emissions, specific fuel consumption, burning rate and fire power is not completely understood. It is also difficult to acquire the necessary experimental data from the existing gasifier stoves and finally, it is not possible to obtain all the data experimentally like superficial air flow in a packed bed reactor.
Objectives
The broad objective was to evaluate the performance of an experimental biomass micro gasifier cook stove.
The specific objectives were:
i. To develop an experimental biomass micro gasifier stove using pressure drop.
ii. To determine the effect of air flow rates on thermal efficiency, emissions, fire power, burning rate and specific fuel consumption of the experimental gasifier stove during cold phase.
iii. To determine the effect of air flow variation on performance of the experimental micro gasifier stove during hot and simmering phases.
Research Questions
i. How does pressure drop vary with change in air flow in a packed bed reactor for the developed micro gasifier stove?
ii. How do thermal efficiency, emissions, firepower, burning rate and specific fuel consumption of the gasifier vary with different air flow rates during cold phases?
iii. How does the developed micro gasifier cook stove perform at different air flow rates during hot and simmering phases?
Justification
Improved biomass cook stoves has been a topic of research for more than 40 years, but still 2.6 billion people globally cook over an open biomass fire. Indoor air pollution resulting from inefficient burning of biomass in traditional cook stoves is a major health hazard affecting around 2.7 billion people globally according to World Health Organization (WHO, 2013). This is expected to grow especially with the continual use of biomass for cooking hence need for efficient energy conversion technologies. These research intents to evaluate the performance of an experimental biomass micro gasifiers cook stove. Most stoves based on the principle of micro gasification of biomass have low emissions with improved heat transfer efficiencies. However, underlying knowledge into the effect of the stoves operation at different air flow rates on thermal efficiency, emissions, fire power, burning rate, temperature and specific fuel consumption is not completely understood. There was also the need to understand the physical and combustion characteristics of the fuel used in order to factor in the parameters during development of the gasifier stove.
1.6 Scope and Limitation
The scope of this work was limited to development of an experimental biomass micro gasifier cook stove and evaluation based on the effect of air flow rates on thermal efficiency, emissions (CO, CO2 and PM), burning rate, specific fuel consumption and time of combustion using water boiling test version 4.2.3 for improvement on the stove performance. Simulation was based on COMSOL Multiphysics which was used to determine the specific draft for saw dust pellets. The diameter and height of the reactor were 140 mm each. The materials used in the fabrication of the prototype were stainless steel, vermiculite mixed with 10% cement and iron sheet gauge 16 and 18. The fuel used was saw dust pellets with moisture content =12 %, ash content = 1 %, carbon content = 20 %, volatile materials = 68%, heat energy calorific value = 4600 kcakg-1, diameter 6-10 mm and length of approximately 40 mm.
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