Biomass Gasification (Small-scale)
Introduction
It appears to be a fascinating solution: The conversion of wood or other carbon-rich dry bio-mass into a combustible gas and then into electricity via a generator set – a perfect solution for remote rural areas with a lack of electricity but an abundance of shrubs, straw, rice and peanut husks or other forms of biomass.
The technology has been well known for more than a hundred years. In light of rising prices of fossil fuels in 2008 and the debate about climate change, this technology has again come under consideration as a renewable energy source in rural areas. In fact, it is possible to convert dry wood or rice husks into gas and electricity. However, it is not as easy as some manufacturers would like to make us believe.
The Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) on behalf of the German Ministry for Economic Cooperation and Development (BMZ) has been searching for sustain-able solutions to provide access to basic energy services in rural areas and has analysed experiences with small-scale applications of the gasification technology over the last decades. This analysis was based on publicly available documents, as well as interviews and email discussions with experts in this field. This text summarises the results, revealing several diffi-culties and challenges. It refers to small-scale applications of less than 100 kW only and focuses on potentials for providing basic energy services to rural households and small busi-nesses.
The Technology
Biomass gasification is basically the conversion of solid fuels like wood and agricultural resi-dues into a combustible gas mixture. In order to produce electricity the generator gas is used as a fuel in an electric generator set with a combustion motor. The gasifier is essentially a chemical reactor that uses wood chips, charcoal, coal or similar carbonaceous materials as fuel and burns them in a process of incomplete combustion due to a limited air supply. Products of the gasification process are • solid ashes • partially oxidized products like soot (which have to be removed periodically from the gasifier) and • generator gas.
The main flammable components of the resulting generator gas are • Carbon monoxide (CO), • Hydrogen (H2), and • Methane (CH4).
Due to its high content of nitrogen (more than 50%) and other incombustible components this producer gas has a low calorific value compared to other fuels. The calorific value of genera-tor gas is only about 5 -6 MJ/kg versus 35-50 MJ/kg for natural gas.
There are many different gasification methods in use or in development, but the downdraft fixed-bed technology is almost exclusively used for small-scale power gasifiers. This is the only economic option on a small-scale that also produces a fairly clean gas. Models with batch or continuous feed are available.
Theoretically, electricity can be produced by various kinds of technical equipment, for exam-ple a combustion unit in combination with a steam turbine, a gas turbine, a Stirling motor, or even a fuel cell. In practice internal combustion piston engines are almost exclusively used to drive electric generators for the small-scale applications discussed here. Apart from some minor adaptations, this generator set is more or less the same as used with other fuels. Spark ignition “Otto” engines as well as compression ignition “diesel” engines can also be used. While the Otto engines can be operated on generator gas only, diesel engines generally need co-fuelling of conventional diesel fuel. However, all internal combustion engines require a very clean gas as a fuel. Otherwise ex-cessive engine wear and low power output will inevitably occur. Therefore a cleaning system is an essential component of a gasifier plant. Cleaning systems that use water to wash out the undesired components are quite efficient. However, they produce a high quantity of toxic and carcinogenic liquid waste. Hence, dry cleaning systems are the preferred solution in non-industrial systems today.
The gasification technology is principally well suited for small power plants ranging from 10 kW to over 100 kW. Appropriate gasifier systems with internal combustion engines can pro-duce 1 kWh of electricity from 1.1 – 1.5 kg wood, 0.7 – 1.3 kg charcoal, or 1.8 – 3.6 kg rice husks. Assuming the wood originates from renewable production – regardless of whether planned forestation or natural regeneration - it would be a perfect, nearly CO2 neutral, re-newable energy source.
Hence, this technology seems to be a very interesting solution for many initiatives and pro-jects in times of the climate change debate. The general features of the technology are in-deed promising: In contrast to a photovoltaic system or a wind generator, electricity can be produced at any desired time given the availability of the required biomass. A generator in the range between 10 and 100 kW provides sufficient energy not only for household lighting, but for televisions, refrigerators and the operation of small machinery as well. In addition, the provision of fuel in the form of wooden sticks or agricultural waste can be a source of income for small farmers and an incentive for reforestation. However, the following documentation of practical experience shows that there are still many obstacles to overcome.
Existing Experience in Different Countries
Gasification of biomass or coal is a relatively old technology. Town gas in Western European cities was produced by the gasification of coal before natural gas became widely available. By 1850, large parts of London had gas lights powered by the gas produced from gasifiers using coal and biomass. With the increasing availability of other energy sources and electrifi-cation the technology lost its importance. In the early years of the 20th century, gasifier systems to power stationary engines and trucks were demonstrated but did not gain general acceptance. The technology reappeared only after petroleum fuels became scarce during World War II. Almost one million gasifier-powered vehicles were in use during that time. However, with increasing availability of diesel and gasoline this rather inconvenient technology was again abandoned. The energy crisis of the 1970s and 1980s again triggered interest in gasification technology. By the 1980s about 15 manufacturers were offering wood and charcoal power gasifiers. Amongst others, DGIS, GTZ, and SIDA began financing and running pilot gasifier power sys-tems in several developing countries. Brazil, China, India, Indonesia, the Philippines and Thailand had gasifier programmes based on locally developed technologies. In some cases the technology was promoted by local entrepreneurs. However due to frequent technical problems and decreasing petrol prices the interest in this technology again disappeared rap-idly.
Only large-scale industrial applications and plants for heat production have achieved some economic success and become fairly common. Biomass gasification is used quite success-fully in Scandinavia, especially using residues of the wood, pulp and paper industry. (IEA, 2004) However, worldwide the development and construction of new small and medium-size gasifi-cation plants has once more gained momentum during the last decade parallel to the discus-sions on climate change. In particular the guaranteed high feed-in tariffs in Germany have triggered the installation of about 50 gasifier power plants.
The experience with the gasifier power plants constructed in the last 30 years is the back-ground for this appraisal summary, even if it is very hard to obtain reliable detailed data es-pecially concerning long-term operation. Manufacturers promote their gasifiers with perform-ance figures. However, these rarely seem to be based on practical operation. The projects that use the gasifiers publish their use as success stories, but apparently only rarely collect reliable long-term data. Tracking the operational history of a gasification plant is in many cases almost impossible. It is for that reason that the information provided may contain minor contradictions. Nevertheless, there are some important studies available that relate personal observations by experts and allow for a conclusive statement.
A comprehensive World Bank study in 1998 examined gasification plants installed in the 1980s and came to the following disillusioning results: Most gasifier plants had been taken out of operation. After just a few years only 11 of the 24 installed gasifiers in Indonesia were still in use. In the Philippines only 1-5 % of the gasifiers installed 3-6 years earlier were still operating. Results from other countries were similar. The detailed analysis of the status of 24 gasifiers in Indonesia revealed: “Almost none of the projects identified became fully commercial, and most proved unsustainable for technical, financial/economic, and institutional reasons” (Stassen, 1995). Only with significant subsidies did some of the examined gasification projects produce some benefits for the users. The reliability of many gasifier systems installed in the 1980s proved to be low compared to conventional options. The study found only very few cases where the gasifier plant operated more or less efficiently, continuously and reliably. But even in those few cases severe technical problems had occurred at the beginning. Only through the steady commitment of the gasifier company or other external experts could the plants be modified and adapted to local conditions in a way that made technically sound operation possible. However, even one of the most promising examples in the study, a gasification plant in Vanuatu, stopped its gasification-based operations eventually. Although permanent technical support had achieved stable operation, it was converted to run exclusively on coconut oil some years later (Schragl, 2007).
Even though proper documentation of operational experience is rare it can be stated that recent projects are struggling with similar difficulties, regardless of whether in developing or industrialised countries.
