Small scale biomass gasification in developing countries

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Small scale biomass gasification in developing countries

Citation for published version (APA):

Kirkels, A. F., & de Boer, A. (2009). Small scale biomass gasification in developing countries. Technische Universiteit Eindhoven.

Document status and date: Published: 07/07/2009

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Small scale biomass gasification in

developing countries

ir. A.F. Kirkels A. de Boer

Eindhoven University of Technology July 7, 2009

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List of Tables

2.1 Typical characteristics of different biomass fuel types presently used com-mercially for energy generation, based on [QSK99] and [Sta95] . . . 18 3.1 Some characteristics of fixed bed gasifiers [Kno05, p.26,32], [btg02] . . . . 23 3.2 General characteristics of down- and updraft gasifiers [Bri94, p.636] . . . . 24 4.1 Fuel requirements versus gasifier design [Kno05] . . . 27 4.2 Feeding system components and their application, based on [Kno05, p.185] 29 4.3 Feeding system components for sealing and injection, based on [Kno05,

p.186] . . . 29 4.4 Different thermal end-uses, based on [VK97] . . . 32 4.5 Performance of gasifier plants, based on [Sta95, p.36] . . . 35 4.6 Capacity factors for various modalities of rural electrification, based on

[Sie02, p.232] . . . 38 5.1 An overview of small scale biomass gasification projects . . . 43 5.2 Operational costs [FAO86, p.101] . . . 50 6.1 Suggested applications for large-scale deployment of biomass gasifiers and

key interventions that will be required for effective deploymenta[GSK06, p.1581] . . . 56 6.2 Information on considering biomass gasification. Adapted from [Sta95, p.58] 57 6.3 Environmental aspects of gasification systems [AB99, p.28] . . . 60 A.1 Overview of existing manufacturers [Nov01] . . . 70 C.1 Required values of producer gas quality for use in gas engines [Kno05, p.190] 76 D.1 Approximate characterisation of diesel and biomass gasification

alterna-tives. [Sie02, p.225] . . . 82 D.2 Investment costs for power generation [Sta95, p.43] . . . 83 D.3 Investment costs for heat generation [Sta95, p.44] . . . 84 D.4 Gasifier system investment costs used in this study (e/kWe) [Sie02, p.230] 85 D.5 Typical fuel prices in selected developing countries [Sie02, p.233] . . . 87 D.6 Current fuel prices . . . 90

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List of Figures

1.1 Flowchart of biomass gasification used for power generation . . . 11

1.2 PV cells or biomass: which technology to prefer? [N. van Onne, TU/e] . . 12

2.1 Rice husk . . . 13

2.2 Biomass composition [QSK99] . . . 14

2.3 Fuel consumption as a function of capacity at different moisture contents, based on [Nov01] . . . 18

3.1 Updraft gasifier [Kno05] . . . 20

3.2 Downdraft gasifier [Kno05] . . . 21

3.3 Crossdraft gasifier [Kno05] . . . 22

4.1 Overview of gasifier system components . . . 26

4.2 Wood biomass preprocessing [Cri08] . . . 27

4.3 A screw conveyer (left) and a chain conveyer (right) . . . 30

4.4 Small scale gasifiers in India. The one on the right is being flared, which is standard procedure during start up [Cri08] . . . 31

4.5 Application domains of the BGSs in India between 1987 and 2007 [Cri08, p.37] . . . 35

4.6 Rural electrification in Bolivia [W. Drinkwaard 08]. Two major market segments identified by [GSK06, p.1575] are captive power and rural elec-trification. . . 36

5.1 Flow diagram for the gasifier system of the power plant at the Sapire sawmill [FAO86, p.94] . . . 46

5.2 Sketch of the wood gasifier at the Sapire sawmill [FAO86, p.95] . . . 47

6.1 Grid electricity in Bangalore [I. de Visser 07] . . . 58

6.2 Making a gasifier is not that difficult, as this practical example shows at the Eindhoven University of Technology. However, manufacturing and operating one without problems over a longer period of time is . . . 64

B.1 Typical pisten briquetting machine [RDU88] . . . 72

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C.1 Cyclone filter schematic [AS96] . . . 77 C.2 ESP schematic . . . 78 C.3 Several wetscrubber schematics . . . 79 D.1 Equi-IRR lines comparing (gasification and diesel fuelling) for varying fuel

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List of Symbols

ηHG Gasifier efficiency on a hot gas basis . . . 23

ηCG Gasifier efficiency on a cold gas basis . . . 23

LHV Lower Heating Value . . . 15

HHV Higher Heating Value . . . 15

M Cw Moisture content on a wet basis . . . 16

ASd Ash content on a dry basis . . . 17

M We Mega Watt electrical . . . 9

kWe kilo Watt electrical . . . 9

kJ kilo Joule . . . 16

N m Newton meter, Joule . . . 21

IC Internal Combustion . . . 31

SF C Specific Fuel Consumption . . . 18

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Chapter 1

Introduction

1.1 Project description

“Energy is central for achieving the interrelated economic, social and environmental aims of sustainable human development. We cannot simply ignore the energy needs of the 2 bil-lion people who have no means of escaping continuing cycles of poverty and deprivation. Nor will the local, regional and global environmental problems linked to conventional ways of using energy go away on their own. Other challenges confront us as well: the high prices of energy supplies in many countries, the vulnerability to interruptions in supply, and the need for more energy services to support continued development.” With these words Jos´e Goldemberg introduced the ‘World Energy Assessment’, a document prepared as input for the Rio+10 meeting in Johannesburg 2002. It stresses the impor-tance of a good energy supply for rural development, which is widely recognized, e.g. in Agenda 21 and in the Millenium Development Goals. Biomass gasification for small scale applications is an interesting option to provide this energy.

Hivos, an NGO based in the Netherlands, has given FACT foundation the assign-ment to perform a literature study on biomass gasification applications in developing countries. This assignment has been executed in collaboration with Eindhoven Univer-sity of Technology. The final goal is to provide an overview on benefits, costs, advantages and disadvantages of these systems. This should include technological aspects, but also economical, social and practical issues. Therefore also (un)successful projects should be taken into consideration. Actors that are active in this field as well as relevant literature should be disclosed.

The scale of this project has been specified as a developing rural village with a input of maximum 1000 farmers (a village or a farmers corporation). This is made operational by starting at 10 kWe uptil an upper limit of 1-2 MWe, which should be more than enough to meet the demands of such a scale.

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1.2 What is biomass gasification?

Biomass as feedstock is especially interesting in the context of developing countries. Current practices in these countries often rely on the use of wood for fuel. Many rural regions mainly rely on agriculture and agriculture-based-industry as source of income. This implies that people with experience in biomass as a fuel.

Biomass holds several other advantages like being renewable and CO2 neutral – although both promises have limitations. It is renewable since it can be grown over and over again in a short time span. However, replanting should be assured, mineral balances should be taken into account (for example for nitrogen as fertiliser and other essential trace minerals) and soil degradation should be prevented. The use of biomass is in principal CO2-neutral – the CO2 that is released during combustion was captured by the plant during growth, making it a closed short carbon cycle. Other major issues regarding biomass as fuel are land use and emissions. Land use is mainly an issue when biomass is planted on a large scale dedicated for use only as fuel. This is not an issue on small scale local applications using residues. The other issue comes from the fact that biomass is a solid fuel and during thermal conversion this will result in the emission of particles, tar, NOx, etc. Whether this poses a problem will mainly depend on the after treatment of the gas and the local conditions.

The best known energy conversion process is of course combustion. In combustion a fuel (a hydrocarbon, like fossil fuels or biomass) reacts at high temperature with oxygen from the air to produce heat, carbon dioxide and water. The process prefers biomass of high heating value and low water content – in general woody biomass, nutshells, straw like material, etc. Gasification is a somewhat similar process at high temperature, but with a limited supply of oxygen. As a result a gas is produced containing carbon monoxide, hydrogen and several contaminants like tar and particles. This gas is called producer gas. This gas should be handled with some care: carbon monoxide is toxic and the gas is combustible. This gas is an intermediate product – it needs a further con-version. Typical applications include: electricity production using an engine-generator (for households, irrigation water pumping, drinking water, agro processing), mechanical drive (irrigation also) and heat applications (drying, ovens, institutional cooking, cre-matoria). [Kis06, Cri08, QSK99] The economic competitiveness of heat gasifiers when compared to conventional alternatives is very attractive. The technical performance is generally proven and reliable. The economics of power gasifiers is highly sensitive to the diesel fuel price. [Kno05] Wood gas can also be used to operate vehicles on. This option is not deliberated on in this report.

For practical applications, not only the gasification process should be taken into account, but the entire energy supply chain. This will include pretreatment of the biomass (drying, reducing size), the gasifier, conditioning of the producer gas, the conversion of the gas to heat, electricity or mechanical power and the end-use of this energy. In figure 1.1 a typical flow chart for electricity production is presented.

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Gasifier Cooler / Cleaner Engine Air

Biomass

Gas

Ash Dust Condensate

Gas Power

Figure 1.1: Flowchart of biomass gasification used for power generation

1.3 Why gasification ? – competing technologies

For generating energy multiple technologies can be considered. Combustion might be the most directly competing technology with gasification – especially because both prefer the same sort of biomass. Combustion for electricity production requires the application of a steam cycle. Combustion systems based on steam cycles are technically mature and commercially available. Gasification systems are commercially available as well, however, small-scale applications need much supervision and suffer from frequent interruptions. For concepts with capacities below 1 MWe, production costs for the gasifier are lower than that of the steam cycle. However, the conclusions are conditional – depending on the assumptions made and feasibility studies must be performed in each case to determine which system is most suitable. [QSK99, p.52] For these small scale systems a micro-turbine is another alternative. This solution is more high-tech and requires extensive gas cleaning equipment and is therefore also not considered any further. Another application is heat. If the heat is applied in an indirect heating system using heat exchangers (e.g. boilers), both gasification and combustion can be used. In general, direct combustion systems may offer the highest overall efficiency, whereas gasification may offer the highest controllability regarding emissions and controlling of the heating. [QSK99, p.40]

Other technologies that are often applied in the context of rural electrification are solar cells and diesel engine-generator sets. Solar cells are reliable and based on the widely available energy from the sun. However, these systems tend to be expensive and produce limited power – having a hard time to provide enough energy for use in a local grid or industry.

Diesel engine-generator sets are probably the most used technology for local electric-ity production. The technology is widely applied and well proven. The major disadvan-tage is the continuous need for diesel supply and the related costs. These are important issues for developing countries, with a limited and sometimes unreliable energy infras-tructure and a small budget.

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1.4 Outline report

The outline of this report is as follows: first an introduction on biomass as a fuel will be given, introducing basic and essential concepts of fuels (chapter 2). A list of commonly used biomass fuels and their characteristics is displayed here as well. Next, an overview of existing gasification technologies will be presented in chapter 3. In chapter 4, gasifier systems and common components will be discussed to give more information on how biomass is converted into energy. Special attention is paid to different end-use application and sectors. Next, more or less successful projects are introduced. These provide detailed information and gives the reader a good feeling for what such a project is all about (chapter 5). Chapter 6.1 presents an overview of the performance of the technology on a wide variety of criteria, e.g. technological, environmental, economical, etc. Chapter 7 finally presents the overall conclusions of the research.

Contact information of active actors (manufacturers, consultants, research institutes, projects leaders) is included, as well as disclosure of relevant literature (appendix A). Ap-pendix B will give some information on briquetting and pelletisation machines, whereas appendix C will thoroughly discuss gas cleaning technologies. A chapter on gasifier eco-nomics has been added in appendix D. Last but not least, some copyright information is presented.

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Chapter 2

Biomass as a fuel

It is a common mistake to assume that any type of biomass which fits into the opening of the refueling lid of a gasifier can be used as fuel. Many of the operational difficulties which face inexperienced users of gasifiers are caused by the use of unsuitable fuels. In order to avoid bridging in the fuel bunker, reduced power output because of large pressure losses, or “weak” gas, slag cakes, tar in the engine and damage to the gasifier caused by overheating, it is necessary for most designs that the fuel properties are kept within fairly narrow ranges. The responsibility for quality control of the fuel rests with the operator. [FAO86, p.5]

An assessment of the use of biomass as a fuel requires a basic understanding of the types of suitable biomass and of their basic composition and characteristics. These characteristics will be discussed in this chapter, that is based on [QSK99, p.2-6]. Also, an overview is given for some “common” biomass fuel types.

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2.1 Properties of biomass

Each type of biomass has specific properties that determine its performance as a fuel in gasification devices. The most important properties relating to the thermal conversion of biomass are:

• Moisture content • Ash content • Heating value • Bulk density

• Volatile matter content • Elemental composition

• Particle size and size distribution

In the available literature, different indicators are often used to quantify the prop-erties listed above, causing confusion. Hence, the definition of these indicators and the relationships between them are emphasized here. In defining the properties of biomass, it is important to note that it consists of water, ash, and ash-free matter (figure 2.2), and that the proportion of each is critical in evaluating the suitability of biomass as a fuel. These characteristics, as well as the heating value, bulk density and particle size are discussed below.

2.2 Moisture content

Figure 2.2: Biomass composition [QSK99]

The moisture content of biomass is the quantity of water in the material, expressed as a percentage of the material’s weight. This weight can be referred to on a wet basis, on a dry basis, and on a dry-and-ash-free basis. If the moisture content is de-termined on a wet basis, the water’s weight is expressed as a percentage of the sum of the weight of the water, ash, and dry-and-ash-free matter. Similarly, when calculating the moisture content on a dry basis (however contradictory that may seem), the water’s weight is expressed as a percentage of the weight of the ash and dry-and-ash-free matter. The moisture content can also be expressed as a percentage of the dry-and-ash-free matter content. In that case, the water’s weight is related to the weight of the dry biomass. Because the moisture content affects the value of biomass as a fuel, the basis on which the

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ranging from less than 10% for cereal grain straw up to 50-70% for forest residues. Al-though it is physically possible to gasify moderately high-moisture fuels in some gasifiers, fuel moisture reduces the quality of the gas, the throughput of the gasifier and increases tar production. Biomass can contain more than 50% moisture (wet basis) when it is cut; it is generally desirable to dry biomass containing more than 25% moisture (wet basis) before gasification. [QSK99, p.18-19]

2.3 Ash content

The inorganic component (ash content) can be expressed in the same way as the moisture content on a wet, dry, or dry-and-ash-free basis. In general, the ash content is expressed on a dry basis. The inherent ash value – an integral part of the plant structure, which consists of a wide range of elements – represents less than 0.5% in wood, 5-10% in diverse agricultural crop materials, and up to 30-40% in rice husks and milfoil. The total ash content in the biomass and the chemical composition of the ash are both important. [FAO86, p.29] The composition of the ash affects its behavior under the high temperatures of gasification. For example, molten ash may cause problems like clogged ash caused by slagging.

2.4 Heating value

The heating value of a fuel is an indication of the energy chemically bound in the fuel with reference to a standardized environment. The standardization involves the temperature, state of water (vapor or liquid), and the combustion products (CO2, H2O, etc.). These standard conditions are widely available in the literature on the measurement of heating values. The energy chemically bound in the fuel is given by the heating value of the fuel in energy (J) per amount of matter (kg). This energy cannot be measured directly, but only with respect to a reference state. Reference states may differ, so a number of different heating values exist. The best known are the lower heating value (LHV) and higher heating value (HHV). For the LHV, the reference state of water is its gaseous state; for the HHV, the reference state of water is its liquid state. Both LHV and HHV can be defined on a wet and on a dry basis. The HHV is a correct but not realistic way of presenting fuel heating values for gasification purposes. Biomass always contains some water, which is released as vapor upon heating. At a moisture content of approximately 87% (wet basis) the LHV would be zero. In practice, the maximum allowable moisture content is 55% (wet basis) for fuel to ignite. However, several gasifiers pose much stricter conditions on the fuel; MCw = 10-20%, as stated in chapter 3.

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2.5 Bulk density

Bulk density refers to the weight of material per unit of volume. For biomass this is commonly expressed on an oven-dry-weight basis (MC = 0%) or an as-is basis, with a corresponding indication of moisture content (MCw). Similar to biomass moisture contents, biomass bulk densities show extreme variation, from lows of 150 to 200 kg/m3 for cereal grain straws and shavings to highs of 600 to 900 kg/m3_{for solid wood. Together,} heating value and bulk density determine the energy density – that is, the potential energy available per unit volume of the biomass in kJ/m3. In general, biomass energy densities are approximately one-tenth of fossil fuels such as petroleum or high-quality coal.

2.6 Particle size and size distribution

Up and downdraft gasifiers are limited in the range of fuel size acceptable in the feed stock. Fine grained and/or fluffy feedstock may cause flow problems in the bunker section of the gasifier as well as an inadmissible pressure drop over the reduction zone and a high proportion of dust in the gas. Large pressure drops will lead to reduction of the gas load of downdraft equipment, resulting in low temperatures and increased tar production. Excessively large sizes of particles or pieces reduces reactivity of the fuel, resulting in startup problems, poor gas quality and transport problems through the equipment. A large range in size distribution of the feedstock will generally aggravate the above phenomena. Too large particle sizes can cause gas channeling problems, especially in updraft gasifiers. In chapter 3, indications of particle size for different types of gasifiers will be presented. The particle size can be influenced by pretreatment of the raw biomass, i.e. cutting or densification by briquetting and pelletisation.

2.7 Overview of biomass fuels and fuel consumption

Many agricultural residues can be used as fuels. They include straw from grains; husks from rice, coconuts, or coffee; stalks from maize or cotton; and bagasse from sugar cane. In addition, forestry and landscape conservation activities generate biomass such as thinnings and verge grass. Using these biomass residues as fuels may solve the environ-mental problem of how to dispose of them. Moreover, the potential for using residues as a source of energy may create new incentives to grow crops that are now only marginally profitable. Cultivation of biomass specifically for direct use as a fuel (known as energy-cropping) may create new incentives for the agricultural sector, particularly in countries that suffer from overproduction of crops. However, energy cropping or setting up a sus-tainable wood-fuel forest, will probably not be the starting point for rural electrification. Future energy-cropping activities may involve cultivation of fast-growing wood species such as poplar, willow, or miscanthus in moderate climates, and sugar cane or sweet sorghum or other suitable species in tropical areas.

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Although similar with regard to higher heating values, biomass fuels have large dif-ferences with respect to physical (moisture content and bulk density), chemical (volatile matter content and ash content), and morphological (size and size distribution) charac-teristics. These fuel characteristics affect the choice of conversion technology: ‘easy’ fuels such as charcoal or wood blocks can be made to work in a large variety of equipment, whereas ‘difficult’ fuels such as rice husks or bagasse call for very specific and often expensive technological solutions, either in conversion equipment or in fuel preparation facilities. Some biomass types that are presently used commercially for energy gener-ation, together with their natural moisture content (MCw), ash content (ACd), lower heating values (LHV’s) and specific fuel consumption are listed in table 2.1.

The list presented here is by no means exhaustive. For more information we refer to the ECN Phyllis database on the composition of biomass and waste. Phyllis can be used to get all accessible information on a single sample, or to get average values for certain biomass fuels, of a limited number of parameters (moisture, volatiles and ash content, heating value and elemental composition). The database is accessible on the internet for free. [ECN]

Fuels considered most acceptable for gasifiers include lump charcoal, dry wood blocks, dry coconut shells and rice husk. [Sta95, p.12], [FAO86, p.31-33] Because good quality charcoal contains almost no tars it is a feasible fuel for all types of gasifiers. Good gasifier charcoal is low in mineral matter and does not crumble or disintegrate easily. The major disadvantages are the relatively high cost of charcoal and the energy losses which occur during charcoal manufacture (up to 70%).

Most dry wood species are suitable for gasification. Due to high a content of volatiles, tar will be produced during gasification. Application of sawdust generally requires pel-letisation. For agricultural residues, most experience has been with gasifying coconut shells, maize cobs and rice husk. Gasification of rice husk requires different measures, like special gasifiers, due to the high ash content.

The most right column in table 2.1 gives an indication on specific fuel consumption, SFC – that is the amount of fuel (kg) fuel required to generate a kWh of electrical output. The specific fuel consumption does not only depend on the type of fuel, but also on the gasifier used and whether the gasifier is operated on full load or not. However, it is included in the table since it offers the possibility to make a first estimate of the amount of biomass required to fuel a given gasifier capacity – or given the amount of biomass available, a first estimate can me made of the electrical power that could be produced. Take the example of a 30 kW rice husk gasifier. To operate this gasifier at full capacity, 30 ∗ (1.8 − 3.6) kg/hour rice husk is needed, that is 54 - 108 kg/hour rice husk. This only holds for continuous operation on full load. Note that in practice operation is often discontinuous and in partial load. The specific fuel consumption is mentioned in kg/kWh electrical output. These data do not apply for other applications, like mechanical shaft power and heat. In [Sta95], some values are mentioned for these as well, e.g. 0.16 - 0.37 kg/kWhth for heat production by wood gasification. In addition, in figure 2.3, the required fuel consumption is presented as a function of the capacity at different moisture contents. Assumed is an efficiency of the gasifier and respectively 80%

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and 25% and an ash content of 5% (∼wood). 1-2 kg/hr of fuel wood per kW seems to be a good approximation of fuel wood consumption.

Table 2.1: Typical characteristics of different biomass fuel types presently used commercially for energy generation, based on [QSK99] and [Sta95]

Type LHVw [MJ/kg] MCw [%] ACd[%] SFC [kg/kWhe] Bagasse 7.7 - 8.0 40 - 60 1.7 - 3.8 Cocoa husks 13 - 16 7 - 9 7 - 14 Coconut shells 18 8 4 Coffee husks 16 10 0.6 Cotton residues Stalks 16 10 - 20 0.1 Gin trash 14 9 12 Maize Cobs 13 - 15 10 - 20 2 Stalks 3 - 7 Palm-oil residues Fruit stems 5 63 5 Fibres 11 40 Shells 15 15 Debris 15 15 Peat 9 - 15 13 - 15 1 - 20 Rice husks 14 9 19 1.8 - 3.6 Straw 12 10 4.4 Wood 8.4 - 17 10 - 60 0.25 - 1.7 1.1 - 1.4 Charcoal 25 - 32 1 - 10 0.5 - 6 0.8 - 1.4 Capacity kWe 10% MC 30% MC 50% MC

Figure 2.3: Fuel consumption as a function of capacity at different moisture contents, based on [Nov01]

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Chapter 3

Gasifiers

3.1 Introduction

Gasifiers are relatively simple devices. The mechanics of their operation, such as feeding and gas cleanup, are also not too complex. The successful operation of gasifiers however, is not so simple. Biomass fuels used in gasifiers vary widely, and hence include many reactants and many possible reaction paths. The reaction rates are relatively high. All these factors contribute to the complex nature of gasification and make the process hard tot control and operate satisfactorily (Stassen 1995; van Swaay and others 1994; both in: [QSK99, p.26]). This report will not go into more detail on the gasification process itself (the different reactions taking place, the conditions and how this effects the producer gas composition). We considered that this is not of primary interest for our client. For these details we refer to [RDU88, p.21-27], [Kno05, p.14-16] and [FAO86, p.16-21].

A biomass gasification system consists primarily of a reactor or container into which fuel is fed along with a limited supply of air (less than stoichiometric which is required for complete combustion). Heat for gasification is generated through partial combustion of the feed material. The resulting chemical breakdown of the fuel and internal reactions result in a combustible gas usually called producer gas. The heating value of this gas varies between 4.0 and 6.0 MJ/Nm3_{, or about 10 to 15 % of the heating value of natural} gas. Producer gas from different fuels and different gasifier types may considerably vary in composition, but it consists always of a mixture of the combustible gases hydrogen (H2), carbon monoxide (CO), and methane (CH4) and the incombustible gases carbon dioxide (CO2) and nitrogen (N2). Because of the presence of CO, producer gas is toxic. In its raw form, the gas tends to be dirty, containing significant quantities of tars, soot, ash, and water. [Sta95, p.5]

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Biomass gasification reactors are the vessels in which solid biomass is converted into producer gas. Because this report is limited to small-scale gasification, only reactors of the fixed-bed type are considered (larger biomass gasifiers are usually of the fluidized-bed or entrained-flow type). The different fixed-bed reactor types are often characterized by the direction of the gas flow through the reactor (upward, downward, or horizontal) or by the respective directions of the solid flow and gas stream (co-current, counter-current, or cross-current). In the next paragraphs, different types of gasifiers are characterised, citing the ‘Handbook Biomass Gasification’ by Knoef. [Kno05, p.23-26]

3.2 Updraft (or counter-current) gasifiers

Figure 3.1: Updraft gasifier [Kno05]

The most straight forward type of gasifier is the fixed bed updraft gasifier, see figure 3.1. The biomass is fed at the top of the reactor and moves downwards as a result of the conversion of the biomass and the removal of ashes. The air intake is at the bottom and the gas leaves at the top. The biomass moves counter-currently to the gas flow, and passes through the drying zone, the pyrolysis zone, the reduction zone and the oxidation zone.

The major advantages of this type of gasifier are its simplic-ity, high charcoal burn-out and internal heat exchange leading to relatively low gas exit temperatures and high gasification efficiencies. Because of the internal heat exchange the fuel is dried in the top of the gasifier and therefore fuels with a high moisture content (up to 60%wb) can be used. Furthermore this type of gasifier can even process relatively small sized fuel par-ticles and accepts some size variation in the fuel feedstock.

Major drawbacks are the high amounts of tar and pyrolysis products, because the pyrolysis gas is not combusted. This is of minor importance if the gas is used for directs heat

applica-tions, in which the tars are simply burnt. In case the gas is used for power production, extensive gas cleaning is required.

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3.3 Downdraft (or co-current) gasifiers

In a downdraft reactor biomass is fed at the top and the air intake is also at the top or from the sides, see figure 3.2. The gas leaves at the bottom of the reactor, so the fuel and the gas move in the same direction. The same zones can be distinguished as in the updraft gasifier, although the order is somewhat different. Adding air to the char zone is an excellent approach for achieving low tar gas (<100 mg tar/Nm3_{). In effect, this is} a twin-zone or double-fire gasifier with a plug flow reactor configuration, which is why tar production is often very low.

Figure 3.2: Downdraft gasifier [Kno05]

The main advantage of a downdraft gasifier is the produc-tion of a gas with a low tar content, which is nearly suitable for engine applications. Downdraft gasifiers produce the lowest level of tar and are therefore the best option for engine appli-cations. Scaling-up of this type of gasifier is however limited. At low load levels, the temperature is decreasing and more tars are produced because the tar cracking becomes less efficient at low temperatures. Advantages of low load levels is the lower entrainment of particles in the gas. At high load levels, the tar cracking capability is higher which results in lower tar levels. However, more particles are entrained with the gas. At too high load levels, the residence time for tar cracking becomes too short which will increase the tar level again, along with the particle level. In practice, a tar-free gas is seldom if ever achieved over the whole operating range of the equipment.

Drawbacks of the downdraft gasifier are the high amounts of ash and dust particles in the gas due to the fact that the gas has to pass the oxidation zone where small ash particles are entrained. This leads also to a relative high temperature of the leaving gases resulting in a lower gasification efficiency. Downdraft gasifiers demand relatively strict requirements of

the fuel like a moisture content less than 25% (on a wet basis) and uniform in size in the range of 4 - 10 cm to realise regular flow, no blocking in the throat, enough “open space” for the pyrolysis gases to flow downwards and to allow heat transport from the hearth zone upwards; therefore pelletisation or briquetting of the biomass is often necessary.

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3.3.1 Downdraft gasifiers: open-core

Open-core gasifiers are especially designed to gasify fine materials with low bulk density. Because of the low bulk density of the fuel no throat can be applied in order to avoid bridging of the fuel which causes hampering or even stopping of the fuel flow. Special devices, like rotating grates, may be included to stir the fuel and to remove the ash. Particularly rice husk gasifiers require continuous ash removal systems because of the high ash content of rice husks resulting in large volumes of ash (approximately 55%v of the initial fuel volume). The bottom of the gasifier is set in a basin of water by which the ash is removed.

3.3.2 Downdraft gasifiers: multi-stage

In fixed bed gasifiers different zones can be distinguished of which the sequence depends on the flow direction of the gasification fuel and agent. The zones are not physically fixed and move up- or downwards dependent on operating conditions; they are to some extent overlapping.

In order to optimise each zone, several designs were developed where the combus-tion (oxidacombus-tion zone), gasificacombus-tion (reduccombus-tion zone) and/or pyrolysis zone are separated physically in different vessels. The basic idea is to further decrease the tar production by combusting of the pyrolysis gasses since combusting a gas-gas mixture is more effective than gas-solid.

3.4 Crossdraft gasifiers

Figure 3.3: Crossdraft gasifier [Kno05]

Crossdraft gasifiers (figure 3.3) are originally designed for the use of charcoal. Charcoal gasification results in very high tem-peratures (1500◦C and higher) in the hearth zone which can lead to material problems. Advantages of the system lie in the very small scale at which it can be operated. In developing countries installations for shaft power under 10 kWe are used. This is due to the very simple gas-cleaning train (cyclone and a bed filter). A drawback is the minimal tar-converting capa-bility, resulting in the need for high quality charcoal.

3.5 Gasifier overview

Some major characteristics of updraft, downdraft, crossdraft and open-core gasifiers are presented in table 3.1 applying wood as feedstock. Because of the variety of gasifier designs, which have been developed for each type of gasifier, the men-tioned data are only rough indications, which can hardly be called typical. But they give at least an indication of, in some

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extent, typical differences between the three basic fixed bed gasifier types.

Table 3.1: Some characteristics of fixed bed gasifiers [Kno05, p.26,32], [btg02]

Downdraft Updraft Crossdraft Open-core Operating T [◦C] 700 - 1200 700 - 900

Control easy very easy

Scale (MWth) < 5 < 20

Feedstock very critical critical

Fuel (wood) (charcoal)

moist. content (% wet basis) 12 (max. 25) 43 (max. 60) 10-20 7-15 (max. 20) ash content (% dry basis) 0.5 (max.6) 1.4 (max. 25) 0.5-1.0 1-2 (max. 20)

size (mm) 20-100 5-100 5-20 1-5

Gas exit temp (◦C) 700 200-400 1250 250-500

Tars (g/Nm3) 0.015-0.5 30-150 0.01-0.1 2-10

Sensitivity to load fluctuations sensitive not sensitive sensitive not sensitive

Turndown ratio 3-4 5-10 2-3 5-10

ηHG full load (%)a 85-90 90-95 75-90 70-80

ηCG full load (%)b 65-75 40-60 70-85 35-50

Producer gas LHV (MJ/Nm3) 4.5-5.0 5.0-6.0 4.0-4.5 5.5-6.0 a_η

HGHot gas efficiency, taking into account the heat contained in the gas. To be applied for heat

applications.

b_η

LGCold gas efficiency. The gas will be cooled after leaving the gasifier to ambient temperature. To be

applied for engine (and power) applications.

Some of the characteristics are important for the feedstock (fuel), others are gasi-fier characteristics (temperature, efficiency), and others are important for the gasigasi-fier- gasifier-application (exit temperature, tar level, lower heating value). The turndown ratio is an important concept in sizing gasifiers. It is a measure of the extent to which the gasifier can be operated at partial load. It is defined as the ratio of the highest practical gas generation rate at which trouble free operation with acceptable tar production is pos-sible (maximum gas flow for which the reactor is designed) to the lowest rate. As can be seen in the table, this is especially important for downdraft and crossdraft gasifiers with low turndown ratios. A downdraft gasifier cannot function well at levels below 25-33% of maximum design capacity. Low loads might result in disproportionate heat losses and might also cause tar formation problems. [RDU88, p.36], [Sta95, p.7] ηHG refers to hot gas efficiency of the gasifier that takes the heat contained in the gas into account. This efficiency should be applied for heat applications. ηCG is the cold gas efficiency. In this case the gas will be cooled after leaving the gasifier to ambient temperature. This efficiency is to be applied for engine and power applications. [Kno05, p.26]

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Electric power production and irrigation demand minimal requirements on turndown and response time of gasifiers. Updraft gasifiers have the slowest response time of the gasifier types and cannot be expected to follow changing loads with favourable results. The fastest response time is obtained from crossdraft gas producers, but these are only suitable for low tar fuels such as charcoal. Downdraft gasifiers also have a rapid response time. Table 3.1 gives an overview of some of the characteristics of fixed bed gasifiers, including turndown ratio and efficiencies. These figures can be translated into general characteristics, which are listed in table 3.2

Table 3.2: General characteristics of down- and updraft gasifiers [Bri94, p.636]

Downdraft Updraft

Simple, reliable and proven for certain fuels Very simple and robust construction

Relatively simple construction Low exit gas temperature

Close size specification required on feedstock High thermal efficiency

Low moisture fuels required Product gas is dirty with high levels of tar Relatively clean gas is produced High carbon conversion

High gas exit temperature Low ash carryover

Possible ash fusion and clinker formation on grate High residence time of solids

Low specific capacity Product gas suitable for direct firing

High residence time of solids Extensive gas cleanup needed for engines

Potential high carbon conversion Good turndown

Low ash carryover Very high conversion efficiency

Limited turndown Good scale-up potential

High conversion efficiency

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Chapter 4

System components

Operation of modern spark ignition or compression ignition stationary engines with gasoline or diesel fuel is generally characterized by high reliability and minor efforts from the operator. Under normal circumstances the operator’s role is limited to refueling and maintenance. Anyone expecting something similar for wood gas operation of engines will be disappointed. Preparation of the system for starting can require half an hour or more. The fuel is bulky and difficult to handle. Frequent feeding of fuel is often required and this limits the time the engine can run unattended. Taking care of residues such as ashes, soot and tarry condensates is time-consuming and dirty.

To convert raw biomass into energy, a chain of various components is needed. To-gether these form the entire gasifier system, from feedstock to electricity or heat. This chapter will clarify the most important components of this system and some of its gen-eral characteristics. Raw biomass needs to be processed before it can enter the gasifier (section 4.1). When processed, it needs to be fed into the gasifier (section 4.2). After the biomass has been gasified, the gas needs to be cleaned before it can be used (sec-tion 4.4). The gasifier itself is only shortly men(sec-tioned, since it has already widely been discussed in chapter 3. The end-use applications are given special attention in section 4.5. Main issues are the identification of applications and their respective requirements. Figure 4.1 presents a schematic overview of the components of a gasifier system that will be discussed in this chapter.

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Storage Gasifier Cyclone Char / ash reservoir Flare Feeder Cooler Filter Engine

Figure 4.1: Overview of gasifier system components

The surface area required for a 50 kWe gasifier system has been estimated at 50-150 m2. However, these are rough estimates and depend highly on the gas cleaning equipment and feeder. Biomass storage has also not been taken into account, as this can vary a lot according to the operating circumstances. For example, there is a 7 kWe power gasifier that fits on a car trailer. Heat gasifiers used as stoves can be even smaller, as these don’t require any gas treatment. In general, gasifiers can be located outside or under a roof in a well ventilated area.

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4.1 Feedstock preparation

One of the major bottlenecks of commercialisation of small scale gasification is the dif-ficulty to keep the process continuously going due to little control over the fuel quality. Feedstock preparation is required for almost all types of biomass materials. Table 4.1 gives an indication for fuel requirements for fixed bed gasifiers. The sequence of pre-treatment technologies depends on the type and characteristics of the biomass material and the requirements of the gasifier fuel. Feedstock preparation already starts at the storage. A closed bin, silo or hopper must be supplied to hold the biomass feedstock and to prevent it from getting wet. In many cases, industrial or agricultural containers are available in appropriate sizes at low cost. Densification and drying are other important operations in feedstock preparation, they have been given special consideration below. More information on pretreatment can be found in [Kno05, p.21-22, p.181-188].

Table 4.1: Fuel requirements versus gasifier design [Kno05]

Gasifier type Downdraft Updraft Moisture content (%wb) < 15 − 20 < 50 Ash content (%db) < 5 < 15

Morphology uniform almost uniform

Bulk density [kg/m3] > 500 > 400 Ash melting point (◦C) > 1250 > 1000

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Densification

Biomass fuels usually have bulk densities ranging from one-half to one-tenth of that of coal. Biomass fuels also come in a wide spectrum of sizes, many of which are not suitable for fixed bed gasification (such as sawdust, sander dust, shredder fines, straw and husks). However, biomass residues can be used in fixed-bed gasifiers if they are first densified to suitably sized pellets or briquettes using commercially available equipment. Biomass densification means the use of some form of mechanical pressure to reduce the volume of vegetable matter and its conversion to a solid form which is easier to handle and store than the original material. The produced pellets make excellent gasifier fuels and allow the fuel to be stored at much higher densities. Densification typically consumes 1-2% of the energy contained in the biomass. Some biomass forms, with high ash or dirt contents, are difficult to densify because they cause excessive wear of the die. [RDU88, p.17]

The briquetting of organic materials requires significantly higher pressures as addi-tional force is needed to overcome the natural springiness of these materials. Essentially, this involves the destruction of the cell walls through some combination of pressure and heat. The need for higher pressures means that the briquetting of organic materials is inherently more costly than for inorganic fuels. There are several types of machines that can be used for briquetting or pelletisation, the most common are: piston, screw, pel-let and manual presses. More information on these type of machines can be found in appendix B, based on [EP90].

Drying

Biomass can contain more than 50% moisture (wet basis) when it is cut; it is generally desirable to dry biomass containing more than 25% moisture (wet basis) before gasifi-cation – although updraft gasifiers can operate under higher moisture contents. Drying often can be accomplished using waste heat or solar energy. Natural drying (for instance on the field) is cheap but requires longer drying times. Artificial drying is more expensive but also more effective. In practice, artificial drying is often integrated with the gasi-fication plant using “waste heat”, to ensure a feedstock of constant moisture content. Drying of coarse materials requires more time compared to fine materials. Commercial dryers are available in many forms and sizes, and it is beyond the scope of this report to recommend such equipment for commercial scale operation.

4.2 Feeding

A biomass feeding system is a device that transports the feedstock from the long-term storage into the unit in which actual gasification takes place. Four functions can be attributed to feeding systems: transportation over a certain distance and height; dosage with which load conditions can be determined (process control mechanism); sealing from the outside world, preventing back firing or carbon monoxide release; and fuel injection. Feeding systems bring biomass into the gasification unit at a specific position in a specific way, as demanded by the applied gasification technology. The importance of distribution

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of fuel across and within the fixed bed increases in some proportion to the bed volume. Non-uniform heat release will occur in non-uniformly fed fixed beds and cause operation irregularities and worsened control of the gasifier.

Feeding of fixed bed gasifiers can be realised either manually or automatically. Manual feeding is very robust, almost any type of fuel can be fed. It is the preferred feeding method for small gasifiers. Because of labour costs and possible health risk, the common practice for larger systems is automated feeding – at least in European countries. In a state of the art feeding system biomass is screwed from a fuel bunker on a belt conveyor or elevator to an intermediate storage, from which the fuel is fed via a lock hopper or a rotary valve into the gasifier reactor. Straw, bagasse and rice husk still require manual feeding. Different components that can be applied in the feeding system are identified in table 4.2 and shown in figure 4.2. [Kno05, p.181-188] Additionally, table 4.3 shows which feeding system components should be used to seal and inject different types of biomass fuels.

Table 4.2: Feeding system components and their application, based on [Kno05, p.185]

Feeding system Function in feeding system Type of fuel Remark

Screw Injection into gasification unit, High and compacted low Particles size < 8 cm, sealing by

transportation, sealing density materials, powder formation of plug, but not to high pressures

Conveyor belt (Upward) transportation of non High density and Dosage capabilities; pressurised materials low density materials for abrasive materials;

not sticky material only if belt cleaners installed

Vibrating feeder Transportation of non pressurised Dry high density materials high capacity potential, no dosage capabilities;

materials fine materials cause dust formation;

sticky materials cannot be fed

Lock hopper Sealing, downward transportation All types Sealing to high pressures; loss of purge gas

Rotary valve Sealing, downward transportation All types, except fluffy or Sealing up to 10 bar, oversized materials (bridging) good dosage capabilities

Piston feeder Injection into gas unit High density and compacted low Low inert gas consumption, density materials, powder risk of bridge formation or

jamming with large particles

Table 4.3: Feeding system components for sealing and injection, based on [Kno05, p.186]

Fuel type Examples Feeding system component (fixed bed) Fluffy low density materials (uncut fibers) Straw, bagasse Manual

Fluffy low density materials (short fibers) Rice husk, cut straw, cut bagasse Manual, or cylindric lock hopper Powder type materials Sawdust, dried slurries, chicken manure n.a.

High density materials (5-50 mm) Wood chips Rotary valve, or lock hopper High density materials (> 50 mm) Wood blocks Lock hopper, or manual

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Figure 4.3: A screw conveyer (left) and a chain conveyer (right)

4.3 Gasifiers

The gasifier is the component in which the gasification takes place. Several types of gasi-fiers are extensively discussed in chapter 3. Be aware that gasigasi-fiers come with auxiliary equipment. Proper seals are important to ensure both gas quality and safe operation. It is also important to provide a suitable method for pulling or pushing the gas through the gasifier, and since the mass of gas and air being moved is much larger than the mass of fuel being fed, considerable power may be required. When an engine operates on producer gas, it can also provide suction and compression for the gasifier. In the process of gasification char-ash is produced and must be removed. An air-tight char-ash receiver should be provided, since this char material is combustible and may reignite spontaneously. In addition, it may be necessary to cool the receiver. The receiver can contain explosive gas even when cold. They have been known to ignite on startup, and precautions should be taken against it. [RDU88, p.93-95]

4.4 Gas cleaning

When biomass is gasified, the resulting producer gas needs to be cleaned if its end-use is to power an IC engine. Especially start-up and shut down requires special attention and procedures since a lot of tar is formed in these phases and equipment is still cold or cooling down. Often a flare is connected to the gas cleaning equipment, to flare off producer gas when starting up the gasification process, or in case of engine malfunction. A flare can also be used to inspect the quality of the producer gas, the color of the flare reveals important information like flame temperature and particle contents. In engine applications in developing countries, in general a cyclone is applied in combination with scubbers and/or filters [Sta95, p.20]. Thermal applications of producer gas usually don’t require extensive gas cleaning. Appendix C will go in detail on gas cleaning specifications and equipment, based on [Kno05, p.189-210].

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Figure 4.4: Small scale gasifiers in India. The one on the right is being flared, which is standard procedure during start up [Cri08]

4.5 End-use applications

At the end of the gasifier system, several components can be used. The most basic op-tion for heat applicaop-tions is to use a combustor. An internal combusop-tion (IC) engine can be used for shaft power generation (e.g. the shaft connected to an irrigation pump, for sawing timber or for grain milling), as well as electricity generation (the shaft con-nected to a generator). The options of heat and power generation will be elaborated in the following paragraphs. Cristiaens concluded that for India, irrigation applications, rural electrification and captive power have not yet developed to market niches, whereas thermal applications that replaced petroleum based fuels, gasifier based crematoria and larger scale grid connected systems did.

4.5.1 Heat applications

A major application is to generate heat for boilers, dryers, ovens and kilns. There is a broad range of applications for heat, like tea drying, tobacco leaf curing, institutional cooking, road tarring, brick making etc. [Cri08, p.80] An overview is presented in table 4.4. The commercial potential for heat gasifiers is significant. The technical performance is generally proven and reliable. The economic competitiveness of heat gasifiers when compared to conventional alternatives is very attractive. In addition to the excellent prospects in the agro-industrial sector, heat gasifiers can be applied in non-biomass producing industries requiring process heat, if acceptable and affordable biomass fuels are available. Potential heat gasifier markets include retrofits for oil-fired boilers, ovens, kilns and dryers used in industries as varied as food processing to manufacturing. [Kno05,

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p.39]

In heat gasifiers, hot producer gases are conducted through gas pipes to combustors. To prevent problems with tar condensation, it is important that the piping is well in-sulated and that the gas is not transported over a too long distance. A large number of “Waterwide” heat gasifiers have been successfully introduced in Papua New Guinea. These gasifiers have in general been used to replace diesel fuel burners in the copra, cocoa, coffee and tea industries. Another application is the gas production for cooking, something that is daily practice in China. The Biomass Energy Foundation invented a new type of gasifier, the “woodgas stove”. [Kno05, p.46-47]

Table 4.4: Different thermal end-uses, based on [VK97]

End-use Examples

Water boiling cooking cocoons, dyeing fabric, production of magnesium chloride, etc.

Dryers (50 - 130◦_C _{farm products, food, spices (like rubber, tea, coffee, cardamom, tobacco, chemical products)} Kilns (800 - 950◦C) baking of tiles, bricks, potteries or for heat treatment purpose (hardening,annealing) Furnaces (650 - 1600◦C) melting metals in foundries, glass-melting industries etc.

Ghosh and others (2004, 2006) provide an overview of developments in India in the last 20 years and suggest a possible approach for moving forward. They conclude that the most fruitfull strategy for technology diffusion would be the one that initially focuses on thermal productive systems. Two specific applications areas are identified, firstly the heat production for small and medium enterprises (SME) and secondly for the informal sector. A large number of SMEs require substantial amounts of process heat as part of their industrial operations (range 30-200 kW). These include chemicals manufacturers, large brick kilns, steel rerollers, foundries, lime kilns, rubber driers, and ceramic manufacturers, to name a few categories. A number of these rely on liquid fuels, such as diesel and furnace oil, to meet process-heat requirements, thereby adding significantly to production costs due to high costs of fuel. Those enterprises that do not use liquid fuel use biomass, albeit in an inefficient manner. A shift from inefficient biomass burning to biomass gasifiers can increase overall efficiency of the heating process substantially, often by as much as a factor of two. The main barrier to the uptake of gasifiers in such an application is likely to be a lack of information and awareness.

A second application is heat production for the informal sector. Large numbers of small, informal enterprises rely on thermal energy for undertaking many of their oper-ations. Examples of activities in this sector include small sized brick making units, silk reeling, textile dyeing, small agro-processing units, small-scale soap and oil manufac-turing and small bakeries. The market potential is high. In India, it is estimated that there are around 5 million enterprises belonging to this sector having substantial energy consumption. Biomass is often the primary energy source for these units, since it is often the only available energy supply option – given the scale and nature of their operations, often this biomass is used in an extremely inefficient fashion (simply being burnt). This often creates unhealthy workplace conditions. Replacement by gasifier-based heat

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deliv-conditions. Substitution of biomass burning by gasifiers can also result in significant pro-ductivity and product-quality improvements in certain activities, like silk reeling units [Dhi01] and cardamom curing [MK01]. For estimates of firewood consumption for tradi-tional applications, both total and per unit of product, we refer to Ghosh (2004) – this can give an indication of the total potential for biomass gasifiers in different applications. 4.5.2 Engine-generator sets

For small scale applications, engines are usually preferred for power generation, as the costs of these engines are low, maintenance is easy and required gas cleaning is limited (compared to more advanced technologies like micro-turbines) but essential. Spark igni-tion engines, normally used with petrol or kerosene, can be run on producer gas alone. Diesel engines can be converted to full producer gas operation by lowering the compres-sion ratio and installation of a spark ignition system. A normal unconverted diesel engine can run in “dual fuel” mode, whereby the engine anything between 0-90% of its power output draws from producer gas, the remaining diesel oil being necessary for ignition of the combustible gas/air mixture. This is probably the most applied system at the moment and cheaper than using 100% producer gas engines. The advantage lies in its flexibility: in case of malfunctioning of the gasifier or lack of biomass fuel, an immedi-ate change to full diesel operation is generally possible. The disadvantage is mainly the continued use of diesel (added costs). Not all types of diesel engines can be converted to the above mode of operation. Stassen [Sta95] makes a special remark regarding rice husk gasifiers – these were only working with special sturdy low-speed engines that require frequent maintenance.

For power production efficiencies in the range of 25-35% are typical, the span being the result of the engine type and overall size of the installations, ranging from 25 kW up to 5 - 8 MW. [Kno05, p.78-80] In many cases of industrial small scale gasification, a gasifier is used for power generation to operate a factory. Examples include: rice husk gasification to produce power for rice mills, coconut shell gasification in coconut dessicating plants and wood gasification in wood mills.

The temperature of the gas influences the power output of a producer gas engine. The highest power output is realised at the lowest gas temperature. Thus, in power applications it is advantageous to cool the gas as far as practical. The maximum engine power output of a producer gas engine is lower than the output of an equivalent engine operated on conventional liquid fuel, a phenomenon known as derating. The efficiency of a producer-gas engine, however, is still theoretically the same as that of an Otto or diesel engine. A summary of the performance that can be expected from different feedstock, gasifier and engine combinations is provided in table 4.5. More details on engines, engine sizing and conversions can be found in [FAO86, p.8-15] and [RDU88, p.105-118].

Ghosh e.a. (2006) conclude that producer gas engines hold the potential to be more economically attractive, but these need to be developed further in order to reduce their costs. These engines are not specifically designed for producer gas as fuel, but are mostly modified diesel engines. Gasifier manufacturers are primarily undertaking engine

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modi-Ghosh also reflected on most interesting market segments for power generation in India: rural electrification, delivering power to one or several small or medium sized firms, captive power, or rural grid-interfaced applications. Rural electrification brings electricity to remote villages that are depending on traditional energy fuels. Major advantages are social advantages and for the long term improvement of the potential to develop economical activities. NGOs or community based organizations are likely to be best suited to undertake the dissemination of these systems since no individual in a village would have the incentive or the skill to install, operate and maintain such a system. Long term satisfactory operation of the installation and the grid, as well as sustainable supply of biomass are main issues. Due to limited applications the capacity factor is low and therefore electricity supply is relative expensive (as are other options for rural electrification). Possibilities to finance such an installation are limited. Gasifier unit size requirements will be in the range of 10-20 kW for a single village (say having 40 households) and may range up to 50 kW for a cluster of villages.

A gasifier system could be deployed in small and medium sized enterprises or clus-ters of enterprises. For an individual firm the gasifiers could be in the 30-100 kW range, for a cluster of firms it would be in the 100-200 kW range. One enterprise would be the operator of the installation and possibly be delivering electricity to other firms (entrepreneur acting as an energy service company). In general in India these applica-tions are not competitive with the grid. Of course, for any enterprise already relying on liquid-fuel-based generation due to unreliable grid supply, it would be highly economical. Captive power pertains to the use of gasifier based electricity generation systems to uti-lize the excess/waste biomass that is available as by-product of agricultural or industrial processing. Hence the key features of this biomass supply are: large quantities, relative uniformity of composition, and one single supply source. Examples in this category are rice mills that generate rice husk as waste, cashew-nut shells from cashew processing enterprises, residues from rubber and coffee industries, and wood wastes from plywood industries. The market potential for gasifier applications in this category is substantial for example in India operate about 100.000 rice mills. Units would have capacities in the range of 100-500 kW. This option might economically be attractive for large scale gasifiers (500 kW) when a producer gas engine can be applied.

Rural grid-interfaced applications provides similar benefits as rural electrification although only to regions to which the grid is extended. Many rural areas connected to the grid suffer from frequent power shortages due to unreliable power supply. The power supply quality is often poor as well, characterized by fluctuating voltages, due to poor grid infrastructure. In this system it is possible to provide power to multiple villages and feed excess power to the grid. This will significantly increase the capacity factor of the installation and thereby the economical feasibility. The gasifiers for this application would be in the 100-500 kW range. Some intermediary organizations would be required for implementing such a project to ensure biomass supplies, operation of the gasifier and engine, maintenance and troubleshooting; as well as collecting revenues from the villagers. For this size of installation this is easier (more feasible) to do as it is for small scale systems. Ensuring finance availability is crucial, but given the nature of the project

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and the social benefits that accrue from it, government and donor institutions are likely to be interested in supporting it. A critical success factor for projects in this category is that appropriate policies for selling power to the grid need to be in place as viability of the project pivots around this sale. Concerns on biomass sustainability could arise.

Figure 4.5: Application domains of the BGSs in India between 1987 and 2007 [Cri08, p.37]

Table 4.5: Performance of gasifier plants, based on [Sta95, p.36]

System

Engine Specific fuel overall Diesel Engine Problems derating consumption efficiency savings life- Gasifier Health Environ-Gasifier type [%]a _[kg/kWh]b _[%]b _[%]c _time _quality _{and safety} _mental Wood Otto 50-60 1.4 > 16 n.a. normal doubtful none minor Wood diesel 60-90 1.1 > 21 60-90 normal doubtful none minor Charcoal Otto 50 0.9 > 10 n.a. normal doubtful none none Charcoal diesel 50-80 0.9 > 12 40-70 doubtful doubtful none none Rice husk Otto 50-60 > 3.5 > 7 n.a. doubtful normal none major Rice husk diesel 60-90 > 2.0 > 12 50-75 shortened doubtful none major Peat diesel 20-50 high very low very little shortened bad possible major a_{Maximum engine output on producer gas as a percentage of maximum power output on gasoline/diesel.} b_{At full engine load.}

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Figure 4.6: Rural electrification in Bolivia [W. Drinkwaard 08]. Two major market segments identified by [GSK06, p.1575] are captive power and rural electrification.

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4.5.3 Capacity factors

The capacity is crucial for power generation: it is essential for calculating its economical feasibility and fuel requirement. Since the capacity factor is directly affected by the end-use, we will be discuss this here.

[Meu90] reports capacity factors between 6-50% (where the 50% is exceptionally high due to the use of street lighting) achieved with rural electrification in five Asian coun-tries: Bangladesh, India, Pakistan, Philippines, and Thailand. For likely capacity factors, typical conditions for rural electrification are analysed in table 4.6. If households alone are provided with electricity, a capacity of 7% appears to be typical. Higher values may be achieved if additional applications are found. If commercial or industrial activities are also served, the capacity factor may reach a value of 35%. If a battery charging service was to be provided with electricity from the generating system, a capacity factor as high as 55% could be achieved since case battery charging could be continued overnight. Note that systems considered here envisage one single generator set. Under some circum-stances, it may be preferable to install a second diesel engine for peak load provision and back-up. The load factor of the gasifier system could then be increased. For example, in the case of the second application in table 4.6 (households plus commercial or industrial services), from 50% to 90%. As a result, the capacity factor would increase from 35% to 60%.

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Table 4.6: Capacity factors for various modalities of rural electrification, based on [Sie02, p.232]

Option for use of isolated grid Unit Households Households Households

only plus plus battery

commercial charging or industrial service

Parameter values services

Operational period (OP) week/year 52 52 52

day/week 7 6 6

hour/day 4 24 24

Planned maintenance during OP % of 0% 10% 10%

(PM)

Availability factor during % of 90% 90% 90%

(100%-PM)*OP (100%-PM)*OP

Load factora 50% 50% 80%

Capacity factor (CF)b 7% 35% 55%

On-stream time (OST)c _hour/year _1,310 _6,065 _6,065

Full-load equivalent hoursd _{FLE hour/year} ₆₅₅ _3,033 _4,852

a_{The quantity of energy produced by a power plant during the period that it is operational}

divided by the theoretical maximum quantity of energy produced by the power plant during that same operational period (expressed as a fraction or as a percentage).

b_{The quantity of energy produced by a power plant during a year divided by the theoretical}

maximum quantity of energy produced by the power plant during that period (expressed as a fraction or as a percentage). OP ∗(100%−P M )∗availability∗load₈₇₆₀ _hours

c_{Week * day * hour * availability factor * (100%-PM).} d_{OST * load factor}

Small scale biomass gasification in developing countries

Small scale biomass gasification in developing countries

Small scale biomass gasification in

developing countries

Contents

List of Tables

List of Figures

List of Symbols

Chapter 1

Introduction

1.1

Project description

1.2

What is biomass gasification?

1.3

Why gasification ? – competing technologies

1.4

Outline report

Chapter 2

Biomass as a fuel

2.1

Properties of biomass

2.2

Moisture content

2.3

Ash content

2.4

Heating value

2.5

Bulk density

2.6

Particle size and size distribution

2.7

Overview of biomass fuels and fuel consumption

Chapter 3

Gasifiers

3.1

Introduction

3.2

Updraft (or counter-current) gasifiers

3.3

Downdraft (or co-current) gasifiers

3.4

Crossdraft gasifiers

3.5

Gasifier overview

Chapter 4

System components

4.1

Feedstock preparation

4.2

Feeding

4.3

Gasifiers

4.4

Gas cleaning

4.5

End-use applications