2.7.1 Production of fuel gas
2.7.2 Production of mechanical or electrical power in stationary installations
2.7.3 Mobile applications
A review of gasifier applications has been published by Foley and Barnard (12), who discuss the use of gasifiers for production of fuel gas for heat generation as well as the utilization of gasifiers in combination with engines.
Most gasifiers in commercial operation today are used for the production of heat, rather than fuel for internal combustion engines, because of the less stringent requirements for gas heating value and tar content. The fundamental advantage of a gasifier close coupled to a burning system is its ability to produce higher temperatures than can be achieved with conventional grate, combustion, liable to slagging problems at such temperatures, and in consequence its enhancement of boiler efficiency and output.
All types of gasifiers described in Section 2.3 can provide producer gas for combustion purposes, but for the sake of simplicity up-draught gasifiers are preferred in small systems (below 1 MW thermal power), while fluidised bed gasifiers are appropriate in power ranges above this level.
Most conventional oil-fired installations can be converted to producer gas.
The most potential users of low-calorific fuel-gas in the future are expected to be found among the following industries: metallurgy, ceramic, cement, lime and pulp. In these industrial branches the conversion of kilns, boilers and driers from oil to fuel gas operation is in principal a quite simple operation.
Gasifiers connected to stationary engines offer the possibility of using biomass to generate mechanical or electrical power in the range from a few kW up to a few MW.
Producer gas of engine quality needs a sufficiently high heating value (above 4200 Kj/m³ ), must be virtually tar and dust free in order to minimize engine wear, and should be as cool as possible in order to maximize the engine's gas intake and power output.
It is convenient to distinguish between applications in terms of power output. Figure 2.16 shows the power range of the various systems (36).
Figure 2.16 Application of biomass gasification processes
a) Large scale applications (500 kW and above)
This is the domain of the specialized fluidised bed or fixed bed installations.
The equipment is custom built and fully automized. Design and manufacture should be handled by specialized engineering and construction firms.
Equipment costs are likely to be in the range of US$ 1000 per installed kW and upwards.
b) Medium scale applications (30 -500 kW)
Fixed bed equipment fuelled by wood, charcoal and some types of agricultural wastes (maize cobs, coconut shells) is offered by a number of European and US manufacturers.
Adequate and continuing demand for this type of equipment could lead to standardization of parts and designs thus lowering production costs. For the moment quoted costs are in the range of 300 - 800 US$/kW (gasifier only) depending on type and capacity, level of automation and auxiliary equipment.
Full local manufacture is considered possible in countries possessing a well developed metal manufacturing industry. Major parts of the installations could be manufactured in most countries.
Applications are foreseen in small to medium size forestry and agro-allied industries (secondary wood industries, sawmills, coconut desiccating factories, etc.) as well as in power supply to remote communities.
c) Small-scale applications (7 - 30 kW)
This size would be appropriate for a multitude of village applications in developing countries (e.g. village maize and cereal mills, small-scale sugar crushers, looms, etc.).
The equipment must be cheap (less than 150 US$/kW), extremely reliable and should need no special operation and maintenance skills.
Designs suitable for local manufacture are tested and produced in the Philippines (13), Tanzania (48)-and a number of other countries. Documented evidence of their success is for the moment limited, and it should be stressed that training programmes for users and the organization of some type of maintenance service are of paramount importance.
It seems that charcoal gasifiers tend to give less operational problems in this power bracket than gasifiers fuelled by wood or agricultural residues. It is sometimes also believed that charcoal gasifier systems can be made cheaper than wood gasifiers systems in the 7 - 30 kW power range. There is some support for this in the prices charged for vehicle gasifier systems during the Second World War (43). It is not clear however if the difference of about twenty percent was caused by the difference in technology or was a result of better organized production or simply a matter or different profit margins.
d) Micro scale applications (1 - 7 kW)
This is the range Used by small and medium farmers in developing countries for providing power for irrigation systems.
Equipment must be transportable, cheap, simple and light in weight. It is quite possible that only small locally manufactured charcoal gasifiers will be able to meet the above requirements.
The use of down-draught gasifiers fuelled by wood or charcoal to power cars, lorries, buses, trains, boats and ships has proved its value and at least one European country (Sweden) maintains plans for large-scale production in case of an emergency, (see Chapter 3). This technique is currently being studied for powering of tractors (Switzerland, France, Finland, Netherlands) as well as small vans and boats (Philippines) and lorries (Sri Lanka).
However mobile applications present a number of additional difficulties as compared with stationary units.
In the first place the construction needs to be as light as possible in order not to reduce excessively the hauling capacity of the vehicle. Because the filter installations described in Chapter 3 tend to be fairly heavy and voluminous, exacting demands are put on the engineering skills of designers of mobile equipment as well as on the choice of materials.
In the second place mobile applications tend to operate with fairly large variations in engine (and gasifier) load. Under a given set of circumstances (especially long idling periods) this can lead to tar formation and clogging of cooler/cleaners and engines, as commonly occurred during the Second World War.
Applications on trains and boats suffer less from weight and load constraints, and for this reason give better results.
Engines retrofitted with gas producers show an appreciable loss of maximum power, and it will depend very much on the geographical situation (flat or hilly terrain) as well as on the skills of the driver whether the vehicle can be operated satisfactorily.
Whether these disadvantages will be balanced by the better economy of gasifier fuelled transport vehicles depend entirely on the local situation, especially on the cost and availability of petrol and diesel oil.