Cape Town-based industrial boilers designer and manufacturer John Thompson developed the MicroGen watertube boiler – launched this year – to cater for increasing demand for small and medium- size pressure power boilers that are particularly suited to applications in fibrous biomass fuel and coal-fired cogeneration.

The boiler was designed for a steam output of between 20 t/h and 25 t/h, depending on the fuel type, but higher outputs can be achieved using multiple boilers, says John Thompson marketing consultant Roger Dawson.

To match popular steam turbine selections that can achieve good cycle efficiencies at a reasonable cost, the MicroGen watertube boiler can produce steam at pressures of between 31 bar and 61 bar. The design incorporates a single-drum configuration with a membrane- wall furnace construction, he notes.

Energy Cost
Dawson says that, in all industries, energy costs have become a major consideration in the management of existing plants and in the feasibility studies for potential new plants.

In remote areas, or where exorbitant electricity prices from the national grid defeat the feasibility of operating a plant, generating power for private consumption has become a priority, he adds.

Cogeneration
Processes in the breweries, food and tyre industries, for example, that require steam as the heating medium could use cogeneration.

Cogeneration entails using high-pressure steam to turn the back-pressure turbo-alternator of the boiler, while exhaust steam is used as process steam. In this way, for a small increase in thermal input, the plant can generate electricity while generating sufficient heat to meet the requirements of the particular process, says Dawson.

If a process produces combustible waste products that can supplement the fuel feedstock, the case for cogeneration becomes even stronger. Environmentally, cogeneration becomes more attractive and viable if the fuel can be considered as a renewable biomass fuel.

Fuel Types
In addition to coal, a wide variety of biomass fuels can be burnt on the stoker, including bagasse; pellets, chips, and other waste from wood, such as sawdust; sunflower- and cotton- seed husks; nutshells; grape pomace pellets; corncobs; dried hops; and torrefied biomass.

Modular Design
The MicroGen boiler is configured to facilitate the manufacture of the pressure parts in large subassemblies, which can be containerised for transportation. This reduces expensive and time-consuming site construction work.

The furnace design incorporates a continuous ash-discharge stoker, a suitably proportioned furnace and pneumatic fuel spreaders.