The changing world of boilers means companies wishing to improve their thermal energy efficiencies while containing costs and environmental impacts must make careful and informed decisions. South African industry mainly relies on two types of boilers, fire tube boilers, handling lower steam pressures of 20 bar or less and water tube designs, catering for greater steam tonnages, higher operating pressures or unusual fuels sources and novel combustion mechanisms. Water tube boilers are used by large utilities and also the sugar and paper industries.
Fuelling boiler choices
Key considerations are the amount of steam required per hour and the steam pressure followed by specific on-site requirements. Challenges could include everything from configurations and space to environmental issues: “Companies in the Durban South Basin would not get permission for a 50 ton per hour coal-fired boiler installation due to stringent cutbacks on sulphur dioxide. Sulphur content needs to be less than 1%, even in coal. Here, a gas-fired unit would be preferable,” says Dennis Williams, commercial director of expert steam and boiler operations and maintenance service provider, Associated Energy Services (AES).
“Boilers can be catalogue or quite specialised,” Williams continues. In the industrial steam space there is a wide selection. Large utility boilers facilitate more specialised design, while for those processing unique fuels or waste streams, there are craft boilers to provide bespoke solutions which reduce waste while saving costs and boosting efficiencies. A food manufacturer in KwaZulu-Natal, for example, added a boiler to process chicory grounds from coffee-making, avoiding the cost of treating and dumping this wet slurry.
Up until now most boilers have been coal-fired as this fuel was abundant and affordable. Now there are also gas- and liquid- fuel-fired boilers which are perfect for companies operating them intermittently or for short periods: “These allow the fire tube design to operate at a slightly higher steam pressure because the combustion flue is smaller,” Williams explains. In the past, electrical steam generation was popular with breweries, textile and tyre manufacturers installing electrode boilers. Due to loadshedding companies no longer had a reliable power source and began operating with costly heavy furnace oil.
“They had to decide what they were going to do long-term. Many of our clients are former electrode boiler users who converted to coal or biomass, moving away from heavy furnace oil as a backup,” Williams says.
Replace or retrofit: the burning question
Again, a choice between a complete upgrade or a retrofit depends on a client’s individual steam requirements. Williams notes that an original 20 bar boiler cannot be transformed into a 45 bar boiler as the metallurgy and steel thickness are unsuitable for this level of pressure: “Improvements should rather come from the application of the latest control system and combustion technology, as well as waste heat recovery on the back-end,” he advises.
A fire tube, coal-fired boiler with a large chain grate stoker feeding fuel to the boiler can be retrofitted by removing the stoker and placing a burner which uses liquid fuel or gas on the front end. To convert a liquid or gas-fired boiler to biomass, a Dutch oven − essentially a refractory-lined furnace which combusts wood biomass on the outside and then takes heat through the flue into the boiler – can be placed on the front end.
However, William warns that changing the flue gas characteristics may change the output capacity. When converting to biomass with high moisture content of up to 40%, a boiler designed to produce 20 tons of steam per hour could only achieve 12 or 14 tons at maximum output.
Boilers and beyond
Certain fuel types also allow the use of additional equipment to optimise performance on existing and new boilers according to Williams. This could include an economiser, which is a heat exchanger that recovers some of the lower grade heat from the flue gas exiting the stack to return it to reheat boiler feed water reducing fuel usage, or air pre-heaters that raise the temperature of combustion air into the boiler.
Fitting an economiser is possible with biomass but not advisable with coal as the flue gas will contain sulphur dioxide which leads to acid dew point corrosion.
Williams says AES’s rich pool of experience, ability to keep pace with new steam trends and technologies and understanding of their clients’ production processes all assist clients to weigh up different options in a careful and informed manner.
“This is where thermodynamic modelling and project design comes in, as well as understanding what different boiler manufacturers offer. The last thing clients want is a cutting-edge plant which becomes problematic to operate or maintain three or four years down the line. We can provide a viable and optimal boiler solution, not just from a capital cost, but also from an operational perspective,” Williams concludes.
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