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, advises Dennis Williams of steam and boiler operations and maintenance service provider, Associated Energy Services (AES).
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Fire-tube boilers are typically used to handle steam pressures of 20 bar or less.
South African industry relies mainly 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 fuel sources and novel combustion mechanisms. Water tube boilers are used by large utilities, as well as the sugar and paper industries.
Key considerations for making a choice are the amount of steam required per hour and the steam pressure, followed by specific on-site requirements.
Challenges could include everything from configuration and space to environmental issues: “For example, companies in the Durban South Basin would not get permission for a 50 t 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 AES’ Dennis Williams.
“Boilers can be catalogue or quite specialised,” he 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 boilers fired by gas- and liquid-fuels, 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, which used electrode boilers. Due to loadshedding, electricity was no longer a reliable power source, so companies 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
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 thicknesses are unsuitable for this level of pressure. “Improvements should rather come from the application of the latest control systems and combustion technologies, 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 replacing it with a burner that 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 that 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 t of steam per hour may only achieve a maximum output of 12 or 14 t.
Certain fuel types also allow the use of additional equipment to optimise the performance of existing and new boilers, according to Williams. An economiser, for instance, uses a heat exchanger to recover some of the lower grade heat from the flue gas exiting the stack to preheat the boiler feed water. This reduces fuel usage. Recovered heat can also be used to raise the temperature of combustion air entering the boiler.
Fitting an economiser is possible with biomass but not advisable with coal, because the flue gas will contain sulphur dioxide that will cause acid-dewpoint corrosion.
Williams says AES’ rich pool of experience, ability to keep pace with new steam trends and technologies and understanding of their clients’ production processes can 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 can offer.
“The last thing clients want is a cutting-edge plant that becomes problematic to operate or maintain three or four years down the line. We can provide a viable and optimal boiler solution, not just based on capital cost, but also from an operational perspective,” Williams concludes.
