As the price of biomass fuels becomes more competitive, compared with traditional fuels such as coal, oil and gas, the demand for biomass-fired boilers is steadily increasing. Subsequently, this has created a growing need for the manufacturing of boilers for these cleaner and more sustainable fuels, states boiler designer and manufacturer John Thompson (JT).

As a result of the interest in biofuel combustion, for both existing and new boilers, the company has initiated a programme that establishes the combustion characteristics of each particular fuel.

“With this information, our engineers can advise our customers on expected performance and any modifications that an existing plant needs to efficiently burn a particular fuel. The company’s Thompson Europac test boiler is a standard coal-fired design and, with minor software and stoker modifications, we have been able to fire most of the biofuels tested successfully,” says JT Package Boilers technical manager Neville Sharwood.

The test boiler is installed at JT’s Boiler Development & Training Centre, in Bellville, Cape Town. This facility is dedicated to training customers’ staff and developing JT’s products. Between training courses, the boiler is available to the research and development engineers for the development of new products and the testing of various fuels. The boiler features a programmable logic controller-based combustion management system and all the necessary instrumentation to collect test data.

The programme to test the combustion characteristics of each type of fuel involved a series of tests, firing a variety of biomass fuels at sites in South Africa and with JT’s test boiler to enable JT engineers to establish the most efficient combustion settings for each fuel type and understand the limitations of the fuel on existing equipment.

Biomass fuels tested include wood pellets, woodchips, wood pucks, grape pomace, nutshells, sunflower seed husks, corncobs, dried hops and torrefied biomass.

The tests measure the combustion efficiency and fuel gas losses. This information is used to predict the performance of an existing plant. Parameters, such as grate rating and furnace volumetric heat-release rates, are calculated from the test results and are useful in sizing a new plant to ensure that a boiler operates efficiently.

“Under efficient combustion conditions, steam outputs of between 50% and 85% of the coal-fired output were recorded during firing of the biomass fuels. As many boiler installations have surplus capacity, the existing boilers could be converted from coal to biomass firing without having to buy additional boilers,” he highlights.

Sharwood adds that biofuels typically have lower calorific values than coal; therefore, the mass of the fuel has to be increased to generate the same quantity of steam when fired using a coal-fired design.

“On existing coal-fired equipment, maxi- mum output will be reduced when firing a biomass fuel. But there is often spare capacity in an existing plant that allows for a simple fuel switch to take place. Dependent on the biomass, the materials handling system may need changing, but many of the biofuels tested were adequately conveyed by the existing coal handling equipment. JT is looking into designing a range of boilers optimised around the biomass combustion parameters,” he explains.

Biogas is produced from the treatment of wastewater in certain industries and can offset 10% to 25% of fuel requirements in the production of steam. Several JT biogas boilers have been supplied to the local market, which have reduced customers’ carbon emissions and reliance on fossil fuels.

The high outputs achieved during the tests were largely owing to JT’s modified chain- grate stoker and Micropac boiler management system, which can easily be fine-tuned for different fuels. Both of these can be used to retrofit existing coal-fired boilers when changing from coal to biomass.

Upgrades to and conversions of existing plants can be engineered by JT to achieve optimal boiler efficiency and emissions in compliance with the National Environmental Management: Air Quality Act, No 39 of 2004.

“We have several packaged boilers in the field specifically for the burning of biomass. We have supplied more than 50 small wood-fired boilers, ranging from 3 t/h to 6 t/h, in the last ten years, especially to com- panies in sub-Saharan Africa. We have customers operating large sunflower-seed-husk-fired boilers and a large project, coming on line soon, that will burn a mix of coal and husks,” Sharwood concludes.