In order to produce steam for its production processes, the mill at a global paper company had primarily used coal as fuel. With support from Babcock, it was identified that some of the coal could be replaced by tree bark. The mill created 70 000 tons of bark per year, which had, until then, been regarded as a waste product, and at significant cost was being disposed of in a landfill site.
Using coal as fuel
The benefits of combusting the bark, as a renewable bio-fuel source were numerous:
- Reduction of the carbon footprint of the plant
- Reduction of CO2 emissions from the combustion process
- Eliminates methane gas production at the landfill site as the bark decomposes
- Security of an annuity revenue stream from carbon emission credits commodity trading
- Elimination of the cost of bark disposal
- Elimination of the cost of coal consumption displaced by bark combustion
In order to realize these benefits, Babcock designed the conversion of the existing coal fired boiler, to a co-fired bio-mass/coal boiler which has the capacity to combust the 70 000 tons of bark. The converted boiler reduces CO2 emissions by nearly 56 000 tons per annum.
The boiler conversion required the conventional boiler grate to be replaced with a bubbling fluidized sand bed, and new fuel feed system for both coal and bark. Particulate emissions were also targeted for reduction, and a wet gas scrubber was specified and installed. Modifications to the boiler furnace enabled the boiler to produce 28 tons of steam per hour (22 tons of steam produced by the bark and 6 tons produced by the coal, used as the balancing fuel source). The boiler has a rated thermal capacity of 21.7 Megawatt.
The bubbling fluidized sand bed conversion is the first application of its kind for the pulp and paper industry in South Africa. The conversion saves 53 000 tons of coal consumption per year. The converted boiler has an expected operational life of 25 years.
Interesting fact:
The conversion of this boiler was one of the first completed Clean Development Mechanism (CDM) projects in South Africa and received an award for ‘The Most Innovative Co-generation Project’ from African Energy.
