Production of activated carbon from South African sugarcane bagasse.
Loading...
Date
2005
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
South Africa has an annual sugarcane milling capacity of about 22 million tonnes on average
producing about 3.3 million tonnes of dry bagasse, of which one third is surplus to factory
requirements. Currently surplus bagasse is used for furfural, pulp and paper and cogeneration
but significant amounts still remain . This prompted the need to find viable alternative and
appropriate technology to utilize the surplus.
A laboratory pilot plant was used to investigate the production of activated carbon from
bagasse. Experiments were carried out to investigate conditions for making the best activated
carbon in a rotary batch kiln, and also to examine potential ene rgy recovery from process gases
using Gas Chromatography. Derived results from the laboratory experiments were used to
develop a conceptual design for a demonstration plant sited within a sugar mill. The conceptual
design was evaluated for economic and environmental impacts using a robust Excel spreadsheet
and GABI-3 modelling software respectively.
Excellent activated carbon was produced from sugarcane bagasse by a two-stage physical
process involving pyrolysis and gasification with steam. The best operating conditions were
pyrolysis at 700°C for 1 hr and activation at 850°C for 1hr, a heating rate of 10°C/min and a
steam flow of 15g/g of char per hour. The active carbon yield was 7% on dry bagasse basis with
a Methylene Blue Number of 257mglg of carbon. The active carbon had a sugar decolourisation
capacity of 20% at a carbon dosage rate of 0.7 wt% on Brix using clear juice (l2°Brix) and 70%
at 0.5 wt% on Brix using brown liquor (65°Brix) . The Freundlich isotherm showed that the
bagasse-based activated carbon was a suitable adsorbent for sugar colour bodies.
Gas analysis results revealed that the off gases from the pyrolysis and activation stages had
calorific values of about 63MJ and 31MJ per kg of activated carbon respectively . The total
combustion energy of 94 MJ/kg of active carbon was enough to satisfy the process energy
requirements for drying, pyrolysis and activation. By burning combustibles like tar, methane,
carbon monoxide, ethylene and hydrogen for process thermal energy needs, the environmental
impact of the manufacturing process was reduced to a Global Warming Potential of llkg CO2
Equiv per kg of carbon produced.
The demonstration plant requires a capital investment of US$lOA million to give a competitive
bagasse-based activated carbon (BPAC) selling price of US$1.80 per kg and IRR, ROI and
Investment payback time of 17.93%, 23.93% and 3.80 years respectively. A sensitivity analysis
was also carried out to investigate the effect of possible variation in the main project forecasts which are BPAe selling price , bagasse buying price, capital investment and production costs on
IRR, ROI and payback time . The benefits of process integration within a sugar mill would be
expected to improve the business feasibility ; If bagasse was free the IRR would increase to
28.59% and even better to 32.12% if extra boiler and electricity capacity was also available at
the mill.
Description
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2005.
Keywords
Bagasse., Sugar--Manufacture and refining--By-products., Theses--Chemical engineering.