Browsing by Author "Nsaful, Frank"

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    Process modelling of sugar mill biomass to energy conversion processes and energy integration of pyrolysis
    (Stellenbosch : Stellenbosch University, 2012-12) Nsaful, Frank; Gorgens, Johann F.; Knoetze, J. H.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: The sugar industry over the years has been producing sugarcane bagasse as part of the sugar milling process. Currently this sugar mill biomass is incinerated inefficiently as a means of their disposal to produce steam and electricity, which in most cases are only just enough to supply the energy required to run the mills, thereby leaving very little or no extra energy for sale to bring in extra income in addition to sales revenue from sugar. However, the recent instability and uncertainties in the price of sugar and the global call for a green and sustainable environment have necessitated the search for ways of making effective use of this biomass to supply sugar mill energy demands, while producing extra energy in the form of electricity and other energy products for sale and at the same time contributing towards environmental sustainability. The main objective of this work was to develop process models for the processing of sugar mill biomass into energy and energy products. Based on this, biomass to energy conversion process (BMECP) models have been developed for various process configurations of two thermochemical processes; Combustion and Fast Pyrolysis using the Aspen Plus® simulation software. The aim of process modelling was to utilizing sugar cane bagasse as an input energy source to supply the energy requirements of two sugar mill configurations (efficient and less efficient mills), while generating extra electricity and high valued energy products for sale. Four BMECP configurations; 30bar BPST, 40bar CEST, 63bar CEST and 82bar CEST systems were modelled for the combustion thermochemical process. For the fast pyrolysis thermochemical process, two process configurations: Pure Fast Pyrolysis BMECP and Partial Fast Pyrolysis BMECP were modelled. The former BMECP utilizes all available bagasse through fast pyrolysis to produce bio-oil and biochar alongside generating electricity as well as energy to run the sugar mill operations. In the latter BMECP model, only surplus bagasse after separation of the quantity needed to supply the sugar mill energy requirement and electricity production is used to produce bio-oil and biochar. The technical performance of the BMECP models have been analysed and compared based on steam and electricity production rates, process efficiencies and environmental impacts (based on CO2 savings). The effects of boiler operating pressure and bagasse moisture content on the performance of the combustion based BMECP models have also been investigated. Finally, detailed economic models have been developed using the Aspen Process Economic Analyzer (Icarus®) to assess the economic viability of the BMECP models and sensitivity analysis performed to study the response of the BMECP models to variations in economic parameters. Technical performance analysis shows the combustion based BMECP models perform better than the Pure Fast Pyrolysis and Partial Fast Pyrolysis BMECP models with regards to steam and electricity production, thereby giving them higher electrical efficiencies. The electricity generation rate has been shown to increase with increasing boiler operating pressure and decreasing bagasse moisture content while steam production rate has been shown to increase with decreasing bagasse moisture content and decreasing boiler operating pressure. Despite the lower electrical efficiencies of the fast pyrolysis based BMECP models, the analysis shows that their overall process efficiencies compare very well with those of the combustion based BMECP models due to the production of high energy value pyrolysis products. Based on common operating pressure and 50% bagasse moisture content, the Pure Fast Pyrolysis and the Partial Fast Pyrolysis models have proved to be more environmental friendly with hourly CO2 savings of 40.44 and 41.30 tons for the Partial Fast Pyrolysis BMECP and the Pure Fast Pyrolysis BMECP respectively based on a 300 ton of sugarcane/h (81 ton bagasse/h) plant size. From an economic point of view, biomass combustion based on the 63bar CEST BMECP model has proved to be the most economically viable option under current economic conditions. First order total capital investment estimate for this BMECP is about $116 million, producing NPV of $390 million at the end of a 20 year plant life and IRR of 34.51%. The Pure Fast Pyrolysis BMECP model is the least economic viable option. Sensitivity analysis shows this BMECP model is the most sensitive to changes in bagasse and electricity prices; recording -191.61/+446.86% change in NPV for a ±30% change in bagasse price and -91.5/+338.60% for a ±30% change in electricity price.
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    Thermochemical biomass upgrading for co-gasification with coal
    (Stellenbosch : Stellenbosch University, 2018-03) Nsaful, Frank; Gorgens, Johann F.; Collard, Francois-Xavier; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Lignocellulosic biomass is considered as a sustainable and renewable fuel source with the potential to substitute or partially replace coal in applications such as gasification for energy generation due to its sustainable carbon as well as its potential to reduce greenhouse gas emissions. However, raw biomass differs significantly from coal in terms of several important fuel properties, such as low energy density, high moisture, oxygen and volatile matter contents. Due to this the co-utilization of raw biomass with coal in gasification systems has been shown to result in the increase in the production of oxygenated volatile compounds (tar precursors) which impacts negatively on the quality of the gasification products and causes critical operational problems. This challenge has limited the development of biomass-based gasification processes. Hence to ensure the effective and efficient utilization of lignocellulosic biomass with coal, an upgrading process is required to improve some biomass properties to make them more similar to that of coal. The approach of this work consists in a thermal pretreatment in order to generate char products with reduced oxygen and volatile matter contents. The overall aim of the study therefore was to use thermochemical technologies (torrefaction and slow pyrolysis) as methods to pretreat lignocellulosic biomass feedstocks; pine (PN), bamboo (BB), corn cob (CC) and corn stover (CS) to produce upgraded biomass feedstocks (char), with reduced oxygen content as well as improved fuel properties, comparable to coal for use in co-gasification. For this task the study was divided into several objectives. The initial part focused on the characterization of the lignocellulosic chemical composition of the various biomass feedstocks. Next the types and quantities of oxygenated volatile products produced during the devolatilization of raw biomass feedstocks were studied. For this objective a novel analytical method incorporating the use of Thermogravimetric Analysis, thermal desorption and Gas Chromatography–Mass Spectrometry (herein referred as (TGA-TD/GC-MS) was developed and used to analyse and quantify the oxygenated volatile products. The analysis of the volatile composition data by means of principal components analysis (PCA) showed a clear correlation between lignocellulose chemical composition and the type and quantities of oxygenated volatile compounds produced during biomass devolatilization. The influence of thermal pretreatment conditions (temperature and time) on the structural transformation of raw biomass and on the volatile evolution mechanism of the resulting char during subsequent char devolatilization was also studied. Thermal pretreatment was done within the temperature range of 250-400 oC and hold time at pretreatment temperature of 30 and 60 min. It was observed that the temperature had a more profound effect than hold time during thermal pretreatment. The distribution of char devolatilization products was shown to be consistent with the extent of biomass transformation during thermal pretreatment. The biomass composition, particularly cellulose crystallinity, had an impact during thermal pretreatment. It was shown that for biomass feedstock with high degree of crystallinity such as PN a higher temperature (>300 oC) was required to achieve significant cellulose degradation. Hence char produced from such feedstock at temperature ≤300 oC generated high amount of anhydrosugar and furan volatiles during the char devolatilization. With the aim of using pretreated biomass with coal for co-gasification, biomass chars produced at different pretreatment temperatures were compared to coal in terms of fuel properties (proximate and elemental composition and Higher Heating Value), with particular attention given to the composition of oxygenated volatile compounds generated during the devolatilization stage. The result of the study showed that chars produced at temperature ≥350 oC had fuel properties comparable to that of coal. In addition these chars produced mainly aromatic hydrocarbons and phenolics during devolatilization which were similar to the volatiles generated from coal under identical conditions. Hence the pretreatment temperature of at least 350 oC is recommended when considering coal substitution, while 400 °C could be considered in the case of samples with high lignin (softwood) or high inorganic contents. Finally, the reactivity of the biomass chars under gasification condition was investigated. The devolatilization characteristics and CO2 gasification kinetics of biomass/char (produced at 350 oC) and coal at different blend ratios were studied. The devolatilization characteristics of char were found to follow the profile of coal especially at blend ratios of 10 wt% and 20 wt% with no particular synergy detected, while the kinetic parameters were also comparable. This work confirmed the potential of the use of thermally pretreated biomass chars for coal substitution in gasification process and brought decisive insights for the implementation of future tests at pilot scale.