Converting waste to energy through fluidised bed processing

Fluidised bed combustion has been used to burn coal for many years, but using the experience gained at the CSIR to convert industrial waste to energy gives an important new slant to existing technology. CSIR research and development (R&D) activities in this area, span the past three decades. The earliest developments were in low-grade coal combustion, but fluidised bed technology has since been applied to designs for a hot gas generator, a biomass sludge boiler, a high-sulphur pitch incinerator and an industrial deodoriser.

Initial research: low-grade coal combustion

In the mid-1970s, the CSIR commenced with investigations into fluidised bed coal combustion and gasification. For this purpose, an initial refractory-lined bubbling fluidised bed test rig was constructed. Research became more focused in the early 1980s when an investigation was initiated into the use of discard coal, duff coal and coal slurries by means of fluidised bed combustion technology. This resulted in the development of the National Fluidised Bed Combustion (NFBC) boiler for the former Department of Minerals and Energy Affairs in 1984.

This facility was primarily used for thermal and combustion efficiency trials, where the effects of load, cyclone grit refiring and bed temperature on the thermal and combustion efficiencies of the waste coals were investigated. Sulphur reduction trials were conducted to determine the ability of South African sorbents to reduce sulphur dioxide emissions in situ, as the high-ash discard coal generally also contained elevated levels of sulphur.

In 1990, the original small-scale fluidised bed combustor was replaced by a larger one that combined the functionality and freeboard (above-bed) height of the NFBC boiler with the flexibility of the initial test facility. This was dubbed theMulti-Purpose Fluidised Bed Combustor (MPFBC). The MPFBC was used extensively for process development in the fields of waste combustion, calcination and minerals treatment.

Commissioned plants

Experience gained through R&D conducted on the NFBC and the MPFBC has been successfully applied for the benefit of South African industries. The CSIR was commissioned to design fluidised bed combustors for a number of plants in South Africa, to deal with a variety of environmental and industrial challenges.

These included the design of a hot gas generator for Slagment in Vanderbijlpark, Gauteng; a biomass and coal co-fired boiler for a multinational food producer in Estcourt, KwaZulu-Natal; a high-sulphur pitch thermal oxidiser for Sasol; and a coalfired fluidised bed deodorising and steam generation plant for African Products.

In each of these cases, the CSIR was commissioned to design a customised fluidised bed combustion unit to meet the client’s specific and often multiple needs. The technology is licensed through South African engineering companies that manufacture the combustors according to the CSIR’s design specifications.

Fluidised bed technology is robust with unique characteristics, rendering it suitable to a host of different applications. This includes the recovery of precious metals through the incineration of carbon and wood chips, as well as chemical vapour deposition onto valuable substrates.  Utilising this technology not only has environmental benefits, but makes economic and business sense. It enables the manufacturer to generate his own hot gas, steam or electricity, while enabling him to dispose of waste matter easily and cost effectively.

This innovative research in the field of clean energy certainly proves that there is no end to the application potential of FBC technology, which leads to new opportunities for commercialisation and technology transfer.

Hot gas generator earns the CSIR a projects and systems award from the South African Institution of Mechanical Engineers

The hot gas generator that was developed for Slagment in Vanderbijlpark was the first commercial fluidised bed combustor to be designed by the CSIR. The client had an existing coal-fired furnace, which was used to supply hot gas to a slag dryer. High maintenance costs and the need to use a high-quality graded coal product in this furnace prompted the organisation to look for an alternative. The fluidised bed combustor that was designed by the CSIR could utilise low-grade “duff’ coal, which was regarded as a waste product and was available at a low price.

Biomass and coal co-fired boiler receives the Innovation Award of the South African Institution of Chemical Engineers

A unique biomass and coal co-fired boiler was designed for a multinational food producer based in Estcourt. The purpose of this boiler was to develop a reliable system of incineration to dispose of a stream of 12 tons of coffee grounds per hour (comprising 85% water), while raising 26 tons of process steam with the off-gases. A fluidised bed combustor that could burn the coffee grounds in their wet state, rather than drying and treating the press water, provided the best solution to the client’s problem at the lowest capital cost.

Again, past experience was applied. In particular, a deep bed, which creates a ‘thermal flywheel’, allowed for swings in fuel quality and water content. Extensive process development was carried out in the MPFBC and the NFBC to prove the practical application of calculated design parameters. The boiler was designed in 1992, and is still operating successfully.  This project earned the research team the Innovation Award of the South African Institution of Chemical Engineers in 1994.

About fluidised bed combustion

Fluidised beds suspend solid fuels in upward blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids that provides more effective chemical reactions and heat transfer. It is ideal to burn fuels that are difficult to ignite, like low-quality fuel such as coal mine waste.

Disposing of high-sulphur pitch while reducing sulphur emissions

Sasol produces approximately two tons of high-sulphur pitch per hour in its coal-to liquids process. This waste stream has a high calorific value (similar to heavy fuel oil), but contains a number of contaminants and sulphur that renders it unsuitable for disposal in a landfill site. The best disposal option is to thermally oxidise the pitch, while reducing the sulphur emissions in the flue gases. The high sulphur pitch (HSP) thermal oxidiser, which was designed by the CSIR, is a fluidised bed combustor that achieves exactly this.

The design team drew on past experience in the combustion of liquid fuels (coal slurries and biomass sludge) to develop an HSP injection system that would ensure that the pitch was delivered into and burnt in the fluidised bed rather than burning it above the bed. This ensured uniform, controlled temperatures in the bed and freeboard to enable sulphur to be captured in situ using a limestone addition to the bed.

The design of the air distribution system for the Slagment hot gas generator was employed to allow the sulphur to be continuously removed as solid calcium sulphate. The gases leaving the fluidised bed combustor were used to generate 20 tons of process steam per hour in a waste heat boiler.

Environmental claims that can be made for this fluidised bed combustor include a reduction in sulphur oxides through in situ sulphur capture, a reduction in nitrogen oxides due to a relatively low combustion temperature, and a reduction in carbon dioxide since the steam produced displaces steam previously generated by burning coal in a boiler.

Deodorising of air using FBC technology

The 12 MW coal-fired fluidised bed deodorising and steam-generation plant that was commissioned by African Products, one of the largest maize processing plants in the southern hemisphere, is another one of the CSIR’s customised designs. The process air from the plant’s maize-drying circuits contained odorous organics, and the client wished to render this air odourless in order to conform to current world standards in the minimisation of environmental impact.

The CSIR’s fluidised bed thermal treatment was selected as the most appropriate technology for this purpose. The odorous air was used as the fluidising air for the fluidised bed combustor. Coal was burnt in the fluidised bed to maintain it at 900°C, and the odours in the air were thereby destroyed. Process steam was generated using the off-gases to minimise the total CO2 emissions from the plant.