In November 2016, PM Group welcomed Amy O’Keeffe to its offices to present her with the 2016 PM Group Kevin Kelly Design Award. The award is presented annually to the final year student at UCD's School of Chemical and Bioprocess Engineering who makes the highest individual contribution to his or her final year design project.
Amy O’Keeffe, a graduate of the UCD School of Chemical and Bioprocess Engineering, won the award for her contribution to the team project 'Design of a Toxic Waste Incineration Facility'.
Toxic-waste incineration facility for Irish domestic requirements
As the world’s population increases, there is a greater need for safe and efficient waste management methods. The objective of the UCD final-year chemical and bioprocess engineering team design project was to develop an innovative chemical and mechanical design, to meet the requirements of a hypothetical chemical and/or bioprocessing plant. Each individual team member was required to complete the design of one major and one minor piece of equipment as part of the overall plant design.
The main design unit needed to integrate equipment specification, safety and loss prevention, environmental impact assessment, process synthesis, chemical and mechanical design optimisation as well as ethical and wider socio-economic considerations.
The group project, in which Amy participated, detailed the design of a toxic waste incineration plant that facilitated the burning of, at a minimum, the Irish domestic toxic waste requirements. The proposed plant was to be situated on a brownfield site within Ireland where raw materials were readily available.
The plant required a robust specification of the
rotary kiln and the secondary combustion chamber which enabled the plant to be versatile in its operation and the form of waste it could handle. As products of combustion were to include extremely volatile solids and liquids, the rotary kiln needed to be designed to facilitate this kind of treatment.
The team unanimously agreed that, due to its location, the social and health considerations of a plant of this specification were extremely important. For this reason, emphasis was placed on these specific issues during the development of an
Environmental Impact Statement (EIS). The EIS assessed the preferred site, the possible adverse impacts of the plant on the local environs and the most effective means to mitigate these impacts.
Stringent bespoke measures were put in place for the treatment of the flue gas so that it could be safely emitted to the atmosphere at the end of treatment.
Three main aspects of the plant were selected and designed by the group; incineration, energy recovery and flue gas treatment.
Flue gas treatment was split into three steps; particulate removal, SOx removal and NOx removal. A comprehensive risk assessment and
HAZOP study were completed on the rotary kiln and the wider plant. This detailed the extent of controls and the necessary alarms that would be in place and where additional controls were needed. A stringent monitoring and control network was put in place to ensure the plant operated within acceptable constraints and that the safety of all personnel within the vicinity of the plant and beyond were considered.
Despite the expense of the rotary kiln and the secondary combustion chambers, it was predicted that the plant would be a viable investment, however, consideration had to be given to the addition of municipal waste to increase the loading of the incinerator which would contribute to greater profitability of the plant.
Electrostatic precipitator
The aim of the author's work was to complete a full chemical and mechanical design of the
Electrostatic Precipitator (ESP) used in the treatment of the flue gas. In choosing to design the ESP, bag filters were also considered, however, based on available information and the robustness of the ESP, it became the chosen filtration method.
The ESP was constructed to achieve an overall particulate collection efficiency of 99 per cent. This reduced the inlet suspended particulates to within EU acceptable levels before the flue gas passed to the wet scrubber for further purification treatment.
A substantial literature review was conducted as part of the ESP design as well as contacting personnel in industry including
Indaver Waste Management and
CDM Smith Consultants. Two separate efficiency models were used throughout the design to achieve a greater representation of the operation of the system.
The chemical design of the ESP focused on several design considerations including;
- Short-cut calculations and mathematical models;
- Corona discharge requirements;
- Addressing the effects of particle migration;
- Particle collection; and,
- Charge dissipation.
The mechanical design addressed aspects such as duct sizing, vessel wall thickness, combined stress analysis and insulation lagging. Overall the design was considered a success as the theoretical efficiency of the design achieved a 99 per cent removal efficiency using a plate-wire configuration. The ESP vessel was designed for optimum efficiency which included using the maximum voltage supplied to the discharge electrodes (wires). The vessel was also modelled at the minimum design voltage and the expected operating voltage.
The grid electricity supply was stepped up using transformer-rectifier sets designated to each field of the vessel. The ESP was designed with a lifespan of 20-30 years in mind.
The author also designed a secondary unit, an induced draft fan. This was important in order to maintain negative pressure through the ESP, ensuring a constant particulate migration velocity. The pressure drop across the ESP was to be kept at -1.5kPa and the induced draft fan maintained these specifications.
Project conclusion
The overall viability of a toxic incinerator plant was assessed and the use of ESPs in industry was also reviewed. Bag filters are becoming increasingly numerous in industry due to their cost effectiveness and their high collection efficiency. Although ESPs are still used due to their robustness as the loading on incinerators becomes greater, versatility of particulate collection methods is becoming increasingly important requiring further enhancement of ESP design.