One of South Africa’s largest coal-to- gas boiler conversions has been successfully completed at ethanol distiller NCP Alcohols’ Durban distillery. The inconsistent quality of coal supply meant that management of the process performance was becoming increasingly more difficult to achieve and steam demands for distillation were being compromised.
Natural gas supplier Spring Lights Gas was contracted to convert a water tube boiler, used for generating steam in its distillery, from coal to gas. The conversion represents a landmark in the history of both companies.
The planning phase was carried out over a period of two years, with strict adherence to environmental compliance requirements, supported by the necessary approvals and licensing applications. The construction phase was completed over a two-month period and conformed strictly to safety standards and international gas practices.
The first phase of construction involved the connection of the distillery to a 6,3-km local gas supply pipeline. As NCP Alcohols is situated near a residential area, the community and all interested parties along the gas pipeline route were consulted and integrated into the environmental study, before the pipeline was constructed.
Once the pipeline was completed, a 32-t low-pressure gas-fired boiler was installed to replace the 30-t coal-fired low-pressure coal boiler, followed by the complete re- engineering of the coal-fired boiler to burn pipeline gas. Both these processes were controlled by a fully integrated software programme, which ensured the safety and efficiency of the boiler operation.
The coal-fired boiler, which generated 20-bar process steam within the distillery, was retired and replaced with two natural gas-fired dried distiller’s grain (DDG) 12 burners supplied by industrial combustion company Saacke South Africa. The conversion to gas was undertaken by Saacke, in close cooperation with boiler and environmental solutions company Alstom John Thompson South Africa.
Each boiler has an output of 16 MW and is designed to guide the gas to the combustion zone by means of several gas guns that are located in the core air tube. The gas guns are connected to one another by an outer gas ring body, which may be adjusted from the outside by turning the gas guns to optimally adjust the gas flame and achieve the lowest emission values. Alstom John Thompson carried out the assembly under the supervision of Saacke, and commissioning of the burners was completed after two weeks.
To effectively manage the burner control system, NCP Alcohols introduced a computerised process monitoring and management system, which allows for the integration of the safe Ethernet and visuali- sation (Se@vis) burner control system with the current process management interface programmes. The Se@vis system creates a secure Ethernet connection between the control system and the combustion process. All its sensors and actuators are connected to a fail-safe single-touch screen that can be operated and its performance optimised from one location.
The interface provides information on all current process dimensions, such as operating conditions, parameters and alarms. The Se@vis system is able to manage other operations, such as firing range control, pressure control, temperature control and level control, as well as automatic firing sequence control and fuel-to-air ratio control. This newly developed system has extended the operational capability of NCP Alcohols to manage the respective processes by accessing process data through a single- point touch screen.
Spring Light Gas CEO Motsamai Koapeng says that cooperation between all parties involved in the project resulted in the conversion being carried out without delays. “NCP Alcohols has clearly demonstrated its unwavering commitment to making a success of this project by not only looking beyond the immediate obstacles and challenges, but by also focusing on the strategic advantages and the long-term benefits for the company and surrounding communities,” he adds.
The gas boiler system has been in opera- tion since August 2008 and has resulted in several benefits for NCP Alcohols. The storing of coal, and the associated risks, such as fire and groundwater contamination, have been eliminated, and the fuel storage area has been reduced, creating a safer operating environment and higher savings through the elimination of fuel storage costs and ash removal costs.
The boiler’s efficiency since conversion has been consistently at 92% and higher. The main advantage of this has been the enhanced efficiency and elasticity of the 32-t/h boiler, which allows for quicker process start-up times, energy that is clean burning and on tap, improved process efficiency and capacity, increased operational efficiency and improved operational response to meet process demands.
There has been a significant reduction in greenhouse gases, as well as harmful stack emissions, volatile organic compounds and ash, with emissions well below the limited values prescribed by law. There is also potential benefit to be had from the sale of carbon credits.