Your team works for a company that provides electricity to the grid through the incineration of household waste.
Establish the likely aspects and impacts of the plant and carry out an analysis of these considering the likely risks to the environment. In the light of the information critically appraise the overall environmental position of such processing of waste.
The company is investigating the prospect of widening the scope of the types of waste that can be processed at the plant and is looking at new fluidised bed technology. A local University has been studying the impact of such technology on emissions of N2O and NOx. Their findings are shown after carrying out a study at a partner incineration plant in Austria.
Examining the figures provided for the trials, indicate what the ideal conditions might be for minimum N2O production in terms of operating temperature and free O2 levels. The Austrian plant, for cost reasons, runs at an operating temperature of 850 oC. The university has calculated that predicted N2O levels will be that due to temperature plus that due to the level of free O2 present during combustion. In other words:-
Total N2O = N2O due to Temperature + N2O due to free O2
Incineration Plant Background Information
On average, the company produces 550 to 750 kilowatt-hours of electricity per ton of waste, by the incineration of waste from the locality. This waste is termed municipal solid waste or MSW.
Combustion of MSW is the complete oxidation of the combustible materials contained in the
solid waste fuel, and the process is highly exothermic. During combustion of solid waste,
several complex processes happen simultaneously. The chamber containing the waste is heated to evaporate moisture and volatise the components of the waste mixture. Combustion then begins once the gases given off by the waste are ignited in the presence of air. This is highly exothermic. The process leads to the conversion of waste fuel into flue gas, ash and heat. The heat released is used to produce a high-pressure superheated steam from water, which is sent to the steam turbine that is coupled with a generator to produce electricity.
Leftover ash, now a cement-like product, is then taken off to line landfill.
Fluidised bed technology
New incineration plants use circulating fluidised bed (CFB) technology to recover energy from its waste. CFB technology is proven to be better suited for high moisture content waste such as sludges, sewerage waste, agricultural waste etc. The waste has to be pre-dried.
A fluidized bed is a bed of solid particles, through which gas is flowing in order to liquidize it. The principle behind the operation of the beds is that the particles in a vessel offer resistance to the flow of gas inserted in the vessel basin. Waste is mixed with hot circulating sand at lower temperatures compared with conventional incineration. The lower temperatures used, together with the more uniform distribution of temperatures – which eliminates hot spots and high oxygen zones, results in reduced thermal NOx production. The low combustion temperatures promote the formation of the greenhouse gas Nitrous Oxide (N2O). It is also possible to reduce the production of Nitrous Oxide. Emissions decrease with increasing bed temperature, and increase to a certain degree with increasing oxygen concentration.
Studies on the fluidised bed plant in Austria gave the following results with respect to combustion temperature
Trial Temp oC Emissions
mg/Nm3 dry gas
1 800 250
2 810 212
3 820 158
4 830 119
5 840 84
6 850 25
Further trial at a fixed temperature measured the levels of free oxygen when the ratio of air/fuel was varied.
mg/Nm3 dry gas
Performance over the past year, under normal production conditions, is shown in the following table.
Month Av Free O2 N2O
mg/Nm3 dry gas
Aug-17 2.5 45
Sep-17 2.81 52
Oct-17 2.52 20
Nov-17 2.65 24
Dec-17 2.83 320
Jan-18 2.8 280
Feb-18 2.95 330
Mar-18 2.78 370
Apr-18 2.64 25
May-18 2.58 17
Jun-18 2.71 40
Jul-18 2.67 30