A comparison between the two different combustion methods of Grate-firing and Fluidized bed, applied to a CHP-plant with MSW as fuel
In this study, the two most frequently used incineration systems, fluidized bed and grate-firing have been compared and analyzed. The performance of Mälarenergi’s combined heat and power (CHP) plant in Västerås, which consists of a fluidized bed incinerator, has been used as benchmark to elaborate the different calculations made in this study. It extracts electricity and heat amounting to 50 and 100 MW respectively.
On average, 75-85% of the weight of Municipal Solid Waste (MSW) is converted into Refuse Derived Fuel (RDF). One ton of RDF which is used as fuel in fluidized bed boilers has a calorific value of 16.6 MJ/kg while the calorific value of one ton MSW used as fuel in grate-fired boilers was calculated to 9.15 MJ/kg. The total amount of extracted energy in regards of heat and electricity for the grate-fired incineration was computed to 2.54 MWh/ton, while it was calculated to 4.63 MWh/ton using the fluidized bed incineration.
A simple Rankine cycle was used to understand the significance of better steam qualities in the incinerators, and to learn the difference between the total power output and the turbine efficiency. The total power output was provided with enthalpy values of different steam conditions using a constant mass flow in the cycle. For grate-firing, the amount was calculated to 10 829 kJ/s and for fluidized bed it was calculated to 11 245 kJ/s. Using the different enthalpy values, the isentropic turbine efficiency of grate-firing and fluidized bed were calculated to 71 and 80%, respectively.
The yearly amount of waste each incinerator requires to reach the performance of Mälarenergi’s CHP-plant was calculated and used to perform a profitability analysis. The efficiency of this plant using a fluidized bed is 90%, whilst two different efficiencies of 75 and 85% were used for the grate-firing. The higher net present value was concluded belonging to the fluidized bed plant with an amount of 9978 Mkr, whilst the net present value of the grate-firing measured to 8146 and 6407 Mkr.
Emissions of carbon dioxide (CO2) were calculated to be higher when incinerating one ton of MSW in a grate-fire, than of incinerating one ton RDF in a fluidized bed. In comparison, emissions of nitrogen oxides (NOx) became higher in a fluidized bed than in a grate-fire, with the same amount of fuel. CO2 emissions from a grate-fire respectively fluidized bed were measured to 47.24 and 23.75 kg, whilst the NOx emissions were measured to 0.73 respectively 1.16 kg.
The fluidized bed incinerator was considered the preferable alternative when incinerating waste, from all three different regarded perspectives - energy based, financial and environmental.
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