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How can Attero increase the energy efficiency and R1 mark by steam cycle improvements of the plant?

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How can Attero increase the energy efficiency and R1 mark by steam cycle improvements of the plant?

Rechten: Alle rechten voorbehouden

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The waste to energy plant of Attero is paid to process waste in a environmentally way. Part of this waste will be converted into useful energy, in forms such as heat and electricity. This type of waste recovery has been encouraged greatly by the government over the last decade. This type of rewarding creates a strong drive that urges old waste separation plants to transform into a waste to energy plant. Practice shows that this leads to a national increase of waste demand, which will also decrease the waste price. Also of influence are the local initiatives to separate waste on local authority collection points, and the economic crisis. These influences will result in a further decrease of the supply of available amount of waste for the WTE plants. Ultimately, this will lead to a reduced income for Attero.
Attero wants to deal with the increase in waste demand on the one hand, and the decrease in waste supply on the other in the following ways: Firstly investing in optimizing the plant in order to create a higher energy efficiency which will bring a higher energy profit; secondly, by buying international waste to ensure a reliable en sufficient large waste stream. Therefore, it is necessary to achieve permission to purchase international waste by meeting the requirements for a specific energy efficiency - the so-called R1 mark - .
Increasing the heat and electricity output of the plant can increase these two efficiencies, which can be achieved by improving the steam cycle of the plant. The main study question will be: How can Attero increase the energy efficiency and R1 mark by steam cycle improvements of the plant? To answer this question, the study consists of 3 main chapters, which describe the current state, (where process constraints will be found), potential optimizations and the calculated highest potential optimization suggestion.
The current steam cycle is susceptible to several changing variables. The influences of the ambient temperature create in summer conditions a turbine work loss of 9.2[%](3.3[MW]) due to the capacity shortage of the air condenser capacity, and this loss will be reinforced by the heat extraction of the pre-heater 10 and 20. The operation of the boiler is subject to the variation of the furnace load, temperature (<1150[°C]) and the conditions of the heat exchanger surfaces. A changing furnace load will be managed by water injectors and a proportional pressure controller, but will go hand in hand with exergetic losses. The heat exchanger surface condition (pollution), - in a clean condition - creates disproportional large heat extraction in the Drum and lead to an instable boiler balance. The inner drum heat exchanger (IDHE) is able to level this imparity and will beside this prevent that the flue gas temperature does not become too low (>225[°C]). The economiser (first heating step) is subject to the minimum surface temperature of 150[°C] and leads to the need to pre-heat the supply water. This pre-heating process consist of 3 heating stages (PH10 PH20 and supply tank) and will be heated with overheated drained turbine steam. The influence of this part process is that it lowers the plant capacity with 26.5[MJ]. This part process is controlled by the level indicator of the main condensate tank and creates strong fluctuating heat demand on the heating stages, which eventually leads to turbine work variation. Besides, the supply tank, while heating the supply water, also evaporates (on 150[°C]) a certain amount of supply water in order to degas, which creates a heat loss 528[kJ]. The external steam supply (ESS) will currently extract 3[kg/s] MP-steam for an external consumer by bleeding 1.35[kg/s] out of the MP-drain and 1.65[kg/s] out of the turbine driven supply pump. This extraction will decrease the turbine work 1.7[MW] but will supply a heat energy of 6.8[MJ]. This creates a current total plant efficiency (TPE) of 26.2[%] and a R1 mark of 0.632.
The four optimization suggestions will be put to use to improve the steam cycle efficiencies by solving the steam cycle constrains or process boundaries. 1. Enlarging the air condenser capacity would solve the turbine work loss, due to high ambient temperatures, and solve the turbine work loss reinforcement due to the pre-heating heat energy demand variation. 2. Enlarging the MP-steam turbine drain by a new diaphragm valve set point. This valve will increase the outlet pressure of the high-pressure turbine in order to create MP-steam conditions on the current LP-drain. The benefit would be a larger MP-drain (50.1[kg/s] vs. 3.99[kg/s]) for external steam supply. 3. A change of managing the supply water flow through the pre-heater stages. By shifting the leading controller from mean condensate tank level indicator to the supply tank level indicator, it would possibly create a more stable and constant batch flow. 4. The following fourth optimization suggestion is selected as the
optimization with the highest potential for increasing the efficiencies. This optimization is extensively analyzed in order to provide a founded conclusion, which includes the technical feasibility and process benefit according to the TPE and R1 mark. Optimization 4 proposes a pre-heating process optimization, which is necessary to decrease the internal steam extraction for pre-heating, in order to create more space on the MP-drain for ESS and the ability to create more turbine work. An optimized operation set-up in which the supply-tank heater replaces the pre-heater 20 and the supply tank heater is replaced by the IDHE will realize this. The new pre-heater set-up will shift a heat energy demand of 10.35[MJ] from the MP-drain to the drum. The supply-tank will heat the supply water from 80[°C] till 110[°C] with LP-steam where after a significant smaller quantity of heat will leak due to the degassing process (59.7[kJ]). The IDHE heat extraction out of the Drum in order to heat the supply water up to 150[°C] is larger then demanded. The heat transfer coefficient in combination with the surface and the logarithmic average temperature difference between the drum water and the IDHE water (ΔTln) will lead to a heating capacity of 11.25[MJ]. This extra heat extraction out of the Drum will lead to a new boiler load balance and a need to increase furnace capacity. To realize this furnace capacity enlargement, the flue gas flow will increase with 14[kg/s] and the furnace temperature will be decreased to manage exactly on the demanded boiler capacity and the flue gas output temperature (>225[°C]). The boiler load balance will be occurred by enlarging the drum heat transfer surface and slightly decrease of the heat transfer coefficient of the OVO and ECO. A negative outcome of this furnace capacity enlargement is that the flue gas flow and the lower furnace temperature will decrease the boiler efficiency with 1.5[%] to 80.5[%].
The consumer side of the steam cycle will - due this pre-heating optimisation - no longer need to bleed 2.8[kg/s] steam out of the MP-drain to pre-heat. This would make generating 3[MW] electricity or enlarging the ESS with 2.8[kg/s] possible. However, due to the decrease of the boiler efficiency, in both options the TPE nor the R1 will increase. In order to increase the R1, mark is it necessary to increase the ESS supply to a steam supply of 12[kg/s]. The TPE will in that case increase 6.3[%] to an efficiency of 32.5[%].
Apart from the efficiency study, is it noteworthy that the capacity increase of the furnace will lead to a waste handling capacity enlargement, which will benefit the financial income.
Concluding: it is evident that optimizing the pre-heater process will lead till a TPE increase of 6.3[%] and has an unchanged R1 mark at that point. The final recommendation is: optimizing the pre-heater process when there are enough external steam consumers to extract a minimal flow of 12[kg/s].

Toon meer
OrganisatieHanzehogeschool Groningen
AfdelingInstituut voor Engineering
Jaar2013
TypeBachelorscriptie
TaalEngels

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