Converting Waste Toilet Paper into Electricity: A First-Stage Technoeconomic Feasibility Study

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B. Mass and Energy balances
For each piece of equipment mass and energy balances were derived, based on the conversion and removal rates. Important notes: -For all energy balances: the reference temperature is 0°C -Inlet T for air/WTP from outside is 15°C -Qsens is the sensible heat of a compound/gas -Qcond. water is the heat required to evaporate water in a gas/compound and therefore the difference between LHV and HHV

Dryer
The purpose of the dryer was to decrease the water content in the WTP from 60% to 25%. The most important number that had to be obtained was the amount of energy needed to evaporate the water and heat up both water, biomass and air. The mass balance (Table 8) only contains the biomass in-and out-stream and water in air out stream, the amount of air needed was not calculated. The mass balance was solved by using the mass balance for water and for the total streams.   1 The heat supplied by air was calculated by difference to solve the energy balance 2 The heat from condensed water (Q condensed water) is the difference between the HHV and LHV of the wet and dried WTP. 3 The Q water in air contains the energy needed to heat up the water in the biomass from 15°C to 100°C plus the energy needed to evaporate this water, see table 10 for constants used.  Only gas + tar  1 It is assumed that there is no heat loss in the gasifier, but there is heat loss in the combustor. 2 The difference is 1195 kW between the in and outflow. Probably this is caused by using incorrect numbers for the specific heat capacity of tar, char and ash. Those numbers are derived from van der Meijden [1] as if wood is gasified. Also, the cp for sensible heat does not change with temperature. It could also be that there is extra heat required during WTP gasification because the gas/char ratio is higher due to the higher cellulose content [16,17] , but this has to be verified experimentally.

Settling chamber gasifier
In the settling chamber within the MILENA gasifier, 90% of the char & ash are settled and send to the combustor. No energy balance is given here because nothing happens with the energy streams.

HX -2
For heat exchangers, no mass balance is given because nothing changes in the mass flows. In the energy balance, only the sensible heat (Qsens) is given as the energy content in the components stay constant.
The gas is cooled from 800°C to 400°C.

Cyclone
In the cyclone 90% of the ash and char are removed from the gas, heat loss is assumed to be negligible. Therefore only the mass balance is given.

Combustor
In the combustor the char and tar are completely burned. Air for burning comes via OLGA together with the light tars. Heavy tars come via a separate stream from OLGA. Char and ash come from the settling chamber within the MILENA gasifier as well as via the cyclone. 1% of the flue gas is recycled to the gasifier and 0.8% of the product gas enters the combustor.  1 The specific heat capacity (kJ/kg/K) could not be derived from the model of van der Meijden [1] so is left out of the energy balance. 2 Heat loss is assumed to be 2% of the WTP input (LHV) [1] 3 The heat supply to the gasifier is determined by difference to solve the energy balance

HX3
In this heat exchanger the flue gas from the combustor is cooled down from 900°C to 35°C.

OLGA
In the OLGA tar removal system, both light tars are removed and travel together with an air stream to the combustor while heavy tars are removed with scrubbing oil [15,18] . There is a small bleed of oil. In the mass and energy balance of the combustor this oil bleed is not taken into account.  1 The heating value of the producer gas does not change when tar is removed, because tar was not included in the LHV/HHV of the producer gas 2 Is it assumed here that the air is not cooled during stripping and goes to the combustor still at 380°C. 3 In OLGA the gas is cooled down from 400°C to 80°C, which will result in a heat loss.

Water scrubber
In the water scrubber, the water content is reduced and HCl and NH 3 are removed. The gas is also cooled from 80°C to 50°C.

Compressor
In the compressor the pressure of the producer gas rises from 1 bar to 3 bar and therefore also temperature rises from 50°C to 145°C (Royal Dalhman, personal communication). The work of the compressor is calculated by thermodynamic calculations with as assumptions an isentropic efficiency of 80% and a mechanical efficiency of 98% taken from van der Meijden (2010). The mass balance does not change.

HX5
In this heat exchanger the temperature of the gas has to rise from 145°C to 350°C.

HDS
In the HDS reactor thiophene, COS, HCN and hydrocarbons are convertedwith the help of an Cobalt-Molybdenum (Co-Mo) or Nickel-Molymbdenum (Ni-Mo) catalyst. [2,14,15,19] Conversion rates can be found in Table 7. In the HDS, the temperature rises because of the hydrogenation of hydrocarbons. Because the c p of the producer gas was known (see appendix E) as well as the energy content of the gas after treatment in the HDS (see method section), the temperature of the outgoing stream was found to be 553°C.

HX6
In this heat exchanger the gas is cooled from 553°C to 350°C before entering the ZnO reactor.  Because the H 2 S is converted its energy is not part of the product gas anymore

SOFC
In the fuel cell the cleaned gas is converted into heat and electricity. Electrical efficiency is 55% LHV and efficiency to heat 31%. [20]

C. Total table and heat integration
After all separate mass-and energy balances, all energy inflows and outflows are combined (Table 33). Sum 8120 8097 To get a first idea of the heat integration possibilities, Table 34 shows the heat sinks and sources in the gasifier and cleaning system. The rejected heat of the fuel cell system is included. The streams that are matched as heat source and sink have the same colour. So while the air for the OLGA cleaning system is heated up from 15°C to 380°C, the raw product gas from the gasifier can be cooled down from 800°C to 400 °C. The dryer requires such a large amount of heat that two heat sources are needed. It would be most efficient to use the fuel cell flue gas to heat up the dryer air first, and then the combustion flue gas is used to heat up the last part. After heat integration, there is still 573 kW (low temperature heat) left. Of this heat all heat with a temperature of 80°C and higher (555 kW) is assumed to be useful for the district heating system of the AEB. Part II -OPEX and CAPEX specification A. CAPEX The system CAPEX was obtained via different sources. The gasifier and cleaning system investment costs were obtained via a supplier (Royal Dahlman) directly (Table 35). These costs included Engineering, Procurement and Construction (EPC) and 10% contingencies. However, the TPC are not complete yet without the SOFC investment costs. No specific cost data could be obtained from suppliers directly and therefore the following method was used based on. [21,22] 1. Equipment cost estimation SOFC costs were retrieved from as many public sources as possible. [20,[23][24][25][26][27][28] 2. Scaling of the equipment to the required size The system has a 2.8MWel scale. For an SOFC no scaling factors were found, but based on some general information on scaling it was decided to choose a scaling factor of 0.85. [29] Because fuel cell systems are made by assembling many small cells to into stacks to obtain a large system, the costs will not be reduced that much when a larger system is made. Only the stack packaging will be strongly reduced when producing larger systems. [28] Formula used for scaling: Where C 2 are the scaled investment costs with capacity S 2 and C 1 the investment costs with capacity S 1 . n is the scaling factor (0.85 in this case).

Converting to euro
When prices are given in US dollar, they were converted to euro-2015 by using conversions factor from OANDA (2016). Specific conversion rates used can be found in For prices that were at first instance given in dollar, a location factor of 1.23 was used. This value is based on the value of 1.19 for US Gulf Coast to the Netherlands in 2003 and updated with dollar to euro conversion rates in 2003 and 2015 . [29] 6. Cost escalation For fuel cells one factor that include all EPC costs was not found. Instead, an installation factor could be derived of 1.42, which is typical for SOFC systems. [23,25,32] According to Ceasar, equipment cost and installation sum up to the Total Direct Plant Cost. [33] To arrive at the EPC costs an 'indirect cost' factor of 14% (so 1.14) had to be added that included yard improvement, service facilities, engineering/consultancy cost, building and a miscellaneous factor [33] . So cost escalation with these two factors was done to arrive at the EPC costs of the SOFC.
The fuel cell investment cost were gathered from different sources and are summarized in Table 37, including the system parts that were said to be part of the cost estimate. As can be seen, the costs vary highly among the different sources. Prices were derived according to the method as described in above. The dollar-euro exchange rates used (Table 36) were calculated, the average exchange rate over a whole year was taken [30] .