Fig 4.2 Process flow of integrating the torrefaction with AD
4.4 Process simulation (paper IV)
The experimental results from paper II were used to simulate industrial scale operations of 10 t/h of torrefied biomass pellets production integrated with AD.
4.4.1 Process description
The different process configurations considered in this thesis for the technical and economic evaluations were presented in Fig. 4.5. The major difference between the standalone torrefaction process (case 1) and the integrated approaches lays with the application of torrefaction volatiles. For the standalone torrefaction process, combustion of torrefaction volatiles along with wood chips to provide heat
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energy for the drying and torrefaction units was considered. For integrated approaches (cases 2 and 3), the condensation of volatiles to produce torrefaction condensate and later producing biogas through AD of torrefaction condensate was considered. In these cases, combustion of un-condensed volatiles along with wood chips was considered to meet the required heat energy demand.
Again, the difference between case 2 and case 3 was with the end application of the biogas. In case 2, using biogas in a gas engine to produce electrical energy and heat energy was considered and in case 3 upgrading the biogas using high-pressure water scrubbing (HPWS) and pressure swing adsorption (PSA) to use it as a vehicle fuel was considered. The possible heat energy recovery options were also considered for both standalone and integrated approaches.
Fig. 4.5. Different process configurations considered in this thesis for technical and economic evaluation. a) Case 1 (standalone torrefaction) (b) Case 2 (Torrefaction – AD_Engine), (c) Case 3 (Torrefaction –AD_Biomethane) (paper IV).
4.4.2 Process parameters
The operating characteristics of the drying and torrefaction processes are presented in Table 4.3. The wood chips with a moisture content and heating value of 40% and 10 MJ/kg respectively were considered as raw material for torrefaction process. Reducing the moisture content from 40 to 10 % during drying was considered. The total heat energy required at drying unit was calculated considering latent heat of evaporation of water and the sensible heat requirement of wood chips at dryer operating conditions.
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Table 4.3 Properties and operating characteristics (paper IV)
Feedstock properties Raw wood
chips After drying After torrefaction Moisture content (%) 40 10 0
Lower heating value (LHV) (MJ/kg) 10 16.5 22
Operating characteristics
Dryer operating temperature (⁰C) 150
Torrefaction temperature (⁰C) 300
AD operating temperature (⁰C) 35
Engine exhaust gases temperature (⁰C) 480
The operating temperature of the torrefaction was selected as 300 ⁰C. The yield of torrefied biomass, uncondensed torrefaction volatiles and torrefaction condensate was selected as 0.55, 0.2 and 0.25 kg/kg of dry wood chips respectively (based on paper II). The energy balance at torrefaction reactor was evaluated based on the equation (1). The loss of heat energy (radiative) was considered as 3% on the LHV of the dried biomass.
𝑚𝐷𝐵 × [𝐿𝐻𝑉𝐷𝐵 + (𝐶𝑝𝐷𝐵 × 𝑇𝐷𝐵)] + 𝑄𝑖𝑛 = 𝑚𝑇𝐵 × [𝐿𝐻𝑉𝑇𝐵 + (𝐶𝑝𝑇𝐵 × 𝑇𝑇𝐵)] + 𝑚𝑇𝑉 × [𝐿𝐻𝑉𝑇𝑉 + (𝐶𝑝𝑇𝑉 × 𝑇𝑇𝑉)] + 𝑄𝑙𝑜𝑠𝑠 (1)
Where, m, LHV, Cp and T are the mass, lower heating value (kJ/kg),
specific heat capacity (kJ/kg.K) and temperature and DB, TB and TV are the dried biomass, torrefied biomass and torrefaction volatiles respectively. Qin is the heat
energy input to the torrefaction reactor and Qloss is the heat energy loss from the
torrefaction reactor.
As reported in previous studies the heating value of the torrefied biomass varies from 19 – 24 MJ/kg depending on the torrefaction operating condition [20]. However, in this study a heating value of 22 MJ/kg of torrefied biomass was selected. The heating value of the torrefaction volatiles was calculated using the equation (2). It was assumed that the uncondensed gases mainly contain CO2 and CO. The yield of CO2 and CO was calculated based on the correlation presented by [93] i.e. the CO2 and CO ratio is equal to 2.5.
𝐿𝐻𝑉 = ∑𝑛𝑖=1𝑚𝑖. 𝐿𝐻𝑉𝑖 (2) Where, ‘i’ denotes the major compounds present in the torrefaction volatiles.
4.4.4 Process flow for pellets production
The operational procedure for producing torrefied pellets from torrefied biomass reported by Kumar et al. [27] was followed in this study. Initially torrefied biomass is cooled to 50 ⁰C and then ground. Preconditioning of torrefied biomass with 15% moisture was considered as a binding agent. The material loss at different stages during pelletization was negligible.
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4.4.5 Anaerobic digestion
The operating parameters for the AD are presented in Table 4.4. The digester volume was calculated following the methodology presented by [94] and a design factor of 1.25 was selected to allow the calculation errors and safety. The Heat energy required at digester was calculated using equation (3) and (4) [95].
𝑄𝐴𝐷= [𝑚𝑇𝐶 × 𝐶𝑝 × (𝑇2− 𝑇1)] + 𝑄𝐴𝐷−𝑙𝑜𝑠𝑠 (3)
𝑄𝐴𝐷−𝑙𝑜𝑠𝑠= 𝑘𝑑 × 𝐴𝐷 × ∆𝑇𝐷 (4)
Where 𝑄𝐴𝐷, is the heat energy demand for AD, 𝑚𝑇𝐶 is the mass of the torrefaction condensate, 𝐶𝑝 is the specific heat capacity of the torrefaction condensate, T2 is the AD operating temperature, and T1 is the torrefaction condensate inlet temperature. The 𝐶𝑝 of the torrefaction condensate was selected as 2.8 kJ/kg-K [96]. Where, 𝑄𝐴𝐷−𝑙𝑜𝑠𝑠 is the heat loss at digester. 𝑘𝑑 is the k-factor of the digester material, 𝐴𝐷 is the surface area of the digester, and ∆𝑇𝐷 is the average temperature difference between the heating medium and the substrate (torrefaction condensate).
Table 4.4 Operating parameters selected for the process scale-up of anaerobic digestion
AD operating parameter Selected value
Temperature (⁰C) 35 ⁰C
Organic loading rate 3 kg VS/m3
Inlet temperature of the torrefaction condensate
20 ⁰C
k-factor of the digester material 𝑘𝑑 0.5 W/m2 ⁰C
Outside wall temperature -10 ⁰C
Digester height to diameter ratio 0.5
The amount of electrical energy produced was calculated using equation 5 [97]. The energy potential of bio-methane was considered as 10 kWh of electricity.
𝐸𝑒𝑙𝑐.= 𝑉𝑚 𝑥 10 𝑥 η𝑒𝑙𝑐. (5)
Where, 𝑉𝑚 is the volume of the methane produced, η𝑒𝑙𝑐. is the electrical efficiency of the biogas engine (considered to be 45% in this study) [98].
In case of biogas upgrading, the methane yield of 96% for both HPWS and PSA was considered. Currently no published data is available on the properties of the digestate that is produced during the AD of torrefaction condensate. However, the previous studies [96,99] and [100], have established that the release of nitrogen, phosphorous and potassium to the gas phase during the thermal treatment of biomass at 300 ⁰C is limited. This implies that, the possibilities for the application of digestate, as a fertilizer is limited. Thus, in this study the digestate was considered as wastewater that needed to be treated.
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4.4.6 Energy recovery
In this thesis work, the heat energy recovery possibilities with in the torrefaction process were also presented with an aim of improving the overall thermal efficiency and generating additional revenue to improve the economic feasibility of the process. Three different possibilities were considered for heat energy recovery such as from: 1) biogas engine (flue gases, water jacket, and lube oil), 2) dryer exhaust gases and 3) torrefaction products cooling. However, the combination of these heat energy recovery options are different for different cases studied. The combination of heat energy recovery options for different cases were as presented in Table 4.5.
Table 4.5 The combination of heat energy recovery options for different cases.
Heat energy recovery from Case 1 Case 2 Case 3
Dryer exhaust gases
Torrefied biomass cooling
Torrefaction volatiles cooling
Biogas engine
It was considered that the engine’s exhaust gases temperature was reduced from 485 ⁰C to a stack temperature of 135 ⁰C. Previously [101], different process configurations for heat energy recovery from biomass dryer exhaust gases have been reported. However, in this study the recovered heat energy from dryer exhaust gases was considered for district heating and air preheating. It was assumed that combustion air is preheated from 10 ⁰C to 50 ⁰C. It was considered that torrefaction products are cooled to 50 ⁰C in two stages. Initially from 300 ⁰C to 95 ⁰C and later from 95 ⁰C to 50 ⁰C. The high-grade heat energy recovered from torrefaction product cooling (when cooled from 300 ⁰C to 95 ⁰C) was considered for district heating. The heat energy requirement at AD could be meet by using the low-grade heat energy from torrefaction volatiles cooling (when cooled from 95 ⁰C to 50 ⁰C). The district heating water operating temperatures were selected as 60 ⁰C and 90 ⁰C for the inlet and outlets respectively. A process modeling software Aspen hysys® was used to simulate the heat energy recovery.