Energy Analysis and Heat Integration in the Joint Process of Biomass Fast Pyrolysis and In Line Sorption Enhanced Steam Reforming

Biomass Fast Pyrolysis and in line Steam Reforming (PY-SR) is promising alternative for H2 production. However, there are potential strategies for intensifying the process, such as capturing the CO2 in situ in the reforming step, which is so-called Sorption Enhanced Steam Reforming (SESR). Both PY-SR and PY-SESR were simulated using a thermodynamic approach and empirical correlations, and they were compared based on the energy requirements, H2 production, and H2 purity at different temperatures (500–800 °C) and steam to biomass (S/B) ratios (0–4). Then, the energy requirements for the PY-SESR were analyzed in detail for a reforming temperature of 600 °C and several S/B ratios, and a heat integration scheme was proposed, aiming at making the process thermally autosustained. Although the energy requirement of PY-SESR is always higher than that of PY-SR at the same reforming conditions, it allows the use of milder operating conditions, with the process performance being even better. Thus, PY-SESR outshines PY-SR, as it allows obtaining a higher H2 production (0.124 kgH2 kg–1biomass vs 0.118 kgH2 kg–1biomass) and H2 purity (98 mol % vs 67 mol %), with a lower energy requirement, and capturing the CO2 generated, thereby attaining negative emissions. The main energy demands of this process account for water evaporation and sorbent calcination. Nevertheless, a thermally autosustained PY-SESR process may be attained by recovering heat from the product streams, transferring heat from the reforming reactor to the pyrolysis reactor, and burning the char generated in the pyrolysis step.

-  ( ℎ -1 ): energy requirement for heating the pyrolysis products from the temperature of the pyrolysis (500 °C) to the temperature of the reforming.- - ( ℎ -1 ): energy requirement needed for the steam reforming step.
The calculations of these terms are detailed below.

Biomass energy requirement (𝑸 𝒃𝒊𝒐𝒎𝒂𝒔𝒔 )
In order to determine the energy needed for heating the biomass, the next equation was used: where   is the energy needed to heat the biomass from the ambient temperature to the reaction one (MJ h -1 ),    is the biomass specific heat (MJ kg -1 biomass K -1 ),   is the ambient temperature (K),   is the moisture mass fraction of the biomass (kg water kg -1  biomass ), whose value is 0.1 kg water kg -1  biomass ,   is the reaction temperature (K), which was set at 873 K,   is the water latent heat of vaporization at atmospheric pressure (MJ kg -1  water ), which has a value of 2.26 MJ kg -1  water [1] and   is the biomass mass flow rate (kg biomass h -1 ), which has been set to 100 kg biomass h -1 .
The specific heat of the biomass has been determined by using the next equation [2]: where    is the specific heat of the biomass (MJ kg -1 biomass K -1 ) and  is temperature (K).

Water energy requirement (𝑸 𝒘𝒂𝒕𝒆𝒓 )
The energy required for heating the water from the ambient temperature to the reaction temperature was determined as follows: where   is the energy needed to heat the water from the ambient temperature to the reaction one (MJ h -1 ),    is the water specific heat (MJ kg -1 water K -1 ),   is the ambient temperature (K),   is the reaction temperature (K), which was set at 873 K,   is the water latent heat of vaporization at atmospheric pressure (MJ kg -1  water ), which has a value of 2.26 MJ kg -1  water [1], /  is the steam to biomass ratio (kg water kg -1 biomass ) and   is the biomass mass flow rate (kg biomass h -1 ), which has been set to 100 kg biomass h -1 .
The specific heat of the liquid water was determined as follows: where
The specific heat of the water vapor was determined by using the next equation: where

Pyrolysis heat of reaction (𝑸 𝒑𝒚𝒓𝒐𝒍𝒚𝒔𝒊𝒔-𝒓𝒆𝒂𝒄𝒕𝒊𝒐𝒏 )
The pyrolysis heat of reaction (MJ h -1 ) was determined as follows: where  - is the pyrolysis heat of reaction considered, whose value is -0.255MJ kg -1  biomass [2], and   is the biomass mass flow rate (kg biomass h -1 ), which has been set to 100 kg biomass h -1 .

Heating of biomass pyrolysis volatiles (𝑸 𝒗𝒐𝒍𝒂𝒕𝒊𝒍𝒆𝒔 ) and reaction heat of the steam reforming (𝑸 𝒓𝒆𝒇𝒐𝒓𝒎𝒊𝒏𝒈 )
As for the energy input required for heating the pyrolysis volatiles from the pyrolysis temperature (500 °C) to the reforming one (500 °C to 800 °C) (  ) and for the energy input required for the reforming step (  ), they were determined by using the simulation Pro II v.2021 software, as mentioned in the main manuscript.

PY-SESR alternative
The terms included in the energy analysis in the PY-SESR alternative are as follows: The   was already defined in equation (2).
The remaining terms are defined as follows: The terms that have not already described are as follows: - - ( ℎ -1 ): energy involved in the cooling of the CaO from the calcination temperature to the reforming one.- - ( ℎ -1 ): energy involved in the heating of the CaO from the reforming temperature to the calcination one.-  ( ℎ -1 ): energy required for heating the nitrogen from the ambient temperature (25 °C) to the calcination one.- - ( ℎ -1 ): energy required for the calcination reaction.
Next, the calculations are detailed.

𝑸 𝒔𝒐𝒓𝒃𝒆𝒏𝒕-𝒓𝒆𝒇𝒐𝒓𝒎𝒊𝒏𝒈
First, it is important to mention that all the sorbent that comes from the calcination is CaO.
The energy involved in the cooling of the CaO from the calcination temperature to the reforming one was determined as follows: where  - is the energy involved in heating the CaO from the calcination temperature to the reforming one (MJ h -1 ),    is the calcium oxide specific heat (MJ kg -1  CaO K - 1 ), the /  is the calcium oxide to biomass ratio (kg CaO kg -1 biomass ), which has been set as 1.59 kg CaO kg -1  biomass and   is the biomass mass flow rate (kg biomass h -1 ), which has been set to 100 kg biomass h -1 .

𝑸 𝒔𝒐𝒓𝒃𝒆𝒏𝒕-𝒄𝒂𝒍𝒄𝒊𝒏𝒂𝒕𝒊𝒐𝒏
First, it is important to mention that not all the sorbent that comes out from the reforming step is CaCO 3 .Depending on the reforming temperature, the carbonation extent would vary.As consequence, the sorbent that leaves the reformer will be a mixture of CaO and CaCO 3 .
The energy related to the heating of the CaO from the reforming temperature to the calcination one was determined as follows: where   is the energy related to the change in the CaO temperature from the reforming temperature to the calcination one (MJ kg -1  CaO ).
The energy related to the heating of the CaCO 3 from the reforming temperature to the calcination temperature was determined as follows: where   3 is the energy related to the change in the CaCO 3 temperature from the reforming temperature to the calcination one (MJ kg -1 CaCO 3 ) and is the calcium carbonate specific heat (MJ kg -1 CaCO 3 K -1 ).
Finally, the energy related to the heating of the sorbent from the calcination temperature to the reforming temperature was determined as follows: where  - is the energy related to the change in the sorbent temperature from the reforming temperature to the calcination one (MJ h -1 ),   is the energy related to the change in the CaO temperature from the reforming one to the calcination one (MJ kg -1 CaO ),   3 is the energy related to the change in the CaCO 3 temperature from the reforming one to the calcination one (MJ kg -1 CaCO 3 ),   is the calcium oxide mass flow rate from the reforming step to the calcination one (kg CaO h -1 ) and   3 is the calcium carbonate mass flow rate from the reforming step to the calcination one (kg CaCO 3 h -1 ).

𝑸 𝒏𝒊𝒕𝒓𝒐𝒈𝒆𝒏
The energy required for heating up the nitrogen from the ambient temperature to the calcination temperature was determined as follows: where   is the energy needed to heat the nitrogen from the ambient temperature to the calcination one (MJ h -1 ),   2  is the nitrogen specific heat (MJ kg -1 N 2 K -1 ) and ),   2 is the nitrogen mass flow rate fed to the calcination step (kg N 2 h -1 ) (determined so that the partial pressure of CO 2 in the calcination step is 30 % lower than the equilibrium value for the decarbonation reaction at the calcination temperature).
The specific heat of the nitrogen was determined by using the next equation.

𝑸 𝒄𝒂𝒍𝒄𝒊𝒏𝒂𝒕𝒊𝒐𝒏-𝒓𝒆𝒂𝒄𝒕𝒊𝒐𝒏
The calcination reaction is as follows: The enthalpy of the calcination reaction has been determined as follows: where ∆    is the enthalpy of the calcination reaction (MJ kmol -1 ) at the calcination temperature, ∆ 1 is the enthalpy associated with the cooling of the CaCO 3 from the calcination temperature to the standard temperature (298 K) (MJ kmol -1 ), ∆ 0  is the standard enthalpy of the calcination reaction (MJ kmol -1 ), and ∆ 2 is the enthalpy associated with the heating of the reaction products from the standard temperature (298 K) to the calcination one.
∆H 1 has been determined as follows: The term   3  has been described above.
∆H 2 has been determined as follows: where is the specific heat of CO 2 (MJ kmol -1 K -1 ).
The energy required for the calcination reaction, at the calcination temperature, was determined as follows: where  - is the energy required for the calcination reaction at the calcination temperature,   3 is the molecular weight of CaCO 3 (kg kmol -1 ), whose value is 100 kg kmol - 1 and   3 is the mass flow rate of the CaCO 3 to be calcined (kg h -1 ).

PY-SESR heat integration
The process diagram and the overall energy balance for heat integration in the PY-SESR option can be found in the Materials and Methods section in the main manuscript.
Next, the calculations concerning each of the terms of the overall energy balance are addressed.

Pyrolysis
In the pyrolysis step (operation temperature of 500 °C), the energy balance is as follows: I  -- refers to the heat transferred from the SESR step to the pyrolysis step to meet the energy requirement described in equation ( 28).
- - =   +  - = 125.62-25.5 = 100.12 ℎ -1 (28) Furthermore, the term   corresponds to the energy requirement related to the heating of the water from 25 °C to 500 °C.The S/B ratio was set to 2, and the biomass mass flow rate was fixed at 100 kg h -1 , so the water mass flow rate to heat is 200 kg h -1 .
It is to note that the term   was balanced by the terms  -- and  - , as shown in equation ( 29). - -- refers to the heat recovery from the SESR reactor and  - to the heat recovery from the reforming products.

SESR
A temperature of 600 °C was considered for the SESR step.
The overall energy balance for the SESR stage is as follows: The term  - refers to the energy recovered from the SESR reactor to supply the energy needed for the pyrolysis (  +  - ).
The term  -- refers to the energy recovered from the SESR reactor for heating the water fed into the pyrolysis step ( - -- ) by using all the available energy generated in the SESR step.
The energy related to the cooling of the reforming products ( - ) from the reforming temperature to the temperature needed to cover the heat   +  -- has been determined by using the PRO II software.
The energy related to the heating of the pyrolysis volatiles from the pyrolysis temperature to the reforming one (  ) and the energy related to the SESR heat of reaction (  - ) have also been determined by using the PRO II software.

Calcination
As mentioned in the section of Results and Discussion, the calcination temperature selected was 750 °C.
The overall energy balance regarding the calcination step is as follows: It is to note that   has been divided into  -1 and  -2 . -1 refers to the heating of the nitrogen from 25 °C to 745 °C, which is a temperature slightly lower than that of the calcination.It is consequence of the fact that water is integrated into the calcination products, whose temperature is 750 °C, and a thermal gradient is required for the heat transfer. -2 refers to the heating of the nitrogen from 745 °C to 750 °C, and was assumed to take place inside the calcination reactor.
The energy related to the cooling of the calcination products ( - ) from the calcination temperature to the temperature needed to meet the demand of  -1 has been determined by using the PRO II software.
Taking into account that  -1 = - - , and that  - is the heat transferred from the combustion stage to the calcination stage in order to balance the calcination stage (  = 0).The latter has been determined as follows: - - =  - +  - +  -2 (32)

Combustion
As mentioned in the main manuscript, the char generated in the pyrolysis has been used as fuel for the combustion stage.
The combustion temperature was set at 770 °C, i.e., 20 °C higher than that of calcination.
It is to mention that the char mass flow rate has been determined based on the char yield (0.1734 kg char kg -1 biomass [3]) and assuming an inlet mass flow rate of biomass into the process of 100 kg biomass h -1 .The air mass flow rate is the stoichiometric value to burn completely the char, for which the composition determined by Amutio et al. [3] has been assumed The overall energy balance of the combustion stage would be as follows: The term  - corresponds to - - , which was addressed above.
The term   , determined by using the software PRO II, is related to the heating of the air inlet.It has been divided into  -1 and  -2 . -1 refers to the heating of the air from 25 °C to 755 °C, which is a temperature slightly lower than that of combustion.It is consequence of the fact that air is integrated with the combustion products, whose temperature is 770 °C, so a thermal gradient is required for heat transfer. -2 refers to the heating of the air from 755 °C to 770 °C, and was assumed to take place inside of the combustor.
The energy related to the cooling of the combustion products ( - ) from the combustion temperature to the temperature needed to meet the demand of  -1 has been determined by using the PRO II software.Therefore,  -1 = - - , and so  -1 +  - = 0.
As for  - , it was determined as follows: - =   •  ℎ •  ℎ (34) where  - is the energy related to the combustion of the char (MJ h -1 ),   is the biomass mass flow rate (kg biomass h -1 ), whose value was set at 100 kg biomass h -1 ,  ℎ is the char yield (kg char kg -1 biomass ), whose value is 0.1734 kg char kg -1 biomass [3] and  ℎ is the lower heating value of the char (MJ kg -1 char ), whose value is 30.4MJ kg -1 char [3].
It is to note that the same value of  - has being used independently of the combustion temperature.
As for the  ℎ , it was determined as follows: where  ℎ is the energy related to the heating of the char from the pyrolysis temperature to the combustion one (MJ h -1 ),   is the biomass mass flow rate (kg biomass h -1 ), whose value was set at 100 kg biomass h -1 ,  ℎ is the energy needed to heat the char from the pyrolysis temperature to the combustion one per mass of char produced (MJ kg -1 char ), and  ℎ is the char yield (kg char kg -1 biomass ), whose value is 0.1734 kg char kg -1 biomass [3].
The energy required for heating the char from the pyrolysis temperature to the combustion one, per mass of char produced, was determined as follows: where  ℎ is the energy needed to heat the char from the pyrolysis temperature to the combustion one, per mass of char produced (MJ kg -1 char ), and  ℎ  is the char specific heat (MJ kg -1 char K -1 ).
The specific heat of the char was determined by using the next equation [2]: ℎ  = 0.43263 + 0.00209• 1000