# Toward Sustainability of the Aqueous Phase Reforming of Wastewater: Heat Recovery and Integration
## 1 Introduction  
Aqueous-phase reforming (APR) is a catalytic process for the transformation of oxygenated organic compounds into hydrogen; it also generates alkanes as a byproduct [1; 2]. In general terms, the process is relatively similar to steam reforming, but is carried out in the liquid water phase at temperatures around 200 \({}^{\circ}\)C [3; 4]. In the context of hydrogen economy, APR has been studied as a method to produce renewable hydrogen [5]. Many different feedstocks have been studied [6], including sugar alcohols (such as glycerol, xylitol or sorbitol, among others) [7], bio-sils, bio-refinery residues [8] and biomasses (including waste biomass) [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 8 operation [14; 18; 19] were commonly used. The hydrogen content in an APR gas product is highly conditioned by operating conditions (temperature, pressure and feed concentration), influencing not only H\({}_{2}\) but also CO, CO\({}_{2}\) and CH\({}_{4}\) formation, as was revealed by the thermodynamic analysis of glycerol APR [20]. Likewise, the concentration of substrate in the APR feed stream was shown to have an important influence. In general terms, aqueous phase reforming needs a diluted feed stream; however, a moderate concentration increase has been found to lead to higher substrate conversion at the expense of lower hydrogen yield [1; 21; 22]. In this context, APR has also been applied to the treatment of low-concentration wastewater bearing biomass-derived pollutants, which made it possible to decrease wastewater organic load and generate a valuable gas stream from waste [8; 23]. Most works on APR in the literature are based on experimental work and are focused on catalyst development and the influence of feedstock and reaction conditions in organic matter conversion and gas product yield. Several studies can also be found in the literature about process modeling and simulation, with the aim of assessing APR process feasibility from a technical and economic perspective. These works show consensus of the fact that temperature and heat requirements for the APR process require compensation to make the process economically feasible. The most common option has been the use of H\({}_{2}\)-rich gas generated for commodity applications; however, the heat integration and/or the heat vaporization of alkane byproducts produced is also an interesting option [13]. Likewise, the price of hydrogen does not compensate sufficiently for the cost of some marketable feedstocks proposed as substrates in APR [24]. One of the challenges in the application of APR to the treatment of wastewater is that the low organic carbon, in comparison to other proposed feedstocks, limits the amount of gas that can be produced and used to achieve auto-sustainability of heat demand of the process. This aspect has also been studied for other wastewater treatment processes, enabling the production of flue gas but using other mature technologies, such as steam reforming (SR). Thus, [PERSON] et al. [25] studied the integration of heat and power generation in a wastewater treatment plant on the basis of biogas valorization, concluding that ca. 47 and 100% of power and heat demands could be covered, respectively. On the other hand, the production of 80 kg/h hydrogen could be achieved by APR at 250 \({}^{\circ}\)C/50 bar with lower costs than by SR at 550 \({}^{\circ}\)C/1 bar, using glycerol as a feedstock in both cases [26]. This study deals with the evaluation of heat integration and the sustainability of APR applied to the treatment of wastewater.