Mattias Ljunggren

Techno-economic analysis of a two-step biological process producing hydrogen and methane

Biological hydrogen production using dark fermentation has been proposed as an alternative and renewable way of producing hydrogen from biomass. However, theoretically, the conversion of biomass to biohydrogen using dark fermentation only utilizes one third of the energy content of the sugars derived from starch or cellulose; the rest being in the form of organic acids also produced by the microorganisms. To provide a viable alternative, the energy contained in the organic acids produced in dark fermentation must also be utilized. This can be done in several ways, one of which is to produce methane in an anaerobic digestion step. The technical and economical feasibility of this two-step biological process is investigated in the present study employing three base cases reflecting the different strategies that can be used when performing dark fermentation: high productivity, high yield, and low productivity-low yield. The glucose concentrations, hydrogen productivities and yields studied herein ranges from 4-20 g/L, 7.25-45.75 mmol H₂/(lh) and 1.37-3.48 mmol H₂/mole glucose, respectively. The production of pure methane was included as a reference case to investigate how the production of hydrogen affects the production cost. The cost estimates ranged from 50 to 340 €/GJ for the three base cases and the reference case for the process alternatives investigated. The results show that the capital costs and the nutrients used in the two biological steps are the main contributors to the cost in all base cases and the reference case. Furthermore, increasing the substrate concentration and, surprisingly, decreasing the hydrogen yield, reduces the production cost.

Techno-economic comparison of a biological hydrogen process and a 2nd generation ethanol process using barley straw as feedstock

A process combining dark fermentation and photofermentation for production of hydrogen is interesting due to its potential of producing hydrogen at a high yields. In this study, the hydrogen process is compared to a 2nd generation ethanol process with respect to cost and with the aim of increasing our understanding of the pros and cons and giving a clear picture of the present status of the two processes. The hydrogen production cost was found to be about 20 times higher than the ethanol production cost, 421.7 €/GJ compared to 19.5 €/GJ. The main drawbacks of the hydrogen process are its low productivity, low energy efficiency, and the high cost of buffer and base required to control the pH.

Techno-economic evaluation of a two-step biological process for hydrogen production

An integrated biological process for the production of hydrogen based on thermophilic and photo‐heterotrophic fermentation was evaluated from a technical and economic standpoint. Besides the two fermentation steps the process also includes pretreatment of the raw material (potato steam peels) and purification of hydrogen using amine absorption. The study aimed neither at determining the absolute cost of biohydrogen nor at an economic optimization of the production process, but rather at studying the effects of different parameters on the production costs of biohydrogen as a guideline for future improvements. The effect of the key parameters, hydrogen productivity and yield and substrate concentration in the two fermentations on the cost of the hydrogen produced was studied. The selection of the process conditions was based mainly on laboratory data. The process was simulated by use of the software Aspen Plus and the capital costs were estimated using the program Aspen Icarus Process Evaluator. The study shows that the photo‐fermentation is the main contributor to the hydrogen production cost mainly because of the cost of plastic tubing, for the photo‐fermentors, which represents 40.5% of the hydrogen production cost. The costs of the capital investment and chemicals were also notable contributors to the hydrogen production cost. Major economic improvements could be achieved by increasing the productivity of the two fermentation steps on a medium‐term to long‐term scale.

A kinetic model for quantitative evaluation of the effect of hydrogen and osmolarity on hydrogen production by Caldicellulosiruptor saccharolyticus

BackgroundCaldicellulosiruptor saccharolyticus has attracted increased interest as an industrial hydrogen (H2) producer. The aim of the present study was to develop a kinetic growth model for this extreme thermophile. The model is based on Monod kinetics supplemented with the inhibitory effects of H2 and osmotic pressure, as well as the liquid-to-gas mass transfer of H2.ResultsMathematical expressions were developed to enable the simulation of microbial growth, substrate consumption and product formation. The model parameters were determined by fitting them to experimental data. The derived model corresponded well with experimental data from batch fermentations in which the stripping rates and substrate concentrations were varied. The model was used to simulate the inhibition of growth by H2 and solute concentrations, giving a critical dissolved H2 concentration of 2.2 mmol/L and an osmolarity of 0.27 to 29 mol/L. The inhibition by H2, being a function of the dissolved H2 concentration, was demonstrated to be mainly dependent on H2 productivity and mass transfer rate. The latter can be improved by increasing the stripping rate, thereby allowing higher H2 productivity. The experimentally determined degree of oversaturation of dissolved H2 was 12 to 34 times the equilibrium concentration and was comparable to the values given by the model.ConclusionsThe derived model is the first mechanistically based model for fermentative H2 production and provides useful information to improve the understanding of the growth behavior of C. saccharolyticus. The model can be used to determine optimal operating conditions for H2 production regarding the substrate concentration and the stripping rate.

Integration study on a two-stage fermentation process for the production of biohydrogen

In order to make the hydrogen economy fully sustainable, renewable resources have to be employed for its production. Simulation models, developed with Aspen Plus to calculate mass and energy balances, will be used to integrate the process steps necessary to produce pure hydrogen from biomass in a 2-stage fermentation process. The main challenge is the reduction of water and heat demand connected to the low substrate concentration in the fermentation steps; the easiest solution is to partly recirculate outgoing process streams. Electrolyte equilibrium was considered during simulation of different recirculation options to evaluate important effects on the pH and on the system osmolality. The results show that certain recirculation options can reduce the heat and water demand significantly. (Less)

Heat integration of a fermentation-based hydrogen plant connected with sugar factory

The paper is concerned with heat integration of a conceptual hydrogen plant connected with a sugar factory. The sugar factory serves as a source of sucrose-containing thick juice for the hydrogen plant, where this feedstock is processed to hydrogen. Moreover, this connection gives an opportunity to utilize waste heat from the sugar factory. Hydrogen is produced by two-stage fermentation, that is, thermophilic fermentation followed by photofermentation. The gas mixture obtained in the two process stages is supplied to the gas separation system, composed of absorbing and stripping columns for circulating amine solution, to separate hydrogen from carbon dioxide. Using Pinch Technology and considering sugar factory with its CHP plant as an energy source, the hydrogen plant is heat-integrated to minimise the energy consumption. (Less)