Jens Bo Holm-Nielsen

The future of anaerobic digestion and biogas utilization

One of the common tendencies of animal production activities in Europe and in developed countries in general is to intensify the animal production and to increase the size of the animal production units. High livestock density is always accompanied by production of a surplus of animal manure, representing a considerable pollution threat for the environment in these areas. Avoiding over-fertilization is not only important for environmental protection reasons but also for economical reasons. Intensive animal production areas need therefore suitable manure management, aiming to export and to redistribute the excess of nutrients from manure and to optimize their recycling. Anaerobic digestion of animal manure and slurries offers several benefits by improving their fertilizer qualities, reducing odors and pathogens and producing a renewable fuel - the biogas. The EU policies concerning renewable energy systems (RES) have set forward a fixed goal of supplying 20% of the European energy demands from RES by year 2020. A major part of the renewable energy will originate from European farming and forestry. At least 25% of all bioenergy in the future can originate from biogas, produced from wet organic materials such as: animal manure, whole crop silages, wet food and feed wastes, etc.

Hydrothermal liquefaction of Spirulina and Nannochloropsis salina under subcritical and supercritical water conditions

Six hydrothermal liquefaction experiments on Nannochloropsis salina and Spirulina platensis at subcritical and supercritical water conditions (220–375 °C, 20–255 bar) were carried out to explore the feasibility of extracting lipids from wet algae, preserving nutrients in lipid-extracted algae solid residue, and recycling process water for algae cultivation. GC–MS, elemental analyzer, FT-IR, calorimeter and nutrient analysis were used to analyze bio-crude, lipid-extracted algae and water samples produced in the hydrothermal liquefaction process. The highest bio-crude yield of 46% was obtained on N. salina at 350 °C and 175 bar. For S. platensis algae sample, the optimal hydrothermal liquefaction condition appears to be at 310 °C and 115 bar, while the optimal condition for N. salina is at 350 °C and 175 bar. Preliminary data also indicate that a lipid-extracted algae solid residue sample obtained in the hydrothermal liquefaction process contains a high level of proteins.

Transflexive embedded near infrared monitoring for key process intermediates in anaerobic digestion/biogas production

This work reports an off-line method development simulating at-line anaerobic co-digestion process monitoring using a new transflexive embedded near infrared sensor (TENIRS) system as a process analytical chemistry (PAC) facility. The operative focus is on optimising anaerobic digestion biogas production with energy crops as the main feedstock. Results show that several key monitoring intermediates in the anaerobic fermentation process can be quantified directly using near infrared spectroscopy with good results, especially ammonium and total volatile fatty acids. Good feasibility study prediction validations have been obtained for total solids (TS), volatile solids (VS), ammonium, acetic acid and total volatile fatty acids. The TENIRS system is a new option for real-time, at-line/on-line monitoring of biogas fermentation operations, offering a robust, low-budget PAC approach to a rapidly growing bulk volume industry.

Near infrared and acoustic chemometrics monitoring of volatile fatty acids and dry matter during co-digestion of manure and maize silage.

In this study, two process analytical technologies, near infrared spectroscopy and acoustic chemometrics, were investigated as means of monitoring a maize silage spiked biogas process. A reactor recirculation loop which enables sampling concomitant with on-line near infrared characterisation was applied. Near infrared models resulted in multivariate models for total and volatile solids with ratio of standard error of performance to standard deviation (RPD) values of 5 and 5.1, indicating good on-line monitoring prospects. The volatile fatty acid models had slopes between 0.83 and 0.92 (good accuracy) and RPD between 2.8 and 3.6 (acceptable precision). A second experiment employed at-line monitoring with both near infrared spectroscopy and acoustic chemometrics. A larger calibration span was obtained for total solids by spiking. Both process analytical modalities were validated with respect to the total solids prediction. The near infrared model had an RPD equal to 5.7, while the acoustic chemometrics model resulted in a RPD of 2.6.

Dynamic biogas upgrading based on the Sabatier process: thermodynamic and dynamic process simulation.

This study aimed to investigate the feasibility of substitute natural gas (SNG) generation using biogas from anaerobic digestion and hydrogen from renewable energy systems. Using thermodynamic equilibrium analysis, kinetic reactor modeling and transient simulation, an integrated approach for the operation of a biogas-based Sabatier process was put forward, which was then verified using a lab scale heterogenous methanation reactor. The process simulation using a kinetic reactor model demonstrated the feasibility of the production of SNG at gas grid standards using a single reactor setup. The Wobbe index, CO2 content and calorific value were found to be controllable by the H2/CO2 ratio fed the methanation reactor. An optimal H2/CO2 ratio of 3.45-3.7 was seen to result in a product gas with high calorific value and Wobbe index. The dynamic reactor simulation verified that the process start-up was feasible within several minutes to facilitate surplus electricity use from renewable energy systems.

Conceptual design of an integrated hydrothermal liquefaction and biogas plant for sustainable bioenergy production.

Initial process studies carried out in Aspen Plus on an integrated thermochemical conversion process are presented herein. In the simulations, a hydrothermal liquefaction (HTL) plant is combined with a biogas plant (BP), such that the digestate from the BP is converted to a biocrude in the HTL process. This biorefinery concept offers a sophisticated and sustainable way of converting organic residuals into a range of high-value biofuel streams in addition to combined heat and power (CHP) production. The primary goal of this study is to provide an initial estimate of the feasibility of such a process. By adding a diesel-quality-fuel output to the process, the product value is increased significantly compared to a conventional BP. An input of 1000 kg h(-1) manure delivers approximately 30-38 kg h(-1) fuel and 38-61 kg h(-1) biogas. The biogas can be used to upgrade the biocrude, to supply the gas grid or for CHP. An estimated 62-84% of the biomass energy can be recovered in the biofuels.

Monitoring of biogas test plants—a process analytical technology approach

Most studies reported in the literature have investigated near infrared spectroscopy (NIR) in laboratory‐scale or minor pilot biogas plants only; practically no other studies have examined the potential for meso‐scale/full‐scale on‐line process monitoring. The focus of this study is on a meso‐scale biogas test plant implementation of process analytical technologies (PAT) to develop multivariate calibration/prediction models for anaerobic digestion (AD) processes. A 150 L bioreactor was fitted with a recurrent loop at which NIR spectroscopy and attendant reference sampling were carried out. In all realistic bioreactor scales, it is necessary to obtain a fairly constant level of volatile fatty acid (VFA) concentration, which furthers a stable biogas production. Uncontrolled VFA contents have a significant negative impact on biogas production; VFA concentrations should not exceed 5–6000 mg/L lest the microorganism population may suffer fatal reductions. On‐line control and management of VFA concentration levels are therefore critical in order to be able to speed up or slow down the AD processes which produce the desired sustainable bioenergy for combined heat and power production. By calibrating pilot plant NIR spectra to laboratory VFA reference concentrations at the experimental locality Bygholm, it was possible to develop calibration models by partial least squares (PLS) regression, which displayed acceptable to very good prediction performances for total VFA as well as for three other essential individual acids based on test set validations. The average statistics assessing prediction performance, accuracy (slope) and precision (explained variance r2), were both 0.92, which must be considered excellent for this type of significantly heterogeneous systems. The meso‐ to full‐scale feasibility has thus been proven, which has important positive implications for robust, sufficient and reliable, low‐cost PAT monitoring systems in the biogas arena and other continuous fermentation processes. Copyright

Influence of trace substances on methanation catalysts used in dynamic biogas upgrading

The aim of this work was to study the possible deactivation effects of biogas trace ammonia concentrations on methanation catalysts. It was found that small amounts of ammonia led to a slight decrease in the catalyst activity. A decrease in the catalyst deactivation by carbon formation was also observed, with ammonia absorbed on the active catalyst sites. This was via a suppression of the carbon formation and deposition on the catalyst, since it requires a higher number of active sites than for the methanation of carbon oxides. From the paper findings, no special pretreatment for ammonia removal from the biogas fed to a methanation process is required.

On-line near infrared monitoring of ammonium and dry matter in bioslurry for robust biogas production: a full-scale feasibility study.

Heterogeneous substrates fed into agricultural biogas plants originate from many sources with resulting quality fluctuations potentially inhibiting the process. Biogas yield can be substantially increased by optimisation of the organic dry matter load. In this study, near infrared (NIR) spectroscopy was applied on-line in a re-circulating loop configuration operating identically as a full-scale setup. Ammonium could be modelled in the industrially-relevant range 2.42–8.52 gL−1 with an excellent accuracy and precision, slope ∼1.0, r2 = 0.97, corresponding to a relative root mean square error of prediction (RMSEP) of 6.7%. Also, dry matter in the similar plant relevant range 5.8–10.8 weight-percent could be predicted with acceptable accuracy (slope ∼1.0, r2 = 0.83, and a relative RMSEP below 8.0%. Based on these performance characteristics, it was concluded that NIR spectroscopy can be applied for optimising the efficiency of current and future biogas plants, as well as in biorefinery operations converting heterogeneous bioslurry, energy crops, and wastes into value-added products.

Biorefinery plant design, engineering and process optimisation

Abstract: Before new biorefinery systems can be implemented, or the modification of existing single product biomass processing units into biorefineries can be carried out, proper planning of the intended biorefinery scheme must be performed initially. This chapter outlines design and synthesis approaches applicable for the planning and upgrading of intended biorefinery systems, and includes discussions on the operation of an existing lignocellulosic-based biorefinery platform. Furthermore, technical considerations and tools (i.e., process analytical tools) which could be applied to optimise the operations of existing and potential biorefinery plants are elucidated.