ENK6-2001-00555 Biomass and Waste Conversion in Supercritical Water for the Production of Renewable Hydrogen

Contacts
Summary Information



To find similar Items, click on a keyword below:
Chemical Conversion : EESD (Energy, Environment and Sustainable Development) : Liquid Biofuels and Biogas

Proposal No:	ENK6-2001-00555
Date Prepared:	February 2004
Source:	European Bio-Energy Projects (EUR 20808)

Introduction

In all EU-countries large amounts of wet biomass and waste streams (e.g. sewage sludge and vegetables-garden-fruit residues), are produced, and due to (new) environmental regulations, it becomes more and more difficult to dispose of these streams. Landfilling is expensive or even forbidden. Technologies able to convert these streams are scarce, expensive or only provide a partial solution.

A relatively new approach to produce renewable hydrogen from wet biomass and waste is the application of the so-called SuperCritical Water (SCW) gasification process. Supercritical water conditions are achieved at T>374 °C and P > 22 MPa. The main advantages of this technology are:

• suitable to convert very wet biomass (> 80wt% moisture) and waste streams
• the produced gas is very clean, and free of tars and other contaminants
• the raw gas is very rich in hydrogen (50 - 60 vol%)
• the gas becomes available at high pressure, avoiding expensive compression (e.g. for storage).

Scientific and technical challenges within the SuperHydrogen project are mainly related to feedstock preparation, SCW-process development and product upgrading.

The SCW process is operated at 300 bar, and the feedstock needs to be pressurized. Pumping liquids is simple and pumps for liquid feedstocks are readily available. Pumping feedstocks containing solids is much more complex and very expensive. The objective is to develop a feedstock preparation method to convert feedstocks of different nature into a pumpable slurry.

The use of supercritical water to produce hydrogen from biomass has been demonstrated, but a number of process-related problems need to be solved such as:

• Design of the heat exchanger; heat integration is essential for the overall thermal efficiency of the process.
• Fate of minerals; minerals are present in most feedstocks; however, the fate of minerals in the process is unclear.
• High-pressure gas-liquid separator; the high pressure G/L separator is used to separate the product gas and the water.

However, under the prevailing conditions (P = 300 bar) part of the gaseous product will remain in the water phase (e.g. CO₂, NH₃, H₂S, but also some H₂ and CH₂). The aim is to maximize the H₂- production, but contaminants like NH₃ and H₂S should remain in the water phase. Absence of these contaminants avoids the need for downstream gas processing.

The hydrogen content is limited by gas phase equilibrium reactions. In situ removal of H₂ will shift the equilibrium towards H₂. The challenge is to develop a high-pressure, catalytic membrane reactor to maximise the hydrogen yield. A properly selected catalyst will be deposited inside the membrane to enhance the reforming of CH ₄, and the water-gas-shift reaction.

The project covers the whole SCW process-chain from wet bio-waste feedstocks to compressed hydrogen. It starts with a selection of suitable feedstocks in Europe, and ends with a demonstration of the complete chain from wet biomass to renewable hydrogen. In the first phase of the project the individual unit operations (feed preparation, scw-process, and product upgrading) will be further developed. However, to avoid sub-optimization of the unitoperations, an overall process model will be developed. This model will be used for the basic engineering of the process, and provide a rather accurate cost estimate of the whole process. In the last year of the project the unit operations will be combined, and operated simultaneously.

Progress

The project started in late 2001 with the development of the individual unit operations. The work on the feed preparation showed that with simple, conventional pretreatment techniques it seems possible to make a slurry with a solids content of about 20 wt%. A number of tests have been carried out in the SCW pilot plant yielding a gas rich in H₂ and CH₄, but also large quantities of CO were observed. However, by changing the process conditions it is possible to remove nearly all CO avoiding the need for downstream CO conversion.

The work on the upgrading reactor started with the preparation of different membrane samples, which will be tested with artificial gas. The focus now shifts to the simultaneous reforming of CH₄ and H₂ removal as CO conversion seems now less important.

Impact and exploitation

The project aims at the development of a novel process for the production of clean, compressed hydrogen from renewable resources (biomass and waste). Using renewable energy sources will as such contribute to an increase security and diversity of energy supply. The proposed technology can offer an end-solution for the conversion of wet feedstocks. For many industries the wet waste streams become increasingly difficult to dispose of in an environmentally sound way. Currently, high costs are associated with this, and the costs are increasing rapidly.

The technology offers the possibility to produce clean hydrogen and concentrated CO₂ from biomass and waste. The CO₂ stream is also available at elevated pressure (> 150 bar), and offers good opportunities for sequestration. However, hydrogen -and probably in particular renewable hydrogen- is seen as an energy carrier for the long-term.

The gas produced by the SCW process mainly contains H₂ and CH₄, and after minor conditioning it might be very suitable as Substitute Natural Gas (SNG) The first applications of the SCW process are expected for the latter application. In transition phase, the applications will shift from SNG to H₂.