HT Gasification of Biomass and Wastes

by GP Green Energy

DUAL FLUIDIZED BED (DFB) GASIFICATION

The Dual Fluidized bed (DFB) Steam Gasification process is considered as one of the most sophisticated processes in the world. It can convert 1 Ton of biomass or garbage (RDF) to –

• either 1 MWh electricity plus 2.5 MWh thermal energy
• or 2 MWh liquid fuel plus 1.5 MWh thermal energy
• or 2.5 MWh synthetic natural gas plus 0.4 MWh thermal energy
• or 2 MWh Hydrogen plus 1 MWh thermal energy

Process

The resulting raw producer gas is cooled and cleaned by two stage cleaning system. Tar, dust, condensate etc. are removed by an oil scrubber. Scrubber liquid saturated with tar and condensate is vaporized and introduced into combustion zone of the gasifier. Waste heat is available during cooling of the syngas, which is effectively used for drying of the feedstock or any other application. Only dry ash with very low carbon comes out from the system and there is no liquid effluent.

GREEN HYDROGEN (GH2)

From a DFB Gasifier the produced synthesis gas, which has high contents of hydrogen and carbon monoxide, is highly suitable for hydrogen production. High efficiencies of more than 60% can be reached by generating hydrogen from biomass and total efficiencies of up to 80% are possible by utilization of waste heat. Here, 320 kW of extrinsic energy are required for 1000 kWh of H₂. By comparison, generating the same amount of H₂ via electrolysis would need 3000 kWh of energy.

Hydrogen plant through above technology is most effective for production decentrally at low cost. This can be installed close to highways or traffic model centers, where biomass is available locally to avoid transportation of hydrogen. A fully distributed network can be designed at the places where trucks, buses and passenger cars are widely available.

Process

For hydrogen production, the syngas generated out of DFB Gasifier needs further processing. First, filter to remove dust particles, thereafter a water gas shift stage to enhance the hydrogen output, an organic scrubber to remove water and tar out of the syngas, a CO₂ removal stage to separate sour gas components, a hydrogen purification unit to produce high quality hydrogen and finally, a tail gas reforming step to enhance the efficiency of the plant.

The main components of the GH₂ plant are shown in the diagram below.

The tar-rich syngas from DFB gasifier is mixed with recycled gas from the steam reformer and sent over a two-staged water gas shift reactor (orange).

A high temperature sour gas shift catalyst, based on iron and chromium, is used for the water gas shift process. The carbon monoxide and steam in the gas are converted to carbon dioxide and hydrogen according to water gas shift equation:

CO + H₂0   <–>  CO₂ + H₂

The CO shift process is operated at atmospheric pressure and temperature up to 450° C at the outlet of the shift reactors.

After the gas has passed the water gas shift stage, it enters a two-stage organic scrubber (yellow), operated with rape seed methyl ester (RME). In this scrubber, tars are removed and water is condensed down to 3% moisture content in the syngas. The scrubber fluid has to be cooled to ensure the necessary separation efficiency.

Then, the gas is compressed and sour gas components (CO₂, H₂S) are removed by an amine scrubber (green) operated at a pressure of 23 bars. The temperature in the CO₂ absorber is set at 40° C.

With a pressure swing adsorption (blue), the hydrogen of the gas is recovered by adsorption of impurities on a molecular sieve and activated carbon. This hydrogen has high purity and can be further used for energy generation.

The tail gas of the PSA, which has a high content of hydrocarbons, is splitted, the main part is sent in to a steam reformer (red) to covert the hydrocarbons in to carbon monoxide, carbon dioxide and hydrogen according to the general steam reforming equation :

CₓHᵧ  +  ₓH₂O     <–>   (ₓ₊ᵧ/₂) H₂ + CO

The reformed gas is sent back to the water gas shift stage to achieve a better efficiency. The other part of the tail gas is sent back to the combustion zone of the gasifier represent the amount of gas necessary for heating the reformer.  

The impurities were caused by nitrogen, which was used for online cleaning of bag filters in the gasifier. These cleaning activities can be performed with the separated carbon dioxide. This would result in low nitrogen content in the producer gas.

SYNTHETIC NATURAL GAS (Bio-SNG)

Beside environmental aspects, high oil prices increase the necessity of the use of renewable energy sources. Biomass is a renewable energy source, which allows a sustainable utilization for several reasons. Its carbon dioxide neutrality and high availability make economic usage of this source possible. Rather for generating heat and power, a more sophisticated way of using biomass is transforming it into a secondary energy source by liquefaction and gasification. The product of the gasification process – is a medium calorific product gas, which is nearly free from nitrogen and has a H₂ : CO ration favorable for synthesis processes. Therefore, the product gas can be converted into a Synthetic Natural Gas (Bio-SNG). The compressed Bio-SNG can be stored in a fuelling station to supply energy to CNG vehicles. It can also be injected to the natural gas grid.

Process

For the production of Bio-SNG from biomass, syngas from DFB gasifier is very effective as it is high in hydrogen (apprx. 40 vol.% (dry)), low in nitrogen (< 2 vol.% (dry)) and contains a rather high amount of methane (apprx. 9 vol.% (dry)).

The product gas generated by the gasifier is cleaned from dust (ash) and tar after sensible heat recovery. This is carried out by a bag house filter and a subsequent scrubber using rape seed methyl ester (RME) as a solvent for the tars.

In the Bio-SNG production, additional cleaning is done. Here, several scrubbing stages are installed to avoid the transport of substances with the product gas that would damage the methanation catalyst.

Methanation reactions: