SYNGAS CLEAN-UP – THE CORNERSTONE OF A SUCCESSFUL BIOMASS GASIFICATION

PROJECT

Most gasification project shortfalls have been associated with project economics, inconsistent and/or insufficient fuel supply, or the Syngas Clean-Up process.

The south-central Biomass Gasification Combined Cycle Power Project will be located in the middle of its own fuel plantation. This will allow the homogeneous fuel-crop to be harvested on an as needed basis. The in-house fuel supply and size of the project allows for economies of scale to improve project economics. Therefore, the remaining hurdle surrounds a successful Syngas Clean-Up process.

We all have seen how large amounts of moisture condense on the outside of a glass of ice tea on a hot summer day. This moisture, that moments ago was invisible in its gaseous state, suddenly appears as droplets on the outside of the cold surface. However, if the glass is warm or if it is a dry winter day, the amount of condensation is greatly reduced. This phenomenon is called condensation. In this case, the moisture in the air is considered a condensable. Condensables are normally considered liquids. However, some condensables go directly from a gaseous state to a solid (dusts) in a phenomenon called sublimation.

Gas Turbine gaseous fuel specifications address the concentration of four types of Syngas contaminants; dusts, acid gas, condensable liquids (tars), and condensable dusts (alkalis). The BIG Syngas Clean-Up process uses a unique sequence of six steps to exceed biomass Syngas Clean-Up to Gas Turbine gaseous fuel specs:

1. bulk dust separation using a high efficiency cyclone inside the gasifier, 2. Syngas cooling raising process steam,3. fine dust removal using a solvent venturi scrubbing, 4. solvent tars scrubbing,5. further gas cooling to reduce the Syngas temperature, and remove the

condensable alkalis and acid gas constituents using a water quench, and6. pressurizing using a screw compressor.

All these process steps are common in several process industries, including biomass, except the fourth step – solvent tars scrubbing. The tars which need to be cleaned from the Syngas are also gaseous at the gasification temperature. Unless these tars are reduced to concentrations below the dew-point temperature, they will condense out as liquids, waxes and gums in gas turbine inlet valve and combustor causing the equipment to shut-down.

Most mothers prior to the age of industrial high-tech laundry stain removers understood that certain chemicals removed certain stains. They would maintain a supply of ammonia, baking soda, chlorine bleach, vinegar, lemon juice, alcohol, lighter fluid, as well as detergents to clean the blemishes caused by active children. Through trial and error mothers learned that organic solvents removed tar.

Biomass tars are a very broad complex of hydrocarbon compounds which include; volatiles or light oils, different density heavier oils, waxes, and gums.

Some common organic solvents in dissolving tars, in order of effectiveness, are rubbing alcohol, lighter fluid, gasoline, and diesel fuel. Significant Syngas concentrations of the heavier oils, waxes and gums cause problems in the Gas-Turbine. Rubbing alcohol, lighter fluid and gasoline would actually remove more tars than necessary from the Syngas lowering its heat value. Therefore, the solvent that we will incorporate into our solvent scrubber is a diesel fuel. Actually, bio-diesel is much more effective in dissolving tars than petroleum based diesel and is considered a renewable fuel when a small amount is recycled back to the gasification process.

Industrial solvent scrubbers have been around for nearly 100 years with sizes larger and smaller than that required for our project. EPA recognizes solvent scrubbers as a reliable method of removing soluble compounds from gases. Large-scale pilot testing on biomass based steam gasification Syngases has shown that the proposed scrubber will remove over 99.5% of the targeted tars.

It should also be noted that the Syngas Compressor is used to raise the pressure of the syngas from atmosphere to nearly 400 psi (pounds per square inch). This compressor is a liquid ring screw compressor and their manufacturers are actually guaranteeing their performance with a full tars loading. Additionally, they are stating that the equipment will actually remove condensable tars in the liquid ring, thus only requiring a filter system as a back-up tars removal step.

The proposed robust BIG Syngas Clean-Up system utilizes commercially available equipment assembled into a unique process which comfortably exceeds the Gas Turbine manufacturer’s gaseous fuel specification.