Gasification

History of Gasification

The process of converting energy by gasifying organic material has been around for more than 180 years. During much of that time, coal and peat were the primary fuels used to power gasification plants. Initially in the US, gasification technology was used to produce gas from coal or coke for municipal lighting and cooking. By 1850, the major cities of the world had "gaslight." About 1880, the internal combustion engine was invented and “Producer Gas” was used to make electricity. Eventually, natural gas pipelines displaced the municipal plants. Gasification became popular again during the world wars, especially World War II when gasoline became scarce. Wood gas generators helped to power about a million vehicles world-wide in 1945. In the late 1990s, the Department of Energy contracted CPC and three other companies to develop a new generation of small modular biopower systems. CPC was the first to deploy a system under the program and is the world leader in small modular biopower systems.

Current Gasification Technologies

There are five basic gasifier technologies currently in commercial use:

1. “Down Draft” Fixed Bed. In this technology, the oxidizing agent (steam, oxygen and/or air) flows through the feedstock in the same direction as the feedstock is moving through the gasifier. The fuel gas exits the gasifier at a high temperature, and since all tars pass through a very hot bed of char as the gas exits, tar levels are fairly low. A highly advanced proprietary down-draft gasifier is the “heart” of all BioMax® Systems.
2. “Up Draft” Fixed Bed. The up draft gasifier is essentially the same design as the down draft, except the oxidizing agent flows in the opposite direction of the feedstock. Throughput for this type of gasifier is relatively low. Gas exit temperatures generally result in the gas needing extensive cleaning because the process produces large quantities of tar.
3. Fluidized Bed Reactor. Granulated feedstock is used in this process and it becomes fluidized with the introduction of the oxygen or air. Fuel throughput is higher than the fixed bed, however, the tar levels can be higher and conversion efficiencies can be rather low.
4. Entrained Flow. This technology uses atomized, pulverized, dry feedstock which is usually processed with pure oxygen and not air. The process requires high temperature and pressure. It is used primarily for high volume processing.
5. Plasma. High voltage, high current electricity is passed through electrodes creating an electrical arc. An inert gas passes through the feedstock and the arc breaking the feedstock into a fuel gas. This gasification method operates at very high temperatures and can process any kind of waste. This technology is often used in chemical production.

Gasification Chemistry

The organic feedstock goes through several different thermochemical processes:

1. Dehydration or drying. The feedstock, if needed, is dried before the gasification process and the moisture extracted is used in later chemical reactions.
2. Pyrolysis. Next, organic materials are thermo-chemically decomposed at elevated temperatures in the absence of oxygen releasing volatiles and producing char. This prepares the chemically changed feedstock for combustion.
3. Combustion. A carefully controlled burn using small amounts of air allows the volatiles and the char to react with the oxygen to form primarily carbon dioxide, water and trace amounts of carbon monoxide. The heat created in the process is used in the gasification process.
4. Gasification. In this step, the char reacts with the carbon dioxide and the steam produced in previous steps to form carbon monoxide and hydrogen.
5. Equilibrium. A chemical reaction known as the “water gas shift reaction” helps to balance the carbon monoxide, steam, carbon dioxide and hydrogen in the gasifier establishing a chemical equilibrium during the final step of the process.

The gasification process is NOT incineration. Incineration is simply burning. The output is mainly carbon dioxide and heat plus toxic materials that may have been present in the materials. Gasification targets non-toxic carbonaceous feedstocks and the output is clean fuel gas that can be used to make electricity, heat and liquid fuels.