Syngas Production from Coal/ Biomass Feed

Syngas, derived from synthesis gas, is a gas mixture that contains carbon monoxide, carbon dioxide and hydrogen followed by lower amount of sulphuric compounds, nitrogen oxides and methane. The amount of each gas is varying depend on feed, oxidant agent and and the process as well. Syngas is producing from fossil hydrocarbons - oil and coal - or organic biomass such as wood, plant sticks, municipal waste and agricultural manure. The typical use is as an intermediate to produce other chemicals or to be burnt and used as a energy source. In these combustion reactions, carbon combines with water or oxygen to transform to carbon dioxide. Generally, any material that has hydrocarbons may be converted to a syngas. In fact, that a wide range of resources is a major advantages of syngas. When the biomass being used as a resource, it reduces greenhouse gas emissions over 85% compared to the same amount of fossil fuel being burned.
The energy content of the syngas depends on the feed as well as the gasification agent and fuel used for reaction. The feed must be high in carbon and hydrogen content. When burn is supplied by air, nitrogen in the air dilutes the product. The produced syngas has lower energy content, up to 8MJ/Nm3. Syngas produced by pure oxygen has higher energy content, up to 20MJ/Nm3.

Syngas Production Process

Syngas is being produced by burning hydrocarbons (coal, oil, biomass) in the gasification process at high temperatures with a controlled amount of injected oxygen or steam as a process gasification agent. The syngas obtained during this process has approximate 50% the energy density of natural gas (methane mostly) which depend of process parameters and feedstock. Properly processed syngas is more efficient then original energy source because the thermodynamic upper limit to the efficiency defined by Carnot's rule is higher. Further, it is more applicable due to easer storage and transport. It is quite difficult to use either coal or biomass from home rubbish for combustion to run a vehicle, but syngas as a specific type of energy carrier avoids the offensives. Coal gasification and the proper processing of the collected syngas before the combustion can reduce emissions through new power generation systems such as Integrated Gasification Combined Cycle (IGCC). It removes pollutions from the coal gas before combustion that results in lower emissions of sulphur-oxides, hydrocarbon dust and metals. The main problem for IGCC is its extremely high cost per obtained energy unit, although it being used in several refineries.
Syngas obtained from biomass is an elegant path to avoid greenhouse gas emissions. It has many great application. Some studies observed that syngas production based on biomass feedstock is cheaper by all means, but coal much easier to handle.

• Syngas composition:

Typical chemical reactions of coal, biomass or any other feed combustion:
water gas: C + H2O → CO + H2
water gas shift: CO + H2O → CO2 + H2
total oxidation: C + O2 → CO2
an excess of carbon: 2C + O2 → 2 CO
These are the basic reactions. Higher hydrocarbons such as biomass or plastics being burned more complex, but the products are CO, CO2 and H2 also. Content of each of them depends on feed (coal, oil, gas, biomass), operation temperature, oxidant - gasification agent and construction of gasifier.

There are three basic types of gasifiers which being used for syngas production:

• The fixed-bed, mostly used for high-energy coal

• Fluidized-bed, used for varying types of feed, mostly low-energy coal and biomass

• Entrained flow – mostly used for heavy oil or strength types of coal, generally for hydrocarbons consists significant amount of tar

Each of them are characterised by different processes, but yield same product.

The fixed-bed syngas production process is being performed in gasifier which consists of a fixed bed of fuel that is maintained at a constant depth by the addition of fuel from the top of the gasifier. Fixed bed gasifiers have a stationary reaction zone. In order to maintain a permeable bed, the fuel itself must have mechanically strength. Therefore this process is usual for the coal or solid biomass (example: wood) as a feed. The steam/ oxygen/ air which are used as the oxidant for gasification burning has two methods of delivery for this type of process. When gasification agent-oxidant flows from the bottom in counter-current configuration is referred to as an “up draft” configuration. Opposite, when gasification agent-oxidant flows from the top in co-current configuration is named as a “down draft”. The temperature in the gasifier must be kept below that of ash fusion 1000C in order to keep high 'steam to carbon' ratio. The tar and methane production is significant at typical process temperatures, but product gas must be washed before use. This type of gasifier has the advantages of high thermal efficiency and low temperatures for produced gas.
Fluidised-bed syngas production process is being performed in gasifiers which are recognised by their method of fluidised fuel in upward flow of oxygen/ air and steam using as an gasification agent. The turbulent mixing of feedstock and gasification agent take a maximum heat and mass transfer between the gases and solids and uniformly distributed temperature throughout the reactor. The result of turbulence inside is very low tar production. Process temperatures ought to be maintained between 600 -1000C for direct heating and slightly lower for indirect heating. The great advantage of this type of gasifiers is possibility for sulphur capture using limestone. This type of gasifier is able to accept varied particularised and composed feedstock, but the lower process temperatures require highly reactive fuels. The most suitable fuel biomass or low-grade coals particularly. Unburned feed and tar are being relatively easy separated via recycling cyclones and reload for the next production which improve production sustaining and efficiency. This type of gasifiers are recommended for green syngas production based on biomass feedstock.
Entrained Flow Syngas Production Process is the most efficient carbon conversion process, but require pre-treatment of the feed and gasification agent. Pre-treatment process pulverize the fuel up to 600 μm. Each fuel, coal or biomass can be used and it being gasified within a second at high temperatures. The process temperature is 1600C -1900C which is well above the ash fusion temperature. Therefore, no unburned solid content-caked fuel and tar. However, entered flow gasifiers has highly efficient carbon conversion, although thermal efficiency is lower due to pre-treatment and generated syngas cooling before cleaning. The feed is in-flowed with oxygen (or air) and steam in a co-current flow. The process itself require additional equipment such as pumps and air separation unit increasing costs and energy consumption. Entrained Flow Syngas Production Process is not preferred for biomass gasification, due to forming corrosive slag which can damage ceramic inner walls of the gasifier.