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Fixed-Bed Gasifier

The EERC developed mobile and stationary industrial biomass gasification systems to provide a test bed for different biomass feeds. Numerous biomass feedstocks, including wood and grasses, have been tested.

The fixed-bed gasifiers were designed on the premise of converting forest and agricultural residues into a valuable resource for distributed heat and power. This application is especially valuable to remote or off-grid-type locations or areas where industrial-scale heat and power needs coexist with cheap biomass resources. Testing of other biomass feedstocks such as switchgrass, corn stover, straws, and animal manures has also been conducted. The systems are designed to match typical power requirements of various manufacturing industries, generating 10 kW of power, made to simulate a system of 1 MW.

Operating Conditions

  • Nominal feed rate: 30 lb/hr
  • Syngas production: 25 scfm
  • System pressure: atmospheric or slight vacuum
  • Gasifier nominal temperature: 1000°F (540°C)
  • Warm-gas cleanup capability: full stream

Two systems are housed in EERC pilot facilities: the mobile biomass gasifier and the pilot-scale advanced fixed-bed gasifier.

Demonstration-Scale Mobile Gasifier

The 150-kW mobile biomass gasifier is a fixed-bed downdraft gasifier typically used for biomass gasification relative to heat and power production. The unit operates at low pressure and consists of a feed system, gasifier, and gas filtration system. The gasifier is used to demonstrate material balance, energy balance, emissions, and gas utilization.

Pilot-Scale Advanced Fixed-Bad Gasifier

A pilot-scale advanced fixed-bed gasifier is used to evaluate the operating parameters necessary to achieve self-sustained gasification from different feedstocks (including high-moisture feedstock) and operating conditions.

Operating Conditions

  • Nominal feed rate: 33 lb/hr
  • Syngas production: 25 scfm
  • System pressure: atmospheric or slight vacuum
  • Gasifier nominal temperature: 1000°F (540°C)
  • Warm-gas cleanup capability: full stream

Process

Fixed Bed Gasifier
High-moisture forestry residue or complex carbonaceous waste is fed to the gasifier to produce heat, electricity, hydrogen, or liquid fuels. The syngas that is generated from these feeds must be clean and contain the appropriate gas composition for the desired application. The pilot-scale gasifier is used to conduct tests to understand the effect of feedstock composition on gasifier operation, syngas composition, and contaminant concentration.

The pilot plant gasifier is designed to achieve a long and uniform high-temperature reaction zone in the fixed bed. This results in high carbon conversion while simultaneously achieving high H2/CO and CO/CO2 ratios. Optimizing these ratios is critical for liquid fuel synthesis because properly optimized syngas composition results in maximum liquid yields and a reduction or elimination of unconverted char. Unlike most commercial fixed-bed gasifiers, the advanced gasifier is capable of utilizing green or high-moisture biomass. An earlier feasibility study determined that high-temperature heat injection into the gasifier, when operated in a thermally integrated gasification mode, may achieve favorable syngas composition, provided that an adequate heat-transfer rate in the reacting bed is achieved. The new gasifier design is capable of overcoming the heat-transfer difficulties observed in earlier research.

The main components of the gasifier system include the fuel feed system, the fixed-bed gasifier reactor, the residue extraction system, the syngas scrubber, the induced draft (ID) fan, the syngas exhaust system, and the operating parameter monitor and control system. The system is classified for Class 1, Division 2, Group B operation. Gasification air is injected from the top of the gasifier. The gasifier is equipped with a solid fuel igniter. An exothermic heat profile is established in the fuel bed soon after ignition, and steady-state gasification can be achieved within 30 minutes of ignition, depending on the fuel moisture content and reactivity. Syngas leaves the gasifier reactor from the bottom. The syngas is scrubbed in a two-stage scrubber prior to being exhausted through a flare. The ID fan located downstream of the syngas scrubber maintains a pressure drop across all upstream components, including the gasifier and the syngas-scrubbing system. The positive pressure maintained downstream of the ID fan helps exhaust the syngas through the flare. The syngas flow rate is measured with the help of an orifice flowmeter located downstream of the blower. A similar arrangement is provided to determine the gasification airflow rate upstream of the gasifier. The flare system consists of a hot-surface igniter. The flare combustion air is induced by the ejector effect caused by the flow of syngas. The flare contains a gas-sampling port to measure exhaust emissions. The clean syngas composition is determined using an online gas analyzer capable of measuring CO, CO2, O2, H2, and CH4. A separate sampling arrangement is utilized to determine the amount of tar and particulate matter in raw or hot as well as clean syngas.