The cleaner coal option

With the backdrop of volatile natural gas prices, tightening emissions regulations, and an abundant world coal supply, Integrated Gasification Combined Cycle (IGCC) technology provides power producers with a cleaner, economical solution for coal in power generation.

Robert Rigdon & Kevin Miles, GE Energy, USA

Gasification uses a partial oxidation reaction to convert low-cost hydrocarbons, such as coal, into a high-value fuel called synthesis gas (syngas), which is primarily made up of hydrogen and carbon monoxide. The syngas fuels a gas turbine combined cycle system for power generation. Advantages of IGCC include reduced emissions, greater fuel diversity and increased siting and permitting flexibility.

With its acquisition of a major gasification technology provider in 2004, GE became the first equipment manufacturer with the ability to supply both key components of an IGCC plant – the gasification and power generation technology. Shortly thereafter, GE announced its IGCC alliance with Bechtel – a major engineering, procurement and construction (EPC) firm – and a turnkey, 630 MW IGCC offering. Taking IGCC from a “process” business to a “product” business helps remove many of the historical barriers to mainstream, large-scale commercialization of these power plants.

Pollutant control

GE supports IGCC as one of the major energy technologies to help power producers reduce their emissions, as IGCC plants approach emissions levels of natural gas combined cycle plants. The IGCC process removes a significant portion of the key pollutants – sulphur dioxide, mercury and particulate matter – from the syngas prior to combustion. Lower nitrogen oxide emission levels are reached via gas turbine combustion technology and/or selective catalytic reduction (SCR) devices in the turbine exhaust.

Because gasifying coal creates a syngas instead of burning the coal directly to generate energy, the majority of pollution-causing emissions are captured efficiently with minimal cost during gasification, rather than treating the emissions after combustion as is done in supercritical pulverized coal (SCPC) plants. The syngas stream is cleaned at a higher pressure and, therefore, a much smaller volume than the exhaust gas a SCPC plant would scrub and treat.


Figure 1. The reference plant block design
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Another important advantage of IGCC is its ability to be configured to remove much of the carbon dioxide (CO2) from the syngas prior to combustion in the gas turbine. IGCC plants today can be configured for CO2 capture as high as 90 per cent. IGCC plants can be more efficiently and cost-effectively designed for CO2 capture compared to SCPC plants.

Capturing CO2 in an IGCC plant requires the addition of a water-gas shift reactor, additional heat recovery, CO2 removal, compression and solvent regeneration equipment. According to a study from the DOE and EPRI1, designing an IGCC plant for CO2 capture requires about a 30 per cent higher initial capital investment for a greenfield plant, and results in modest reduction in plant output. However, IGCC is more cost effective at capturing CO2 than an SCPC plant. GE is investigating design options for 90 per cent capture of CO2, as well as quantifying the impact of the addition of CO2 capture on the net output and efficiency of its IGCC reference plant.

Advantages of alliance

Although the case for IGCC technology has been and continues to be compelling from an experience and environmental performance point of view, historically there have been barriers to entry for IGCC technology in the utility industry:

  • Higher capital cost than traditional pulverized coal
  • Inconsistent initial operating availability
  • No single point of responsibility
  • No system of warranties and guarantees
  • Higher cost of electricity over other coal alternatives.

In order to help overcome these barriers, a sound technology solution had to become part of a complete commercial offering including cost, schedule and performance guarantees. This commercial offering would require single point, “turnkey” responsibility from engineering through construction, commissioning and successful commercial operation.

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To accomplish this, the GE and Bechtel IGCC alliance offers a single source solution, with the guarantees, warranties and certainty of execution power producers require. GE and Bechtel have been investing in and co-developing the reference plant since early 2005. Their IGCC alliance operates its Center of Excellence from Houston, Texas, and draws from a depth of technology and other resources worldwide.

IGCC Alliance reference plant projects will be executed in a project consortium arrangement that encompasses all the gasification, power generation and related technologies required for a complete IGCC offering. This integrated and seamless approach focuses the resources of two experienced companies on the factors that can reduce cost and customer risk through:

  • Single source offering, for guarantees and warranties
  • Reduced Front End Engineering Design (FEED) scope and cost
  • System optimization resulting from transparency between gasification and power generation technologies
  • Supply chain optimization
  • Design replication.

The reference plant

The selection of a nominal 630 MW IGCC platform for the IGCC reference plant product is based on three main factors: proposed coal plant capacity additions in the US, the ability to benefit from economies of scale while remaining small enough to manage first cost, and the ability to replicate for larger capacity applications, in 600 MW incremental additions for projects requiring nominal 1200 MW capacity.

The current IGCC alliance reference plant has been designed to gasify approximately 5000 short tons per day of eastern US bituminous coal through two gasifier trains fueling two GE frame 7FB gas turbines. The nominal net power output of the facility is 630 MW with an estimated efficiency range of 38.5 per cent to 40 per cent depending upon feedstock properties. A reference plant configuration for sub-bituminous coals such as those from the Powder River Basin (PRB) is currently under evaluation.

Some important design configurations are the gasifier design, the acid gas removal system for sulphur removal, the adsorption bed for mercury removal, the power block components and the site.

After evaluating a variety of gasifier and gas cooling configurations as part of the reference plant configuration optimization, GE’s gasifier with a radiant syngas cooler plus quench configuration was determined to provide the best value on a cost-of-electricity basis. To make this determination, the team developed an economic model that included dispatch predictions within the electrical grid system supporting the Ohio River Valley region. As would be expected, this model predicts that the strongest economic drivers for an IGCC plant are capital cost, net plant output and availability. Three GE gasifier heat recovery systems were evaluated – quench, radiant plus quench, and radiant plus convective cooler. Results clearly show that for today’s IGCC power plant application, both radiant type configurations offer significant value over the quench-only case. The justification of the radiant plus quench case over the radiant plus convective case is demonstrated via its lower capital cost, equivalent net output and higher availability, all of which contributed to its lowest overall cost of electricity.


Figure 2. GE and Bechtel’s IGCC Alliance reference plant plan
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A physical solvent-based acid gas removal (AGR) process is included for deep sulphur removal from the syngas stream. The deep removal enables the use of SCR catalyst with an acceptable rate of ammonium sulphate deposition and fouling in the HRSGs. This deep sulphur removal target was the leading driver for the final selection of a physical solvent.

As customers in the USA must meet Clean Air Mercury Rule (CAMR) emissions regulations for mercury, a mercury removal adsorption bed is included in the reference plant design. This is an activated carbon bed that will remove from 90-95 per cent of mercury contained in the syngas. This system provides very effective mercury removal with low associated investment and operating cost.

The power island is a multi-shaft combined cycle system consisting of two separate trains of GE’s syngas fueled frame 7FB 60 Hz heavy duty gas turbine generators, and multi-pressure level HRSGs combined with a single GE D11 reheat steam turbine generator. The power island incorporates conventional combined cycle features for high efficiency and reliability, with additional gasification island integration to enable operation on syngas.

GE’s IGCC 7FB gas turbine is specifically configured to combust the cleaned syngas from the gasification island. This includes modifications to its hot gas path to enable high mass throughput associated with combustion of the low-BTU syngas with diluent nitrogen. The GE 7FB gas turbines are designed to emit no more than 15 parts per million by volume (ppmv) of NOx corrected to 15 per cent oxygen, and the HRSGs will have provisions for the addition of an SCR device to offer further NOx removal (5 ppmv), if desired.

The core process and power block areas of the reference plant are intended to remain relatively unchanged from site to site and require about 10 ha. Site-specific units such as the switchyard, water treatment, coal handling, certain buildings and the flare typically add 12 to 16 ha to the overall plot plan. The reference plant design facilitates slide-along addition of units, and includes space for CO2 capture capability.

Fuel characteristics

An important feature of the reference plant design is its ability to handle a broad range of feedstocks containing up to four per cent sulphur and 14 per cent ash (both by weight and dry basis). Higher sulphur (>four per cent) coals can be accommodated as an option by increasing the AGR and SRU capacities, with additional front-end engineering and plant capital cost.

The reference plant performance has been optimized for a range of bituminous coals, such as those from the Illinois Basin. It is estimated that petroleum coke can be blended with the coal feedstock up to 30 per cent by weight with no configuration modification. Up to 70 per cent by weight petroleum coke blend can be used with minimal modification to the design.

The chloride content of coals can affect the material selections in an IGCC plant. As the concentration of chlorides increases, the water system pH can drop, leading ultimately to the need to upgrade from carbon steel materials in certain areas of the plant. The reference plant design allows for a robust 2500 ppmw chlorides in the feed coal, with an option to design for higher chlorides.

A bright future

Today, the prospects for IGCC have never looked brighter. GE is significantly increasing its investment in gasification technology. The attractiveness of IGCC and gasification has also been evident by the variety of professional talent this business has attracted.

Reference

  1. DOE- EPRI Report #1000316 Dec 2000.

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