Air Blown Gasification Cycle Research Articles

The formation of ammonia in air-blown gasification: does char-derived NO act as a precursor? ☆

Air-blown gasification of coal has received considerable attention worldwide and especially in the UK in conjunction with the Air-Blown Gasification Cycle, an integrated combined cycle which has been developed in recent decades. The cycle has the potential to produce electricity at higher efficiencies and lower emissions than conventional pf plants. However, the formation of ammonia in the gasifier has been identified as a potential problem, as it would contribute to NO x emissions. The formation of ammonia has been studied in a laboratory-scale rig designed and constructed specifically for that purpose. The reactor was a fluidised bed of the spouted bed variety. NH 3, NO and HCN levels could be monitored at different bed heights to trace their formation and conversion. Four coals were gasified at 1000 °C and 0.2 MPa. It was found that the bulk of the NH 3 was formed in the spout and the NH 3 concentration increased progressively with reactor height. Formation of NO was relatively minor and, although both NO and HCN may act as precursors for NH 3 formation in the devolatilisation/partial combustion zone, no evidence was found to suggest they make a significant contribution to any subsequent increase in NH 3 emissions.

The effect of coal particle size on pyrolysis and steam gasification

For future power generation from coal, one preferred option in the UK is the air-blown gasification cycle (ABGC). In this system coal particles sized up to 3 mm, perhaps up to 6 mm in a commercial plant, are pyrolysed and then gasified in air/steam in a spouted bed reactor. As this range of coal particle sizes is large it is of interest to investigate the importance of particle size for those two processes. In particular the relation between the coal and the char particle size distribution was investigated to assess the error involved in assuming the coal size distribution at the on-set of gasification. Different coal size fractions underwent different changes on pyrolysis. Smaller coal particles were more likely to produce char particles larger than themselves, larger coal particles had a greater tendency to fragment. However, for the sizes investigated in this study ranging from 0.5 to 2.8 mm, the pyrolysis and gasification behaviour was found not to vary significantly with particle size. The coal size fractions showed similar char yields, irrespective of the different char size distributions resulting from pyrolysis. Testing the reactivity of the chars in air and CO 2 did not reveal significant differences between size fractions of the char, nor did partial gasification in steam in the spouted bed reactor. From the work undertaken, it can be concluded that pyrolysis and gasification within the range of particle sizes investigated are relatively insensitive to particle size.

Investigation of Ammonia Formation during Gasification in an Air-Blown Spouted Bed: Reactor Design and Initial Tests

A laboratory scale, pressurized gasifier has been developed to simulate the operating conditions of air-blown spouted bed gasifiers. The new 28 mm i.d. reactor operates with a continuous coal feed of typically 3 g min-1, at temperatures up to 980 °C and pressures up to 20 bara. Fluidizing gases can be combinations of air, steam, and nitrogen. The gases and coal enter the gasifier through the inverted conical base of the reactor, as a high velocity jet, which induces the rapid mixing of the fresh coal with the bed char. At the pilot scale this mode of operation had been found to avoid the operating difficulties associated with the agglomeration of sticky coal particles. Gases within the reactor may be sampled from the spout jet (using a specially designed water-cooled probe) and from the exit of the reactor. In the present study, the reactor has been used to study NH3 formation under conditions relevant to those of the pilot-scale gasifier that was used during the process development stage of the Air-Blown Gasification Cycle. The pilot-scale work had found high, variable, and at times difficult-to-predict concentrations of NH3 in the fuel gas. The present stage of the work was aimed to help understand the pilot-scale data by focusing on the effect of operating conditions on NH3 formation reactions. The gasifier has been shown to run with continuous feed for periods of up to 30 min, before sinter builds up on the base and prevents normal operation. Within this time data are collected on the spout and exit gas compositions. In this paper we report preliminary findings, which suggest that the NH3 is produced during pyrolysis and by the effect of steam on the char-N. The latter is the most important reaction and the mechanism is at present unclear. High concentrations of NH3 appear to form through the latter mechanism and to vary with operating conditions, temperature, steam content, presence of sorbent, and coal:air ratio.

The ALSTOM benchmark challenge on gasifier control

Integrated gasification combined cycle power plants are being developed around the world to provide environmentally clean and efficient power generation from coal. As part of the UK's Clean Coal Power Generation Group, ALSTOM (formerly GEC ALSTHOM) has undertaken a detailed feasibility study on the development of a small-scale prototype integrated plant (PIP), based on the air-blown gasification cycle. In pursuit of this goal the ALSTOM Power Technology Centre (formerly the GEC ALSTHOM Mechanical Engineering Centre) has produced a comprehensive dynamic model and control philosophy for the PIP. The gasifier is one component of the model which, being a highly coupled multi-variable system with five inputs (coal, limestone, air, steam and char extraction) and four outputs (pressure, temperature, bed mass and gas quality), has been found to be particularly difficult to control. For this reason the gasifier, together with its associated control specification, operating constraints and various disturbance characteristics, has been selected as the subject for this control challenge. This paper provides a brief background to the problem and describes the control specification and closed-loop tests to be performed.

Factors governing reactivity in low temperature coal gasification. Part 1. An attempt to correlate results from a suite of coals with experiments on maceral concentrates

This paper reports on the first stage of a study attempting to develop laboratory scale tests for the reliable determination and eventual prediction, of the effect of coal properties on the performance of coals in air blown gasifiers. The pyrolysis and gasification behaviour of a suite of coals have been matched, using a high-pressure wire-mesh reactor (WMR), with those of maceral enriched samples. Reaction conditions were selected to simulate those of the pilot-scale gasifier stage of the Air Blown Gasification Cycle operated by British Coal. Subsequent stages of the study have examined the role of mineral matter and char morphology. The suite of coals previously tested in the pilot scale reactor were originally selected to represent coals of a wide geographical spread, a limited range of properties and were commercially available in the EU. The maceral enriched samples were obtained from different coals, but were of similar rank to those examined in this study. Short hold time experiments (10 s) have been carried out in the laboratory scale reactor. Three distinct types of behaviour have been identified—arising from competing influences of the reactivity of the base char, the impact of secondary char deposition and the effect of melting on the reactivity of the ageing particles. The relative combustion reactivity of the residual chars was measured in an atmospheric pressure TGA test. The reactivities were all low, but differences were apparent between the different samples and were consistent with the observed gasification behaviour. The use of maceral analysis to predict the behaviour of the whole coals has been examined. Extrapolation from maceral behaviour was found to give reasonable estimates of the behaviour under pyrolysis conditions, but predictions of gasification behaviour were not reliable.

99/03998 Dynamic modeling and simulation of the air blown gasification cycle prototype integrated plant: Pike, A. W. et al. IEE Conf. Publ., 1998, 457, (Simulation '98), 354–361

99/03998 Dynamic modeling and simulation of the air blown gasification cycle prototype integrated plant: Pike, A. W. et al. IEE Conf. Publ., 1998, 457, (Simulation '98), 354–361

99/03127 Overview of clean coal technologies and current status of the air blown gasification cycle

99/03127 Overview of clean coal technologies and current status of the air blown gasification cycle

98/02039 The air blown gasification cycle

The Air Blown Gasification Cycle (ABGC) is a hybrid partial gasification cycle based on a novel, air blown pressurized fluidized bed gasifier (PFBG) with a circulating fluidized bed combustor (CFBC) to burn the residual char from the PFBG. The ABGC has been developed primarily as a clean coal generation system and embodies a sulfur capture mechanism based on the addition of limestone, or other sorbent, to the PFBG where it is sulfided in the reducing atmosphere, followed by oxidation to a stable sulfate residue in the CFBC. In order to achieve commercialization, certain key technological issues needed to be addressed and an industry-led consortium was established to develop the components of the system through the prototype plant to commercial exploitation. The consortium, known as the Clean Coal Power Generation Group (CCPGG), is undertaking a program of activity aimed at achieving a design specification for a 75 MWe prototype integrated plant by March, 1996. Component development consists of both the establishment of new components, such as the PFBG and the hot gas clean up system, and specific development of already established components, such as the CFBC, raw gas cooler, heat recovery steam generator (HRSG) and gas turbine. This paper discusses themore » component development activities and indicates the expected performance and economics of both the prototype and commercial plants. In addition, the strategy for component development and achievement of the specification for a 75 MWe prototype integrated plant is described.« less

97/04597 Fuel behaviour studies in the air-blown gasification cycle: Paterson, N. Fuel, 1997, 76, (13), 1319–1325

Coal chars of four coal types were gasified with carbon dioxide using a PDTF or TGA at high temperature and pressure. Test conditions of temperature and partial pressure of the gasifying agent were determined to simulate the conditions in air-blown or oxygen-blown entrained flow coal gasifiers. Coal chars were produced by rapid pyrolysis of pulverized bituminous coals using a DTF with a nitrogen gas flow at 1670 K. In gasification tests with the PDTF, gasification temperatures were 1670 K or below and partial pressures of carbon dioxide were 0.7 MPa or below. Carbon monoxide of 0.6 MPa or below was supplied for the gasification tests with the TGA.As a result, coal types showed a large difference in the char gasification rate with carbon dioxide, and this difference remained large without decreasing even in the high-temperature area when the gasification rate was controlled by pore diffusion the same as in entrained flow gasifiers. Inhibition of the gasification reaction by carbon monoxide was also observed. Reaction rate equations of both the nth order and Langmuir–Hinshelwood type were applied to the char gasification reaction with the random pore model and the effectiveness factor, and the applicability of these rate equations to air-blown and oxygen-blown entrained flow gasifiers evaluated. Gasification rate equations and kinetic parameters applicable to a pore diffusion zone at high temperature were obtained for each coal.

97/04867 An evaluation of the United Kingdom clean coal power generation group's air-blown gasification cycle: Wheeldon, J. M. et al. Proc. Annu. Int. Pittsburgh Coal Conf., 1996, 13, (1), 279–287

The Electric Power Research Institute (EPRI) is conducting an engineering and economic study of various pressurized fluidized-bed combustor (PFBC) designs. Studies have been completed on bubbling and circulating PFBC technologies and on an advanced PFBC power plant technology, in which the feed coal is partially gasified and the residual char burned in a PFBC. The United Kingdom Clean Coal Power Generation Group`s (CCPGG) air-blown gasification cycle (ABGC), known formerly as the British Coal Topping Cycle, also partially gasifies the feed coal, but uses a circulating atmospheric fluidized-bed combustor (AFBC) to burn the residual char. Although not a PFBC plant, the study was completed to effect a comparison with the advanced PFBC cycle.

Fuel behaviour studies in the air-blown gasification cycle

The air-blown gasification cycle (ABGC) is being developed in the UK and this paper gives an overview of the different parts of the programme. Pilot-scale investigations of the fuel flexibility of the gasification process have been conducted. A selection of world coals have been gasified at elevated pressure. The performance and operability of the gasifier with these coals is discussed. The successful development of the components of the ABGC is nearing completion, although there are some outstanding areas that need further investigation.

97/01858 Development of hot gas filtration for air blown gasification plant

This paper describes some of the development work carried out on hot gas filtration for the Air Blown Gasification Cycle (ABGC). The ABGC comprises partial gasification of coal at elevated pressure with combustion of the fuel gas produced in a gas turbine. The residual carbon from gasification is burned in an atmospheric pressure circulating fluidized bed combustor raising steam to drive a steam turbine. A critical requirement in the ABGC is to ensure that the fuel gas is free of dust, in order to avoid damage to the gas turbine. Ceramic filter elements are the preferred technology for this clean-up. The required operating temperature is 400--600 C, based on optimizing efficiency and to allow use of other hot gas clean-up systems, for instance for sulfur polishing. A development program on hot gas filtration has been carried out at CTDD in order to ensure that this component of the cycle can be used with minimum risk. To date, over 2,000 h of operation at up to 600 C has been achieved on two pilot scale hot gas filters, each taking full flow of gas from air blown gasifiers. The filters have operated with high availability and there have been no incidentsmore » of breakage of filter elements. Information has been generated for effect of filtration velocity and temperature, cleaning gas requirements, changing dust and gas composition, and for design of critical components such as fast opening valves, venturi ejectors and sealing mechanisms. The effect of different operating conditions on filter element strength has been evaluated for a range of filter elements.« less

97/01360 Development issues for the char combustor component of an integrated partial gasification combined cycle system

The Air Blown Gasification Cycle (ABGC), formerly known as the Topping Cycle, is an integrated partial gasification combined cycle system which offers potential advantages over other advanced coal-fired processes under development. Within the United Kingdom a development program is underway to develop the components of the ABGC leading to the establishment of demonstration and commercial units. The ABGC comprises an air blown partial gasifier which produces a fuel gas and a char residue. The gas after cooling and cleaning is fired in a gas turbine, while the char is burned in a CFBC to raise steam for the steam cycle. There are several char combustor component development issues which are under investigation at British Coal. The objective of this work is to produce combustion performance and emissions data for what is an unusual fuel having a very low volatile matter content, a bimodal size distribution and containing CaS which needs to be fully converted to CaSO{sub 4} before disposal. The R and D program has involved test work on the 0.12MWth CFBC test facility at the University of British Colombia (UBC) and the construction and extended operation of a 1.8MWth CFBC test facility at British Coal. In support of themore » CFBC test runs laboratory studies are being undertaken on the CaS oxidation reaction and a cold model of the British Coal CFBC test facility has been constructed to assist in scale up of the test facility data. Preliminary test work carried out at UBC is described. It showed that the British Coal gasifier residues can be successfully burned in a CFBC environment with the minimum generation of environmentally contentious species, giving a combustion efficiency of 99%, and a sulphur retention efficiency of 96% with a calcium to sulphur ratio of 2:1. Indications for the special requirements for the design of the CFBC for use in an ABGC plant are discussed.« less

97/01253 An evaluation of high pressure coal dust explosions

In the United Kingdom an industry-led consortium has been set up to continue the development of a coal-based advanced power generation system. The program primarily addresses the development of the key components for the Air Blown Gasification Cycle (ABGC), previously known as the British Coal Topping Cycle. One of the main features of the ABGC process is the use of an air blown pressurized fluidized bed gasifier which has the advantage over alternative oxygen blown systems, of not requiring air separation equipment. However, as a consequence the ABGC process does not have an available source of nitrogen for purging and pressurizing. Coal in the ABGC process is fed to the gasifier through lock hoppers pressurized up to 25 bar. The storage of coal in air at elevated pressures is associated with an increased propensity for spontaneous heating and dust explosion. This paper describes the experimental work commissioned by the Coal Technology Development Division of British Coal (and undertaken by TNO Prins Maurits Laboratory, Netherlands) to determine the explosive characteristics of a lignite, an anthracite, and a bituminous coal from UK sources over a range of elevated pressures up to 20 bar. Data on the maximum oxygen content, maximum explosion pressuremore » and dust explosion constant are presented. This information will be used to consider the feasibility of alternatives to expensive nitrogen inerting. This will include partial inertization and high pressure dust explosion suppression systems.« less

97/01249 Coal gasification and its relevance to electricity generation

The air-blown gasification cycle (ABGC) is being developed in the UK and this paper gives an overview of the different parts of the programme. Pilot-scale investigations of the fuel flexibility of the gasification process have been conducted. A selection of world coals have been gasified at elevated pressure. The performance and operability of the gasifier with these coals is discussed. The successful development of the components of the ABGC is nearing completion, although there are some outstanding areas that need further investigation.

Grimethorpe filter element performance—the final analysis

A demonstration hot gas filter, funded by the Electric Power Research Institute and installed in a pressurised fluidised bed combustor at Grimethorpe, South Yorkshire, UK was operated for a second period in 1991–92 for the Grimethorpe Topping Cycle Project (GTCP). This project was aimed at assessing the feasibility of running a gas turbine on coal-derived gas as part of the development programme for the Air Blown Gasification Cycle (formerly known as the British Coal Topping Cycle).As in previous studies operation of the filter led to progressive changes to the mechanical properties of the tubular clay-bonded silicon carbide filter elements. In order to explain the mechanisms behind these changes and thus determine operating conditions which influence element properties and performance detailed element characterisation and microstructural studies have been carried out. It has been found that slow bond devitrification and the development of a bond-grit interlayer of cristobalite led to changes in the integrity of the interface, with loss of strength and elastic modulus and a decrease in ultrasonic pulse velocity through the material. Microstructural studies suggest that the combination of the upper temperature of operation (≈ 830°C) and the thermal cycling during reverse pulse cleaning to remove the dust cake results in microcracking. While the strength losses experienced did not result in in-plant element failures, the reliability of the elements would be improved if operation can be limited to a temperature at which devitrification of the element's bond material is minimised.