Jon Gibbins

Residual carbon from pulverized coal fired boilers: 1. Size distribution and combustion reactivity

Abstract The amount of residual, or unburned, carbon in fly ash is an important concern in the design and operation of pulverized coal fired boilers. Char oxidation is the slowest step in the coal combustion process, and the rate at which this heterogeneous reaction proceeds has an important effect on the degree of carbon burnout. There is an extensive literature on char combustion kinetics based on data in the early and intermediate stages of carbon conversion. A critical fundamental question is whether the small fraction of the fuel carbon that passes unreacted through a boiler is representative of the char during the main portion of the combustion process. This article addresses that question through a detailed characterization of eight carbon-containing fly ash samples acquired from commercial scale combustion systems. The fly ash characterization included measurement of joint carbon-size distribution and determination of the combustion reactivity of the residual carbon. To minimize mineral matter interactions in the reactivity tests, the technique of incipient fluidization was developed for separation of carbon-rich extracts (∼75 wt% carbon) from the inorganic portion of the fly ash. Reactivity measurements were made at 1400–1800 K to represent conditions in pulverized coal fired boilers. Measurements were also made at 700–1100 K to minimize transport effects and to isolate the influence of char chemistry and microstructure. In both temperature regimes, the residual carbon extracts were significantly less reactive than chars extracted from a laboratory scale laminar flow reactor in the early to intermediate stages of combustion. It is concluded that the boiler environment deactivates chars, making high carbon burnout more difficult to achieve than is predicted by existing char combustion kinetic models that were developed from data on laboratory chars. Finally, the results are used to discuss potential char deactivation mechanisms, both thermal and oxidative, in coal fired boilers.

Development of TG measurements of intrinsic char combustion reactivity for industrial and research purposes

Measurements of the intrinsic reactivity to oxygen of chars are increasingly being sought as an indicator of the combustion potential of fuels, particularly with respect to residual char burn-out. A TG-based method is described which is suitable for both routine industrial use and for research studies. Ramp heating, typically at 15 K/min, offers the significant advantages of allowing the entire sample to be assessed, requiring only a short time (approximately 1 h) to complete and being able to use the same conditions to test almost any sample. Reduced oxygen concentrations (6.3% O2 v/v) minimise uncontrolled exothermic heating of the sample; this is also comparable to oxygen levels encountered in the later stages of pulverised fuel (PF) combustion. Data processing methods, made possible by developments in computer hardware and software, have been developed to give fundamental, quantitative measurements of char reactivity, allowing direct comparison of the overall reactivities of different chars. Information on the heterogeneity of chars can also be derived. Typical results for a range of coal chars and comparison with other TG-based char characterisation methods are presented. Reactivity values measured on two TG instruments with different geometries, both of which operated using the methods developed, have been compared and found to be in agreement.

Variable‐heating‐rate wire‐mesh pyrolysis apparatus

An electrically heated wire‐mesh apparatus for pyrolysis studies has been developed which uses computer‐driven feedback control for the heating system and thus can apply virtually any time‐temperature history to the sample. Internal components are water cooled to prevent heat buildup during long runs. Using this system, coal pyrolysis has been studied at heating rates from 0.1 to about 5000 K/s and temperatures up to 1000 °C. Alternating current is used for heating; this allows the thermocouples to be attached directly to the sample holder and also makes power regulation relatively simple. For atmospheric‐pressure experiments, a gas sweep can be forced through the sample holder to remove products from the heated zone and also to concentrate them in a trap which can be removed from the apparatus and weighed to establish tar yields directly. Although the design is optimized for atmospheric‐pressure operation, relatively simple modifications allow operation under vacuum or at pressures of up to 160 bars in i...

Flexible Operation of Coal Fired Power Plants with Postcombustion Capture of Carbon Dioxide

Carbon capture and storage is one family of technologies that could be used to significantly reduce global carbon dioxide (C O2 ) emissions. This paper reviews the likely flexibility of power plants with postcombustion capture, with a focus on an improved characterization of the dynamic performance of power plants with C O2 capture. The literature has focused on design and optimization for steady state operation of power plants with capture, often at a single design point. When dynamic behavior is considered, it is possible that designs should be altered for best overall plant performance. Economic trade-offs between improving transport and storage scheme flexibility and constraining power plant operations should also be carefully analyzed, particularly if the captured C O2 is to be used in another process such as enhanced oil recovery. Another important aspect of real plant operation will be adhering to legislative requirements. Further work is required to identify mechanisms that allow flexible operatio...

Oxy-combustion processes for CO2 capture from advanced supercritical PF and NGCC power plant

Publisher Summary This chapter represents a reasonable baseline of oxy-combustion; the process evaluation was based on proven technology, where possible, with oxygen production based on cryogenic separation technology and the recycle system utilizing either flue gas or CO2. Process flow diagrams (PFDs) were developed and heat and mass balances (HMBs) compiled to model the overall oxy-combustion process. The PFDs/HMBs analysis produced data allows specifications for the major plant items to be developed; this in turn allows budget capital costs for the major components to be assessed and operating philosophies to be investigated. Aspects of the oxy-combustion process which impact on the safety and operability of the power plant were also highlighted in this chapter. Finally an economic analysis was performed similar to that undertaken on integrated gasification combined cycle (IGCC) power generation to arrive at the power cost per kWh. The basic cost elements being capital cost, operating cost, and agreed economic parameters. A sensitivity analysis was undertaken to compare the influence of discount rate and fuel costs on power costs.

The effect of variations in time-temperature history on product distribution from coal pyrolysis

An electrically-heated wire-mesh pyrolysis reactor is described. The apparatus is capable of heating rates ranging from virtually zero to 5000 K s−1 as well as multi-step heating with variable heating rates and holding times at intermediate temperatures and at the peak temperature. An alternating current power supply is used, allowing thermocouples to be attached directly to the wire mesh to give close tracking of temperature changes. The heating circuit is driven by a micro-computer for real-time logging and control. A continuous stream of carrier gas is forced through the sample holder, giving mean volatiles residence times of around 2ms in the heated zone. The gas sweep gives sample cooling rates of 500–1000 K s−1; slower cooling rates may be selected by pre-programming the heating circuit. Pyrolysis yields of tar and total volatiles from a low rank British bituminous coal (Linby) obtained with single-step heating, show clear changes in the product distribution with increasing heating rate. The effect of holding times, relatively unimportant during slow heating (1 K s−1) runs, is quite pronounced for fast heating rates (l000 K s−1) at peak temperatures below 650–700 °C. Because of this, unless pyrolysis reactions are allowed to run to completion, the effect of the heating rate on product distributions may be masked.

Scope for reductions in the cost of CO2 capture using flue gas scrubbing with amine solvents

Abstract A previous IEA GHG study of CO2 capture from a pulverized coal (PC) power plant using an amine solvent for flue gas scrubbing has been used as the basis for calculations of the reductions in electricity and CO2 avoidance costs that might be achievable through improved thermodynamic integration between the CO2 capture plant and the steam cycle, the use of a solvent with lower regeneration energy requirements and solvent storage to allow rapid load changes and electricity ‘storage’. It was predicted that the efficiency penalty for CO2 capture for PC plant with flue gas scrubbing using an amine solvent can be reduced to 20 per cent (from 28 per cent) with improved thermodynamic integration and lower-energy solvent, giving a 10 per-cent reduction in electricity costs, from

Comparison of coal pyrolysis product distributions from three captive sample techniques

63.5/MW h to

Structural ordering in high temperature coal chars and the effect on reactivity

57.4/MW h, and a 25 per cent reduction in the cost of CO2 avoid from

Retrofitting CO2 capture ready fossil plants with post-combustion capture. Part 1: Requirements for supercritical pulverized coal plants using solvent-based flue gas scrubbing:

45/t to