Low-temperature Flue Gas Research Articles

Development of a Laboratory-Scale Pilot for Studying Corrosion on MSWI Heat Exchangers

The efficiency of Waste-to-Energy (W-t-E) boilers is affected by fireside corrosion of the heat exchangers that involve unexpected shutdown of facilities for repairs and limit the increase of steam conditions used to produce electricity. The parameters governing fireside corrosion are various and mechanisms are very complex, nevertheless, they are relatively well documented in the literature. In this paper, a laboratory-scale corrosion pilot, which reproduces MSWI boilers conditions, is described. The specificity of our approach includes simultaneous simulation of the temperature gradient at flue-gas/tube interface, the velocity of flue-gas and ashes. Corrosion rates obtained on Tu37C carbon steel at a metal temperature equal to 400°C and a flue gas temperatures of 650°C and 850°C (1100 ppm HCl, 110 ppm SO2 and synthetic ashes free of heavy metals) are respectively around 1.6 2m/hour and 5.6 2m/hour. Preferential metal loss, attributed to erosion-corrosion phenomena, is also observed at low flue-gas temperature (T=650°C) on the face exposed at 90° to the flue-gas. The analysis of corrosion scales demonstrates the reproducibility of results and the reliability of corrosion mechanisms determined from experiments, with degradation observed similar to superheater tubes from EfW facilities. Thus, the corrosion pilot developed can be used as an accurate simulator of the environment encountered in MSWI.

Demonstration of a Condensing Economizer

ABSTRACT Many industries and commercial operations depend on steam to provide thermal energy. Most small- to medium-sized installations are configured on low-pressure packaged boilers. These facilities cannot always use the same energy conservation measures (ECMs) as larger plants, and the application of an economizer is often overlooked. Economizers are frequently used to heat feedwater in large, high-pressure boiler plants and are an effective means to recover waste heat from flue gas. Considerable energy can be recovered, although it is normal to limit this practice to sensible cooling only. This prevents the formation of condensate and reduces the potential for any degradation of the boiler stack or economizer itself, though a significant amount of latent heat is lost. Economizers are not often used with small packaged boilers because the flue gas temperatures tend to be lower and the potential for heat recovery is limited. If designed appropriately, condensing economizers can alleviate these shortcomings and allow both sensible and latent heat to be recovered from the effluent gases. This article describes the assessment, installation and performance monitoring of a condensing economizer that was installed at a dairy in upstate New York with the assistance of the New York State Energy Research and Development Authority (NYSERDA). The low flue gas temperature (~350°F) from the boiler made the production of hot water for the plant's clean-in-place (CIP) systems more attractive than heating feedwater. Data obtained from the site showed that enough energy was recovered to offset 86 percent of the normal heating requirement and fuel consumption was reduced by more than 4,000 dekatherms. As a result, the flue gas temperature was reduced to an average of 140°F. The system required a minimum of maintenance and parasitic electricity to operate. Simple payback should be achieved in approximately four years.

Improvement of the cascading closed loop cycle system

Aspen Plus was used to simulate and get more information about the cascading closed loop cycle (CCLC) system [1–3]. Following evaluation of the variable temperature heat source (e.g. gas turbine flue gas) utilized by the CCLC, both qualitative and quantitive comparisons between the system and simple steam Rankine cycle, were made. The results indicate that CCLC has the advantage in recuperating exergy from flue gas, but it cannot sufficiently convert the recuperated exergy to useful work. To improve the utilization of low temperature flue gas heat, the properties and parameters of the working substance must match conditions of the low temperature heat source. A better cycle scheme and pressure distribution was proposed to raise the efficiency of the CCLC. In addition, the multifunction system concept was introduced to improve the performance of CCLC with solar energy.

Effect of NaOH Addition on the Reactivities of Iron Blast Furnace Slag/Hydrated Lime Sorbents for Low-Temperature Flue Gas Desulfurization

NaOH was added to iron blast furnace slag (BFS)/hydrated lime (HL) slurries to prepare the sorbents for semidry and dry flue gas desulfurization (FGD) processes. For sorbents prepared without NaOH, both the Ca utilization and the SO2 capture increased with increasing slurrying temperature and varied with the BFS/HL weight ratio. With NaOH addition, the sorbents contained much less high surface area products of pozzolanic reaction. Their reactivities were insensitive to the slurrying temperature and greatly enhanced when the NaOH/(BFS + HL) ratio was about 10/100. NaOH is deliquescent, and its presence increases the amount of water collected by a sorbent, which is required for the sulfation of a sorbent at low temperatures. The use of NaOH addition at or below the optimal ratio can raise the sorbent reactivity and reduce the cost of sorbent for a semidry or dry FGD process.

Characteristics and reactivities of Ca(OH) 2/silica fume sorbents for low-temperature flue gas desulfurization

Ca(OH) 2/silica fume sorbents were prepared with various Ca(OH) 2/silica fume weight ratios and slurrying times at 65°C and a water/solid ratio of 10/1. Dry sorbents prepared were characterized, and their reactivities toward SO 2 were measured in a differential fixed-bed reactor at the conditions similar to those in the bag filters in the dry and semidry flue gas desulfurization (FGD) processes. The reaction between Ca(OH) 2 and silica fume in the slurry was very fast. The formation of calcium silicate hydrates, which were mainly C–S–H(I), resulted in sorbent particles with a highly porous structure that seemed compressible under high pressures. The sorbents were mesoporous, and their specific surface areas and pore volumes were much larger than those of Ca(OH) 2 alone. The utilization of Ca of sorbent increased with increasing silica fume content mainly due to the increase in the specific surface area of sorbent. The sorbent with 70 wt% Ca(OH) 2 had the maximum 1 h SO 2 capture. Sorbents with Ca(OH) 2 contents less than 100 wt% and greater than 21 wt% would have a SO 2 capture greater than that of Ca(OH) 2 alone. Both the 1 h utilization of Ca and SO 2 capture per unit specific surface area of sorbent decreased in general with increasing specific surface area. At the same Ca(OH) 2 content, the 1 h utilization of Ca or SO 2 capture of the Ca(OH) 2/silica fume sorbent was greater than that of the Ca(OH) 2/fly ash sorbent; however, the amount of SO 2 captured per unit surface area of the former sorbent was smaller than that of the latter sorbent. The results of this study are useful to the preparation of silica-enhanced sorbents for use in the dry and semidry FGD processes.

The prediction of mercury retention in ash from pulverised combustion of coal and sewage sludge

Coal-fired utility boilers and waste incinerators are the principal anthropogenic sources of atmospheric mercury. In order to provide a greater understanding of the behaviour of mercury in and downstream of a combustor, a novel mathematical model has been developed to predict mercury vaporisation and partitioning. The model has been validated in a 0.5 MW furnace, fired with pulverised coal and a coal/dried sewage sludge blend. Flue gas fly ash was sampled to ascertain the removal of mercury through fly ash adsorption and condensation at low flue gas temperatures (ca. 333 K). Mercury vaporisation, speciation, adsorbence and ash enrichment have been successfully predicted.

An advanced fluidized-bed swirl incinerator for dioxin control during municipal waste disposal

This report discusses the principles and performance of an advanced fluidized-bed incinerator installed with an independently controlled secondary swirl combustion chamber. The system we have developed employs a secondary combustion swirl chamber to incinerate unburned combustibles from the initial burning. The combustibles are carried into the secondary chamber by bed material. Discussion includes promotion of the combustion of char as a typical difficult-to-burn matter, and the successful control of carbon monoxide and dioxin emissions. Also we were studied the behavior of dioxins and their precursors as well as their homologue distribution along the gas flow. Good prospects of achieving an equivalent dioxin toxicity concentration of 0.5 TEQ ng/m 3 N or less at the induced draft fan (IDF) outlet were realized through optimizing combustion conditions and maintaining a low flue gas temperature in the bag filter.

Dry, low temperature flue gas desulfurization by alumina-CaO regenerable sorbents

Abstract On the basis of laboratory experimental data on SO2 removal by sol-gel (SG) sorbents in a fixed bed within the temperature range 120–240 °C, both the reaction rate and the attainable conversion were found to depend strongly on three factors: quality of SG sorbent, water vapour concentration in the gas phase, and temperature. The ratio CaSO 3 CaSO 4 in spent SG sorbent is strongly related to its reactivity and the conditions of sorption. Regeneration of sulfated sorbents in a N2 + H2 gas mixture proved the feasibility of the process in the temperature range 750–850 °C with >90% conversion to CaO.

Flue Gas NOX Reduction Using Ammonia Radical Injection

A novel technique has been developed for flue gas NOX reduction through the injection of plasma-treated ammonia and its decomposition products. Numerical investigation of the chemical kinetics shows nearly complete NO removal when ammonia radicals are injected into the flue gas. The feasibility of this new technique was experimentally explored on a small-scale laboratory combustor at Carnegie Mellon University and concurrently on a larger-scale combustor at the DOE Pittsburgh Energy Technology Center. Preliminary experimental results have shown that ammonia plasma-injection is more effective than simple ammonia-injection at low flue gas temperatures. NOX reduction of 85 to 90 percent was achieved at a low plasma power input. This technique is expected to provide additional opportunities for inexpensive and effective NOX reduction in stationary sources.

A new lining for Drax main chimney

The unique chimney design for Drax power station resulted in problems with conventional linings. Low exit flue gas temperatures made it necessary for the chimney flues to be protected from deposited acid attack. A cheap plastic material was initially used to protect the flues, but it failed during precommissioning tests. An alternative protection had to be found in the short period before the chimney became operational. N.E. Region Scientific Services Department together with a manufacturer have developed a new fluoroelastomer plastic lining. The new lining has been installed in two of the flues in the Drax chimney and its performance is being monitored. Life expectancy predictions have been made, based on theoretical treatments of experimental data.