BioMatNet Item: FAIR-CT95-0270 - The Application of Fluidised Bed Technology to Improve the Combustion of Cork Waste

FAIR-CT95-0270
The Application of Fluidised Bed Technology to Improve the Combustion of Cork Waste

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FAIR Area 1.1 - Biomass and Bioenergy Chain : Process Engineering : Solid Biofuels : Thermochemical Conversion : Wood (Lignocellulose)

	Contract No:	FAIR-CT95-0270
	Date Prepared:	February 1999, July 1998
	Source:	Final Report Abstract and Executive Summary First Annual Progress Report

Final Report Executive Summary

Final Report Abstract

Introduction

This was project was of particular relevance to Portugal and Spain because these two countries account for more than 80% of cork production and processing. There are many products made from cork that are known worldwide and are very much part of daily life. This industry provides jobs for many people both in Portugal and Spain through out the whole chain from planting cork trees, debarking and final processing. Its importance to both Portugal and Spain cannot be underestimated. The cork processing plants need energy, mainly heat, for the process that is mostly supplied as a hot fluid. There is also a steam requirement for the production of agglomerated corkboards. As cork processing produces large amounts of wastes they can be used as fuel to provide the energy for thermal equipment.

The existing systems used for burning cork wastes have several problems, mainly resulting from the fusion of cork ash, which leads to shutting down of boilers quite regularly for maintenance, thus giving rise to production losses. The aim of the proposed study was to investigate the fluidised bed combustion behaviour of cork wastes, so as to overcome the operational difficulties due to ash fusion as well as improving combustion efficiency and finally reducing maintenance costs. The eventual objective was to develop fluidised bed technology for boiler applications for both steam raising and thermal fluid heating in cork processing industries.

In order to meet variations from plant to plant, that depend on the thermal capacity required for industrial application, the work proposed included an investigation of the combustion behaviour of cork waste in two different types of fluidised beds, namely:

Bubbling fluidised bed
Circulating fluidised bed.

Activities

Because the scope of the project included experimental trials using two different modes of fluidised bed operation, it was decided that the bubbling bed work would be carried out by one partner (CIEMAT) whilst the experimental runs involving circulating beds were undertaken by another (RNETI). The cork waste used by both groups was supplied by MJO.

It was found that cork, as received following the debarking of trees, had a composition very similar to any forestry biomass with low chlorine content. However, cork dust produced during processing at MJO contained much higher amounts of chlorine due to the chemical additives used. The analysis of ash also revealed that there were reasonable amounts of K and Na, which raised concern about possible slag formation during combustion, particularly in the presence of chlorine. However, in most cases it was found that the levels of K and Na were not significant enough to give rise to any considerable slagging, particularly if the temperature was maintained below 900 ºC. In the combustion studies undertaken at INETI no tendency for agglomeration in the bed or any accumulation of solid deposits on heat transfer surfaces was observed. These results were also verified by those obtained from CIEMAT.

It was expected that feeding might have been a source of operational difficulties, particularly when granulated cork particles were present, however, this was not found to be the case that was very agreeable surprise. The University of Ulster in collaboration with MJO undertook a detailed analysis of several CHP options to identify the one that could be the most advantageous to MJO in order to provide the energy for process-heating and to generate some of the electricity needed in the factory. The three options were successfully modelled using ECLUSE process simulation programs, and were adapted to suit the data provided by the other partners in the project. All three options were found to provide enough thermal energy from the quantity and mixture of cork wastes available from a typical cork processing plant. The amount of electricity provided depended on the conversion technology chosen.

Achievements

The combustion of cork waste was successfully carried out in both bubbling and circulating beds with the same observation regarding the presence of a yellow flame, which is highly radiative. The combustion was found to occur mostly in the gas phase due to generation of volatiles. The ash composition suggests the presence of some K although the amount of Fe was quite low. There could be a possibility of K combining with Cl at temperatures above 750 ºC, which could be observed during the volatile combustion that, for reasons of kinetics, is favoured at higher temperatures. The combustion tests found no evidence of the existence of KCl in the gases or on heat exchanger tubes following the analysis of the deposits. The presence of Ca in the ash and its possible oxidation to CaO during combustion may help to reduce the level of Cl by producing CaCl₂, however, in this case temperature has to be maintained steady and below 850 ºC to prevent its deposition. This was the case in the bed, however, the temperatures, in the freeboard/riser, were over 900 ºC.

The general conclusion was that the combustion was very satisfactory and no agglomeration occurs which is a very positive observation. The mass balance on K and Na suggest that there was about 8-12% of what entered unaccounted for, which suggested that they were entrained out of the cyclones. The experimental results suggest that the combustion of cork waste could be partly in the bed section by adjusting the gas flow. However, it is clear that the bed section should contain any heat transfer surface. The bed should serve only for combustion in which the bed temperature could be controlled to minimise the NOx formation, to prevent the vaporisation of alkalis and to encourage the fixation of Cl by CaO.

Three options for incorporating a CHP facility into a cork processing plant have been identified and given technical, environmental and economic analyses. All three systems are capable of providing the thermal energy necessary for the cork processing, and could also provide some of the power necessary to run the plant. The Fluidised Bed Combustion option appears to be the most promising in the short term, whereas an option involving cork waste combustion with direct hot fluid heating with the excess cork waste gasified for electricity generation in a small gas engine could be of interest in the future. This would require that the cork waste was pelletised without energetic or economic penalties for use in a small-scale gasifier.

For MJO, the results were very important, enabling them to evaluate the various technical options for application to a cork waste fired-system. The project represented a value contribution to their continuing search for an adequate technical and economic system to use in their cork processing plant.

Future actions

The results obtained from the combustion studies have been evaluated by evaluated by INETI and MJO for a possible application of the fluidised bed technology to burn the waste and to generate heat and power simultaneously. A demonstration project based on the results of this project will be prepared to be presented to the Portuguese government.

First Annual Progress Report

Introduction
The project is investigating the combustion behaviour of cork wastes produced in the cork processing industries that consume the wastes produced for the energy needs of the plants. The existing systems used for burning wastes have several problems, particularly resulting from the fusion of the cork ash. This leads to the regular need to shut down boilers for maintenance resulting in production losses. The possibility of overcoming such problems through the application of fluidised bed combustion (FBC) technology to burn cork wastes, is being investigated. Two types of of fluidised bed techniques are being investigated:

bubbling fluidised bed
circulating fluidised bed.

Progress

During 1997 INETI and CEMAT carried out combustion tests with the corkwaste. It was noted that cork as received had very low chlorine content. Due to the nature of the chemical additives used during processing the corkdust produced at MJO contained fairly high levels of chlorine. The analysis of ash also revealed significant amounts of K and Na which could cause problems of slagging if combined during combustion with chlorine. However, in the combustion studies undertaken so far at INETI no tendency for agglomeration in the bed or any accumulation on heat transfer surfaces has been observed, nor was any slag formation observed at the point where the fuel was introduced to the fluidised bed. It was anticipated that introducing the feed into the bed would be a source of problems, particularly when granulated cork particles were present. However, no problems were found, which was an agreeable surprise. These results have still to be verified by those to be obtained from CEMAT, where most test have been carried out with dust that has been sent from Portugal.

Results

The work carried out during the second year of the project involved Tasks 3 and 4 which will continue in the first pan of the third year. Task 3 involves combustion tests that are carried out at both INETI and CEMAT Those at lNETI use a circulating bed, whilst the tests of CIEMAT use a bubbling bed. INETI has completed the combustion studies with cork dust and has started those with granulated particles The tests at CEMAT have been delayed due to an operational problem and are currently ongoing.

The results from INETI can be summarised as follows. The general observation is that cork dust burns very well and most of the combustion occurs in the gas phase. About 80% of the dust is rapidly converted to volatiles as the particles enter the combustion zone This behaviour was also observed with granulated cork. A flame was observed in the riser of the fluidised bed for both types of cork particles. With cork dust, at times the flame was found to stretch into the cyclone. The length of the flame was controlled by the addition of secondary air to the riser. It was found necessary to impart some swirl to this air in order to encourage mixing beteeen the air and the volatile gases. The temperatures in the freeboard for the combustion of both types of cork particles were higher than that in the bed indicating the extent of combustion in the freeboard.

The tests undertaken to date do not indicate any problems related to the nature of ash. No agglomeration was observed in the bed due to the presence of alkali metals and no significant deposition of particles were found on the heat transfer surfaces. This was true for both types of cork particles. INETI will redo some of the test runs to verify the reproducibility of the results. The samples collected during the tests of granular cork are currently being analysed to determine their composition This will make it possible to calculate an overall mass balance concerning ash. CEMAT will continue an extensive test programme to make up the delay. Task 3 should be completed by the end of May 1998.

The University of Ulster, during 1996 and ahead of schedule, has carried out a simulation test concerning the applicability of CHP for a typical cork processing company utilising the data from MJO in the scope of Task 4. This work was continued during 1997 in collaboration with MJO, which also supplied a copy of the energy audit study that was undertaken at the factory of MJO. This gave the overall energy balances for the process as well the electrical energy requirements of the factory in great detail. The audit study also established the energy profile in terms of variations in the company during a day, week and month. These results are important in helping to evaluate the potential application of a cogeneration system in the factory using cork dust. Various aspects of CHP were considered. However, taking into consideration the needs of the company, conventional steam systems appear to be more viable Both CEMAT and INETI will supply their data from test runs for University of Ulster to check the models already de eloped for both bubbling and circulating systems This will complete the task.

Discussion and Conclusion

The combustion of cork waste has successfully been carried out in both bubbling and circulating beds. In both cases a radiant yellow flame was observed. The feeding of cork waste into the systems could present problems. Therefore care needs to be taken in finding adequate feeders. Combustion occurs mostly in the gas phase due to rapid production of volatiles as the feed is introduced. The ash analysis indicates the presence of some K although the level of Fe is quite low. There could be a possibility of K combining with Cl at the temperatures above 750°C which were observed during combustion of the volatiles. The kinetics that favour this reaction at higher temperatures. However, during the combustion tests carried out during 1997 at INETI no evidence for the existence of KCl in the gases or in materials that deposit on surfaces was found The presence of Ca in the ash and its possible oxidation to CaO during combustion may help to reduce the level of Cl by producing CaCl2. However, in this case temperature has to be maintained fairly high to prevent deposition. This was the case since the temperatures, particularly in the riser were over 900°C. This study of ash continues. The general conclusion is that the combustion is very satisfactory and no agglomeration occurs which is a very positive observation The mass balance on K and Na suggest that between 8 and 12% of what entered the system was unaccounted for, which suggests this material was entrained out of the cyclones.

Future actions

In the next report the results from the remaining combustion tests will be presented. This will enable participants in the project to evaluate potential for cork waste combustion in fluidised beds. The delay in the combustion tests of CEMAT will be compensated for in the first 4 months of 1998 and the results to be obtained will enable a more comprehensive assessment of the combustion behaviour of cork waste particularly in relation to the expected improvements envisaged in the original proposal. The most important engineering aspects related to upgrading that need to be addressed during the remaining tests are:

Feeding of different cork waste materials to the combustor
More understanding of ash behaviour
Determination of the fate of chlorine during combustion
Operation using less excess air in an optimised manner.

Task 4 will also be completed with the input from studies of INETI and CEMAT. Then the overall techno-economic evaluation of the operating modes of fluidised beds at an industrial scale will be initiated.