Development of coal-feed systems for pressurized fluidized bed processes to eliminate the use of lock hoppers

Development of coal-feed systems for pressurized fluidized bed processes to eliminate the use of lock hoppers

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Commission of the European Communities
technical coal research
Development of coal-feed systems
for pressurized fluidized bed processes
to eliminate the use of lock hoppers Commission of the European Communities
technical coal research
Development of coal-feed systems
for pressurized fluidized bed processes
to eliminate the use of lock hoppers
Coal Research Establishment
British Coal Corporation
Stoke Orchard, Cheltenham
Gloucestershire GL52 4RZ
United Kingdom
Contract No 7220-EA/824
Final report
Directorate-General Energy
1990 EUR 12820 EN Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industries and Innovation
L-2920 Luxembourg
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting
on behalf of then is responsible for the use which might be made of
the following information
Cataloguing data can be found at the end ,of this publication
Luxembourg: Office for Official Publications of the European Communities, 1990
ISBN 92-826-1506-5 Catalogue number: CD-NA-12820-EN-C
© ECSC-EEC-EAEC, Brussels · Luxembourg, 1990
Printed in Belgium SYNOPSIS
The feeding of coal to a pressurised fluidised bed combustor or
gasifier requires the fuel to be taken at atmospheric pressure, raised to
the process operating pressure and then injected into the process. To
achieve these steps, two groups of feeding systems have been exploited;
they are lock hoppers feeding dry coal, and pumps feeding coal water
slurries. Both of these systems have an impact on the efficiency of the
process in the use of heat to dry the coal and power to crush, mill and
convey it. Lock hoppers are perceived as having to carry the larger
capital cost and reliability penalty.
A reduction in the costs associated with coal feeding was seen as one
method of encouraging the commercial exploitation of pressurised processes.
In response to this a study of coal feeding methods was proposed to the
European Coal and Steel Community, with the following objective;
The development of coal feed systems for pressurised fluidised bed
processes to eliminate the use of lock hoppers".
The work would concentrate on two areas:
1. The development of a rugged pumping system.
2. The manufacture of a coal water mixture requiring a minimum of
production stages.
The project was divided into four main tasks:
1. Literature and product survey
2. Laboratory studies
3. Pump and slurry testing
A. Cost assessment studies
Ίο enable all data to be referred back to a common, and previously
existing, base line the process into which the coal has to be fed was
assumed to be a combined cycle power generation facility incorporating a
pressurised fluidised bed combustor.
Initial pumping trials showed that coal, after a single stage of
crushing and screening to yield a top size of 6 mm, could be slurried and
pumped at a water content of 24%.
Laboratory studies indicated that if sufficient fines were present a
thin lubricating layer of a fines water mixture could be formed and enable
much dryer mixes to be pumped. They also showed that lower water contents
would be achievable if stainless steel or plastic lined pipe were to be
used.
A further study investigated the use of a previously untapped source
of fines material, coal preparation plant froth flotation fines. This
material is available at no greater cost than the UK standard power
generation blend feedstock; in some instances it may even be at a lower
cost. It was found that by incorporating 30% of this material, on a dry
basis, slurries having a minimum viscosity were produced.
When pumped, these modified slurries were still pumpable at 17%
moisture content when working into a pressure vessel at 14 bar. The pumping system developed demonstrated that, at these low moisture
contents, the bulk of the coal in the pipeline was acting as a solid mass,
sliding on a lubricating layer. If this solid mass was sheared, for
example by a change in pipe diameter, pumping became impossible.
The system design criteria developed can be summarised as follows:
1. The pump, valves and pipeline should form a continuous, smooth
bore straight line.
2. The pipeline should be of stainless steel and of the largest
practical diameter.
3. Compressed air should be used to accelerate and disperse the
feedstock into the fluidised bed.
4. The slurry should contain 30% froth flotation fines and unless
perfect mixing can be assured a minimum of 18% water or 14% free
water.
The capital cost and operating cost assessment studies have confirmed
that a coal water mixture of 20% moisture content or less has a financial
advantage over other feed systems.
- IV -CONTENTS
Page
SYNOPSIS m
1. INTRODUCTION 1
1.1. Objectives of the project 2
2. AREAS OF INVESTIGATION
2.1. Feeding methods 3
2.2. Pump manufacturers survey
2.3. Laboratory work
2.3.1. Coal source 4
2.3.2. Selection of coal for testing
2.3.3. Prediction of the pumpability of slurries 4
2.3.4. BHRA testwork
2.3.5. Investigation of modification of a coarse
crushed coal slurry by the addition of fines 5
2.3.6. Further testwork to determine a coal's
pumpability 5
2.4. Pump testing 6
2.5. Capital cost study
2.6. Cycle efficiency study
2.7. Feasibility study for pumping coal water mixtures
into fluidised bed combustore 7
3. RESULTS
3.1. Literature survey
3.1.1. Lock hopper feed systems
3.1.2. Coal water mixture feed systems 8
3.2. Pump manufacturers survey 10
3.2.1. Types of pump identified as being possibly
suitable H
3.2.2. Pumps in use on thick slurries 14
3.3. Collection of coals5
3.4. Rheology studies6
3.4.1. Annular shear cell
3.4.2. Pipe friction apparatus8
3.4.3. BHRA testwork 21
3.4.4.A analysis of laboratory results 22
3.4.5. BHRA results and discussions
3.4.6.A conclusions
3.4.7. Modified slurries7
3.4.8. Pipe viscometer9
- V 3.5. Pump testing 30
3.5.1. Outline
3.5.2. Slurry preparation1
3.5.3. Test method2
3.5.4. Moyno pump
3.5.5. Moineau pump3
3.5.6. Putzmeister pump
3.5.7. MMD pump
3.5.8. Pumping results - The Moyno progressing
cavity pump 34
3.5.9.g results - The Moineaug
cavity pump6
3.5.10. Pumping results - The Putzmeister EKO pump 37
3.5.11.gs - The EKO piston pumo with a
stainless steel pipe 41
3.5.12. Pumping results - The MMD piston pump 4
3.6. Capital cost assessment 5
3.6.1. Foster wheeler study
3.6.2. Study results
3.7. Efficiency assessment3
3.8. Other considerations4
3.9. ABEL feasibility study6
4. DISCUSSION 5
5. CONCLUSIONS9
6. RECOMMENDATIONS FOR FUTURE WORK 60
7. REFERENCES 6
TABLES I to IV5
FIGURES 1 to 76 71
VI -DEVELOPMENT OF COAL FEED SYSTEMS FOR PRESSURISED FLUIDISED BED PROCESSES
TO ELIMINATE THE USE OF LOCK HOPPERS
1 INTRODUCTION
Demands for higher efficiency and environmentally benign coal fired power
generation have led to the proposal of several advanced power generation
systems, most of which incorporate pressurised gasifiers or combustors.
The first of these systems to be commercially exploited is the
Pressurised Fluidised Bed Combustor (PFBC). Plants using this process are
now being constructed in Sweden, Spain and the United States of America by
ABB Carbon. The PFBC process was first described in 1968 at the First
International Conference, on Fluidised Bed Combustion. Two papers included
1 2
reference to PFBC as part of their subject matter ' . Other processes now
3
being developed include pressurised gasifiers and pressurised circulating
4
fluidised bed combustors . All of these processes require the coal fuel to
be taken from atmospheric pressure, raised to process pressure or slightly
above, and then injected or conveyed into the reactor. In most cases this
has been achieved using lock hoppers, where great reliance is placed on
valves and vessels operating in a regular cycle of filling, pressurising,
emptying and depressurising. Because of the arduous conditions valve life
is limited; also the requirements for inert pressurising gas make the
system expensive to operate.
Several plants have now been successfully operated using a coal water
slurry and a positive displacement pump as the fuel and feed system. In
most cases the slurry has been a "prepared" slurry, that is some special
treatment such as crushing, grinding or blending has been required to
produce a relatively free flowing mix that is relatively easy to pump and
pipe around the process area. To obtain this degree of fluidity the water
content of the mix has tended to be high, generally around 30%. The ASEA
Brown Boveri coal feed system, to be used on the PFBCs mentioned above and
now being constructed in Stockholm in Sweden and Tidd in the United States
of America, uses a slightly thicker mix containing 25% water.
One of the disadvantages of coal water mixtures is the detrimental
effect of the water on the overall efficiency of the cycle. Although the
water vapour produced increases the mass throughput of the gas turbine, the
total energy required to convert the water to steam cannot be recovered.
The latent heat of vaporisation is lost with the off gases. Economic
studies on PFBC cycles have shown that, considering efficiency alone, the break even point is reached when slurries containing 20% water or less are
fired.
1.1 Objectives of the Project
For the continued development of advanced pressurised gasifiers and
combustors, the need to improve the overall efficiency of the system is an
important factor towards encouraging commercial exploitation. One area in
which savings in capital and operational costs are possible and which could
also yield an increase in cycle efficiency is coal feeding.
The feeding of coal into any pressurised reaction system imposes a
penalty on the overall efficiency of the process. This penalty results
from the demand for electrical energy to drive pumps, compressors, crushers
and other mechanical equipment and thermal energy, as process heat, steam,
etc for drying the coal.
Of the several coal feeding methods developed either using lock hopper
or slurry feed systems, most are now probably incapable of much further
development. However, one scheme that does show promise is the positive
displacement piston pump. It is possible that, by careful plant layout and
engineering, a coal with a minimum water content could be pumped.
The objectives of this project were to develop a process to feed coal
to a pressurised system, with a balance of capital and operating costs that
would minimise the overall charge to the process for coal feeding.
2 AREAS OF INVESTIGATION
The project can be divided into four topics, as listed below.
1. Surveys of present coal feeding practices and pump manufacturers
2. Rheology studies on a range of coals
3. lumping trials
4. Economic assessment
From the results of the two surveys it was expected that areas would
be identified where improvements to the feeding process could be made. The
pumping trials were then to be used to produce data that would confirm the
practicality of the proposed improvements. When pumping slurries, most
pumps are limited by the requirement for a carrier fluid in which the
coarser solid particles are held. This carrier fluid would normally be
very fine coal (less than 100 microns) in suspension in water. In this
project the carrier fluid was reduced to an absolute minimum or even
absent. It was found that different coals, coal pretreatments and coal
crushing processes also affected the pumping characteristics. Rheology