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Urea From Underground Coal Gas.

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Dev. Chem. Eng. Mineral Process., 11(3/4), pp. 189-200, 2003.
Urea From Underground Coal Gas
C.J. Williamson
Department of Chemical and Process Engineering,
University of Canterbury, Private Bag 4800, Christchurch,
New Zealand
(c. williatii.Fo,i~~ccrpe.
catiterhzin*.uc.ti?)
New Zealand has a dwindling suppb of natural gas currenth used as both a
petrochemical feedstock and as a fuel for thermal power stations. However, the
country has large reserves of coal, much of it at depths that make it diflcult to mine
using conventional methods. It is possible to gasi& the coal underground and produce
a gaseous mixture of hydrogen, carbon dioxide, carbon monoxide and water that can
be used as a replacement for natural gas as a feehtock or fuel. This paper describes
a design study on the production of urea from underground coal gas in New Zealand.
Introduction
New Zealand has an agricultural economy and an internal market for urea of between
200,000 and 300,000 tonnedyear for use as a fertiliser and for urea-formaldehyde
resin manufacture. There is one urea production plant in New Zealand converting feed
natural gas from the Maui gas field off the western coast of the North Island of New
Zealand, to ammonia and subsequently, urea. The Maui field will be exhausted in the
first 10 years of the 21st century and although there are other smaller gas fields
around the country none are large enough to supply thermal power stations, domestic
and industrial gas users, and large-scale petrochemical plants. If urea production
within New Zealand is to be sustained in the long term then an alternative feedstock is
needed.
C.J Williamson
New Zealand has reserves of coal estimated at 8.6 billion tonnes in three regions
around the country; the southern and western parts of the South Island, and in the
Waikato region of the North Island. Conventional mining is used on the West Coast
producing high quality coal for export, and in the Waikato area for steel production
and general industrial and domestic markets. However, much of the Waikato coal is at
depths that are uneconomical for conventional mining, typically between 200-500
metres below ground level. Pilot-scale underground coal gasification has shown that it
is feasible to produce sufficient fuel for a 340 MW combined-cycle power plant from
the deep Waikato coal seams. An alternative to producing a fuel for a power station
would be to use the underground coal gas as feed for a urea plant.
In the test bum, the Controlled Retracting Injection Point (CRIP) configuration
was used for production of underground coal gas. This technology is the best choice
for the mainly horizontal coal seams that exist in Waikato, and involves directionally
drilling two wells horizontally along the bottom of the coal seam, also the injection
and production wells which intersect at the base of a third vertical well, the ignition
well. This vertical well is used to ignite the coal and also to develop a gas flow path
between the injection and production wells. As gasification proceeds, in order to
maintain gas quality, the operator retracts the gas injection point and reignites at the
new point.
These pilot scale tests supplied enough information to be able to cost the
production of raw underground coal gas in New Zealand. The following process
description and costings concentrate on the equipment required after the production of
the underground coal gas, allowing a full assessment of the economic feasibility of
urea production from this feedstock. One difference between the experimental work
and this study was that air was used in the pilot-scale work as the coal oxidant, while
a steadoxygen mixture is assumed in this work.
Process Description
The process concept is to take underground coal gas, and treat it to produce streams of
hydrogen and carbon dioxide suitable as feedstocks for an ammonidurea complex
190
Ureafiom Underground Coal Gas
producing 200,000 tonnedyear of urea (plant production capacity 600 tonnedday
operating for 333 days per m u m ) . An air separation plant would also be required to
supply oxygen for coal gasification, and nitrogen for ammonia production.
Figure 1 shows a block diagram of the proposed process and Table 1 contains the
associated mass balance.
Stretford sulphur
Gas compression
UCG Feed and coo’ing
Waste water to
treatment
I
I
Rectisol 1
Sulphur removal
WaterGasShifl
I
Waste water to
treatment
Waste water
urea & ammonia losses
Nitrogen
I
Makeup MeOH
P
1
Ammonia
Production
Production
I
Waste
gases
Rectisol2 and
Gas Purification
Q
Figure I . Block diagramfor the production of urea*om underground coal gas.
191
C. J Williamson
Table 1. Mass balance for the ureafrom underground coal gas process
(basis = tonnes/day).
I
1
Components
--1
2
3
Methane
46
0
46
Ethane
0 14.5
14.54
Propane
4.33 0.32 4.0 1
Nitrogen
0
27
27
Methanol
0
0
0
Carbon Diox
564 0.84
563
Hydrogen Su 5.53 0.05 5.48
Carbon Monc 485.7 0.27 485
Hydrogen
0 46.1
46.12
Water
0
695 695
Total
Components
1888
9
Total
I92
5
6
7
8
1 .8
2.85
0.12
0
164
5.46
3.29
0.15
0
12.74
0
(I
1.16
0 0.63
26.88
0
0
0
0
0
199.2
0 22.8
0.02
(I
0
182.1
0 0.01
t5.97
0 0.04
0
2103
1794
-
12.7
0.53
26.9
0
1131
0
1.94
80.2
0
14
16
--- ---696 1192 185 1007 2103 1817 1292
-
;
Methane
0
Ethane
0
Propane
0
Nitrogen
0
Methanol
9.58
Carbon D i o 1
Hydrogen Su
Carbon Mon
Hydrogen
Water
Ammonia
0
Urea
4
---6.99 19.01
0 0.03
39
10
11
12
13
--39
0
12.7
0
0
0.53
0
26.9
0
9.58
0
689
0
0
0
1.94
19.4 60.9
0
0
0
0
0
0
0
0
0
0
0
442
0
0
0
0
-0
0
0
284
0
0
0
0
0
15
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0 3 44.9
0 0
0
-
0
0
0
0
0
0
0
181
3.5
2.4
0
0
0
0
0
0
0
0
0
0
0
600
--- -60.9 799 442 284 344.9
186 600
--
Ureaporn Underground Coal Gas
I
Underground Coal Gas
The underground coal gas (UCG) is produced by igniting the coal seam and then
injecting a mixture of steam and oxygen in order to gasify the coal. The oxygen
would be supplied fiom a cryogenic air separation plant which also provides highpurity nitrogen for ammonia production. The CRIP technology for UCG production
using oxygedsteam injection was developed by Energy International and
demonstrated at a commercial scale during the Rocky Mountain-I burn at Hanna,
Wyoming. In 1994, Energy International collaborated with the New Zealand
organizations of Glencoal Energy Ltd and the Electricity Corporation of New Zealand
Ltd in a field test of a small UCG reactor on the Huntly coal field in the Waikato
region (Huntly 5-Spot Field Test Burn). Approximately 80 tonnes of coal were
gasified over a period of 13 days and producing UCG of acceptable quality, thus
successfully demonstrating the feasibility of applying their technology in Waikato.
Energy International’s experience with the technology indicated that UCG could be
produced commercially fiom the Waikato coal fields for between NZ$4 and $6.80 per
GJ (Davis and McGimpsey, 1996). Raw UCG contains liquid water, particulates,
steam, and heavy hydrocarbons, as well as the usual synthesis gas components. It can
be processed, either at the wellhead or downstream, to the quality required by the
final user. The extent of wellhead processing will increase the price of UCG. In this
study the urea plant operator is assumed to purchase wet gas fiom the gasifier at the
lower price of $4/GJ because the gas cooling, compression and cleaning are all
included in the capital and manufacturing cost estimates for the gas processing section
of the urea production plant.
The composition of the Waikato underground coal gas is proprietory to the
electricity generating company responsible for the studies. However, Waikato coal is
of the subbituminous variety, very similar to the coal used in a United States
Department of Energy study on methanol production fiom UCG (Pritchard
Corporation, 1982). The composition of UCG used in that study is also used here.
Typical proximate and ultimate analyses for Waikato coal are shown in Table 2.
193
C. J. Williamson
Table 2. Proximate and ultimate analysis of typical Waikato coal.
Proximate Analysis
Moisture, %
Volatile Matter, %
Ash, %
Sulphur, %
Calorific Value, MJkg
Density, kg/m3
15.1
36.8
44.2
0.29
24.7
1280
UltimateAnalysis
%C
%H
75.7
%S
0.3
%N
1.04
Yo 0
5.3
17.6
11 Gas Cleaning and Compression
The UCG feed (stream 1 in Figure 1) is largely a mixture of hydrogen, carbon
monoxide, carbon dioxide and water but it also contains sulphur compounds, mainly
H2S with trace amounts of COS and SO2, as well as ammonia, nitrogen and oxygen.
Any solids present are removed in an electrostatic precipitator then the gas is cooled,
first by generating steam, and subsequently with cooling water condensing the water
vapour present and thus reducing compression costs. The gas is compressed to the
Rectisol gas wash pressure of 2700 kPa and contacted with cold methanol to remove
sulphur compounds. The methanol is regenerated and refrigerated and the waste gases
sent to a Stretford sulphur recovery process. This was chosen because the low
concentration of H2S in the waste gas stream (stream 4 in Figure 1) make it unsuitable
for treatment by the conventional Claus process.
111 Water Gas Shijit and CO;,Removal
The gas is preheated to 4OOOC and passed through a zinc oxide bed to remove any
traces of HIS remaining in the stream. Steam is added and the composition is adjusted
using a water gas shift reactor, reducing the concentration of CO and producing
hydrogen and carbon dioxide. The shifted gas is then contacted with cold methanol in
194
Ureafrorn Underground Coal Gas
a second Rectisol process operating at 2500 kPa to remove the C02. The treated gas,
mainly hydrogen, is sent to a pressure swing absorption unit to produce a high-purity
hydrogen feed for ammonia production (stream 11 in Figure 1). The mainly C02
stream, flashed from the methanol liquid, is purified and used as feed to the urea
production section of the process (stream 12 in Figure 1).
IV Ammonia and Urea Production
The purified hydrogen stream is mixed with nitrogen from the air separation plant in
stoichiometric proportions and compressed to the ammonia synthesis pressure of
14,000 kPa. The ammonia synthesis reactor contains a bed of conventional magnetite
and subsequent beds of ruthenium-based catalyst, effectively producing ammonia
even at this comparatively low pressure. Effluent from the reactor is cooled, and the
ammonia condensed and separated as a product stream from the unreacted gases that
are recycled to the reactor. Ammonia reacts with purified carbon dioxide to produce
urea in a process based on Stamicarbon’s Urea 2000 process.
V Process Energy and Utility Requirements
The process requires significant amounts of power for gas compression in the gas
treatment section and also to supply nitrogen, hydrogen and carbon dioxide to the
ammonidurea complex. The choice was made to use steam turbines to drive these
compressors as there are significant amounts of waste energy available at
temperatures sufficient to generate high pressure steam. For example, the waste gas
stream from the second Rectisol wash and gas purification section (stream 10 on
Figure 1) of the plant contains significant amounts of methane, ethane and hydrogen.
This could be used as fbel gas for steam generation and superheating, and in a fved
heater used to preheat gas to the ZnO beds prior to the shift reactor. The ammonia
production reaction is exothermic and the plant is capable of generating sufficient
high pressure steam to supply steam turbine drives and process steam users around the
plant. The urea plant is almost balanced in steam production and use, requiring only a
small amount of imported steam from the ammonia plant.
The Rectisol gas treatment process operates at low temperatures and requires
refrigeration both of the incoming gas and of the methanol wash liquid. The most
195
C.J. Williamson
significant electricity use is in the cryogenic oxygen plant for air compression to the
required high pressures for liquefaction. Flows of steam and cooling water, and the
refrigeration and process heating requirements are shown in Table 3. The energy
requirements for the UCG processing section of the plant (gas compression and
cooling, Rectisol 1, water gas shift and Rectisol 2 and gas purification) were found
using the HYSYS process simulation software package, while those for the
ammonidurea plant came from a design project report (Roberts et al., 1998).
Table 3. Energy and utility usefor the process.
Plant Area
Steam ir
Gas Compression
Tonnes
ihr
30 bar,
300°C
4 I .6
Fuel gas
produced
Tonneshr
and cooling
Rectisol 1
Water Gas Shift
87.6
(30 bar,
saturated
ymq-r
33.3
gas purification
Air Separation
Economic Feasibility
The capital and manufacturing costs for the described process were calculated
allowing an assessment of the project profitability.
i Capita[ Costs
Capital costs for equipment in the UCG processing section of the plant were estimated
using the equipment bare module approach (Turton et al., 1998). Prices were
corrected to take into account variations in construction materials, operating pressures
196
Ureaporn Underground Coal Gas
and temperatures, and adjustment for inflation to 1998 New Zealand dollars. The
purchased and bare module costs for groups of equipment items in this section of the
plant are shown in Table 4. Individual equipment items were not designed for the air
separation plant but instead the cost of a representative plant was used (Kirk-Other,
1996), and adjusted for the different production rate using the six-tenths relationship.
A similar approach was used for costing the Stretford sulphur-removal plant. but in
this case the representative cost was taken from the United States Department of
Energy report on producing methanol fkom UCG. The cost estimates for the UCG
processing section of the plant were made by the author, and equipment costs for the
ammonia, urea and gas purification sections came
om a design report by Chemical
and Process Engineering Department students fkom the University of Canterbury
(Roberts et al., 1998). The breakdown of capital costs into the different plant areas is
shown in Table 5 .
Table 4. Purchased and bare module costsfor the gas compression, Rectisol 1 and 2,
and water gas shift sections of the plant.
Purchased Cost
Bare Module Cost
NZ$ (000’s)
NZ$ (000’s)
9,133
23,351
Heat Exchangers
1363
5170
Columns
3 70
1749
Separators
562
4496
Pumps
266
960
Reactors (ZnO and Shift)
199
1299
Fired Heater
546
1261
12,439
38,286
Equipment Type
Compressors with
Steam Turbine Drive
Total
197
C.J. Williamson
ii Manufacturing Costs
Manufacturing costs for the process were calculated using the approach recommended
by Turton et al. and are shown in Table 6. Despite recovery of waste heat and steam
generation using waste gases, the process still requires imported steam at a rate of
32.5 tonneskr. Typical steam cost in New Zealand is $18/tonne. Electricity was
priced at $O.l/kWhr, cooling water at $0.05/tonne, and refiigeration at $20/GJ if
supplied at -15OC or $60/GJ if at -60°C.
Cost in NZ$ (000's)
Plant Section
Electrostatic Precipitator
700
Gas Compression, Rectisol 1 & 2 and
38,286
Water Gas Shift
Methanol Charge
97
Stretford Sulphur Plant
3800
Gas Purification
4000
Ammonia
I
Urea
Air Separation
33,164
I
12,489
14,000
Total Bare Module
106,536
Contingency & Fee
19176
(1 8% of total bare module)
Total Module Cost
125712
Allowance for greenfields development
31,961
(30% of total bare module)
Total Capital Cost
I98
157,673
Ureafrom Underground Coal Gas
iii Product Value
A price of NZ$573/tonne was used for urea and $40/tonne for oxygen in calculating
returns from sales. The urea price in New Zealand has ranged from $325 to $575 per
tonne over the past five years. The urea price is therefore the best that can be
expected. No by-product credit is taken for either sulphur (because the flow is small)
or waste carbon dioxide (because the New Zealand market is limited). Oxygen from
the air separation unit can be sold to the coal gasifier and so a credit is included. The
gross sales revenues are:
Urea
NZ$ 114,600,000
Oxygen
NZ$
Total Product Value
NZ$ 115,750,000
1,150,000
Table 6. Process manufacturing costs.
Manufacturing Cosf Ifem
Raw Materials (UCG, Methanol)
-
Utilities Steam, refrigeration,
NZ$ (000 's)
20,3 10
20,4 15
Electricity etc.
Operating Labour
900
Additional Direct Manufacturing Costs
14,172
Fixed Manufacturing Costs
27,126
General Manufacturing Costs
17,655
Total Manufacturing Costs
100,481
iv Proptabilify Analysis
Using a depreciation rate of 10% per year, a tax rate of 50% (typical for New Zealand
conditions), and a project life of fifteen years, then the project has a discounted cash
flow rate of return (DCFROR) of 5.3%. As most companies require rates of return on
investment of greater than 10% this is a marginal project. However, the project
199
C.J. Williamson
economics are sensitive to variations in urea price, for example, if urea increases to
$626 per tonne the project would have a DCFROR of 10%. Prices may escalate when
readily available natural gas supplies diminish, thus making the project more
attractive. Variations in design from this base case would also improve the
economics. For example, the Rectisol gas wash was originally chosen because
methanol is unaffected by impurities in coal gas. However it requires expensive
refi-igeration so an alternative, such as the Selexol process, may be cheaper overall.
Gasifying coal using steam and air instead of oxygen is possible, and would produce a
gas mixture with nitrogen already present as a component. There would be no need
for an air separation plant in this case, therefore reducing capital and electricity costs.
Conclusions
The production of urea fiom coal gasified in situ is technically feasible, but currently
only marginally economic under New Zealand conditions. However, changes in process
design and increases in urea price may make the project a more attractive investment.
Acknowledgements
I would like to acknowledge the work of final year process design students, Bruce
Roberts, James Dinniss, Matthew Cameron and Steve Clark, whose work provided the
capital and operating costs for the a m o n i d u r e a production sections of the process.
References
1. Davis, B.E., and McGimpsey, N. 1996. Underground Coal Gasification: A fbture sustainable energy supply
for Asia, Proceedings of the I l f h Conference of the Elechic Power Supply Indusfry (CEPSI), Kuala
Lumpur, Malaysia.
2. Kirk, R.E., Othmer, D.F., Kroschwitz, J.I., and Howe-Grant, M. 1991-98. Kirk-Othmer Encyclopedia of
Chemical Technology, 4' edition, Volume 17, John Wiley, New York, USA.
3. Pritchard Corporation. 1982. Evaluation of the use of UCG gas to produce 4000 BPD and 12000 BPD of
methanol with conversion to M-gasoline, United States Department of Energy Report, DOE/ET/14372.
4. Roberts, B., Dinniss, J., Cameron, M., and Clark, S. 1998. The Production of Urea from Underground Coal
Gas,ENCH 463 Design Project Report, University of Canterbury, Christchurch, New Zealand.
5 . Turton, R., Bailie, R.C., Whiting, W., and Shaewitz, J. 1998. Analysis, Synthesis and Design of
Chemical Processes, Prentice Hall Inc., New Jersey, USA.
200
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