ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - CHEMICAL EFFECTS: see EFFECTS OF CHEMICALS; AIR POLLUTANT EFFECTS; POLLUTION EFFECTS ON FISH CHEMICAL TREATMENT: see PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS potx

This coal conversion activity involves the commercial integration of process and power systems.. Overcoming both process engineering and environmental problems will be crucial factors in

C

CHEMICAL TREATMENT : see PHYSICAL AND CHEMICAL TREATMENT OF

WASTEWATERS

COAL GASIFICATION PROCESSES

In spite of temporary oil “gluts,” elements of a new

coal-based synthetic fuels industry are slowly emerging in oil

importing nations This coal conversion activity involves

the commercial integration of process and power systems

Overcoming both process engineering and environmental

problems will be crucial factors in the development of coal

liquefaction and gasification plants Depending upon

proj-ect size and complexity, the associated expenditures for the

total compliance effort could require multimillion dollar

budgeting

The concept of gasification of coal is not a new one

John Clayton proved conclusively that gas could be obtained

from coal in the early 1680s His initial experiments were

observations of the products formed upon heating coal In

the presence of air, heat will invariably be generated by

burning a portion of the coal In order to increase the yield of

secondary fuels with higher hydrogen to carbon (H/C) ratio

than that of coal, it is required to gasify the coal in the

pres-ence of steam and an oxygen containing gas The products

formed during high yield gasification are typically hydrogen,

carbon monoxide, and variable amounts of light

hydrocar-bons, especially methane Carbon dioxide may be scrubbed

from the product The coal, steam, air mixtures are contacted

at temperatures above 700°C in fluidized, entrained flow or

moving bed configurations

Liquefaction of coal may be accomplished by reacting

with heavy oil derivative hydrocarbons at temperatures of

400 to 500°C Contaminants are typically hydrogenated

to gases which may be absorbed (sulfur to H 2 S, nitrogen to

ammonia and oxygen to water)

According to Quig and Granger (1983), a coal conver-sion facility impacts the environment through the handling

of large amounts of coal, and discharges from the conver-sion process and associated facilities Also, there will be impacts related to the construction and operation of any large industrial complex The major health concerns for both occupational and offsite populations include potential exposure to particulates, sulfur compounds, trace elements, aromatic amines, and other nitrogenous compounds and radioactive nuclides

Considerations of these issues and concerns for this facility will begin with the coal handling facilities Fugitive dust, consisting mainly of coal fines, is generated by the disturbance of the coal in the unloading, transfer and stor-age facilities Particulates can remain airborne and be trans-ported from the site under certain meteorological conditions and therefore must be evaluated in terms of their potential impacts and control mechanisms Coal pile runoff and coal wetting wastewater contain varying amounts of coal fines and dissolved constituents depending on variables such as rainfall intensity and duration, contact time, coal storage configuration and coal pile sealing techniques Values of over 2000 mg/l total suspended solids and 10,000 mg/l total dissolved solids have been reported by EPA and TVA for runoff from coal piles The magnetic separation of metal-lic materials from the coal during preliminary coal cleaning operations will generate a variable quantity of pyretic solid waste which must be addressed The coal processing facili-ties, that is coal grinding and slurry preparation, include con-trols which minimize the discharges from these operations

Some of the more important coal gasification processes

include those of Texaco, Shell, Dow & British Lurgi These

are carried out at high temperature 600 to 3000°F and high

pressure 25 to 80 atmospheres The most developed process

is Cool Water integrated gasification/combined cycle (IGCC)

described by Holt (1988) and Spencer et al (1986) which

uses a Texaco gasifier Makansi (1987) compares the

per-formance of various systems Important emissions data for

IGCC projects are presented at the end of the current review

Additional information is presented below on the status

of coal gasification environmental effects A comparison of

the impacts on water streams of various processes is given

in Table 1

Pruschek et al (1995) discusses the removal of

pollut-ants from a coal gasification plant in a more efficient and

economical manner than in previous designs by conserving

energy in the cleaning sections of the plant A zinc titanate

catalyst is being tested for hot (1000°F) gas cleanup potential

at Tampa Electric’s 260 MW coal gasification power plant in

Lakeland, Fla

Waste gas emissions are reduced by scrubbing the raw

gases leaving the gasifier in an acid gas removal system

and converting the H 2 S (via a modified Claus process) to

sulfur Sulfur dioxide is thus drastically reduced in the final

stack emissions NOx levels are reduced by saturating the

gas prior to gas turbine combustion (see Spencer 1986) or Makansi (1987) Advances in process efficiency are pos-sible, through the use of a combined cycle configuration the Shell Coal gasification process The product gas would typically be fired in a combustion turbine followed by an HRSG and a steam turbine (i.e., combined cycle) to com-plete the IGCC Heitz (1985) presented data on end uses of

From an economic point of view it is desirable to

con-struct an IGCC in phases, Le et al (1986) In the typical

scenario the first phase would be installation of simple cycle gas turbines for peaking power As of 1989 the maximum single gas turbine output is about 150 MW In the second phase a heat recovery boiler is used to generate steam for either cogeneration or to power a steam turbine (i.e., ordi-nary combined cycle) Zaininger Engineering (Lewis, 1988) indicate that there is an optimum time at which the gasifier plant could be added as fuel cost/availability would dictate Normal combined cycle efficiency can be approximately 50% (LHV) whereas IGCC values range from 37 to 42% However, new hot gas cleanup processes (such as limestone throwaway or metal oxide catalyst) are being developed which may increase IGCC efficiency to about 48%

TABLE 1 Coal gasification wastewater concentrations (mg/l, unless noted otherwise) (Adapted from Epstein, 1987)

Component

KILnGAS (Illinois No

6) Moving Bed

Lurgi Dry Ash (Montana Rosebud) Moving Bed

Lurgi Dry Ash (High-Sulfur Eastern Coal

at Sasol) Moving Bed

Lurgi Dry Ash (Lignite

at Kosovo) Moving Bed

British Gas-Lurgi Slagger (Pittsburgh

No 8) Moving Bed

Grand Forks Slagger (Lignite) Moving Bed

HYGAS (Illinois No 6) Fluidized Bed

Texaco (Illinois

No 6) Entrained Flow Chemical oxygen

demand

(COD)

23,000

Total organic

carbon (TOC)

Cyanides and

thiocynates

Total suspended

solids (TSS)

Total dissolved

solids (TDS)

various gasifier process streams (see Table 2) The analysis

of a typical product gas stream appears in Table 3 and by reducing gasifier energy losses Figure 1 illustrates

Holt’s (1988) review of Cool Water data, are impressively

low for a coal-based plant Sulfur dioxide in the gases

leav-ing the tail gas incinerator is about one fifth the quantity

leaving in the turbine exhaust stream The turbine exhaust

did not contain sulfur compounds other than SO 2

A typical gasification process will generate both solid

and liquid wastes The solid waste retains the natural

impu-rities inherent in the coal such as heavy metals, chlorides

as well as organic compounds formed by the combustion of

coal in the reactor Testing, as defined in rules and

regula-tions under Section 3001 of the 1976 Resource Conservation

and Recovery Act RCRA, must be conducted to determine

the nature of the solid waste generated by the

gasifica-tion of the particular coal with the specified process The

wastewater stream exiting the carbon-ash scrubbers may

contain dissolved and suspended fly ash and unconverted

carbon trapped in the scrubber The treatment of this process

wastewater for discharge will generate sludge which will

have to be controlled and assessed

A gasification process may not have any atmospheric

emissions during normal operating conditions Gases are

processed and recirculated so that the desired discharge from the system is product gas alone During upset conditions, however, venting will occur causing releases of gaseous wastes

Sources of non-gasifier liquid wastes which must be evaluated include blowdown from clarifiers and cooling towers, drainage from equipment and floor areas, demin-eralizer wastes and sanitary wastes Contaminants in these streams are generally a function of the intake water quality and any chemical additives

Coal

Coal Receiving

and Storage

Coal Milling

and Drying

Coal Feed

System Steam to

Utilization

Steam to Utilization

High Temperature Cooling

Solids Removal and Cooling

Acid Gas Removal

Acid Gas to Sulfur Recovery

Water Recycle

Water Treatment To Biotreater

Product Gas

to Power Generation Slag to

Utilization

To Utilization

Oxygen

Gasifier

Quench Gas

FIGURE 1 Shell coal gasifi cation process block fl ow diagram for SCGP-1

TABLE 2 Uses of products, co-products and effluents (from Heitz, 1985)

The stack emission values presented in Table 4, as per

The main problems found with sulfur sorbents involve

mechanical property degradation and/or loss of sulfur

capac-ity over many sulfidation-regeneration cycles The sorbents

receiving the most attention are all zinc based Building on the

Cool Water technology, various zinc titanate formulations and

proprietary materials were developed by the U.S Department

of Energy (DOE)/Morgantown Energy Technology Center

(METC) and used at Tampa Electric—Swisher et al (1995)

A hot gas pilot scale desulfurization is currently operating

at METC It uses a simulated coal gas mixture at volumetric

flow rates of up to 120,000 standard cubic feet per hour and

400 psia and up to 1200°F Fluidized bed technology is ideal

for reactors that continuously circulate reactive and

regen-erated adsorbent For further information and updates the

reader is referred to the following publication and the DOE

websites:

and One of the world’s largest coal gasification plants for

elec-tricity generation, a 253 MWe power plant based on the Shell

IGCC process, was built and started-up in Buggenum, in the Netherlands, in 1994 Since October 2001, the installation has been owned and operated by NUON Power as a fully com-mercial electric generating unit Pulverized coal is gasified at about 400 psi and 2700°F using pure oxygen rather than air Fly ash bearing raw gas exiting the gasifier is passed through cyclones to remove the larger particles The remaining fines are collected in a hot gas ceramic candle filter The filter treats about 1 million cubic feet per hour of syngas at about 500°F and 380 psia The filter contains tube modules with elements made from a structure of silicon carbide supporting a porous Mullite grain membrane with a pore size of about 4 × 10 −4 inch Future plans include co-gasifying waste materials such

as biomass chicken litter, wood products and sewage sludge Further cleanup of the syngas involves removal of acid gases such as COS, H 2 S and chlorides as well as a process to pounds is included in the water treatment process

Sources of non-gasifier solid wastes include sludge from raw water treatment and spent bauxite catalyst from the Claus unit, spent cobalt/molybdenum catalyst from the tail

TABLE 3 Typical treated product gas composition (Adapted from Heitz, 1985 for SCGP-1)

TABLE 4 Cool water stack emissions (adapted from Holt 1988)

HRSG Emissions (Lbs/Million Btu)

Incinerator combustion products Pollutant

U.S EPA NSPS

Cool Water Permits*

Cool Water Test Results**

* Cool Water Permit for Low Sulfur Coal Operation (Low sulfur coal is defined in the permit as coal containing less than 0.7 wt.% sulfur)

** 1987 EPA Performance Test Results for SUFCo Coal, Holt (1988)

recover sulfur (see Fig 1) Also removal of ammonia

http://www.netl.doe.gov/publications/proceedings/96/

96ps/ps_pdf/96ps1_5.pdf, http://www.netl.doe.gov/

publications/proceedings/02/GasCleaning/1.05paper/pdf

gas treatment, silica gel from the air separation plant, and

various resins from the water treatment plant

Sources of non-gasifier atmospheric emissions are

cool-ing tower evaporation and drift, and the gas turbine The

cooling tower releases will cause salt deposition in the

sur-rounding area The nature and extent must be determined

and evaluated using intake water quality data, design

infor-mation and mathematical modeling The on-site gas turbine

will emit low levels of NOx, SO 2 and particulates which will

have to be considered

Other impacts associated with any large scale project are

those related to the construction of the plant There will be

an increase in traffic by the construction work force on the

existing roads There are potential construction work force

impacts related to workers immigration, shortage of

cer-tain skilled labor categories, etc which must be assessed

In addition, there is a potential for noise impacts during the

construction or operation of the facility

The increase in traffic on the roads, rails and rivers near

a proposed site due to coal deliveries will create additional

demands on the system The construction of a facility will

have unavoidable impacts on the land use at the site

Construction and operation may impact on the surface

water bodies and the potable aquifers in the area but due

to the current state of waste water treatment technology,

minimal impact on water quality is expected after control

measures are applied

The authors wish to acknowledge the work of Quig and Granger (1983) in an earlier version of this article, some of which appears here

REFERENCES Epstein, M., Elec Pwr Res Inst Jnl pp 39–42 Advanced Power System, April/May (1987)

Heitz, W.L., Status of the Shell Coal Gasification Process (SCGP) presented

at 5th Annual EPRI Contr Conference, Palo Alto, CA, Oct 30 (1985) Holt, N.A., EPRI Report AP-5931, Oct (1988)

Le, T.T., J.T Smith and M.T Sander, Elec Pwr Res Inst EPRI Report AP

4395 Jan (1986)

Lewis, A., EPRI Report AP-5467, Feb (1988)

Makansi, J., Power, pp 75–80, Apr (1987)

Pruschek, R., G Oeljeklaus and V Brand, “Combined cycle power plant

Conversion and Management, June/September 1995, pp 797–800

Quig, R.H., Chem Eng Prog., 76, 47–54, March (1980)

Quig, R.H and T Granger, Encycl of Envir Sci and Eng Vol 1, 103–113,

(1983).

Spencer, D.F., S.B Alpert and H.H Gilman, Science 232, 609–612,

May (1986)

Swisher, J.H., J Yang, and R.P Gupta, Ind & Eng Chem Research, Vol 34,

4463–4471 (1995)

ROBERT J FARRELL

ExxonMobil

EDWARD N ZIEGLER

Polytechnic University