Global issues such as the depletion of fossil fuels, long-term supply security of fossil fuels, global warming, and greenhouse gas emissions raise concerns on what holds for the future. These issues trigger as to what is needed today in going forward for our future generations. The new directions of using energy-efficient technologies and the use of renewable energy in the industry are a necessity. One common fact is that our sun's heat and light is free and it's an abundant source of energy. Silicon, mainly in the form of quartz and the second most abundant element on our planet, is used to produce polycrystalline silicon (polysilicon) for photovoltaic (PV) applications. Photovoltaic technology converts sunlight into electric energy and it does not involve greenhouse gas emissions. Solar power generation through photovoltaic technology will be a great potential energy source over the next century.
There are various silicon-based technologies for applications developed in recent years termed as energy source of the future. These technologies are and will remain to compete with other forms of renewable energy such as wind, geothermal, etc. silicon-based technologies include purified silicon (mono-crystalline or multi-crystalline) and non-crystalline such as amorphous silicon. Today's large-scale multi-crystalline or polysilicon production plants require several forms of heating and cooling requirements, as well as a high demand for reliable power supply. They will also require ultra-pure hydrogen.
This paper presents the concept of integrating combined heat and power (CHP) and hydrogen production technologies into the chemical production process for solar-grade polysilicon. Benefits of such facility integration will be highlighted in view of operational savings, flexibility, reduction of greenhouse gas emissions and increased power supply reliability for the mass production of purified polysilicon used in solar cell manufacturing.
Chemical process production of polysilicon
There are a number of chemical route polysilicon production technologies available in the industry. To name a few, Union Carbide Process, Siemens Process, and new technologies such as fluidized bed reactor technology (FBR) and free space reactor technology (FSR). Approximately 90% of today's polysilicon production is based on the Siemens process and 75% projected for 2012. Despite the dominant position of the well-proven Siemens technology in the industry, this technology has the following disadvantages [1]:
1. The process is energy intensive (high power consumption);
2. Polysilicon plants are capital intensive;
3. The process is labor intensive due to the batch nature of the chemical vapor deposition (CVD) reactor process;
4. Two main power supplies are needed into plant;
5. Preheating of the seed rods is normally required in the CVD reactor; and
6. Power failure (especially when starting the process) causes run abortion.
Process integration
For the manufacture of polysilicon, electrical power, steam, and hydrogen are among the many requirements, along with the main raw material metallurgical-grade silicon. Typical values of power, heat, and hydrogen requirements representative of polysilicon production from trichlorosilane via the Siemens process are presented in the table below. A combined heat and power (CHP) system as a source of power for a polysilicon production facility is very attractive because it delivers a variety of energy, environmental and economic benefits. CHP is very much useful as main power source while keeping the connection of electrical grid as back-up supply. A recent report of the International Energy Agency [2] cited the following benefits of installing CHP:
-- Lower fuel cost through improved efficiency of providing heat and electricity;
-- Improved power reliability through removing or reducing load on the electricity grid;
-- Lower emissions of nitrous oxides, sulfur dioxide, mercury, particulate matter, and carbon dioxide compared with conventional power plants; and
-- Reduced demand for fossil fuel resources for a given energy output.
Some general criteria of installing CHP systems [3] are as follows:
-- A ratio of electricity to fuel costs of at least 2.5:1;
-- Relatively high requirement of heating and/or cooling, i.e., annual demand of at least 5000 hours;
-- Ability option to connect to the grid at reasonable price with the availability of back-up and top-up power at reasonable price; and
-- Space availability at the site for equipment and heat transport.
 |
| Typical power, steam (heat) and hydrogen consumption values for polysilicon. (Values based from Fluor experience) |
Natural gas conversion paths for CHP and hydrogen production
In a gas-fueled combined heat and power system, natural gas is fed
to the gas turbine generator and its flue gas energy is recovered at
the heat recovery steam generator (HRSG) to produce steam. The
mechanical energy is used to spin a generator producing electricity.
The HRSG is an essential part of the CHP installation as it recovers
the heat exhausted by the prime mover and generator to produce
steam. Steam produced is sent to the steam turbine generator (STG)
to produce an additional amount of electricity. A proposed scheme of
gas-fueled CHP system with back-pressure type STG for a polysilicon
facility is shown in Figure 1.
 |
| Figure 1. Gas-fueled CHP scheme. |
The proposed CHP system as presented above would fit into a larger
scheme to fulfill other needs for the polysilicon plant.
Figure 2 shows the proposed natural gas conversion paths
that also includes the manufacturing of hydrogen.
In this concept, hydrogen is produced via steam methane reforming of
natural gas. Furthermore, there are opportunities to integrate and
generate the hot oil from within the cogeneration system. Hot oil is
used as heating medium in the trichlorosilane production process.
 |
| Figure 2. Natural gas conversion paths for CHP and hydrogen production. * Feedstock/utility essential to chemical route polysilicon process |
Clean coal technology conversion paths for CHP and hydrogen
production
Coal can be used for combined heat and power systems and hydrogen
generation. However, clean coal technology -- i.e., integrated
gasification combined cycle (IGCC) as shown in Figure 3
-- requires significant capital investment as an island power
generating facility. The high investment cost for IGCC can only be
justified for a large-scale polysilicon plants. Hence, small
capacity polysilicon plants will need to be part of an industrial
complex which includes an IGCC power generator, taking the products
such as electricity, hydrogen and steam with long-term contracts.
Biomass conversion paths for CHP and hydrogen production
Biomass as a raw material for CHP and hydrogen generation is a
developing technology with some established aspects. A typical size
of these biomass power plants is ten times smaller, from 1 to 100MW,
than conventional coal-fired plants
[4]. The two biomass conversion technologies for power and heat
are direct-fired and gasification systems. Application of biomass
integrated gasification combined cycle (B-IGCC) will further reduce
the feedstock fuel required for the power production as generating
efficiency will increase, thus resulting in greater generating
capacity.
 |
| Figure 3. Clean coal technology conversion paths. * Feedstock/utility essential to chemical route polysilicon process. |
As shown in Figure 4, there are various process
routes of hydrogen production from biomass namely: thermochemical
gasification of solid biomass coupled with water gas shift,
pyrolysis of solid biomass followed by reforming of carbohydrate
fractions of bio-oil, and microbial conversion of wet biomass under
anaerobic condition. The solid biomass gasification coupled with
water gas shift is the most widely practiced process route for
biomass to hydrogen. This thermochemical gasification of solid
biomass consists of unit operations such as feed handling, drying,
gasification and tar reforming, gas clean up and conditioning, shift
conversion and hydrogen purification
[5]. Pressure swing adsorption (PSA) is utilized for hydrogen
purification.
 |
| Figure 4. Biomass conversion paths for CHP and hydrogen. * Feedstock/utility essential to chemical route polysilicon process. (Source: IEA 2007) |
The cost of power and heat generation from biomass depends on
feedstock quality and cost, CHP technology, availability and
transportation cost, location, and power plant size. If biomass is
abundant in a certain project location, biomass power generation via
CHP provides a clean and reliable power solution for base-load
service.
Conclusion
CHP and hydrogen generation methods presented in this paper could
provide significant benefits to today's large-scale polysilicon
plants. As polysilicon manufacturing consumes significant quantities
of electrical power, the integration of CHP into such a facility
will reduce the operational cost as well as, greenhouse gas
emissions, while at the same time providing increased power supply
reliability and dependability. To reduce global CO2
emissions and the consumption of fossil fuels, the utilization of
energy efficient technologies such as CHP is becoming more essential
in today's aggressive polysilicon manufacturing. Both polysilicon
for photovoltaic solar power and CHP for heat and power generation
will be playing an increased role in the future global energy
supply.
Whether CHP and hydrogen generation can be integrated with
large-scale polysilicon manufacturing will depend on location, local
electricity cost, power supply reliability, and the availability of
the proper (i.e., clean) fuel feedstock.
The global industry has an opportunity of a "perfect marriage"
between well-proven energy efficient CHP technology and the
production of the key raw material for clean solar power generation.
Acknowledgments
The author would like to express sincere gratitude to his advisor,
Marcel Verschuur (e-mail
Marcel.Verschuur@fluor.com), process director of Fluor in
Haarlem, The Netherlands, for valuable advice and great leadership.
And to the following people: Hans van de Ruit (e-mail
Hans.van.de.Ruit@fluor.com),
technical manager-process technology of Fluor in Haarlem, The
Netherlands for providing helpful comments; and to the members of P4
local advisory committee of Fluor Manila process department through
the leadership of Corazon Almirez (e-mail
Corazon.Almirez@fluor.com),
process manager, for their comments, support, and encouragement.
Biography
Louie De los Santos received his chemical engineering degree at
the Jesuit School in Xavier U., Philippines and is a
process/specialty engineer at Fluor, 3rd Floor Asian
Star Bldg., ASEAN Drive Filinvest, Muntinlupa Metro Manila, 1781
Philippines; e-mail
louie.de.los.santos@fluor.com.