Securing Carbon Dioxide Sources for Commercial-Scale Microalgae Production

This article is a manuscript originally contributed to the book “Optimization of Large-Scale Cultivation and Downstream Processing in Bioreactors,” published by Technical Information Institute Co., Ltd., and is reprinted here with permission.

Author: Iori Nozawa (Chitose Laboratory Corp.)

Introduction

In recent years, global efforts to reduce greenhouse gas emissions—primarily carbon dioxide (CO₂)—have been actively undertaken as a countermeasure against environmental problems such as global warming. Initiatives aimed at CO₂ emission reduction are classified into three main categories:

1. Efforts to reduce CO₂ emission volumes.
2. Efforts to prevent emitted CO₂ from dispersing into the atmosphere.
3. Efforts to reduce and utilize emitted CO₂ as a valuable resource.

Under the current social structure grounded in the utilization of fossil resources, achieving zero CO₂ emissions remains difficult. Consequently, high expectations are placed on initiatives to reduce and resource-recycle emitted CO₂. Representative among these initiatives are efforts that leverage renewable energy and photosynthesis to reduce CO₂ released into the atmosphere, thereby producing valuable commodities such as fuels, plastics, and paints.

CO₂ utilization through biological photosynthesis is broadly divided into two methods: agricultural production and microalgae (hereinafter “algae”) production. Compared with conventional agricultural production, algae production is expected to achieve significantly higher CO₂ emission reduction effects. According to a 2022 report by the Food and Agriculture Organization of the United Nations (FAO), annual production per hectare in the United States for soybeans and corn is reported as 3.3 and 10.8 metric tons (wet basis), respectively.¹ Conversely, a demonstration study on algae production conducted at the University of Florence in Italy using a 1,500 m² flat-panel photobioreactor (FP-PBR) confirmed a productivity equivalent to 36–54 metric tons (dry basis) of algae per hectare annually.²

There is no significant difference in photosynthetic efficiency between terrestrial plants and algae.³ Therefore, this difference in productivity does not stem from mechanics in photosynthesis, but rather from the harvest index of the harvested material in each production method. The harvest index is defined as the ratio of the dry weight of economically useful harvested parts (such as grains, fruits, and tubers) relative to the dry weight of the entire plant (excluding roots). Reported harvest indices for wheat, sugarcane, and rice are 0.40, 0.27, and 0.50, respectively, with most crops falling within the range of 0.30–0.50.⁴,⁵ On the other hand, in the case of algae, because the entire generated biomass can be harvested, the harvest index is 1.0. Because algae production can efficiently convert light energy per unit area into biomass, it exhibits high productivity and can be considered a valuable commodity production method that contributes to CO₂ emission reduction superior to agricultural production. In order to expand the production of valuable goods using algae, scaling up algae production is essential.

Scaling up algae production requires technological development to achieve long-term, stable outdoor production. While algae production is a suitable method for producing valuable goods using CO₂ as a raw material, its production scale remains small. According to the FAO report, global algae production in 2019 remained at approximately 56,000 metric tons (wet basis)⁶, whereas global corn production in the same year reached approximately 1.1 billion metric tons (wet basis), indicating a vast disparity between the two.⁷

To solve this challenge, Chitose Laboratory Corp. is advancing demonstration trials for large-scale algae production. Launched in 2020 upon selection by the New Energy and Industrial Technology Development Organization (NEDO), the project established a production facility (4.56 ha) near Kuching, Sarawak State, Malaysia (latitude approx. 1° N) to conduct production demonstrations of algal biomass under a tropical outdoor environment. Through this trial, a suite of technologies for long-term stable algae production will be established, which is expected to enable commercial production exceeding 2,000 ha. Moving forward, establishing a CO₂ supply system represents a critical technical subject for achieving such scaling up.

CO₂ supply is indispensable for algae production, and securing a stable supply source is essential for commercial-scale production. For instance, consider a facility producing 70 metric tons (dry basis) of algal biomass per hectare annually with a carbon content of 50 wt% (dry basis⁹), assuming all carbon in the algal biomass originates from CO₂ reduced via photosynthesis. Furthermore, assuming a CO₂ reduction efficiency of 50%, operating a 2,000 ha algae production facility for one year requires an estimated CO₂ volume of approximately 5.1 × 10⁵ metric tons.

Thus, commercial-scale algae production requires a stable supply of a large volume of CO₂. Therefore, it is essential to understand the types and available supply capacities of major CO₂ emission sources in each region and construct an appropriate supply system. Taking Sarawak State, Malaysia as an example assuming the expansion of algae production, this section presents prospective CO₂ emission sources.

Major CO₂ emission sources in Sarawak State include coal-fired power plants, cement factories, and metal refineries. Much of the CO₂ emitted from cement factories and metal refineries originates from the combustion of coal used as a heat source. For this reason, coal-fired power plants serve as a representative example of CO₂ emissions associated with such coal combustion. On the other hand, while not currently existing, municipal waste incineration facilities and biomass power plants under evaluation by the Sarawak State Government represent potential future CO₂ emission sources. This section outlines these three target categories: coal-fired power plants, municipal waste incineration facilities, and biomass power plants.

1. Coal-Fired Power Plants

Sarawak State hosts three major coal-fired power plants: Sejingkat (120 MW), Mukah (243 MW), and Balingian (624 MW).¹⁰,¹¹ According to Sarawak Energy Berhad’s (SEB) 2023 report, total power generation from coal-fired power in that year was 3,952 GWh.¹² On the other hand, if these three power plants were operated continuously at rated capacity for one year, the theoretical power generation is calculated at 8,646 GWh, placing the average capacity factor at approximately 45.7%. According to a report by the Japan Organization for Metals and Energy Security (JOGMEC), coal-fired power plants consume 301.6 g (wet basis) of coal per 1 kWh of power generation.¹³ Because the CO₂ and CO concentrations in combustion flue gas are 10–18 vol% and 0.01 vol% respectively, carbon can be regarded as fully converted to CO₂.¹⁴,¹⁵ Assuming thermal coal has a carbon content of approximately 70 wt% (wet basis), CO₂ emissions per 1 kWh are calculated at 774.1 g.¹⁶ Calculating with this value and the average capacity factor of 45.7%, annual CO₂ emissions from each power plant are estimated at 3.7 × 10⁵, 7.5 × 10⁵, and 1.9 × 10⁶ metric tons, respectively. Therefore, it became clear that the annual CO₂ emissions of Mukah and Balingian power plants alone exceed the annual CO₂ volume required by a 2,000 ha algae production facility (approx. 5.1 × 10⁵ metric tons).

Flue gas from coal-fired power plants contains impurities such as SOₓ (mainly SO₂), NOₓ (mainly NO₂), and fly ash (soot and dust), which may adversely affect algae production. SO₂ converts to sulfuric acid in water, and NO₂ converts to nitric acid and nitrous acid, creating a risk of acidifying the culture medium. Indeed, under conditions where flue gas contains 30 mg L⁻¹ SO₂ and 60 mg L⁻¹ NO, the medium pH drops to approximately 4.0, which is reported to cause growth arrest and cell death in Chlorella vulgaris.¹⁷ Furthermore, fly ash contains heavy metals such as cadmium, nickel, lead, chromium, arsenic, and selenium, which also exhibit toxicity toward algae.¹⁸ For example, cadmium is known to inhibit enzymes involved in chlorophyll biosynthesis (protochlorophyllide reductase and δ-aminolevulinic acid dehydratase), lowering chlorophyll synthesis.¹⁹,²⁰ Moreover, cadmium has been reported to increase intracellular reactive oxygen species, causing damage to cell membranes and organelles.²¹ In trials using Scenedesmus obliquus, exposure to cadmium at ≥ 0.5 mg L⁻¹ confirmed a reduction in pigment content and growth arrest.²² Therefore, to utilize flue gas from coal-fired power plants as a CO₂ source in algae production, equipment to remove impurities such as SOₓ, NOₓ, and heavy metals is indispensable.

To remove SOₓ, dedicated desulfurization equipment must be installed. There are several types of desulfurization methods, but the most widely adopted globally is the “Wet Limestone-Gypsum process” (Table 1).²3⁻²⁷ In this method, flue gas exiting the boiler is introduced into an absorption tower after particulate removal and cooling. Inside the absorption tower, limestone slurry (a suspension of calcium carbonate) is sprayed from above, absorbing SOₓ in the gas. Absorbed SOₓ reacts with calcium carbonate to form calcium sulfite, which is further oxidized by blowing air from the bottom of the tower into calcium sulfate dihydrate (hereinafter “gypsum”). Generated gypsum is subjected to solid-liquid separation in a settling tank or filter, completing the desulfurization process. The wet limestone-gypsum process features an extremely high SOₓ removal efficiency of 97.5%–99.7%, outperforming other methods. Furthermore, the gypsum obtained as a byproduct can be effectively utilized in construction materials and other applications.

Table 1: Summary of Representative Desulfurization Methods

To remove NOₓ, dedicated denitration equipment must be installed. Primary methods for directly removing NOₓ from flue gas after combustion include “Selective Catalytic Reduction (SCR)” and “Selective Non-Catalytic Reduction (SNCR)” (Table 2). In the SCR method, ammonia or urea is injected as a reducing agent, reacting over a vanadium pentoxide (V₂O₅)-titanium dioxide (TiO₂) based or zeolite-based catalyst to reduce NOₓ to nitrogen and water. The NOₓ removal efficiency is 80%–95%.²⁸,²⁹ On the other hand, in the SNCR method, ammonia or urea is sprayed directly into high-temperature flue gas (870–1,100°C) without using catalysts to reduce NOₓ. The removal efficiency of this method is 30%–70%, which is lower compared to the SCR method.³⁰,³¹

Table 2: Summary of Representative Denitration Methods

Among methods for removing fly ash from flue gas, the most common is the Electrostatic Precipitator (ESP). In this device, when flue gas passes between discharge and collection electrodes, a high negative voltage is applied to the discharge electrode. This partially ionizes surrounding gas molecules, causing corona discharge. Generated electrons and ions collide with fly ash particles, charging them negatively so that they are attracted to and collected on the positively charged collection plate. This is the operating principle of the ESP. Because performance degrades as fly ash accumulates on the collection plate, a vibrating mechanism called a rapping hammer periodically strikes the collection plate to peel off the fly ash layer. Detached fly ash falls into a lower hopper for collection and is reused as cement raw material or landfill material.³² The particulate removal efficiency of ESP is reported at 80%–99%. In addition, methods such as wet scrubbers, baghouse filters, and cyclone separators are also utilized, as summarized in Table 3.³³⁻³⁵

Table 3: Summary of Representative Particulate Removal Methods

When utilizing coal-fired power plants as a CO₂ source for algae production in Sarawak State, operation deadlines set by government policy pose a challenge. The Federal Government of Malaysia has announced a policy not to build new coal-fired power plants while setting a plan to completely phase out existing facilities by 2044.³⁶ Furthermore, SEB, the sole power utility in the state, has indicated a plan to phase out the Sejingkat coal-fired power plant starting in 2026.³⁷ For this reason, while coal-fired power plants are effective as a short-term CO₂ supply source, long-term utilization is considered difficult. On the other hand, even after coal-fired power plants are decommissioned, coal-fueled facilities such as cement factories and metal refineries will remain within Sarawak State, which could potentially be utilized as CO₂ supply sources similar to coal-fired power plants.

2. Municipal Waste Incineration Facilities

In Sarawak State, waste treatment currently relies overwhelmingly on landfilling. Landfill sites are established in each region, including the Kuching Integrated Waste Management Park in southern Kuching City; however, capacity constraints limit long-term operation. Furthermore, under the influence of a hot and humid climate, environmental problems such as leachate generation, odor, and methane emissions have become prominent. Reflecting this situation, the Sarawak State Government announced plans in 2024 to introduce two high-tech municipal waste incineration facilities, one each in the northern and southern regions of the state.³⁸ Therefore, utilizing CO₂ generated from waste incineration facilities for algae production represents a realistic future option.

Major urban centers in Sarawak State—Kuching, Sibu, and Miri—are located in the southern, central, and northern regions of the state, respectively. Daily per capita waste generation in each city is reported as 0.88, 0.83, and 0.82 kg (wet basis), respectively,³⁹ and annual waste generation based on urban populations (609,205; 248,877; and 248,064) is calculated at 2.0 × 10⁵, 7.5 × 10⁴, and 7.4 × 10⁴ metric tons (wet basis).

Generally, combustible municipal waste has a moisture content of 46.8 wt% (wet basis) and a carbon content of approximately 50 wt% (dry basis).⁴⁰ Estimating with these parameters, annual CO₂ emissions when incinerating waste generated in each city are calculated at 1.9 × 10⁵, 7.4 × 10⁴, and 7.2 × 10⁴ metric tons, respectively (assuming CO₂ and CO concentrations in flue gas are approx. 6–15 vol% and 0.001 vol% respectively, with carbon fully converted to CO₂).41 These results suggest the possibility of operating algae production facilities on scales of approximately 745, 290, and 282 ha near each respective city.

Flue gas from waste incineration facilities contains SOₓ and NOₓ similar to coal-fired power plants. While SOₓ and NOₓ concentrations in coal-fired power plants are 0.03–0.30 vol% and 0.015–0.020 vol% respectively,⁴²⁻⁴⁴ concentrations in waste incineration facilities are reported as 0.03 vol% and 0.04 vol%.⁴⁵ For this reason, while denitration equipment requires performance equivalent to or higher than that of coal-fired power plants, desulfurization equipment could potentially operate with smaller specifications.

Furthermore, flue gas from waste incineration facilities contains dioxins. Dioxins are a general term for over 200 chemical compounds, including polychlorinated dibenzo-p-dioxins and polychlorinated biphenyls. These compounds possess chemical stability and heat resistance, decompose poorly in the environment, and tend to accumulate in biological adipose tissue. Within the author’s search, no direct reports evaluating the effect of dioxins on algae culture were identified. However, due to their chemical structure, their water solubility is extremely low, suggesting their impact on algae culture is limited. For example, the water solubility of 2,3,7,8-TCDD, a representative compound, is extremely low at 12.5–19.3 ng L⁻¹.⁴⁷ While dioxins are primarily generated at 250–450°C during waste incineration, thermal decomposition progresses above 600°C, making them difficult to form.⁴⁸,⁴⁹ In addition, international legal regulations mandate high-temperature incineration, reducing the likelihood of dioxins being contained in flue gas.⁵⁰ Therefore, when utilizing flue gas from waste incineration facilities for algae production, although it is necessary to verify impacts through small-scale pilot trials, dioxins are currently unlikely to present a major impediment.

3. Biomass Power Plants

While large-scale biomass power generation is not currently performed, this section evaluates a prospective scenario where new biomass power plants are constructed in the state. SEB previously constructed a pilot biomass power plant with a rated capacity of 10 MW in the Mukah region. This power plant utilized palm oil mill byproducts—Empty Fruit Bunches (EFB), Palm Kernel Shells (PKS), and Palm Mesocarp Fiber (PMF)—as fuel.⁵¹ Total power generation in Sarawak State reached 33,011 GWh as of 2023; even if this 10 MW plant were operated continuously at rated capacity, annual power generation would remain at 87.6 GWh, representing less than 1% of the state’s total. This indicates that the facility served as a trial implementation. On the other hand, the Sarawak State Government aims to utilize 1.0–1.5 GW of biomass energy by 2030,⁵² suggesting that construction of large-scale power plants will likely progress moving forward. Therefore, in the future, utilizing CO₂ generated from biomass power plants for algae production also represents a realistic choice.

The rated capacity of a biomass power plant required to supply CO₂ to a commercial-scale algae production facility (2,000 ha) can be calculated based on the lower heating value (LHV) of the fuel and power generation efficiency. In Malaysia, palm residues represent the most abundant biomass resource. Among these, PKS is exported to countries such as Japan and South Korea, while PMF is utilized for internal power generation and heat supply within palm oil mills. Therefore, securing a stable supply of these residues within the state is not easy. Thus, this section evaluates a scenario utilizing EFB, which offers relatively higher availability.

Generally, EFB has a moisture content of approximately 50 wt% (wet basis) and an LHV of approximately 10 MJ per 1 kg (wet basis).⁵³ Furthermore, boiler efficiency in palm oil mills (the proportion of input energy effectively used as steam) is approximately 70%, and steam turbine efficiency (the efficiency of converting steam to electricity via turbine) is approximately 20%, yielding an estimated overall power generation efficiency of approximately 14%.⁵⁴ In this context, generating 1 kWh (3.6 MJ) requires 25.7 MJ of thermal input. Because the LHV of 1 kg (wet basis) EFB is approximately 10 MJ, the required EFB volume per 1 kWh is approximately 2.6 kg (wet basis). The carbon content in EFB is approximately 45 wt% (dry basis),⁵⁵ estimating CO₂ emissions per 1 kWh of biomass power generation at approximately 4.3 kg (assuming CO₂ and CO concentrations in flue gas are approx. 4–7 vol% and 0.01 vol% respectively,⁵⁶ with carbon fully converted to CO₂). Assuming a capacity factor of 50% for the biomass power plant (since algae fix CO₂ only during light periods, assuming 12 operating hours per day), supplying the CO₂ volume required by a 2,000 ha algae production facility requires a power plant with a rated capacity of approximately 110 MW.

EFB represents the fibrous portion of fruit bunches remaining after palm oil extraction, forming the most abundant byproduct among palm residues. Generally, processing 1 metric ton (wet basis) of Fresh Fruit Bunches (FFB) generates approximately 0.2 metric tons (wet basis) of EFB. In 2023, annual FFB production in Sarawak State reached 2.3 × 10⁷ metric tons (wet basis), from which annual EFB generation is estimated at approximately 4.6 × 10⁶ metric tons (wet basis).⁵⁷ Under a 110 MW rated capacity and 50% capacity factor, requiring 2.6 kg (wet basis) EFB per 1 kWh, annual operation requires a total EFB volume of approximately 3.3 × 10⁶ metric tons (wet basis). Thus, supplying the CO₂ required for a commercial-scale algae facility (2,000 ha) requires utilizing approximately 72% of all EFB generated within the state as fuel. Because EFB has a high moisture content and low transport efficiency, long-distance transport and bulk utilization in its raw form are impractical. However, applying pretreatment such as drying or pelletization can enhance fuel efficiency and potentially establish EFB as a stable CO₂ supply source.

Flue gas from biomass power plants contains SOₓ and NOₓ similar to coal-fired power plants. While SOₓ and NOₓ concentrations in coal-fired power flue gas are 0.03–0.30 vol% and 0.015–0.020 vol% respectively, concentrations in biomass power plants are reported as 0.005 vol% and 0.01 vol%.⁵⁸ Therefore, denitration equipment requires performance equivalent to that used in coal-fired power plants. Conversely, because SOₓ concentrations in biomass power flue gas are approximately 1/10th those of coal-fired power plants, desulfurization equipment could potentially operate with smaller specifications.

Conclusion

As global CO₂ emission reduction becomes an urgent priority in recent years, initiatives producing valuable commodities using atmospheric CO₂ as a carbon source are drawing significant attention. Biological production methods utilizing photosynthesis for direct CO₂ reduction include agricultural crop production and algae production, with algae production proving superior from a productivity standpoint. On the other hand, current algae production scale remains remarkably small compared to agriculture, making securing a stable CO₂ supply one of the primary bottlenecks toward commercialization.

This section focused on Sarawak State, Malaysia, selecting three representative CO₂ emission sources—coal-fired power plants, municipal waste incineration facilities, and biomass power plants—and summarized their respective characteristics. Sarawak State hosts three major coal-fired power plants, two of which were demonstrated to produce annual emissions exceeding the CO₂ volume required by a 2,000 ha algae production facility. However, coal-fired power generation is one of the highest CO₂ emitting generation methods, and its operation is expected to be progressively restricted and phased out under upcoming government policies. Thus, while effective as a short-term CO₂ source, long-term utilization will likely prove difficult.

On the other hand, calculations based on urban waste generation revealed that no single city’s municipal waste incinerator could individually supply the CO₂ demand of a commercial-scale (2,000 ha) algae facility. Furthermore, in the case of EFB-fueled biomass power plants, supplying the required CO₂ would demand utilizing approximately 72% of all EFB generated in the state, which is realistically challenging.

These results indicate that in order to stably supply the CO₂ required for commercial-scale algae production facilities, constructing an integrated supply system that combines multiple emission sources is essential. Individual CO₂ point sources—coal-fired power plants, municipal waste incinerators, and biomass power plants—must be efficiently integrated according to facility location conditions and operational plans. Realizing this vision requires comprehensively advancing initiatives such as optimizing CO₂ supply systems, developing supply infrastructure, and establishing stable procurement networks for biomass fuels. To address these challenges, Chitose Laboratory Corp. is driving the “Large-Scale Algae Production MATSURI Project” to advance R&D aimed at commercializing and scaling up the algae industry. Moving forward, researchers and relevant organizations interested in algae production technologies and CO₂ utilization are expected to join this project and collaboratively contribute to the development of the algae industry.

This article was published in “Optimization of Large-Scale Cultivation and Downstream Processing in Bioreactors,” Chapter 7: Large-scale Cultivation Technology for Microalgae Using Photobioreactors, Section 3 (Publisher: Technical Information Institute Co., Ltd., published on January 30, 2026). For detailed references, please refer to the main book volume.