Carbon-removal firms have one very big backer. That’s a problem.

The tech giant has the deep pockets to back these expensive removal projects. More companies will need to get on board if the sector is to reach a scale that could eventually lower costs for smaller businesses looking to buy the offsets.

To this end, a nascent industry has formed aimed at helping firms without big budgets to get in on the act. Companies such as Patch, CUR8 and CarbonX are working as a kind of market maker, linking corporate buyers with carbon project developers to get new projects off the ground and scale the market.

CUR8, based in London, creates portfolios of carbon-removal projects for its clients, freeing them from the task of having to perform due diligence on the projects themselves. It counts British Airways, Coca-Cola and Standard Chartered among its customers.

The company sets up multiyear offtake agreements—in which a buyer pays for each ton of carbon removed once it has been verified that a project has removed it—between its clients and developers to finance projects. Once a deal is in place, a developer can say that it has been contracted to produce a certain amount of credits to a lender and gain the financing it needs to get the project off the ground.

Removal credits, for projects that pull carbon dioxide out of the atmosphere, are seen as higher quality than credits for projects that simply reduce emissions by preventing their release, known as avoidance credits. Because of that, they are also much more expensive. While an avoidance credit for a project promoting, say, smokeless stoves can cost less than $20 a ton, a removal credit for the latest direct-air capture technology can cost as much as $1,100 a ton, according to BloombergNEF.

Investment in removal projects should start to lower the cost, according to Kyle Harrison, head of sustainability research at BloombergNEF. “Signing these deals gets economies of scale, developers are going to gain skills and bring down the cost over time," he said.

Marta Krupinska, chief executive of CUR8, says by creating portfolios of projects, companies can invest in a wider range of projects, spreading risk and avoiding just a few types of technologies. It also allows companies to work to a specific budget.

Patch, based in San Francisco, also invests in carbon offsets but includes projects that aren’t limited to just carbon removal, such as forestry protection. “We provide an education on how you should budget for CO2 removal," said Brennan Spellacy, CEO and co-founder of Patch. The platform allows companies to purchase credits on the spot or sign up to multiyear offtake agreements.

Large businesses say buying removal credits represents one of the only ways in which they can hit net-zero goals, especially when considering supply chain emissions, also known as scope 3.

Microsoft is using removal credits to meet its 2050 target of being “carbon negative" for its historical emissions, going back to its founding in 1975.

The tech giant has made more than two thirds of all carbon-removal purchases to date, committing to at least 8 million tons of carbon dioxide removal worldwide. In the process it has funded the launch of many startup developers. Brian Marrs, senior director of carbon removal and energy at Microsoft, says that by buying removals from young companies, innovation in the industry gets a kick-start. “You’re pushing solutions into the market and then pulling them forward into mature companies," he said.

Frontier, a group of buyers that includes Google-parent Alphabet, consulting firm McKinsey and Co. and payments giant Stripe, has committed to purchasing $1 billion of carbon removals by 2030 and has invested about $317 million to date, largely through long-term agreements or advanced market commitments.

Nan Ransohoff, head of climate at Stripe who also leads Frontier, said one of the main issues for startups is working out whether or not there will be demand for their removals. “The value of an advanced market commitment is that it sends a really loud signal to entrepreneurs and investors that there is going to be a market for the technologies that they are building and this is the value of pooling funds."

One company in which both Frontier and Microsoft has invested is Lithos, a startup that is developing a technology known as enhanced rock weathering, where crushed basalt rock is spread over soil to capture carbon dioxide from the air in a reaction between the air, rain and minerals in the rock. Lithos says that the funding it has received has accelerated its development by years.

“Something that’s very challenging in our market is you’re not buying the car, you’re not buying a desk, it’s not physically tangible in the same way and so it’s really hard to define," said Mary Yap, CEO of Lithos. “It allows us to prove to our suppliers like the quarries that we work with, the farmers that we work with, that we are a legitimate group, that we’re not just like a fun upstart two-and-a-half-year-old company with some research."

Write to Yusuf Khan at yusuf.khan@wsj.com